ddl_strategy flags
ddl_strategy
accepts flags in command line format. The flags can be vitess-specific, or, if unrecognized by Vitess, are passed on the underlying online schema change tools.
Vitess flags #
Vitess respects the following flags. They can be combined unless specifically indicated otherwise:
--allow-concurrent
: allow a migration to run concurrently to other migrations, rather than queue sequentially. Some restrictions apply, see concurrent migrations.--allow-zero-in-date
: normally Vitess operates with a strictsql_mode
. If you have columns such asmy_datetime DATETIME DEFAULT '0000-00-00 00:00:00'
and you wish to run DDL on these tables, Vitess will prevent the migration due to invalid values. Provide--allow-zero-in-date
to allow either a fully zero-date or a zero-in-date inyour schema. See also NO_ZERO_IN_DATE and NO_ZERO_DATE documentation for sql_mode.--analyze-table
: forALTER TABLE
operation, and inonline/vitess
strategy, run anANALYZE NO_WRITE_TO_BINLOG TABLE
just before starting the migration process, to get better estimation of the number of rows on the migrated table.cut-over-threshold=<duration>"
: set an explicit threshold and timeout for avitess
ALTER TABLE
cut-over phase. The default cut-over threshold, if not specified, is10s
. Avitess
migration will not attempt to cut-over if the vstream, or replication lag, is more than the cut-over threshold. Also, during cut-over, table locks will timeout after the same cut-over threshold (aborting the operation). Normal values are in the range5s
..30s
. Too low and cut-over may never succeed because of the inherent async nature ofvitess
migrations. Too high and table locks will be placed for too long, effectively rendering the table inaccessible.--declarative
: mark the migration as declarative. You will define a desired schema state by supplyingCREATE
andDROP
statements, ad Vitess will infer how to achieve the desired schema. If need be, it will generate anALTER
migration to convert to the new schema. See declarative migrations.--fast-range-rotation
: when the migration runs on a table partitioned byRANGE
, and the migration either runs a singleDROP PARTITION
or a singleADD PARTITION
, and nothing other than that, then this flags instructs Vitess to run theALTER TABLE
statement directly against MySQL, as opposed to running an Online DDL with a shadow table. ForDROP PARTITION
, this flag is actually always desired, and will possibly become default/redundant in the future. If all conditions are indeed met, then the migration is not revertible.--force-cut-over-after=<duration>
: applicable toALTER TABLE
migrations invitess
strategy and on MySQL8.0
. The final step of the migration, the cut-over, involves acquiring locks on the migrated table. This operation can time out when the table is otherwise locked by the user or the app, in which case Vitess retries it later on, until successful. When--force-cut-over-after
is specified, then counting the time since the very first cut-over attempt, and for any further cut-over attempt, Vitess will aggressivelyKILL
queries and transactions that are using or locking the migrated table.For example, say
--force-cut-over-after=2h
, and that the migration takes7h
to run. Say there is constant workload/locking that prevents the migration from cutting over. The first cut-over attempt takes place at the end of the7h
run, and fails due to lock wait timeout. During the next 2 hours there will be multiuple additional attempt to cut-over, and say they all continue to fail. At the2h
mark (9h
since starting the migration), give or take, Vitess runs a cut-over thatKILL
s existing queries and transactions on the table. This is likely to make the cut-over successful. Should this still fail, Vitess will continue to forcefullyKILL
queries and transactions in all additional cut-over attempts.See also forcing a migration cut-over
--in-order-completion
: a migration that runs with this DDL strategy flag may only complete if no prior migrations are still pending (pending means eitherqueued
,ready
orrunning
states).--in-order-completion
considers the order by which migrations were submitted. Note that--in-order-completion
still allows concurrency. In fact, it is designed to work with concurrent migrations. The idea is that while many migrations may run concurrently, they must complete in-order.- This lets the user submit multiple migrations which may have some dependencies (for example, introduce two views, one of which reads from the other). As long as the migrations are submitted in a valid order, the user can then expect
vitess
to complete the migrations successfully (and in that order). - This strategy flag applies to any
CREATE|DROP TABLE|VIEW
statements, and toALTER TABLE
withvitess|online
strategy. - It does not apply to
ALTER TABLE
when using thegh-ost
,pt-osc
,mysql
, ordirect
strategies.
- This lets the user submit multiple migrations which may have some dependencies (for example, introduce two views, one of which reads from the other). As long as the migrations are submitted in a valid order, the user can then expect
--postpone-completion
: initiate a migration that will only cut-over per user command, i.e. will not auto-complete. This gives the user control over the time when the schema change takes effect. See postponed migrations.--declarative
migrations are only evaluated when scheduled to run. If a migrations is both--declarative
and--postpone-completion
then it will remain inqueued
state until the user issues aALTER VITESS_MIGRATION ... COMPLETE
. If it turns out that Vitess should run the migration as anALTER
then it is only at that time that the migration starts.--postpone-launch
: initiate a migration that remainsqueued
and only launches per user command. See postponed migrations.--retain-artifacts=<duration>
: set an explicit artifact retention for this migration. If nonzero, this value overrides thevttablet --retain_online_ddl_tables
value.--singleton
: only allow a single pending migration to be submitted at a time. From the moment the migration is queued, and until either completion, failure or cancellation, no other new--singleton
migration can be submitted. New requests will be rejected with error.--singleton
works as a an exclusive lock for pending migrations. Note that this only affects migrations with--singleton
flag. Migrations running without that flag are unaffected and unblocked.--singleton-context
: only allow migrations submitted under same context to be pending at any given time. Migrations submitted with a different context are rejected for as long as at least one of the initially submitted migrations is pending.It does not make sense to combine
--singleton
and--singleton-context
.
Pass-through flags #
Flags unrecognized by Vitess are passed on to the underlying schema change tools. For example, a gh-ost
migration can run with:
set @@ddl_strategy='gh-ost --max-load Threads_running=200'
Since Vitess knows nothing about --max-load
it will pass it on as a command line argument to gh-ost
. Consult gh-ost documentation for supported command line flags.
Similarly, a pt-online-schema-change
migration can run with:
set @@ddl_strategy='pt-osc --null-to-not-null'
Consult pt-online-schema-change documentation for supported command line flags.
The vitess
strategy (formerly known as online
) uses Vitess internal mechanisms and is not a standalone command line tool. therefore, it has no particular command line flags. For internal testing/CI purposes, the vitess
strategy supports --vreplication-test-suite
, and this flag must not be used in production as it can have destructive consequences.