diff --git a/go/base/context.go b/go/base/context.go
index 6e28cb0..5f1876b 100644
--- a/go/base/context.go
+++ b/go/base/context.go
@@ -7,7 +7,9 @@ package base
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"time"
 
@@ -31,9 +33,13 @@ const (
 // MigrationContext has the general, global state of migration. It is used by
 // all components throughout the migration process.
 type MigrationContext struct {
-	DatabaseName                        string
-	OriginalTableName                   string
-	AlterStatement                      string
+	DatabaseName      string
+	OriginalTableName string
+	AlterStatement    string
+
+	Noop          bool
+	TestOnReplica bool
+
 	TableEngine                         string
 	CountTableRows                      bool
 	RowsEstimate                        int64
@@ -43,21 +49,31 @@ type MigrationContext struct {
 	OriginalBinlogRowImage              string
 	AllowedRunningOnMaster              bool
 	InspectorConnectionConfig           *mysql.ConnectionConfig
-	MasterConnectionConfig              *mysql.ConnectionConfig
-	MigrationRangeMinValues             *sql.ColumnValues
-	MigrationRangeMaxValues             *sql.ColumnValues
-	Iteration                           int64
-	MigrationIterationRangeMinValues    *sql.ColumnValues
-	MigrationIterationRangeMaxValues    *sql.ColumnValues
-	UniqueKey                           *sql.UniqueKey
+	ApplierConnectionConfig             *mysql.ConnectionConfig
 	StartTime                           time.Time
 	RowCopyStartTime                    time.Time
 	CurrentLag                          int64
 	MaxLagMillisecondsThrottleThreshold int64
 	ThrottleFlagFile                    string
+	ThrottleAdditionalFlagFile          string
 	TotalRowsCopied                     int64
+	isThrottled                         bool
+	throttleReason                      string
+	throttleMutex                       *sync.Mutex
+	MaxLoad                             map[string]int64
+
+	OriginalTableColumns             *sql.ColumnList
+	OriginalTableUniqueKeys          [](*sql.UniqueKey)
+	GhostTableColumns                *sql.ColumnList
+	GhostTableUniqueKeys             [](*sql.UniqueKey)
+	UniqueKey                        *sql.UniqueKey
+	SharedColumns                    *sql.ColumnList
+	MigrationRangeMinValues          *sql.ColumnValues
+	MigrationRangeMaxValues          *sql.ColumnValues
+	Iteration                        int64
+	MigrationIterationRangeMinValues *sql.ColumnValues
+	MigrationIterationRangeMaxValues *sql.ColumnValues
 
-	IsThrottled      func() bool
 	CanStopStreaming func() bool
 }
 
@@ -71,8 +87,10 @@ func newMigrationContext() *MigrationContext {
 	return &MigrationContext{
 		ChunkSize:                           1000,
 		InspectorConnectionConfig:           mysql.NewConnectionConfig(),
-		MasterConnectionConfig:              mysql.NewConnectionConfig(),
+		ApplierConnectionConfig:             mysql.NewConnectionConfig(),
 		MaxLagMillisecondsThrottleThreshold: 1000,
+		MaxLoad:       make(map[string]int64),
+		throttleMutex: &sync.Mutex{},
 	}
 }
 
@@ -86,6 +104,11 @@ func (this *MigrationContext) GetGhostTableName() string {
 	return fmt.Sprintf("_%s_New", this.OriginalTableName)
 }
 
+// GetOldTableName generates the name of the "old" table, into which the original table is renamed.
+func (this *MigrationContext) GetOldTableName() string {
+	return fmt.Sprintf("_%s_Old", this.OriginalTableName)
+}
+
 // GetChangelogTableName generates the name of changelog table, based on original table name
 func (this *MigrationContext) GetChangelogTableName() string {
 	return fmt.Sprintf("_%s_OSC", this.OriginalTableName)
@@ -96,10 +119,11 @@ func (this *MigrationContext) RequiresBinlogFormatChange() bool {
 	return this.OriginalBinlogFormat != "ROW"
 }
 
-// IsRunningOnMaster is `true` when the app connects directly to the master (typically
-// it should be executed on replica and infer the master)
-func (this *MigrationContext) IsRunningOnMaster() bool {
-	return this.InspectorConnectionConfig.Equals(this.MasterConnectionConfig)
+// InspectorIsAlsoApplier is `true` when the both inspector and applier are the
+// same database instance. This would be true when running directly on master or when
+// testing on replica.
+func (this *MigrationContext) InspectorIsAlsoApplier() bool {
+	return this.InspectorConnectionConfig.Equals(this.ApplierConnectionConfig)
 }
 
 // HasMigrationRange tells us whether there's a range to iterate for copying rows.
@@ -141,3 +165,42 @@ func (this *MigrationContext) ElapsedRowCopyTime() time.Duration {
 func (this *MigrationContext) GetTotalRowsCopied() int64 {
 	return atomic.LoadInt64(&this.TotalRowsCopied)
 }
+
+func (this *MigrationContext) GetIteration() int64 {
+	return atomic.LoadInt64(&this.Iteration)
+}
+
+func (this *MigrationContext) SetThrottled(throttle bool, reason string) {
+	this.throttleMutex.Lock()
+	defer func() { this.throttleMutex.Unlock() }()
+	this.isThrottled = throttle
+	this.throttleReason = reason
+}
+
+func (this *MigrationContext) IsThrottled() (bool, string) {
+	this.throttleMutex.Lock()
+	defer func() { this.throttleMutex.Unlock() }()
+	return this.isThrottled, this.throttleReason
+}
+
+func (this *MigrationContext) ReadMaxLoad(maxLoadList string) error {
+	if maxLoadList == "" {
+		return nil
+	}
+	maxLoadConditions := strings.Split(maxLoadList, ",")
+	for _, maxLoadCondition := range maxLoadConditions {
+		maxLoadTokens := strings.Split(maxLoadCondition, "=")
+		if len(maxLoadTokens) != 2 {
+			return fmt.Errorf("Error parsing max-load condition: %s", maxLoadCondition)
+		}
+		if maxLoadTokens[0] == "" {
+			return fmt.Errorf("Error parsing status variable in max-load condition: %s", maxLoadCondition)
+		}
+		if n, err := strconv.ParseInt(maxLoadTokens[1], 10, 0); err != nil {
+			return fmt.Errorf("Error parsing numeric value in max-load condition: %s", maxLoadCondition)
+		} else {
+			this.MaxLoad[maxLoadTokens[0]] = n
+		}
+	}
+	return nil
+}
diff --git a/go/cmd/gh-osc/main.go b/go/cmd/gh-osc/main.go
index de7f976..09aa337 100644
--- a/go/cmd/gh-osc/main.go
+++ b/go/cmd/gh-osc/main.go
@@ -19,11 +19,6 @@ import (
 func main() {
 	migrationContext := base.GetMigrationContext()
 
-	// mysqlBasedir := flag.String("mysql-basedir", "", "the --basedir config for MySQL (auto-detected if not given)")
-	// mysqlDatadir := flag.String("mysql-datadir", "", "the --datadir config for MySQL (auto-detected if not given)")
-	internalExperiment := flag.Bool("internal-experiment", false, "issue an internal experiment")
-	binlogFile := flag.String("binlog-file", "", "Name of binary log file")
-
 	flag.StringVar(&migrationContext.InspectorConnectionConfig.Key.Hostname, "host", "127.0.0.1", "MySQL hostname (preferably a replica, not the master)")
 	flag.IntVar(&migrationContext.InspectorConnectionConfig.Key.Port, "port", 3306, "MySQL port (preferably a replica, not the master)")
 	flag.StringVar(&migrationContext.InspectorConnectionConfig.User, "user", "root", "MySQL user")
@@ -35,9 +30,20 @@ func main() {
 	flag.BoolVar(&migrationContext.CountTableRows, "exact-rowcount", false, "actually count table rows as opposed to estimate them (results in more accurate progress estimation)")
 	flag.BoolVar(&migrationContext.AllowedRunningOnMaster, "allow-on-master", false, "allow this migration to run directly on master. Preferably it would run on a replica")
 
-	flag.Int64Var(&migrationContext.ChunkSize, "chunk-size", 1000, "amount of rows to handle in each iteration")
-	flag.StringVar(&migrationContext.ThrottleFlagFile, "throttle-flag-file", "", "operation pauses when this file exists")
+	executeFlag := flag.Bool("execute", false, "actually execute the alter & migrate the table. Default is noop: do some tests and exit")
+	flag.BoolVar(&migrationContext.TestOnReplica, "test-on-replica", false, "Have the migration run on a replica, not on the master. At the end of migration tables are not swapped; gh-osc issues `STOP SLAVE` and you can compare the two tables for building trust")
 
+	flag.Int64Var(&migrationContext.ChunkSize, "chunk-size", 1000, "amount of rows to handle in each iteration (allowed range: 100-100,000)")
+	if migrationContext.ChunkSize < 100 {
+		migrationContext.ChunkSize = 100
+	}
+	if migrationContext.ChunkSize > 100000 {
+		migrationContext.ChunkSize = 100000
+	}
+	flag.Int64Var(&migrationContext.MaxLagMillisecondsThrottleThreshold, "max-lag-millis", 1500, "replication lag at which to throttle operation")
+	flag.StringVar(&migrationContext.ThrottleFlagFile, "throttle-flag-file", "", "operation pauses when this file exists; hint: use a file that is specific to the table being altered")
+	flag.StringVar(&migrationContext.ThrottleAdditionalFlagFile, "throttle-additional-flag-file", "/tmp/gh-osc.throttle", "operation pauses when this file exists; hint: keep default, use for throttling multiple gh-osc operations")
+	maxLoad := flag.String("max-load", "", "Comma delimited status-name=threshold. e.g: 'Threads_running=100,Threads_connected=500'")
 	quiet := flag.Bool("quiet", false, "quiet")
 	verbose := flag.Bool("verbose", false, "verbose")
 	debug := flag.Bool("debug", false, "debug mode (very verbose)")
@@ -75,23 +81,16 @@ func main() {
 	if migrationContext.AlterStatement == "" {
 		log.Fatalf("--alter must be provided and statement must not be empty")
 	}
+	migrationContext.Noop = !(*executeFlag)
+	if migrationContext.AllowedRunningOnMaster && migrationContext.TestOnReplica {
+		log.Fatalf("--allow-on-master and --test-on-replica are mutually exclusive")
+	}
+	if err := migrationContext.ReadMaxLoad(*maxLoad); err != nil {
+		log.Fatale(err)
+	}
 
 	log.Info("starting gh-osc")
 
