37 lines
2.0 KiB
Markdown
37 lines
2.0 KiB
Markdown
# How?
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### How does the cut-over work? Is it really atomic?
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The cut-over phase, where the original table is swapped away, and the _ghost_ table takes its place, is an atomic, blocking, controlled operation.
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- Atomic: the tables are swapped together. There is no gap where your table does not exist.
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- Blocking: all app queries involving the migrated (original) table are either operate on the original table, or are blocked, or proceed to operate on the _new_ table (formerly the _ghost_ table, now swapped in).
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- Controlled: the cut-over times out at pre-defined threshold, and is atomically aborted, then re-attempted. Cut-over only takes place when no lags are present, and otherwise no throttling reason is found. Cut-over step itself gets high priority and is never throttled.
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Read more on [cut-over](cut-over.md) and on the [cut-over design Issue](https://github.com/github/gh-ost/issues/82)
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# Is it possible to?
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### Is it possible to add a UNIQUE KEY?
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Adding a `UNIQUE KEY` is possible, in the condition that no violation will occur. That is, you must make sure there aren't any violating rows on your table before, and during the migration.
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At this time there is no equivalent to `ALTER IGNORE`, where duplicates are implicitly and silently thrown away. The MySQL `5.7` docs say:
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> As of MySQL 5.7.4, the IGNORE clause for ALTER TABLE is removed and its use produces an error.
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It is therefore unlikely that `gh-ost` will support this behavior.
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### Run concurrent migrations?
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Yes. TL;DR if running all on same replica/master, make sure to provide `--replica-server-id`. [Read more](cheatsheet.md#concurrent-migrations)
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# Why
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### Why Is the "Connect to Replica" mode preferred?
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To avoid placing extra load on the master. `gh-ost` connects as a replication client. Each additional replica adds some load to the master.
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To monitor replication lag from a replica. This makes the replication lag throttle, `--max-lag-millis`, more representative of the lag experienced by other replicas following the master (perhaps N levels deep in a tree of replicas).
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