Managed, Online Schema Changes

Note: this feature is EXPERIMENTAL. The syntax for online DDL statements is subject to change.

Vitess offers managed, online schema migrations, via gh-ost and pt-online-schema-change. As a quick breakdown:

  • Using the standard MySQL ALTER TABLE syntax.
  • With either @@ddl_strategy session variable set, or -ddl_strategy command line flag.
  • Vitess responds to an online schema change request with a job ID.
  • Vitess resolves affected shards.
  • A shard’s primary tablet schedules the migration to run when possible.
  • The tablets run migrations via gh-ost or pt-online-schema-change.
  • Vitess provides the user a mechanism to view migration status, cancel or retry migrations, based on the job ID.

Syntax

Note: the syntax is subject to change while this feature is in experimental state.

Use the standard MySQL ALTER TABLE syntax to run online DDL. Whether your schema migration runs synchronously (the default MySQL behavior) or asynchronously (aka online), is determined by the value of command line flags or a session variable.

We assume we have a keyspace (schema) called commerce, with a table called demo, that has the following definition:

CREATE TABLE `demo` (
  `id` int NOT NULL,
  `status` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

Consider the following schema migration statement:

ALTER TABLE demo MODIFY id bigint UNSIGNED;

This statement can be executed as:

  • a gh-ost, managed online migration
  • a pt-online-schema-change, managed online migration
  • a synchronous, unmanaged schema change

gh-ost and pt-online-schema-change (abbreviated as pt-osc) are the supported online DDL methods. Specifics about gh-ost and pt-online-schema-change follow later on.

How the migration is executed depends on your ddl_strategy configuration, and how you run your query: via vtctl/vtctlclient or VTGate. Details follow.

ddl_strategy

You will set either @@ddl_strategy session variable, or -ddl_strategy command line flag, to control your schema migration strategy, and specifically, to enable and configure online DDL. Details follow in next sections. Some initial examples:

  • Empty value ('') means the migration is synchronous, not an online DDL.
  • The value "gh-ost" instructs Vitess to run an online DDL via gh-ost.
  • The value "pt-osc" instructs Vitess to run an online DDL via pt-online-schema-change.
  • You may specify arguments for your tool of choice, e.g. "gh-ost --max-load Threads_running=200". Details follow.

ApplySchema

You may use vtctl or vtctlclient (the two are interchangeable for the purpose of this document) to apply schema changes. The ApplySchema command supports both synchronous and online schema migrations. To run an online schema migration you will supply the -ddl_strategy command line flag:

$ vtctlclient ApplySchema -ddl_strategy "gh-ost" -sql "ALTER TABLE demo MODIFY id bigint UNSIGNED" commerce
a2994c92_f1d4_11ea_afa3_f875a4d24e90

Notice how ApplySchema responds to an online DDL request with a UUID output. When the user indicates online schema change (aka online DDL), vtctl registers an online-DDL request with global topo. This generates a job ID for tracking. vtctl does not try to resolve the shards nor the primary tablets. The command returns immediately, without waiting for the migration(s) to start. It prints out the job ID (a2994c92_f1d4_11ea_afa3_f875a4d24e90 in the above)

If we immediately run SHOW CREATE TABLE, we are likely to still see the old schema:

SHOW CREATE TABLE demo;

CREATE TABLE `demo` (
  `id` int NOT NULL,
  `status` varchar(32) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

That’s because the migration is to be processed and executed in the future. We discuss how the migration jobs get scheduled and executed shortly. We will use the job ID for tracking.

ApplySchema will have Vitess run some validations:

$ vtctlclient ApplySchema -ddl_strategy "gh-ost" -sql "ALTER TABLE demo add column status int" commerce
E0908 16:17:07.651284 3739130 main.go:67] remote error: rpc error: code = Unknown desc = schema change failed, ExecuteResult: {
  "FailedShards": null,
  "SuccessShards": null,
  "CurSQLIndex": 0,
  "Sqls": [
    "ALTER TABLE demo add column status int"
  ],
  "ExecutorErr": "rpc error: code = Unknown desc = TabletManager.PreflightSchema on zone1-0000000100 error: /usr/bin/mysql: exit status 1, output: ERROR 1060 (42S21) at line 3: Duplicate column name 'status'\n: /usr/bin/mysql: exit status 1, output: ERROR 1060 (42S21) at line 3: Duplicate column name 'status'\n",
  "TotalTimeSpent": 144283260
}

Vitess was able to determine that the migration is invalid because a column named status already exists. vtctld generates no job ID, and does not persist any migration request.