-	if *internalExperiment {
-		log.Debug("starting experiment with %+v", *binlogFile)
-
-		//binlogReader = binlog.NewMySQLBinlogReader(*mysqlBasedir, *mysqlDatadir)
-		// binlogReader, err := binlog.NewGoMySQLReader(migrationContext.InspectorConnectionConfig)
-		// if err != nil {
-		// 	log.Fatale(err)
-		// }
-		// if err := binlogReader.ConnectBinlogStreamer(mysql.BinlogCoordinates{LogFile: *binlogFile, LogPos: 0}); err != nil {
-		// 	log.Fatale(err)
-		// }
-		// binlogReader.StreamEvents(func() bool { return false })
-		// return
-	}
 	migrator := logic.NewMigrator()
 	err := migrator.Migrate()
 	if err != nil {
diff --git a/go/logic/applier.go b/go/logic/applier.go
index b4f63a9..5e7c83f 100644
--- a/go/logic/applier.go
+++ b/go/logic/applier.go
@@ -12,6 +12,7 @@ import (
 	"time"
 
 	"github.com/github/gh-osc/go/base"
+	"github.com/github/gh-osc/go/binlog"
 	"github.com/github/gh-osc/go/mysql"
 	"github.com/github/gh-osc/go/sql"
 
@@ -19,10 +20,6 @@ import (
 	"github.com/outbrain/golib/sqlutils"
 )
 
-const (
-	heartbeatIntervalSeconds = 1
-)
-
 // Applier reads data from the read-MySQL-server (typically a replica, but can be the master)
 // It is used for gaining initial status and structure, and later also follow up on progress and changelog
 type Applier struct {
@@ -33,7 +30,7 @@ type Applier struct {
 
 func NewApplier() *Applier {
 	return &Applier{
-		connectionConfig: base.GetMigrationContext().MasterConnectionConfig,
+		connectionConfig: base.GetMigrationContext().ApplierConnectionConfig,
 		migrationContext: base.GetMigrationContext(),
 	}
 }
@@ -63,7 +60,7 @@ func (this *Applier) validateConnection() error {
 	return nil
 }
 
-// CreateGhostTable creates the ghost table on the master
+// CreateGhostTable creates the ghost table on the applier host
 func (this *Applier) CreateGhostTable() error {
 	query := fmt.Sprintf(`create /* gh-osc */ table %s.%s like %s.%s`,
 		sql.EscapeName(this.migrationContext.DatabaseName),
@@ -82,7 +79,7 @@ func (this *Applier) CreateGhostTable() error {
 	return nil
 }
 
-// CreateGhostTable creates the ghost table on the master
+// AlterGhost applies `alter` statement on ghost table
 func (this *Applier) AlterGhost() error {
 	query := fmt.Sprintf(`alter /* gh-osc */ table %s.%s %s`,
 		sql.EscapeName(this.migrationContext.DatabaseName),
@@ -101,16 +98,16 @@ func (this *Applier) AlterGhost() error {
 	return nil
 }
 
-// CreateChangelogTable creates the changelog table on the master
+// CreateChangelogTable creates the changelog table on the applier host
 func (this *Applier) CreateChangelogTable() error {
 	query := fmt.Sprintf(`create /* gh-osc */ table %s.%s (
-			id int auto_increment,
+			id bigint auto_increment,
 			last_update timestamp not null DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
 			hint varchar(64) charset ascii not null,
-			value varchar(64) charset ascii not null,
+			value varchar(255) charset ascii not null,
 			primary key(id),
 			unique key hint_uidx(hint)
-		) auto_increment=2
+		) auto_increment=256
 		`,
 		sql.EscapeName(this.migrationContext.DatabaseName),
 		sql.EscapeName(this.migrationContext.GetChangelogTableName()),
@@ -126,31 +123,50 @@ func (this *Applier) CreateChangelogTable() error {
 	return nil
 }
 
-// DropChangelogTable drops the changelog table on the master
-func (this *Applier) DropChangelogTable() error {
+// dropTable drops a given table on the applied host
+func (this *Applier) dropTable(tableName string) error {
 	query := fmt.Sprintf(`drop /* gh-osc */ table if exists %s.%s`,
 		sql.EscapeName(this.migrationContext.DatabaseName),
-		sql.EscapeName(this.migrationContext.GetChangelogTableName()),
+		sql.EscapeName(tableName),
 	)
-	log.Infof("Droppping changelog table %s.%s",
+	log.Infof("Droppping table %s.%s",
 		sql.EscapeName(this.migrationContext.DatabaseName),
-		sql.EscapeName(this.migrationContext.GetChangelogTableName()),
+		sql.EscapeName(tableName),
 	)
 	if _, err := sqlutils.ExecNoPrepare(this.db, query); err != nil {
 		return err
 	}
-	log.Infof("Changelog table dropped")
+	log.Infof("Table dropped")
 	return nil
 }
 
+// DropChangelogTable drops the changelog table on the applier host
+func (this *Applier) DropChangelogTable() error {
+	return this.dropTable(this.migrationContext.GetChangelogTableName())
+}
+
+// DropGhostTable drops the ghost table on the applier host
+func (this *Applier) DropGhostTable() error {
+	return this.dropTable(this.migrationContext.GetGhostTableName())
+}
+
 // WriteChangelog writes a value to the changelog table.
 // It returns the hint as given, for convenience
 func (this *Applier) WriteChangelog(hint, value string) (string, error) {
+	explicitId := 0
+	switch hint {
+	case "heartbeat":
+		explicitId = 1
+	case "state":
+		explicitId = 2
+	case "throttle":
+		explicitId = 3
+	}
 	query := fmt.Sprintf(`
 			insert /* gh-osc */ into %s.%s
 				(id, hint, value)
 			values
-				(NULL, ?, ?)
+				(NULLIF(?, 0), ?, ?)
 			on duplicate key update
 				last_update=NOW(),
 				value=VALUES(value)
@@ -158,56 +174,51 @@ func (this *Applier) WriteChangelog(hint, value string) (string, error) {
 		sql.EscapeName(this.migrationContext.DatabaseName),
 		sql.EscapeName(this.migrationContext.GetChangelogTableName()),
 	)
-	_, err := sqlutils.Exec(this.db, query, hint, value)
+	_, err := sqlutils.Exec(this.db, query, explicitId, hint, value)
 	return hint, err
 }
 
+func (this *Applier) WriteAndLogChangelog(hint, value string) (string, error) {
+	this.WriteChangelog(hint, value)
+	return this.WriteChangelog(fmt.Sprintf("%s at %d", hint, time.Now().UnixNano()), value)
+}
+
+func (this *Applier) WriteChangelogState(value string) (string, error) {
+	return this.WriteAndLogChangelog("state", value)
+}
+
 // InitiateHeartbeat creates a heartbeat cycle, writing to the changelog table.
 // This is done asynchronously
-func (this *Applier) InitiateHeartbeat() {
-	go func() {
-		numSuccessiveFailures := 0
-		query := fmt.Sprintf(`
-			insert /* gh-osc */ into %s.%s
-				(id, hint, value)
-			values
-				(1, 'heartbeat', ?)
-			on duplicate key update
-				last_update=NOW(),
-				value=VALUES(value)
-		`,
-			sql.EscapeName(this.migrationContext.DatabaseName),
-			sql.EscapeName(this.migrationContext.GetChangelogTableName()),
-		)
-		injectHeartbeat := func() error {
-			if _, err := sqlutils.ExecNoPrepare(this.db, query, time.Now().Format(time.RFC3339)); err != nil {
-				numSuccessiveFailures++
-				if numSuccessiveFailures > this.migrationContext.MaxRetries() {
-					return log.Errore(err)
-				}
-			} else {
-				numSuccessiveFailures = 0
+func (this *Applier) InitiateHeartbeat(heartbeatIntervalMilliseconds int64) {
+	numSuccessiveFailures := 0
+	injectHeartbeat := func() error {
+		if _, err := this.WriteChangelog("heartbeat", time.Now().Format(time.RFC3339)); err != nil {
+			numSuccessiveFailures++
+			if numSuccessiveFailures > this.migrationContext.MaxRetries() {
+				return log.Errore(err)
 			}
-			return nil
+		} else {
+			numSuccessiveFailures = 0
 		}
-		injectHeartbeat()
+		return nil
+	}
+	injectHeartbeat()
 
-		heartbeatTick := time.Tick(time.Duration(heartbeatIntervalSeconds) * time.Second)
-		for range heartbeatTick {
-			// Generally speaking, we would issue a goroutine, but I'd actually rather
-			// have this blocked rather than spam the master in the event something
-			// goes wrong
-			if err := injectHeartbeat(); err != nil {
-				return
-			}
+	heartbeatTick := time.Tick(time.Duration(heartbeatIntervalMilliseconds) * time.Millisecond)
+	for range heartbeatTick {
+		// Generally speaking, we would issue a goroutine, but I'd actually rather
+		// have this blocked rather than spam the master in the event something
+		// goes wrong
+		if err := injectHeartbeat(); err != nil {
+			return
 		}
-	}()
+	}
 }
 
