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246 lines
8.8 KiB
Go
246 lines
8.8 KiB
Go
/*
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Copyright 2017 Google Inc. All Rights Reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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package spanner
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import (
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"fmt"
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"time"
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pbd "github.com/golang/protobuf/ptypes/duration"
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pbt "github.com/golang/protobuf/ptypes/timestamp"
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sppb "google.golang.org/genproto/googleapis/spanner/v1"
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)
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// timestampBoundType specifies the timestamp bound mode.
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type timestampBoundType int
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const (
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strong timestampBoundType = iota // strong reads
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exactStaleness // read with exact staleness
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maxStaleness // read with max staleness
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minReadTimestamp // read with min freshness
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readTimestamp // read data at exact timestamp
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)
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// TimestampBound defines how Cloud Spanner will choose a timestamp for a single
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// read/query or read-only transaction.
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//
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// The types of timestamp bound are:
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//
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// - Strong (the default).
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// - Bounded staleness.
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// - Exact staleness.
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//
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// If the Cloud Spanner database to be read is geographically distributed, stale
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// read-only transactions can execute more quickly than strong or read-write
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// transactions, because they are able to execute far from the leader replica.
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//
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// Each type of timestamp bound is discussed in detail below. A TimestampBound
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// can be specified when creating transactions, see the documentation of
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// spanner.Client for an example.
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//
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// Strong reads
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//
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// Strong reads are guaranteed to see the effects of all transactions that have
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// committed before the start of the read. Furthermore, all rows yielded by a
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// single read are consistent with each other - if any part of the read
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// observes a transaction, all parts of the read see the transaction.
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//
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// Strong reads are not repeatable: two consecutive strong read-only
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// transactions might return inconsistent results if there are concurrent
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// writes. If consistency across reads is required, the reads should be
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// executed within a transaction or at an exact read timestamp.
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//
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// Use StrongRead() to create a bound of this type.
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//
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// Exact staleness
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//
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// These timestamp bounds execute reads at a user-specified timestamp. Reads at
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// a timestamp are guaranteed to see a consistent prefix of the global
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// transaction history: they observe modifications done by all transactions
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// with a commit timestamp less than or equal to the read timestamp, and
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// observe none of the modifications done by transactions with a larger commit
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// timestamp. They will block until all conflicting transactions that may be
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// assigned commit timestamps less than or equal to the read timestamp have
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// finished.
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//
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// The timestamp can either be expressed as an absolute Cloud Spanner commit
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// timestamp or a staleness relative to the current time.
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//
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// These modes do not require a "negotiation phase" to pick a timestamp. As a
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// result, they execute slightly faster than the equivalent boundedly stale
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// concurrency modes. On the other hand, boundedly stale reads usually return
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// fresher results.
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//
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// Use ReadTimestamp() and ExactStaleness() to create a bound of this type.
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//
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// Bounded staleness
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//
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// Bounded staleness modes allow Cloud Spanner to pick the read timestamp, subject to
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// a user-provided staleness bound. Cloud Spanner chooses the newest timestamp within
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// the staleness bound that allows execution of the reads at the closest
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// available replica without blocking.
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//
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// All rows yielded are consistent with each other -- if any part of the read
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// observes a transaction, all parts of the read see the transaction. Boundedly
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// stale reads are not repeatable: two stale reads, even if they use the same
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// staleness bound, can execute at different timestamps and thus return
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// inconsistent results.
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//
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// Boundedly stale reads execute in two phases: the first phase negotiates a
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// timestamp among all replicas needed to serve the read. In the second phase,
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// reads are executed at the negotiated timestamp.
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//
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// As a result of the two phase execution, bounded staleness reads are usually
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// a little slower than comparable exact staleness reads. However, they are
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// typically able to return fresher results, and are more likely to execute at
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// the closest replica.
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//
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// Because the timestamp negotiation requires up-front knowledge of which rows
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// will be read, it can only be used with single-use reads and single-use
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// read-only transactions.
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//
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// Use MinReadTimestamp() and MaxStaleness() to create a bound of this type.
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//
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// Old read timestamps and garbage collection
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//
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// Cloud Spanner continuously garbage collects deleted and overwritten data in the
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// background to reclaim storage space. This process is known as "version
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// GC". By default, version GC reclaims versions after they are four hours
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// old. Because of this, Cloud Spanner cannot perform reads at read timestamps more
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// than four hours in the past. This restriction also applies to in-progress
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// reads and/or SQL queries whose timestamp become too old while
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// executing. Reads and SQL queries with too-old read timestamps fail with the
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// error ErrorCode.FAILED_PRECONDITION.
