Online DDL strategies

Vitess supports both managed, online schema migrations (aka Online DDL) as well as unmanaged migrations. How Vitess runs a schema migration depends on the DDL strategy. Vitess allows these strategies:

  • direct: the direct apply of DDL to your database. This is not an online DDL. It is a synchronous and blocking operation. This is the default strategy.
  • online: utilizes Vitess’s built in VReplication mechanism.
  • gh-ost: uses 3rd party GitHub’s gh-ost tool.
  • pt-osc: uses 3rd party Percona’s pt-online-schema-change as part of Percona Toolkit

CREATE and DROP are managed in the same way, by Vitess, whether strategy is online, gh-ost or pt-osc.

Specifying a DDL strategy #

You will set either @@ddl_strategy session variable, or -ddl_strategy command line flag. Examples:

Via vtctl/vtctlclient #

$ vtctlclient ApplySchema -ddl_strategy "online" -sql "ALTER TABLE demo MODIFY id bigint UNSIGNED" commerce
a2994c92_f1d4_11ea_afa3_f875a4d24e90
$ vtctlclient ApplySchema -ddl_strategy "gh-ost --max-load Threads_running=200" -sql "ALTER TABLE demo add column status int" commerce

Via VTGate #

$ mysql -h 127.0.0.1 -P 15306 commerce
Welcome to the MySQL monitor.  Commands end with ; or \g.

mysql> SET @@ddl_strategy='online';
Query OK, 0 rows affected (0.00 sec)

mysql> ALTER TABLE demo ADD COLUMN sample INT;
+--------------------------------------+
| uuid                                 |
+--------------------------------------+
| fa2fb689_f1d5_11ea_859e_f875a4d24e90 |
+--------------------------------------+
1 row in set (0.00 sec)

Choosing a DDL strategy #

Different strategies have different behavior for ALTER statements. Sections below first break down specific handling and notes for each strategy, followed by an evaluation of the differences.

online/VReplication #

The online strategy invokes Vitess’s built in VReplication mechanism. It is the mechanism behind resharding, materialized views, imports from external databases, and more. VReplication migrations use the same logic for copying data as do other VReplication operations, and as such the online strategy is known to be compatible with overall Vitess behavior. VReplication is authored by the maintainers of Vitess.

VReplication migrations enjoy the general features of VReplication:

  • Seamless integration with Vitess.
  • Seamless use of the throttler mechanism.
  • Visibility into internal working and status of VReplication.
  • Agnostic to planned reparenting and to unplanned failovers. A migration will resume from point of interruption shortly after a new primary is available.

VReplication migrations further:

  • Are revertible: you may switch back to the pre-migration schema without losing any data accumulated during and post migration.
  • Support a wider range of schema changes. For example, while gh-ost has a strict requirement for a shared unique key pre/post migration, online migrations may work with different keys, making it possible to modify a table’s PRIMARY KEY without having to rely on an additional UNIQUE KEY.

gh-ost #

gh-ost was developed by GitHub as a lightweight and safe schema migration tool.

To be able to run online schema migrations via gh-ost:

  • If you’re on Linux/amd64 architecture, and on glibc 2.3 or similar, there are no further dependencies. Vitess comes with a built-in gh-ost binary, that is compatible with your system. Note that the Vitess Docker images use this architecture, and gh-ost comes pre-bundled and compatible.
  • On other architectures:
    • Have gh-ost executable installed
    • Run vttablet with -gh-ost-path=/full/path/to/gh-ost flag

Vitess automatically creates a MySQL account for the migration, with a randomly generated password. The account is destroyed at the end of the migration.

Vitess takes care of setting up the necessary command line flags. It automatically creates a hooks directory and populates it with hooks that report gh-ost’s progress back to Vitess. You may supply additional flags for your migration as part of @@ddl_strategy session variable (using VTGate) or -ddl_strategy command line flag (using vtctl). Examples:

  • set @@ddl_strategy='gh-ost --max-load Threads_running=200';
  • set @@ddl_strategy='gh-ost --max-load Threads_running=200 --critical-load Threads_running=500 --critical-load-hibernate-seconds=60 --default-retries=512';
  • vtctl ApplySchema -ddl_strategy "gh-ost --allow-nullable-unique-key --chunk-size 200" ...

Note: Do not override the following flags: alter, database, table, execute, max-lag, force-table-names, serve-socket-file, hooks-path, hooks-hint-token, panic-flag-file. Overriding any of these may cause Vitess to lose control and track of the migration, or even to migrate the wrong table.

gh-ost throttling is done via Vitess’s own tablet throttler, based on replication lag.

Using pt-online-schema-change #

pt-online-schema-change is part of Percona Toolkit, a set of Perl scripts. To be able to use pt-online-schema-change, you must have the following setup on all your tablet servers (normally tablets are co-located with MySQL on same host and so this implies setting up on all MySQL servers):

  • pt-online-schema-change tool installed and is executable
  • Perl libdbi and libdbd-mysql modules installed. e.g. on Debian/Ubuntu, sudo apt-get install libdbi-perl libdbd-mysql-perl
  • Run vttablet with -pt-osc-path=/full/path/to/pt-online-schema-change flag.