 // ReadMigrationMinValues
 func (this *Applier) ReadMigrationMinValues(uniqueKey *sql.UniqueKey) error {
 	log.Debugf("Reading migration range according to key: %s", uniqueKey.Name)
-	query, err := sql.BuildUniqueKeyMinValuesPreparedQuery(this.migrationContext.DatabaseName, this.migrationContext.OriginalTableName, uniqueKey.Columns)
+	query, err := sql.BuildUniqueKeyMinValuesPreparedQuery(this.migrationContext.DatabaseName, this.migrationContext.OriginalTableName, uniqueKey.Columns.Names)
 	if err != nil {
 		return err
 	}
@@ -216,7 +227,7 @@ func (this *Applier) ReadMigrationMinValues(uniqueKey *sql.UniqueKey) error {
 		return err
 	}
 	for rows.Next() {
-		this.migrationContext.MigrationRangeMinValues = sql.NewColumnValues(len(uniqueKey.Columns))
+		this.migrationContext.MigrationRangeMinValues = sql.NewColumnValues(uniqueKey.Len())
 		if err = rows.Scan(this.migrationContext.MigrationRangeMinValues.ValuesPointers...); err != nil {
 			return err
 		}
@@ -228,7 +239,7 @@ func (this *Applier) ReadMigrationMinValues(uniqueKey *sql.UniqueKey) error {
 // ReadMigrationMinValues
 func (this *Applier) ReadMigrationMaxValues(uniqueKey *sql.UniqueKey) error {
 	log.Debugf("Reading migration range according to key: %s", uniqueKey.Name)
-	query, err := sql.BuildUniqueKeyMaxValuesPreparedQuery(this.migrationContext.DatabaseName, this.migrationContext.OriginalTableName, uniqueKey.Columns)
+	query, err := sql.BuildUniqueKeyMaxValuesPreparedQuery(this.migrationContext.DatabaseName, this.migrationContext.OriginalTableName, uniqueKey.Columns.Names)
 	if err != nil {
 		return err
 	}
@@ -237,7 +248,7 @@ func (this *Applier) ReadMigrationMaxValues(uniqueKey *sql.UniqueKey) error {
 		return err
 	}
 	for rows.Next() {
-		this.migrationContext.MigrationRangeMaxValues = sql.NewColumnValues(len(uniqueKey.Columns))
+		this.migrationContext.MigrationRangeMaxValues = sql.NewColumnValues(uniqueKey.Len())
 		if err = rows.Scan(this.migrationContext.MigrationRangeMaxValues.ValuesPointers...); err != nil {
 			return err
 		}
@@ -266,12 +277,12 @@ func (this *Applier) __unused_IterationIsComplete() (bool, error) {
 		return false, nil
 	}
 	args := sqlutils.Args()
-	compareWithIterationRangeStart, explodedArgs, err := sql.BuildRangePreparedComparison(this.migrationContext.UniqueKey.Columns, this.migrationContext.MigrationIterationRangeMinValues.AbstractValues(), sql.GreaterThanOrEqualsComparisonSign)
+	compareWithIterationRangeStart, explodedArgs, err := sql.BuildRangePreparedComparison(this.migrationContext.UniqueKey.Columns.Names, this.migrationContext.MigrationIterationRangeMinValues.AbstractValues(), sql.GreaterThanOrEqualsComparisonSign)
 	if err != nil {
 		return false, err
 	}
 	args = append(args, explodedArgs...)
-	compareWithRangeEnd, explodedArgs, err := sql.BuildRangePreparedComparison(this.migrationContext.UniqueKey.Columns, this.migrationContext.MigrationRangeMaxValues.AbstractValues(), sql.LessThanComparisonSign)
+	compareWithRangeEnd, explodedArgs, err := sql.BuildRangePreparedComparison(this.migrationContext.UniqueKey.Columns.Names, this.migrationContext.MigrationRangeMaxValues.AbstractValues(), sql.LessThanComparisonSign)
 	if err != nil {
 		return false, err
 	}
@@ -311,11 +322,11 @@ func (this *Applier) CalculateNextIterationRangeEndValues() (hasFurtherRange boo
 	query, explodedArgs, err := sql.BuildUniqueKeyRangeEndPreparedQuery(
 		this.migrationContext.DatabaseName,
 		this.migrationContext.OriginalTableName,
-		this.migrationContext.UniqueKey.Columns,
+		this.migrationContext.UniqueKey.Columns.Names,
 		this.migrationContext.MigrationIterationRangeMinValues.AbstractValues(),
 		this.migrationContext.MigrationRangeMaxValues.AbstractValues(),
 		this.migrationContext.ChunkSize,
-		fmt.Sprintf("iteration:%d", this.migrationContext.Iteration),
+		fmt.Sprintf("iteration:%d", this.migrationContext.GetIteration()),
 	)
 	if err != nil {
 		return hasFurtherRange, err
@@ -324,7 +335,7 @@ func (this *Applier) CalculateNextIterationRangeEndValues() (hasFurtherRange boo
 	if err != nil {
 		return hasFurtherRange, err
 	}
-	iterationRangeMaxValues := sql.NewColumnValues(len(this.migrationContext.UniqueKey.Columns))
+	iterationRangeMaxValues := sql.NewColumnValues(this.migrationContext.UniqueKey.Len())
 	for rows.Next() {
 		if err = rows.Scan(iterationRangeMaxValues.ValuesPointers...); err != nil {
 			return hasFurtherRange, err
@@ -332,17 +343,10 @@ func (this *Applier) CalculateNextIterationRangeEndValues() (hasFurtherRange boo
 		hasFurtherRange = true
 	}
 	if !hasFurtherRange {
-		log.Debugf("Iteration complete: cannot find iteration end")
+		log.Debugf("Iteration complete: no further range to iterate")
 		return hasFurtherRange, nil
 	}
 	this.migrationContext.MigrationIterationRangeMaxValues = iterationRangeMaxValues
-	log.Debugf(
-		"column values: [%s]..[%s]; iteration: %d; chunk-size: %d",
-		this.migrationContext.MigrationIterationRangeMinValues,
-		this.migrationContext.MigrationIterationRangeMaxValues,
-		this.migrationContext.Iteration,
-		this.migrationContext.ChunkSize,
-	)
 	return hasFurtherRange, nil
 }
 
@@ -354,12 +358,12 @@ func (this *Applier) ApplyIterationInsertQuery() (chunkSize int64, rowsAffected
 		this.migrationContext.DatabaseName,
 		this.migrationContext.OriginalTableName,
 		this.migrationContext.GetGhostTableName(),
-		this.migrationContext.UniqueKey.Columns,
+		this.migrationContext.SharedColumns.Names,
 		this.migrationContext.UniqueKey.Name,
-		this.migrationContext.UniqueKey.Columns,
+		this.migrationContext.UniqueKey.Columns.Names,
 		this.migrationContext.MigrationIterationRangeMinValues.AbstractValues(),
 		this.migrationContext.MigrationIterationRangeMaxValues.AbstractValues(),
-		this.migrationContext.Iteration == 0,
+		this.migrationContext.GetIteration() == 0,
 		this.migrationContext.IsTransactionalTable(),
 	)
 	if err != nil {
@@ -371,21 +375,22 @@ func (this *Applier) ApplyIterationInsertQuery() (chunkSize int64, rowsAffected
 	}
 	rowsAffected, _ = sqlResult.RowsAffected()
 	duration = time.Now().Sub(startTime)
-	this.WriteChangelog(
-		fmt.Sprintf("copy iteration %d", this.migrationContext.Iteration),
-		fmt.Sprintf("chunk: %d; affected: %d; duration: %d", chunkSize, rowsAffected, duration),
-	)
 	log.Debugf(
 		"Issued INSERT on range: [%s]..[%s]; iteration: %d; chunk-size: %d",
 		this.migrationContext.MigrationIterationRangeMinValues,
 		this.migrationContext.MigrationIterationRangeMaxValues,
-		this.migrationContext.Iteration,
+		this.migrationContext.GetIteration(),
 		chunkSize)
 	return chunkSize, rowsAffected, duration, nil
 }
 
 // LockTables
 func (this *Applier) LockTables() error {
+	if this.migrationContext.Noop {
+		log.Debugf("Noop operation; not really locking tables")
+		return nil
+	}
+
 	query := fmt.Sprintf(`lock /* gh-osc */ tables %s.%s write, %s.%s write, %s.%s write`,
 		sql.EscapeName(this.migrationContext.DatabaseName),
 		sql.EscapeName(this.migrationContext.OriginalTableName),
@@ -412,3 +417,44 @@ func (this *Applier) UnlockTables() error {
 	log.Infof("Tables unlocked")
 	return nil
 }
+
+func (this *Applier) ShowStatusVariable(variableName string) (result int64, err error) {
+	query := fmt.Sprintf(`show global status like '%s'`, variableName)
+	if err := this.db.QueryRow(query).Scan(&variableName, &result); err != nil {
+		return 0, err
+	}
+	return result, nil
+}
+
+func (this *Applier) buildDMLEventQuery(dmlEvent *binlog.BinlogDMLEvent) (query string, args []interface{}, err error) {
+	switch dmlEvent.DML {
+	case binlog.DeleteDML:
+		{
+			query, uniqueKeyArgs, err := sql.BuildDMLDeleteQuery(dmlEvent.DatabaseName, this.migrationContext.GetGhostTableName(), this.migrationContext.OriginalTableColumns, &this.migrationContext.UniqueKey.Columns, dmlEvent.WhereColumnValues.AbstractValues())
+			return query, uniqueKeyArgs, err
+		}
+	case binlog.InsertDML:
+		{
+			query, sharedArgs, err := sql.BuildDMLInsertQuery(dmlEvent.DatabaseName, this.migrationContext.GetGhostTableName(), this.migrationContext.OriginalTableColumns, this.migrationContext.SharedColumns, dmlEvent.NewColumnValues.AbstractValues())
+			return query, sharedArgs, err
+		}
+	case binlog.UpdateDML:
+		{
+			query, sharedArgs, uniqueKeyArgs, err := sql.BuildDMLUpdateQuery(dmlEvent.DatabaseName, this.migrationContext.GetGhostTableName(), this.migrationContext.OriginalTableColumns, this.migrationContext.SharedColumns, &this.migrationContext.UniqueKey.Columns, dmlEvent.NewColumnValues.AbstractValues(), dmlEvent.WhereColumnValues.AbstractValues())
+			args = append(args, sharedArgs...)
+			args = append(args, uniqueKeyArgs...)
+			return query, args, err
+		}
+	}
+	return "", args, fmt.Errorf("Unknown dml event type: %+v", dmlEvent.DML)
+}
+
+func (this *Applier) ApplyDMLEventQuery(dmlEvent *binlog.BinlogDMLEvent) error {
+	query, args, err := this.buildDMLEventQuery(dmlEvent)
+	if err != nil {
+		return err
+	}
+	log.Errorf(query)
+	_, err = sqlutils.Exec(this.db, query, args...)
+	return err
+}
diff --git a/go/logic/inspect.go b/go/logic/inspect.go
index 31f74ec..902ccbe 100644
--- a/go/logic/inspect.go
+++ b/go/logic/inspect.go
@@ -44,6 +44,9 @@ func (this *Inspector) InitDBConnections() (err error) {
 	if err := this.validateGrants(); err != nil {
 		return err
 	}
+	if err := this.restartReplication(); err != nil {
+		return err
+	}
 	if err := this.validateBinlogs(); err != nil {
 		return err
 	}
@@ -69,15 +72,54 @@ func (this *Inspector) ValidateOriginalTable() (err error) {
 	return nil
 }
 