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type TimestampBound struct {
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mode timestampBoundType
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d time.Duration
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t time.Time
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}
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// StrongRead returns a TimestampBound that will perform reads and queries at a
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// timestamp where all previously committed transactions are visible.
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func StrongRead() TimestampBound {
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return TimestampBound{mode: strong}
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}
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// ExactStaleness returns a TimestampBound that will perform reads and queries
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// at an exact staleness.
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func ExactStaleness(d time.Duration) TimestampBound {
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return TimestampBound{
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mode: exactStaleness,
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d: d,
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}
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}
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// MaxStaleness returns a TimestampBound that will perform reads and queries at
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// a time chosen to be at most "d" stale.
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func MaxStaleness(d time.Duration) TimestampBound {
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return TimestampBound{
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mode: maxStaleness,
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d: d,
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}
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}
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// MinReadTimestamp returns a TimestampBound that bound that will perform reads
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// and queries at a time chosen to be at least "t".
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func MinReadTimestamp(t time.Time) TimestampBound {
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return TimestampBound{
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mode: minReadTimestamp,
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t: t,
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}
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}
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// ReadTimestamp returns a TimestampBound that will peform reads and queries at
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// the given time.
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func ReadTimestamp(t time.Time) TimestampBound {
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return TimestampBound{
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mode: readTimestamp,
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t: t,
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}
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}
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// String implements fmt.Stringer.
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func (tb TimestampBound) String() string {
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switch tb.mode {
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case strong:
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return fmt.Sprintf("(strong)")
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case exactStaleness:
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return fmt.Sprintf("(exactStaleness: %s)", tb.d)
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case maxStaleness:
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return fmt.Sprintf("(maxStaleness: %s)", tb.d)
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case minReadTimestamp:
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return fmt.Sprintf("(minReadTimestamp: %s)", tb.t)
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case readTimestamp:
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return fmt.Sprintf("(readTimestamp: %s)", tb.t)
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default:
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return fmt.Sprintf("{mode=%v, d=%v, t=%v}", tb.mode, tb.d, tb.t)
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}
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}
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// durationProto takes a time.Duration and converts it into pdb.Duration for
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// calling gRPC APIs.
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func durationProto(d time.Duration) *pbd.Duration {
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n := d.Nanoseconds()
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return &pbd.Duration{
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Seconds: n / int64(time.Second),
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Nanos: int32(n % int64(time.Second)),
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}
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}
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// timestampProto takes a time.Time and converts it into pbt.Timestamp for calling
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// gRPC APIs.
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func timestampProto(t time.Time) *pbt.Timestamp {
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return &pbt.Timestamp{
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Seconds: t.Unix(),
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Nanos: int32(t.Nanosecond()),
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}
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}
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// buildTransactionOptionsReadOnly converts a spanner.TimestampBound into a sppb.TransactionOptions_ReadOnly
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// transaction option, which is then used in transactional reads.
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func buildTransactionOptionsReadOnly(tb TimestampBound, returnReadTimestamp bool) *sppb.TransactionOptions_ReadOnly {
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pb := &sppb.TransactionOptions_ReadOnly{
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ReturnReadTimestamp: returnReadTimestamp,
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}
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switch tb.mode {
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case strong:
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pb.TimestampBound = &sppb.TransactionOptions_ReadOnly_Strong{
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Strong: true,
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}
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case exactStaleness:
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pb.TimestampBound = &sppb.TransactionOptions_ReadOnly_ExactStaleness{
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ExactStaleness: durationProto(tb.d),
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}
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case maxStaleness:
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pb.TimestampBound = &sppb.TransactionOptions_ReadOnly_MaxStaleness{
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MaxStaleness: durationProto(tb.d),
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}
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case minReadTimestamp:
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pb.TimestampBound = &sppb.TransactionOptions_ReadOnly_MinReadTimestamp{
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MinReadTimestamp: timestampProto(tb.t),
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}
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case readTimestamp:
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pb.TimestampBound = &sppb.TransactionOptions_ReadOnly_ReadTimestamp{
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ReadTimestamp: timestampProto(tb.t),
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}
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default:
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panic(fmt.Sprintf("buildTransactionOptionsReadOnly(%v,%v)", tb, returnReadTimestamp))
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}
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return pb
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}
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