Note that on Vitess Docker images, pt-online-schema-change and dependencies are pre-installed.

Vitess automatically creates a MySQL account for the migration, with a randomly generated password. The account is destroyed at the end of the migration.

Vitess takes care of supplying the command line flags, the DSN, the username & password. It also sets up PLUGINS used to communicate migration progress back to the tablet. You may supply additional flags for your migration as part of @@ddl_strategy session variable (using VTGate) or -ddl_strategy command line flag (using vtctl). Examples:

  • set @@ddl_strategy='pt-osc --null-to-not-null';
  • set @@ddl_strategy='pt-osc --max-load Threads_running=200';
  • vtctl ApplySchema -ddl_strategy "pt-osc --alter-foreign-keys-method auto --chunk-size 200" ...

Vitess tracks the state of the pt-osc migration. If it fails, Vitess makes sure to drop the migration triggers. Vitess keeps track of the migration even if the tablet itself restarts for any reason. Normally that would terminate the migration; Vitess will cleanup the triggers if so, or will happily let the migration run to completion if not.

Do not override the following flags: alter, pid, plugin, dry-run, execute, new-table-name, [no-]drop-new-table, [no-]drop-old-table.

pt-osc throttling is done via Vitess’s own tablet throttler, based on replication lag, and via a pt-online-schema-change plugin.

Comparing the options #

There are pros and cons to using any of the strategies. Some notable differences:

General #

  • All three options mimic an ALTER TABLE statement by creating and populating a shadow/ghost table behind the scenes, slowly bringing it up to date, and finally switching between the original and shadow tables.
  • All three options utilize the Vitess throttler.

Support #

  • VReplication (online strategy) is internal to Vitess and supported by the Vitess maintainers.
  • gh-ost enjoys partial, informal support from Vitess maintainers.
  • pt-online-schema-change is out of the maintainers control.

Setup #

  • VReplication is part of Vitess
  • A gh-ost binary is embedded within the Vitess binary, compatible with glibc 2.3 and Linux/amd64. The user may choose to use their own gh-ost binary, configured with -gh-ost-path.
  • pt-online-schema-change is not included in Vitess, and the user needs to set it up on tablet hosts.
    • Note that on Vitess Docker images, pt-online-schema-change and dependencies are pre-installed.

Load #

  • pt-online-schema-change uses triggers to propagate changes. This method is traditionally known to generate high load on the server. Both VReplication and gh-ost tail the binary logs to capture changes, and this approach is known to be more lightweight.
  • When throttled, pt-online-schema-change still runs trigger actions, whereas both VReplication and gh-ost cease transfer of data (they may keep minimal bookkeeping operations).

Cut-over #

  • Both pt-online-schema-change and gh-ost have an atomic cut-over: at the end of the migration, the tables are switched, and incoming queries are momentarily blocked, but not lost.
  • VReplication causes a brief outage at time of cut-over (subject to change): apps will not be able to write to the original table during cut-over, and will return with error.
  • VReplication cut-over is only safe when all traffic comes through Vitess/VTGate (subject to change). Any DML query running on migrated table at time of cut-over, and which executes directly on the MySQL server without going through Vitess, might lose its data.

MySQL compatibility #

  • pt-online-schema-change partially supports foreign keys. Neither gh-ost nor VReplication support foreign keys.

Vitess functionality comparison #

StrategyManagedOnlineTrackableDeclarativeRevertibleRecoverableTraffic
directNoMySQL*NoNoNoNoAny
pt-oscYesYes*YesYesCREATE,DROPNo*Any
gh-ostYesYes*Yes+YesCREATE,DROPNo*Any
onlineYesYes*YesYesCREATE,DROP,ALTERYesVitess
  • Managed: whether Vitess schedules and operates the migration
  • Online:
    • MySQL supports limited online (“In place”) DDL and instant DDL. See support chart.
    • gh-ost, online do not support foreign keys
    • pt-osc has support for foreign keys (may apply collateral blocking operations)
  • Trackable: able to determine migration state (ready, running, complete etc)
    • gh-ost also makes available progress % and ETA seconds
  • Declarative: support -declarative flag
  • Revertible: online strategy supports revertible ALTER statements (or ALTERs implied by -declarative migrations). All managed strategies supports revertible CREATE and DROP.
  • Recoverable: an online migration interrupted by planned/unplanned failover, automatically resumes work from point of interruption. gh-ost and pt-osc will not resume after failover, but Vitess will automatically retry the migration (by marking the migration as failed and by initiating a RETRY), exactly once for any migration.
  • Traffic: online migration cut-over uses Vitess specific blocking of traffic, and is therefore only safe when write traffic to the tables runs entirely through Vitess/VTGate. gh-ost and pt-osc use generic MySQL blocking/locking mechanisms, and it is safe to run some write traffic on the migrated table outside Vitess.