-func (this *Inspector) InspectOriginalTable() (uniqueKeys [](*sql.UniqueKey), err error) {
-	uniqueKeys, err = this.getCandidateUniqueKeys(this.migrationContext.OriginalTableName)
+func (this *Inspector) InspectTableColumnsAndUniqueKeys(tableName string) (columns *sql.ColumnList, uniqueKeys [](*sql.UniqueKey), err error) {
+	uniqueKeys, err = this.getCandidateUniqueKeys(tableName)
 	if err != nil {
-		return uniqueKeys, err
+		return columns, uniqueKeys, err
 	}
 	if len(uniqueKeys) == 0 {
-		return uniqueKeys, fmt.Errorf("No PRIMARY nor UNIQUE key found in table! Bailing out")
+		return columns, uniqueKeys, fmt.Errorf("No PRIMARY nor UNIQUE key found in table! Bailing out")
 	}
-	return uniqueKeys, err
+	columns, err = this.getTableColumns(this.migrationContext.DatabaseName, tableName)
+	if err != nil {
+		return columns, uniqueKeys, err
+	}
+
+	return columns, uniqueKeys, nil
+}
+
+func (this *Inspector) InspectOriginalTable() (err error) {
+	this.migrationContext.OriginalTableColumns, this.migrationContext.OriginalTableUniqueKeys, err = this.InspectTableColumnsAndUniqueKeys(this.migrationContext.OriginalTableName)
+	if err == nil {
+		return err
+	}
+	return nil
+}
+
+func (this *Inspector) InspectOriginalAndGhostTables() (err error) {
+	this.migrationContext.GhostTableColumns, this.migrationContext.GhostTableUniqueKeys, err = this.InspectTableColumnsAndUniqueKeys(this.migrationContext.GetGhostTableName())
+	if err != nil {
+		return err
+	}
+	sharedUniqueKeys, err := this.getSharedUniqueKeys(this.migrationContext.OriginalTableUniqueKeys, this.migrationContext.GhostTableUniqueKeys)
+	if err != nil {
+		return err
+	}
+	if len(sharedUniqueKeys) == 0 {
+		return fmt.Errorf("No shared unique key can be found after ALTER! Bailing out")
+	}
+	this.migrationContext.UniqueKey = sharedUniqueKeys[0]
+	log.Infof("Chosen shared unique key is %s", this.migrationContext.UniqueKey.Name)
+	if !this.migrationContext.UniqueKey.IsPrimary() {
+		if this.migrationContext.OriginalBinlogRowImage != "full" {
+			return fmt.Errorf("binlog_row_image is '%s' and chosen key is %s, which is not the primary key. This operation cannot proceed. You may `set global binlog_row_image='full'` and try again")
+		}
+	}
+
+	this.migrationContext.SharedColumns = this.getSharedColumns(this.migrationContext.OriginalTableColumns, this.migrationContext.GhostTableColumns)
+	log.Infof("Shared columns are %s", this.migrationContext.SharedColumns)
+	// By fact that a non-empty unique key exists we also know the shared columns are non-empty
+	return nil
 }
 
 // validateConnection issues a simple can-connect to MySQL
@@ -136,6 +178,32 @@ func (this *Inspector) validateGrants() error {
 	return log.Errorf("User has insufficient privileges for migration.")
 }
 
+// restartReplication is required so that we are _certain_ the binlog format and
+// row image settings have actually been applied to the replication thread.
+// It is entriely possible, for example, that the replication is using 'STATEMENT'
+// binlog format even as the variable says 'ROW'
+func (this *Inspector) restartReplication() error {
+	log.Infof("Restarting replication on %s:%d to make sure binlog settings apply to replication thread", this.connectionConfig.Key.Hostname, this.connectionConfig.Key.Port)
+
+	masterKey, _ := getMasterKeyFromSlaveStatus(this.connectionConfig)
+	if masterKey == nil {
+		// This is not a replica
+		return nil
+	}
+
+	var stopError, startError error
+	_, stopError = sqlutils.ExecNoPrepare(this.db, `stop slave`)
+	_, startError = sqlutils.ExecNoPrepare(this.db, `start slave`)
+	if stopError != nil {
+		return stopError
+	}
+	if startError != nil {
+		return startError
+	}
+	log.Debugf("Replication restarted")
+	return nil
+}
+
 // validateBinlogs checks that binary log configuration is good to go
 func (this *Inspector) validateBinlogs() error {
 	query := `select @@global.log_bin, @@global.log_slave_updates, @@global.binlog_format`
@@ -264,27 +332,28 @@ func (this *Inspector) countTableRows() error {
 	return nil
 }
 
-func (this *Inspector) getTableColumns(databaseName, tableName string) (columns sql.ColumnList, err error) {
+func (this *Inspector) getTableColumns(databaseName, tableName string) (*sql.ColumnList, error) {
 	query := fmt.Sprintf(`
 		show columns from %s.%s
 		`,
 		sql.EscapeName(databaseName),
 		sql.EscapeName(tableName),
 	)
-	err = sqlutils.QueryRowsMap(this.db, query, func(rowMap sqlutils.RowMap) error {
-		columns = append(columns, rowMap.GetString("Field"))
+	columnNames := []string{}
+	err := sqlutils.QueryRowsMap(this.db, query, func(rowMap sqlutils.RowMap) error {
+		columnNames = append(columnNames, rowMap.GetString("Field"))
 		return nil
 	})
 	if err != nil {
-		return columns, err
+		return nil, err
 	}
-	if len(columns) == 0 {
-		return columns, log.Errorf("Found 0 columns on %s.%s. Bailing out",
+	if len(columnNames) == 0 {
+		return nil, log.Errorf("Found 0 columns on %s.%s. Bailing out",
 			sql.EscapeName(databaseName),
 			sql.EscapeName(tableName),
 		)
 	}
-	return columns, nil
+	return sql.NewColumnList(columnNames), nil
 }
 
 // getCandidateUniqueKeys investigates a table and returns the list of unique keys
@@ -361,17 +430,9 @@ func (this *Inspector) getCandidateUniqueKeys(tableName string) (uniqueKeys [](*
 	return uniqueKeys, nil
 }
 
-// getCandidateUniqueKeys investigates a table and returns the list of unique keys
-// candidate for chunking
-func (this *Inspector) getSharedUniqueKeys() (uniqueKeys [](*sql.UniqueKey), err error) {
-	originalUniqueKeys, err := this.getCandidateUniqueKeys(this.migrationContext.OriginalTableName)
-	if err != nil {
-		return uniqueKeys, err
-	}
-	ghostUniqueKeys, err := this.getCandidateUniqueKeys(this.migrationContext.GetGhostTableName())
-	if err != nil {
-		return uniqueKeys, err
-	}
+// getSharedUniqueKeys returns the intersection of two given unique keys,
+// testing by list of columns
+func (this *Inspector) getSharedUniqueKeys(originalUniqueKeys, ghostUniqueKeys [](*sql.UniqueKey)) (uniqueKeys [](*sql.UniqueKey), err error) {
 	// We actually do NOT rely on key name, just on the set of columns. This is because maybe
 	// the ALTER is on the name itself...
 	for _, originalUniqueKey := range originalUniqueKeys {
@@ -384,43 +445,77 @@ func (this *Inspector) getSharedUniqueKeys() (uniqueKeys [](*sql.UniqueKey), err
 	return uniqueKeys, nil
 }
 
-func (this *Inspector) getMasterConnectionConfig() (masterConfig *mysql.ConnectionConfig, err error) {
-	visitedKeys := mysql.NewInstanceKeyMap()
-	return getMasterConnectionConfigSafe(this.connectionConfig, this.migrationContext.DatabaseName, visitedKeys)
+// getSharedColumns returns the intersection of two lists of columns in same order as the first list
+func (this *Inspector) getSharedColumns(originalColumns, ghostColumns *sql.ColumnList) *sql.ColumnList {
+	columnsInGhost := make(map[string]bool)
+	for _, ghostColumn := range ghostColumns.Names {
+		columnsInGhost[ghostColumn] = true
+	}
+	sharedColumnNames := []string{}
+	for _, originalColumn := range originalColumns.Names {
+		if columnsInGhost[originalColumn] {
+			sharedColumnNames = append(sharedColumnNames, originalColumn)
+		}
+	}
+	return sql.NewColumnList(sharedColumnNames)
 }
 
-func getMasterConnectionConfigSafe(connectionConfig *mysql.ConnectionConfig, databaseName string, visitedKeys *mysql.InstanceKeyMap) (masterConfig *mysql.ConnectionConfig, err error) {
-	log.Debugf("Looking for master on %+v", connectionConfig.Key)
+func (this *Inspector) readChangelogState() (map[string]string, error) {
+	query := fmt.Sprintf(`
+		select hint, value from %s.%s where id <= 255
+		`,
+		sql.EscapeName(this.migrationContext.DatabaseName),
+		sql.EscapeName(this.migrationContext.GetChangelogTableName()),
+	)
+	result := make(map[string]string)
+	err := sqlutils.QueryRowsMap(this.db, query, func(m sqlutils.RowMap) error {
+		result[m.GetString("hint")] = m.GetString("value")
+		return nil
+	})
+	return result, err
+}
 
-	currentUri := connectionConfig.GetDBUri(databaseName)
+func (this *Inspector) getMasterConnectionConfig() (applierConfig *mysql.ConnectionConfig, err error) {
+	visitedKeys := mysql.NewInstanceKeyMap()
+	return getMasterConnectionConfigSafe(this.connectionConfig, visitedKeys)
+}
+
+func getMasterKeyFromSlaveStatus(connectionConfig *mysql.ConnectionConfig) (masterKey *mysql.InstanceKey, err error) {
+	currentUri := connectionConfig.GetDBUri("information_schema")
 	db, _, err := sqlutils.GetDB(currentUri)
 	if err != nil {
 		return nil, err
 	}
-
-	hasMaster := false
-	masterConfig = connectionConfig.Duplicate()
 	err = sqlutils.QueryRowsMap(db, `show slave status`, func(rowMap sqlutils.RowMap) error {
-		masterKey := mysql.InstanceKey{
+		masterKey = &mysql.InstanceKey{
 			Hostname: rowMap.GetString("Master_Host"),
 			Port:     rowMap.GetInt("Master_Port"),
 		}
-		if masterKey.IsValid() {
-			masterConfig.Key = masterKey
-			hasMaster = true
-		}
 		return nil
 	})
+	return masterKey, err
+}
+
+func getMasterConnectionConfigSafe(connectionConfig *mysql.ConnectionConfig, visitedKeys *mysql.InstanceKeyMap) (masterConfig *mysql.ConnectionConfig, err error) {
+	log.Debugf("Looking for master on %+v", connectionConfig.Key)
+
+	masterKey, err := getMasterKeyFromSlaveStatus(connectionConfig)
 	if err != nil {
 		return nil, err
 	}
-	if hasMaster {
-		log.Debugf("Master of %+v is %+v", connectionConfig.Key, masterConfig.Key)
-		if visitedKeys.HasKey(masterConfig.Key) {
-			return nil, fmt.Errorf("There seems to be a master-master setup at %+v. This is unsupported. Bailing out", masterConfig.Key)
-		}
-		visitedKeys.AddKey(masterConfig.Key)
-		return getMasterConnectionConfigSafe(masterConfig, databaseName, visitedKeys)
+	if masterKey == nil {
+		return connectionConfig, nil
 	}
-	return masterConfig, nil
+	if !masterKey.IsValid() {
+		return connectionConfig, nil
+	}
+	masterConfig = connectionConfig.Duplicate()
+	masterConfig.Key = *masterKey
+
+	log.Debugf("Master of %+v is %+v", connectionConfig.Key, masterConfig.Key)
+	if visitedKeys.HasKey(masterConfig.Key) {
+		return nil, fmt.Errorf("There seems to be a master-master setup at %+v. This is unsupported. Bailing out", masterConfig.Key)
+	}
+	visitedKeys.AddKey(masterConfig.Key)
+	return getMasterConnectionConfigSafe(masterConfig, visitedKeys)
 }
diff --git a/go/logic/migrator.go b/go/logic/migrator.go
index 451cb96..62fc6e3 100644
--- a/go/logic/migrator.go
+++ b/go/logic/migrator.go
@@ -8,7 +8,10 @@ package logic
 import (
 	"fmt"
 	"os"
+	"os/signal"
+	"regexp"
 	"sync/atomic"
+	"syscall"
 	"time"
 