VTGate

When connected to VTGate, your schema migration strategy is determined by:

  • -ddl_strategy command line flag, or:
  • @@ddl_strategy session variable.

As a simple example, you may control the execution of a migration as follows:

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

mysql> SET @@ddl_strategy='pt-osc';
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)

Just like in the ApplySchema example, VTGate identifies that this is an online schema change request, and persists it in global topo, returning a job ID for tracking. Migration does not start immediately.

@@ddl_strategy behaves like a MySQL session variable, though is only recognized by VTGate. Setting @@ddl_strategy only applies to that same connection and does not affect other connections. You may subsequently set @@ddl_strategy to different value.

The initial value for @@ddl_strategy is derived from the vtgate -ddl_strategy command line flag. Examples:

  • If you run vtgate without -ddl_strategy, then @@ddl_strategy is empty, which implies schema migrations are synchronous. You will need to set @@ddl_strategy='gh-ost' to run followup ALTER TABLE statements via gh-ost.
  • If you run vtgate -ddl_strategy "gh-ost", then @@ddl_strategy defaults to 'gh-ost' in each new session. Any ALTER TABLE will run via gh-ost. You may set @@ddl_strategy='pt-osc' to make migrations run through pt-online-schema-change, or set @@ddl_strategy='' to run migrations synchronously.

Migration flow and states

We highlight how Vitess manages migrations internally, and explain what states a migration goes through.

  • Whether via vtctlclient ApplySchema or via VTGate as described above, a migration request entry is persisted in global topo (e.g. the global etcd cluster).
  • vtctld periodically checks on new migration requests.
  • vtctld resolves the relevant shards, and the primary tablet for each shard.
  • vtctld pushes the request to all relevant primary tablets.
  • If not all shards confirm receipt, vtctld periodically keeps retrying pushing the request to the shards until all approve.
  • Internally, tablets persist the request in a designated table in the _vt schema. Do not manipulate that table directly as that can cause inconsistencies.
  • A shard’s primary tablet owns running the migration. It is independent of other shards. It will schedule the migration to run when possible. A tablet will not run two migrations at the same time.
  • A migration is first created in queued state.
  • If the tablet sees queued migration, and assuming there’s no reason to wait, it picks the oldest requested migration in queued state, and moves it to ready state.
  • Tablet then prepares for the migration. It creates a MySQL account with a random password, to be used by this migration only. It creates the command line invocation, and extra scripts if possible.
  • The tablet then runs the migration. Whether gh-ost or pt-online-schema-change, it first runs in dry run mode, and, if successful, in actual execute mode. The migration is then in running state.
  • The migration will either run to completion, fail, or be interrupted. If successful, it transitions into complete state, which is the end of the road for that migration. If failed or interrupted, it transitions to failed state. The user may choose to retry failed migrations (see below).
  • The user is able to cancel a migration (details below). If the migration hasn’t started yet, it transitions to cancelled state. If the migration is running, then it is interrupted, and is expected to transition into failed state.

By way of illustration, suppose a migration is now in running state, and is expected to keep on running for the next few hours. The user may initiate a new ALTER TABLE... statement. It will persist in global topo. vtctld will pick it up and advertise it to the relevant tablets. Each will persist the migration request in queued state. None will run the migration yet, since another migration is already in progress. In due time, and when the executing migration completes (whether successfully or not), and assuming no other migrations are queued, the primary tablets, each in its own good time, will execute the new migration.

At this time, the user is responsible to track the state of all migrations. VTTablet does not report back to vtctld. This may change in the future.

At this time, there are no automated retries. For example, a failover on a shard causes the migration to fail, and Vitess will not try to re-run the migration on the new primary. It is the user’s responsibility to issue a retry. This may change in the future.