 	"github.com/github/gh-osc/go/base"
@@ -27,7 +30,12 @@ const (
 type tableWriteFunc func() error
 
 const (
-	applyEventsQueueBuffer = 100
+	applyEventsQueueBuffer        = 100
+	heartbeatIntervalMilliseconds = 1000
+)
+
+var (
+	prettifyDurationRegexp = regexp.MustCompile("([.][0-9]+)")
 )
 
 // Migrator is the main schema migration flow manager.
@@ -45,6 +53,8 @@ type Migrator struct {
 	//  excessive work happens at the end of the iteration as new copy-jobs arrive befroe realizing the copy is complete
 	copyRowsQueue    chan tableWriteFunc
 	applyEventsQueue chan tableWriteFunc
+
+	handledChangelogStates map[string]bool
 }
 
 func NewMigrator() *Migrator {
@@ -54,40 +64,138 @@ func NewMigrator() *Migrator {
 		rowCopyComplete:            make(chan bool),
 		allEventsUpToLockProcessed: make(chan bool),
 
-		copyRowsQueue:    make(chan tableWriteFunc),
-		applyEventsQueue: make(chan tableWriteFunc, applyEventsQueueBuffer),
-	}
-	migrator.migrationContext.IsThrottled = func() bool {
-		return migrator.shouldThrottle()
+		copyRowsQueue:          make(chan tableWriteFunc),
+		applyEventsQueue:       make(chan tableWriteFunc, applyEventsQueueBuffer),
+		handledChangelogStates: make(map[string]bool),
 	}
 	return migrator
 }
 
-func (this *Migrator) shouldThrottle() bool {
+func prettifyDurationOutput(d time.Duration) string {
+	if d < time.Second {
+		return "0s"
+	}
+	result := fmt.Sprintf("%s", d)
+	result = prettifyDurationRegexp.ReplaceAllString(result, "")
+	return result
+}
+
+// acceptSignals registers for OS signals
+func (this *Migrator) acceptSignals() {
+	c := make(chan os.Signal, 1)
+
+	signal.Notify(c, syscall.SIGHUP)
+	go func() {
+		for sig := range c {
+			switch sig {
+			case syscall.SIGHUP:
+				log.Debugf("Received SIGHUP. Reloading configuration")
+			}
+		}
+	}()
+}
+
+func (this *Migrator) shouldThrottle() (result bool, reason string) {
 	lag := atomic.LoadInt64(&this.migrationContext.CurrentLag)
 
-	shouldThrottle := false
 	if time.Duration(lag) > time.Duration(this.migrationContext.MaxLagMillisecondsThrottleThreshold)*time.Millisecond {
-		shouldThrottle = true
-	} else if this.migrationContext.ThrottleFlagFile != "" {
+		return true, fmt.Sprintf("lag=%fs", time.Duration(lag).Seconds())
+	}
+	if this.migrationContext.ThrottleFlagFile != "" {
 		if _, err := os.Stat(this.migrationContext.ThrottleFlagFile); err == nil {
-			//Throttle file defined and exists!
-			shouldThrottle = true
+			// Throttle file defined and exists!
+			return true, "flag-file"
 		}
 	}
-	return shouldThrottle
+	if this.migrationContext.ThrottleAdditionalFlagFile != "" {
+		if _, err := os.Stat(this.migrationContext.ThrottleAdditionalFlagFile); err == nil {
+			// 2nd Throttle file defined and exists!
+			return true, "flag-file"
+		}
+	}
+
+	for variableName, threshold := range this.migrationContext.MaxLoad {
+		value, err := this.applier.ShowStatusVariable(variableName)
+		if err != nil {
+			return true, fmt.Sprintf("%s %s", variableName, err)
+		}
+		if value > threshold {
+			return true, fmt.Sprintf("%s=%d", variableName, value)
+		}
+	}
+
+	return false, ""
+}
+
+func (this *Migrator) initiateThrottler() error {
+	throttlerTick := time.Tick(1 * time.Second)
+
+	throttlerFunction := func() {
+		alreadyThrottling, currentReason := this.migrationContext.IsThrottled()
+		shouldThrottle, throttleReason := this.shouldThrottle()
+		if shouldThrottle && !alreadyThrottling {
+			// New throttling
+			this.applier.WriteAndLogChangelog("throttle", throttleReason)
+		} else if shouldThrottle && alreadyThrottling && (currentReason != throttleReason) {
+			// Change of reason
+			this.applier.WriteAndLogChangelog("throttle", throttleReason)
+		} else if alreadyThrottling && !shouldThrottle {
+			// End of throttling
+			this.applier.WriteAndLogChangelog("throttle", "done throttling")
+		}
+		this.migrationContext.SetThrottled(shouldThrottle, throttleReason)
+	}
+	throttlerFunction()
+	for range throttlerTick {
+		throttlerFunction()
+	}
+
+	return nil
+}
+
+// throttle initiates a throttling event, if need be, updates the Context and
+// calls callback functions, if any
+func (this *Migrator) throttle(onThrottled func()) {
+	for {
+		if shouldThrottle, _ := this.migrationContext.IsThrottled(); !shouldThrottle {
+			return
+		}
+		if onThrottled != nil {
+			onThrottled()
+		}
+		time.Sleep(time.Second)
+	}
+}
+
+// retryOperation attempts up to `count` attempts at running given function,
+// exiting as soon as it returns with non-error.
+func (this *Migrator) retryOperation(operation func() error) (err error) {
+	maxRetries := this.migrationContext.MaxRetries()
+	for i := 0; i < maxRetries; i++ {
+		if i != 0 {
+			// sleep after previous iteration
+			time.Sleep(1 * time.Second)
+		}
+		err = operation()
+		if err == nil {
+			return nil
+		}
+		// there's an error. Let's try again.
+	}
+	return err
 }
 
 func (this *Migrator) canStopStreaming() bool {
 	return false
 }
 
-func (this *Migrator) onChangelogStateEvent(dmlEvent *binlog.BinlogDMLEvent) (err error) {
-	// Hey, I created the changlog table, I know the type of columns it has!
-	if hint := dmlEvent.NewColumnValues.StringColumn(2); hint != "state" {
+func (this *Migrator) onChangelogState(stateValue string) (err error) {
+	if this.handledChangelogStates[stateValue] {
 		return
 	}
-	changelogState := ChangelogState(dmlEvent.NewColumnValues.StringColumn(3))
+	this.handledChangelogStates[stateValue] = true
+
+	changelogState := ChangelogState(stateValue)
 	switch changelogState {
 	case TablesInPlace:
 		{
@@ -102,16 +210,12 @@ func (this *Migrator) onChangelogStateEvent(dmlEvent *binlog.BinlogDMLEvent) (er
 			return fmt.Errorf("Unknown changelog state: %+v", changelogState)
 		}
 	}
-	log.Debugf("---- - - - - - state %+v", changelogState)
+	log.Debugf("Received state %+v", changelogState)
 	return nil
 }
 
-func (this *Migrator) onChangelogHeartbeatEvent(dmlEvent *binlog.BinlogDMLEvent) (err error) {
-	if hint := dmlEvent.NewColumnValues.StringColumn(2); hint != "heartbeat" {
-		return nil
-	}
-	value := dmlEvent.NewColumnValues.StringColumn(3)
-	heartbeatTime, err := time.Parse(time.RFC3339, value)
+func (this *Migrator) onChangelogHeartbeat(heartbeatValue string) (err error) {
+	heartbeatTime, err := time.Parse(time.RFC3339, heartbeatValue)
 	if err != nil {
 		return log.Errore(err)
 	}
@@ -132,18 +236,29 @@ func (this *Migrator) Migrate() (err error) {
 	if err := this.inspector.ValidateOriginalTable(); err != nil {
 		return err
 	}
-	uniqueKeys, err := this.inspector.InspectOriginalTable()
-	if err != nil {
+	if err := this.inspector.InspectOriginalTable(); err != nil {
 		return err
 	}
 	// So far so good, table is accessible and valid.
-	if this.migrationContext.MasterConnectionConfig, err = this.inspector.getMasterConnectionConfig(); err != nil {
+	// Let's get master connection config
+	if this.migrationContext.ApplierConnectionConfig, err = this.inspector.getMasterConnectionConfig(); err != nil {
 		return err
 	}
-	if this.migrationContext.IsRunningOnMaster() && !this.migrationContext.AllowedRunningOnMaster {
+	if this.migrationContext.TestOnReplica {
+		if this.migrationContext.InspectorIsAlsoApplier() {
+			return fmt.Errorf("Instructed to --test-on-replica, but the server we connect to doesn't seem to be a replica")
+		}
+		log.Infof("--test-on-replica given. Will not execute on master %+v but rather on replica %+v itself",
+			this.migrationContext.ApplierConnectionConfig.Key, this.migrationContext.InspectorConnectionConfig.Key,
+		)
+		this.migrationContext.ApplierConnectionConfig = this.migrationContext.InspectorConnectionConfig.Duplicate()
+	} else if this.migrationContext.InspectorIsAlsoApplier() && !this.migrationContext.AllowedRunningOnMaster {
 		return fmt.Errorf("It seems like this migration attempt to run directly on master. Preferably it would be executed on a replica (and this reduces load from the master). To proceed please provide --allow-on-master")
 	}
-	log.Infof("Master found to be %+v", this.migrationContext.MasterConnectionConfig.Key)
+
+	log.Infof("Master found to be %+v", this.migrationContext.ApplierConnectionConfig.Key)
+
+	go this.initiateChangelogListener()
 
 	if err := this.initiateStreaming(); err != nil {
 		return err
@@ -159,33 +274,30 @@ func (this *Migrator) Migrate() (err error) {
 	// When running on replica, this means the replica has those tables. When running
 	// on master this is always true, of course, and yet it also implies this knowledge
 	// is in the binlogs.
+	if err := this.inspector.InspectOriginalAndGhostTables(); err != nil {
+		return err
+	}
 
-	this.migrationContext.UniqueKey = uniqueKeys[0] // TODO. Need to wait on replica till the ghost table exists and get shared keys
 	if err := this.applier.ReadMigrationRangeValues(); err != nil {
 		return err
 	}
-	go this.initiateStatus()
+	go this.initiateThrottler()
 	go this.executeWriteFuncs()
 	go this.iterateChunks()
+	this.migrationContext.RowCopyStartTime = time.Now()
+	go this.initiateStatus()
 
 	log.Debugf("Operating until row copy is complete")
 	<-this.rowCopyComplete
 	log.Debugf("Row copy complete")
 	this.printStatus()
 
-	throttleMigration(
-		this.migrationContext,
-		func() {
-			log.Debugf("throttling before LOCK TABLES")
-		},
-		nil,
-		func() {
-			log.Debugf("done throttling")
-		},
-	)
+	this.throttle(func() {
+		log.Debugf("throttling on LOCK TABLES")
+	})
 	// TODO retries!!
 	this.applier.LockTables()
-	this.applier.WriteChangelog("state", string(AllEventsUpToLockProcessed))
+	this.applier.WriteChangelogState(string(AllEventsUpToLockProcessed))
 	log.Debugf("Waiting for events up to lock")
 	<-this.allEventsUpToLockProcessed
 	log.Debugf("Done waiting for events up to lock")
@@ -228,34 +340,71 @@ func (this *Migrator) printStatus() {
 		return
 	}
 
-	status := fmt.Sprintf("Copy: %d/%d %.1f%% Backlog: %d/%d Elapsed: %+v(copy), %+v(total) ETA: N/A",
+	eta := "N/A"
+	if isThrottled, throttleReason := this.migrationContext.IsThrottled(); isThrottled {
+		eta = fmt.Sprintf("throttled, %s", throttleReason)
+	}
+	status := fmt.Sprintf("Copy: %d/%d %.1f%%; Backlog: %d/%d; Elapsed: %+v(copy), %+v(total); ETA: %s",
 		totalRowsCopied, rowsEstimate, progressPct,
 		len(this.applyEventsQueue), cap(this.applyEventsQueue),
-		this.migrationContext.ElapsedRowCopyTime(), elapsedTime)
+		prettifyDurationOutput(this.migrationContext.ElapsedRowCopyTime()), prettifyDurationOutput(elapsedTime),
+		eta,
+	)
+	this.applier.WriteChangelog(
+		fmt.Sprintf("copy iteration %d at %d", this.migrationContext.GetIteration(), time.Now().Unix()),
+		status,
+	)
 	fmt.Println(status)
 }
 
+func (this *Migrator) initiateChangelogListener() {
+	ticker := time.Tick((heartbeatIntervalMilliseconds * time.Millisecond) / 2)
+	for range ticker {
+		go func() error {
+			changelogState, err := this.inspector.readChangelogState()
+			if err != nil {
+				return log.Errore(err)
+			}
+			for hint, value := range changelogState {
+				switch hint {
+				case "state":
+					{
+						this.onChangelogState(value)
+					}
+				case "heartbeat":
+					{
+						this.onChangelogHeartbeat(value)
+					}
+				}
+			}
+			return nil
+		}()
+	}
+}
+
+// initiateStreaming begins treaming of binary log events and registers listeners for such events
 func (this *Migrator) initiateStreaming() error {
 	this.eventsStreamer = NewEventsStreamer()
 	if err := this.eventsStreamer.InitDBConnections(); err != nil {
 		return err
 	}
+	if this.migrationContext.Noop {
+		log.Debugf("Noop operation; not really listening on binlog events")
+		return nil
+	}
 	this.eventsStreamer.AddListener(
-		false,
+		true,
 		this.migrationContext.DatabaseName,
-		this.migrationContext.GetChangelogTableName(),
+		this.migrationContext.OriginalTableName,
 		func(dmlEvent *binlog.BinlogDMLEvent) error {
-			return this.onChangelogStateEvent(dmlEvent)
-		},
-	)
-	this.eventsStreamer.AddListener(
-		false,
-		this.migrationContext.DatabaseName,
-		this.migrationContext.GetChangelogTableName(),
-		func(dmlEvent *binlog.BinlogDMLEvent) error {
-			return this.onChangelogHeartbeatEvent(dmlEvent)
+			applyEventFunc := func() error {
+				return this.applier.ApplyDMLEventQuery(dmlEvent)
+			}
+			this.applyEventsQueue <- applyEventFunc
+			return nil
 		},
 	)
+
 	go func() {
 		log.Debugf("Beginning streaming")
 		this.eventsStreamer.StreamEvents(func() bool { return this.canStopStreaming() })
@@ -281,17 +430,24 @@ func (this *Migrator) initiateApplier() error {
 		return err
 	}
 
-	this.applier.WriteChangelog("state", string(TablesInPlace))
-	this.applier.InitiateHeartbeat()
+	this.applier.WriteChangelogState(string(TablesInPlace))
+	go this.applier.InitiateHeartbeat(heartbeatIntervalMilliseconds)
 	return nil
 }
 
 func (this *Migrator) iterateChunks() error {
-	this.migrationContext.RowCopyStartTime = time.Now()
 	terminateRowIteration := func(err error) error {
 		this.rowCopyComplete <- true
 		return log.Errore(err)
 	}
+	if this.migrationContext.Noop {
+		log.Debugf("Noop operation; not really copying data")
+		return terminateRowIteration(nil)
+	}
+	if this.migrationContext.MigrationRangeMinValues == nil {
+		log.Debugf("No rows found in table. Rowcopy will be implicitly empty")
+		return terminateRowIteration(nil)
+	}
 	for {
 		copyRowsFunc := func() error {
 			hasFurtherRange, err := this.applier.CalculateNextIterationRangeEndValues()
@@ -306,7 +462,7 @@ func (this *Migrator) iterateChunks() error {
 				return terminateRowIteration(err)
 			}
 			atomic.AddInt64(&this.migrationContext.TotalRowsCopied, rowsAffected)
-			this.migrationContext.Iteration++
+			atomic.AddInt64(&this.migrationContext.Iteration, 1)
 			return nil
 		}
 		this.copyRowsQueue <- copyRowsFunc
@@ -315,30 +471,29 @@ func (this *Migrator) iterateChunks() error {
 }
 
 func (this *Migrator) executeWriteFuncs() error {
+	if this.migrationContext.Noop {
+		log.Debugf("Noop operation; not really doing writes")
+		return nil
+	}
 	for {
-		throttleMigration(
-			this.migrationContext,
-			func() {
-				log.Debugf("throttling writes")
-			},
-			nil,
-			func() {
-				log.Debugf("done throttling writes")
-			},
-		)
+		this.throttle(nil)
 		// We give higher priority to event processing, then secondary priority to
 		// rowcopy
 		select {
 		case applyEventFunc := <-this.applyEventsQueue:
 			{
-				retryOperation(applyEventFunc, this.migrationContext.MaxRetries())
+				if err := this.retryOperation(applyEventFunc); err != nil {
+					return log.Errore(err)
+				}
 			}
 		default:
 			{
 				select {
 				case copyRowsFunc := <-this.copyRowsQueue:
 					{
-						retryOperation(copyRowsFunc, this.migrationContext.MaxRetries())
+						if err := this.retryOperation(copyRowsFunc); err != nil {
+							return log.Errore(err)
+						}
 					}
 				default:
 					{
diff --git a/go/sql/builder.go b/go/sql/builder.go
index b8f0358..4b73a5a 100644
--- a/go/sql/builder.go
+++ b/go/sql/builder.go
@@ -32,6 +32,20 @@ func EscapeName(name string) string {
 	return fmt.Sprintf("`%s`", name)
 }
 
+func buildPreparedValues(length int) []string {
+	values := make([]string, length, length)
+	for i := 0; i < length; i++ {
+		values[i] = "?"
+	}
+	return values
+}
+
+func duplicateNames(names []string) []string {
+	duplicate := make([]string, len(names), len(names))
+	copy(duplicate, names)
+	return duplicate
+}
+
 func BuildValueComparison(column string, value string, comparisonSign ValueComparisonSign) (result string, err error) {
 	if column == "" {
 		return "", fmt.Errorf("Empty column in GetValueComparison")
@@ -64,6 +78,22 @@ func BuildEqualsComparison(columns []string, values []string) (result string, er
 	return result, nil
 }
 
+func BuildEqualsPreparedComparison(columns []string) (result string, err error) {
+	values := buildPreparedValues(len(columns))
+	return BuildEqualsComparison(columns, values)
+}
+
+func BuildSetPreparedClause(columns []string) (result string, err error) {
+	if len(columns) == 0 {
+		return "", fmt.Errorf("Got 0 columns in BuildSetPreparedClause")
+	}
+	setTokens := []string{}
+	for _, column := range columns {
+		setTokens = append(setTokens, fmt.Sprintf("%s=?", EscapeName(column)))
+	}
+	return strings.Join(setTokens, ", "), nil
+}
+
 func BuildRangeComparison(columns []string, values []string, args []interface{}, comparisonSign ValueComparisonSign) (result string, explodedArgs []interface{}, err error) {
 	if len(columns) == 0 {
 		return "", explodedArgs, fmt.Errorf("Got 0 columns in GetRangeComparison")
@@ -121,10 +151,7 @@ func BuildRangeComparison(columns []string, values []string, args []interface{},
 }
 
 func BuildRangePreparedComparison(columns []string, args []interface{}, comparisonSign ValueComparisonSign) (result string, explodedArgs []interface{}, err error) {
-	values := make([]string, len(columns), len(columns))
-	for i := range columns {
-		values[i] = "?"
-	}
+	values := buildPreparedValues(len(columns))
 	return BuildRangeComparison(columns, values, args, comparisonSign)
 }
 
@@ -135,6 +162,7 @@ func BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName strin
 	databaseName = EscapeName(databaseName)
 	originalTableName = EscapeName(originalTableName)
 	ghostTableName = EscapeName(ghostTableName)
+	sharedColumns = duplicateNames(sharedColumns)
 	for i := range sharedColumns {
 		sharedColumns[i] = EscapeName(sharedColumns[i])
 	}
@@ -171,12 +199,8 @@ func BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName strin
 }
 
 func BuildRangeInsertPreparedQuery(databaseName, originalTableName, ghostTableName string, sharedColumns []string, uniqueKey string, uniqueKeyColumns []string, rangeStartArgs, rangeEndArgs []interface{}, includeRangeStartValues bool, transactionalTable bool) (result string, explodedArgs []interface{}, err error) {
-	rangeStartValues := make([]string, len(uniqueKeyColumns), len(uniqueKeyColumns))
-	rangeEndValues := make([]string, len(uniqueKeyColumns), len(uniqueKeyColumns))
-	for i := range uniqueKeyColumns {
-		rangeStartValues[i] = "?"
-		rangeEndValues[i] = "?"
-	}
+	rangeStartValues := buildPreparedValues(len(uniqueKeyColumns))
+	rangeEndValues := buildPreparedValues(len(uniqueKeyColumns))
 	return BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartValues, rangeEndValues, rangeStartArgs, rangeEndArgs, includeRangeStartValues, transactionalTable)
 }
 
@@ -198,6 +222,7 @@ func BuildUniqueKeyRangeEndPreparedQuery(databaseName, tableName string, uniqueK
 	}
 	explodedArgs = append(explodedArgs, rangeExplodedArgs...)
 
+	uniqueKeyColumns = duplicateNames(uniqueKeyColumns)
 	uniqueKeyColumnAscending := make([]string, len(uniqueKeyColumns), len(uniqueKeyColumns))
 	uniqueKeyColumnDescending := make([]string, len(uniqueKeyColumns), len(uniqueKeyColumns))
 	for i := range uniqueKeyColumns {
@@ -244,6 +269,7 @@ func buildUniqueKeyMinMaxValuesPreparedQuery(databaseName, tableName string, uni
 	databaseName = EscapeName(databaseName)
 	tableName = EscapeName(tableName)
 
+	uniqueKeyColumns = duplicateNames(uniqueKeyColumns)
 	uniqueKeyColumnOrder := make([]string, len(uniqueKeyColumns), len(uniqueKeyColumns))
 	for i := range uniqueKeyColumns {
 		uniqueKeyColumns[i] = EscapeName(uniqueKeyColumns[i])
@@ -262,3 +288,125 @@ func buildUniqueKeyMinMaxValuesPreparedQuery(databaseName, tableName string, uni
 	)
 	return query, nil
 }
+
+func BuildDMLDeleteQuery(databaseName, tableName string, tableColumns, uniqueKeyColumns *ColumnList, args []interface{}) (result string, uniqueKeyArgs []interface{}, err error) {
+	if len(args) != tableColumns.Len() {
+		return result, uniqueKeyArgs, fmt.Errorf("args count differs from table column count in BuildDMLDeleteQuery")
+	}
+	if uniqueKeyColumns.Len() == 0 {
+		return result, uniqueKeyArgs, fmt.Errorf("No unique key columns found in BuildDMLDeleteQuery")
+	}
+	for _, column := range uniqueKeyColumns.Names {
+		tableOrdinal := tableColumns.Ordinals[column]
+		uniqueKeyArgs = append(uniqueKeyArgs, args[tableOrdinal])
+	}
+	databaseName = EscapeName(databaseName)
+	tableName = EscapeName(tableName)
+	equalsComparison, err := BuildEqualsPreparedComparison(uniqueKeyColumns.Names)
+	if err != nil {
+		return result, uniqueKeyArgs, err
+	}
+	result = fmt.Sprintf(`
+			delete /* gh-osc %s.%s */
+				from
+					%s.%s
+				where
+					%s
+		`, databaseName, tableName,
+		databaseName, tableName,
+		equalsComparison,
+	)
+	return result, uniqueKeyArgs, nil
+}
+
+func BuildDMLInsertQuery(databaseName, tableName string, tableColumns, sharedColumns *ColumnList, args []interface{}) (result string, sharedArgs []interface{}, err error) {
+	if len(args) != tableColumns.Len() {
+		return result, args, fmt.Errorf("args count differs from table column count in BuildDMLInsertQuery")
+	}
+	if !sharedColumns.IsSubsetOf(tableColumns) {
+		return result, args, fmt.Errorf("shared columns is not a subset of table columns in BuildDMLInsertQuery")
+	}
+	if sharedColumns.Len() == 0 {
+		return result, args, fmt.Errorf("No shared columns found in BuildDMLInsertQuery")
+	}
+	databaseName = EscapeName(databaseName)
+	tableName = EscapeName(tableName)
+
+	for _, column := range sharedColumns.Names {
+		tableOrdinal := tableColumns.Ordinals[column]
+		sharedArgs = append(sharedArgs, args[tableOrdinal])
+	}
+
+	sharedColumnNames := duplicateNames(sharedColumns.Names)
+	for i := range sharedColumnNames {
+		sharedColumnNames[i] = EscapeName(sharedColumnNames[i])
+	}
+	preparedValues := buildPreparedValues(sharedColumns.Len())
+
+	result = fmt.Sprintf(`
+			replace /* gh-osc %s.%s */ into
+				%s.%s
+					(%s)
+				values
+					(%s)
+		`, databaseName, tableName,
+		databaseName, tableName,
+		strings.Join(sharedColumnNames, ", "),
+		strings.Join(preparedValues, ", "),
+	)
+	return result, sharedArgs, nil
+}
+
+func BuildDMLUpdateQuery(databaseName, tableName string, tableColumns, sharedColumns, uniqueKeyColumns *ColumnList, valueArgs, whereArgs []interface{}) (result string, sharedArgs, uniqueKeyArgs []interface{}, err error) {
+	if len(valueArgs) != tableColumns.Len() {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("value args count differs from table column count in BuildDMLUpdateQuery")
+	}
+	if len(whereArgs) != tableColumns.Len() {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("where args count differs from table column count in BuildDMLUpdateQuery")
+	}
+	if !sharedColumns.IsSubsetOf(tableColumns) {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("shared columns is not a subset of table columns in BuildDMLUpdateQuery")
+	}
+	if !uniqueKeyColumns.IsSubsetOf(sharedColumns) {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("unique key columns is not a subset of shared columns in BuildDMLUpdateQuery")
+	}
+	if sharedColumns.Len() == 0 {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("No shared columns found in BuildDMLUpdateQuery")
+	}
+	if uniqueKeyColumns.Len() == 0 {
+		return result, sharedArgs, uniqueKeyArgs, fmt.Errorf("No unique key columns found in BuildDMLUpdateQuery")
+	}
+	databaseName = EscapeName(databaseName)
+	tableName = EscapeName(tableName)
+
+	for _, column := range sharedColumns.Names {
+		tableOrdinal := tableColumns.Ordinals[column]
+		sharedArgs = append(sharedArgs, valueArgs[tableOrdinal])
+	}
+
+	for _, column := range uniqueKeyColumns.Names {
+		tableOrdinal := tableColumns.Ordinals[column]
+		uniqueKeyArgs = append(uniqueKeyArgs, whereArgs[tableOrdinal])
+	}
+
+	sharedColumnNames := duplicateNames(sharedColumns.Names)
+	for i := range sharedColumnNames {
+		sharedColumnNames[i] = EscapeName(sharedColumnNames[i])
+	}
+	setClause, err := BuildSetPreparedClause(sharedColumnNames)
+
+	equalsComparison, err := BuildEqualsPreparedComparison(uniqueKeyColumns.Names)
+	result = fmt.Sprintf(`
+ 			update /* gh-osc %s.%s */
+ 					%s.%s
+				set
+					%s
+				where
+ 					%s
+ 		`, databaseName, tableName,
+		databaseName, tableName,
+		setClause,
+		equalsComparison,
+	)
+	return result, sharedArgs, uniqueKeyArgs, nil
+}
diff --git a/go/sql/builder_test.go b/go/sql/builder_test.go
index b101bbf..3e0a8c6 100644
--- a/go/sql/builder_test.go
+++ b/go/sql/builder_test.go
@@ -68,6 +68,35 @@ func TestBuildEqualsComparison(t *testing.T) {
 	}
 }
 
+func TestBuildEqualsPreparedComparison(t *testing.T) {
+	{
+		columns := []string{"c1", "c2"}
+		comparison, err := BuildEqualsPreparedComparison(columns)
+		test.S(t).ExpectNil(err)
+		test.S(t).ExpectEquals(comparison, "((`c1` = ?) and (`c2` = ?))")
+	}
+}
+
+func TestBuildSetPreparedClause(t *testing.T) {
+	{
+		columns := []string{"c1"}
+		clause, err := BuildSetPreparedClause(columns)
+		test.S(t).ExpectNil(err)
+		test.S(t).ExpectEquals(clause, "`c1`=?")
+	}
+	{
+		columns := []string{"c1", "c2"}
+		clause, err := BuildSetPreparedClause(columns)
+		test.S(t).ExpectNil(err)
+		test.S(t).ExpectEquals(clause, "`c1`=?, `c2`=?")
+	}
+	{
+		columns := []string{}
+		_, err := BuildSetPreparedClause(columns)
+		test.S(t).ExpectNotNil(err)
+	}
+}
+
 func TestBuildRangeComparison(t *testing.T) {
 	{
 		columns := []string{"c1"}
@@ -143,7 +172,7 @@ func TestBuildRangeInsertQuery(t *testing.T) {
 		rangeStartArgs := []interface{}{3}
 		rangeEndArgs := []interface{}{103}
 
-		query, explodedArgs, err := BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartValues, rangeEndValues, rangeStartArgs, rangeEndArgs, true)
+		query, explodedArgs, err := BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartValues, rangeEndValues, rangeStartArgs, rangeEndArgs, true, false)
 		test.S(t).ExpectNil(err)
 		expected := `
 				insert /* gh-osc mydb.tbl */ ignore into mydb.ghost (id, name, position)
@@ -162,7 +191,7 @@ func TestBuildRangeInsertQuery(t *testing.T) {
 		rangeStartArgs := []interface{}{3, 17}
 		rangeEndArgs := []interface{}{103, 117}
 
-		query, explodedArgs, err := BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartValues, rangeEndValues, rangeStartArgs, rangeEndArgs, true)
+		query, explodedArgs, err := BuildRangeInsertQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartValues, rangeEndValues, rangeStartArgs, rangeEndArgs, true, false)
 		test.S(t).ExpectNil(err)
 		expected := `
 				insert /* gh-osc mydb.tbl */ ignore into mydb.ghost (id, name, position)
@@ -186,13 +215,13 @@ func TestBuildRangeInsertPreparedQuery(t *testing.T) {
 		rangeStartArgs := []interface{}{3, 17}
 		rangeEndArgs := []interface{}{103, 117}
 
-		query, explodedArgs, err := BuildRangeInsertPreparedQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartArgs, rangeEndArgs, true)
+		query, explodedArgs, err := BuildRangeInsertPreparedQuery(databaseName, originalTableName, ghostTableName, sharedColumns, uniqueKey, uniqueKeyColumns, rangeStartArgs, rangeEndArgs, true, true)
 		test.S(t).ExpectNil(err)
 		expected := `
 				insert /* gh-osc mydb.tbl */ ignore into mydb.ghost (id, name, position)
 				(select id, name, position from mydb.tbl force index (name_position_uidx)
 				  where (((name > ?) or (((name = ?)) AND (position > ?)) or ((name = ?) and (position = ?))) and ((name < ?) or (((name = ?)) AND (position < ?)) or ((name = ?) and (position = ?))))
-				)
+				lock in share mode )
 		`
 		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
 		test.S(t).ExpectTrue(reflect.DeepEqual(explodedArgs, []interface{}{3, 3, 17, 3, 17, 103, 103, 117, 103, 117}))
@@ -202,7 +231,7 @@ func TestBuildRangeInsertPreparedQuery(t *testing.T) {
 func TestBuildUniqueKeyRangeEndPreparedQuery(t *testing.T) {
 	databaseName := "mydb"
 	originalTableName := "tbl"
-	chunkSize := 500
+	var chunkSize int64 = 500
 	{
 		uniqueKeyColumns := []string{"name", "position"}
 		rangeStartArgs := []interface{}{3, 17}
@@ -262,3 +291,191 @@ func TestBuildUniqueKeyMinValuesPreparedQuery(t *testing.T) {
 		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
 	}
 }
+
+func TestBuildDMLDeleteQuery(t *testing.T) {
+	databaseName := "mydb"
+	tableName := "tbl"
+	tableColumns := NewColumnList([]string{"id", "name", "rank", "position", "age"})
+	args := []interface{}{3, "testname", "first", 17, 23}
+	{
+		uniqueKeyColumns := NewColumnList([]string{"position"})
+
+		query, uniqueKeyArgs, err := BuildDMLDeleteQuery(databaseName, tableName, tableColumns, uniqueKeyColumns, args)
+		test.S(t).ExpectNil(err)
+		expected := `
+			delete /* gh-osc mydb.tbl */
+				from
+					mydb.tbl
+				where
+					((position = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{17}))
+	}
+	{
+		uniqueKeyColumns := NewColumnList([]string{"name", "position"})
+
+		query, uniqueKeyArgs, err := BuildDMLDeleteQuery(databaseName, tableName, tableColumns, uniqueKeyColumns, args)
+		test.S(t).ExpectNil(err)
+		expected := `
+			delete /* gh-osc mydb.tbl */
+				from
+					mydb.tbl
+				where
+					((name = ?) and (position = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{"testname", 17}))
+	}
+	{
+		uniqueKeyColumns := NewColumnList([]string{"position", "name"})
+
+		query, uniqueKeyArgs, err := BuildDMLDeleteQuery(databaseName, tableName, tableColumns, uniqueKeyColumns, args)
+		test.S(t).ExpectNil(err)
+		expected := `
+			delete /* gh-osc mydb.tbl */
+				from
+					mydb.tbl
+				where
+					((position = ?) and (name = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{17, "testname"}))
+	}
+	{
+		uniqueKeyColumns := NewColumnList([]string{"position", "name"})
+		args := []interface{}{"first", 17}
+
+		_, _, err := BuildDMLDeleteQuery(databaseName, tableName, tableColumns, uniqueKeyColumns, args)
+		test.S(t).ExpectNotNil(err)
+	}
+}
+
+func TestBuildDMLInsertQuery(t *testing.T) {
+	databaseName := "mydb"
+	tableName := "tbl"
+	tableColumns := NewColumnList([]string{"id", "name", "rank", "position", "age"})
+	args := []interface{}{3, "testname", "first", 17, 23}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		query, sharedArgs, err := BuildDMLInsertQuery(databaseName, tableName, tableColumns, sharedColumns, args)
+		test.S(t).ExpectNil(err)
+		expected := `
+			replace /* gh-osc mydb.tbl */
+				into mydb.tbl
+					(id, name, position, age)
+				values
+					(?, ?, ?, ?)
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{3, "testname", 17, 23}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"position", "name", "age", "id"})
+		query, sharedArgs, err := BuildDMLInsertQuery(databaseName, tableName, tableColumns, sharedColumns, args)
+		test.S(t).ExpectNil(err)
+		expected := `
+			replace /* gh-osc mydb.tbl */
+				into mydb.tbl
+					(position, name, age, id)
+				values
+					(?, ?, ?, ?)
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{17, "testname", 23, 3}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"position", "name", "surprise", "id"})
+		_, _, err := BuildDMLInsertQuery(databaseName, tableName, tableColumns, sharedColumns, args)
+		test.S(t).ExpectNotNil(err)
+	}
+	{
+		sharedColumns := NewColumnList([]string{})
+		_, _, err := BuildDMLInsertQuery(databaseName, tableName, tableColumns, sharedColumns, args)
+		test.S(t).ExpectNotNil(err)
+	}
+}
+
+func TestBuildDMLUpdateQuery(t *testing.T) {
+	databaseName := "mydb"
+	tableName := "tbl"
+	tableColumns := NewColumnList([]string{"id", "name", "rank", "position", "age"})
+	valueArgs := []interface{}{3, "testname", "newval", 17, 23}
+	whereArgs := []interface{}{3, "testname", "findme", 17, 56}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{"position"})
+		query, sharedArgs, uniqueKeyArgs, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNil(err)
+		expected := `
+			update /* gh-osc mydb.tbl */
+			  mydb.tbl
+					set id=?, name=?, position=?, age=?
+				where
+					((position = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{3, "testname", 17, 23}))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{17}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{"position", "name"})
+		query, sharedArgs, uniqueKeyArgs, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNil(err)
+		expected := `
+			update /* gh-osc mydb.tbl */
+			  mydb.tbl
+					set id=?, name=?, position=?, age=?
+				where
+					((position = ?) and (name = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{3, "testname", 17, 23}))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{17, "testname"}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{"age"})
+		query, sharedArgs, uniqueKeyArgs, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNil(err)
+		expected := `
+			update /* gh-osc mydb.tbl */
+			  mydb.tbl
+					set id=?, name=?, position=?, age=?
+				where
+					((age = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{3, "testname", 17, 23}))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{56}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{"age", "position", "id", "name"})
+		query, sharedArgs, uniqueKeyArgs, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNil(err)
+		expected := `
+			update /* gh-osc mydb.tbl */
+			  mydb.tbl
+					set id=?, name=?, position=?, age=?
+				where
+					((age = ?) and (position = ?) and (id = ?) and (name = ?))
+		`
+		test.S(t).ExpectEquals(normalizeQuery(query), normalizeQuery(expected))
+		test.S(t).ExpectTrue(reflect.DeepEqual(sharedArgs, []interface{}{3, "testname", 17, 23}))
+		test.S(t).ExpectTrue(reflect.DeepEqual(uniqueKeyArgs, []interface{}{56, 17, 3, "testname"}))
+	}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{"age", "surprise"})
+		_, _, _, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNotNil(err)
+	}
+	{
+		sharedColumns := NewColumnList([]string{"id", "name", "position", "age"})
+		uniqueKeyColumns := NewColumnList([]string{})
+		_, _, _, err := BuildDMLUpdateQuery(databaseName, tableName, tableColumns, sharedColumns, uniqueKeyColumns, valueArgs, whereArgs)
+		test.S(t).ExpectNotNil(err)
+	}
+}
diff --git a/go/sql/types.go b/go/sql/types.go
index 942bd37..e82720c 100644
--- a/go/sql/types.go
+++ b/go/sql/types.go
@@ -11,21 +11,62 @@ import (
 	"strings"
 )
 
+// ColumnsMap maps a column onto its ordinal position
+type ColumnsMap map[string]int
+
+func NewColumnsMap(orderedNames []string) ColumnsMap {
+	columnsMap := make(map[string]int)
+	for i, column := range orderedNames {
+		columnsMap[column] = i
+	}
+	return ColumnsMap(columnsMap)
+}
+
 // ColumnList makes for a named list of columns
-type ColumnList []string
+type ColumnList struct {
+	Names    []string
+	Ordinals ColumnsMap
+}
+
+// NewColumnList creates an object given ordered list of column names
+func NewColumnList(names []string) *ColumnList {
+	result := &ColumnList{
+		Names: names,
+	}
+	result.Ordinals = NewColumnsMap(result.Names)
+	return result
+}
 
 // ParseColumnList parses a comma delimited list of column names
 func ParseColumnList(columns string) *ColumnList {
-	result := ColumnList(strings.Split(columns, ","))
-	return &result
+	result := &ColumnList{
+		Names: strings.Split(columns, ","),
+	}
+	result.Ordinals = NewColumnsMap(result.Names)
+	return result
 }
 
 func (this *ColumnList) String() string {
-	return strings.Join(*this, ",")
+	return strings.Join(this.Names, ",")
 }
 
 func (this *ColumnList) Equals(other *ColumnList) bool {
-	return reflect.DeepEqual(*this, *other)
+	return reflect.DeepEqual(this.Names, other.Names)
+}
+
+// IsSubsetOf returns 'true' when column names of this list are a subset of
+// another list, in arbitrary order (order agnostic)
+func (this *ColumnList) IsSubsetOf(other *ColumnList) bool {
+	for _, column := range this.Names {
+		if _, exists := other.Ordinals[column]; !exists {
+			return false
+		}
+	}
+	return true
+}
+
+func (this *ColumnList) Len() int {
+	return len(this.Names)
 }
 
 // UniqueKey is the combination of a key's name and columns
@@ -40,6 +81,10 @@ func (this *UniqueKey) IsPrimary() bool {
 	return this.Name == "PRIMARY"
 }
 
+func (this *UniqueKey) Len() int {
+	return this.Columns.Len()
+}
+
 func (this *UniqueKey) String() string {
 	return fmt.Sprintf("%s: %s; has nullable: %+v", this.Name, this.Columns, this.HasNullable)
 }
diff --git a/go/sql/types_test.go b/go/sql/types_test.go
new file mode 100644
index 0000000..177b8cf
--- /dev/null
+++ b/go/sql/types_test.go
@@ -0,0 +1,29 @@
+/*
+   Copyright 2016 GitHub Inc.
+	 See https://github.com/github/gh-osc/blob/master/LICENSE
+*/
+
+package sql
+
+import (
+	"testing"
+
+	"github.com/outbrain/golib/log"
+	test "github.com/outbrain/golib/tests"
+	"reflect"
+)
+
+func init() {
+	log.SetLevel(log.ERROR)
+}
+
+func TestParseColumnList(t *testing.T) {
+	names := "id,category,max_len"
+
+	columnList := ParseColumnList(names)
+	test.S(t).ExpectEquals(columnList.Len(), 3)
+	test.S(t).ExpectTrue(reflect.DeepEqual(columnList.Names, []string{"id", "category", "max_len"}))
+	test.S(t).ExpectEquals(columnList.Ordinals["id"], 0)
+	test.S(t).ExpectEquals(columnList.Ordinals["category"], 1)
+	test.S(t).ExpectEquals(columnList.Ordinals["max_len"], 2)
+}