Tracking migrations

You may track the status of a single migration, of all or recent migrations, or of migrations in a specific state. Examples:

$ vtctlclient OnlineDDL commerce show ab3ffdd5_f25c_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000201 | 40-80 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:24:33 | 2020-09-09 05:24:34 | complete         |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
| test-0000000401 | c0-   | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
| test-0000000101 |   -40 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce show 8a797518_f25c_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000401 | c0-   | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | running          |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 | 2020-09-09 05:23:33 | complete         |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | running          |
| test-0000000101 |   -40 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | running          |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce show 8a797518_f25c_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000401 | c0-   | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000101 |   -40 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 | 2020-09-09 05:23:33 | complete         |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce show recent
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000201 | 40-80 | vt_commerce  | demo        | 63b5db0c_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:22:41 | 2020-09-09 05:22:42 | complete         |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 | 2020-09-09 05:23:33 | complete         |
| test-0000000201 | 40-80 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:24:33 | 2020-09-09 05:24:34 | complete         |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 63b5db0c_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:22:41 | 2020-09-09 05:22:42 | complete         |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
| test-0000000401 | c0-   | vt_commerce  | demo        | 63b5db0c_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:22:41 | 2020-09-09 05:22:42 | complete         |
| test-0000000401 | c0-   | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000401 | c0-   | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
| test-0000000101 |   -40 | vt_commerce  | demo        | 63b5db0c_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:22:41 | 2020-09-09 05:22:42 | complete         |
| test-0000000101 |   -40 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000101 |   -40 | vt_commerce  | demo        | ab3ffdd5_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:25:13 | 2020-09-09 05:25:14 | complete         |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce show failed
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000401 | c0-   | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
| test-0000000101 |   -40 | vt_commerce  | demo        | 8a797518_f25c_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 05:23:32 |                     | failed           |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

The syntax for tracking migrations is:

vtctlclient OnlineDDL <keyspace> show <migration_id|all|recent|queued|ready|running|complete|failed|cancelled>

Cancelling a migration

The user may cancel a migration, as follows:

  • If the migration hasn’t started yet (it is queued or ready), then it is removed from queue and will not be executed.
  • If the migration is running, then it is forcibly interrupted. The migration is expected to transition to failed state.
  • In all other cases, cancelling a migration has no effect.

The syntax to cancelling a migration is:

vtctlclient OnlineDDL cancel <migration_id>

Example:

$ vtctlclient OnlineDDL commerce show 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | running          |
| test-0000000101 |   -40 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | running          |
| test-0000000401 | c0-   | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | running          |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | running          |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce cancel 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+--------------+
|     Tablet      | RowsAffected |
+-----------------+--------------+
| test-0000000401 |            1 |
| test-0000000101 |            1 |
| test-0000000201 |            1 |
| test-0000000301 |            1 |
+-----------------+--------------+

$ vtctlclient OnlineDDL commerce show 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000401 | c0-   | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000101 |   -40 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

Retrying a migration

The user may retry running a migration. If the migration is in failed or in cancelled state, Vitess will re-run the migration, with exact same arguments as previously intended. If the migration is in any other state, retry does nothing.

It is not possible to retry a migration with different options. e.g. if the user initially runs ALTER TABLE demo MODIFY id BIGINT with @@ddl_strategy='gh-ost --max-load Threads_running=200' and the migration fails, retrying it will use exact same options. It is not possible to retry with @@ddl_strategy='gh-ost --max-load Threads_running=500'.

Continuing the above example, where we cancelled a migration while running, we now retry it:

$ vtctlclient OnlineDDL commerce show 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000401 | c0-   | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
| test-0000000101 |   -40 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:32:31 |                     | failed           |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce retry 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+--------------+
|     Tablet      | RowsAffected |
+-----------------+--------------+
| test-0000000101 |            1 |
| test-0000000201 |            1 |
| test-0000000301 |            1 |
| test-0000000401 |            1 |
+-----------------+--------------+

$ vtctlclient OnlineDDL commerce show 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+-------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy | started_timestamp | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+-------------------+---------------------+------------------+
| test-0000000201 | 40-80 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   |                   |                     | queued           |
| test-0000000101 |   -40 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   |                   |                     | queued           |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   |                   |                     | queued           |
| test-0000000401 | c0-   | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   |                   |                     | queued           |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+-------------------+---------------------+------------------+

$ vtctlclient OnlineDDL commerce show 2201058f_f266_11ea_bab4_0242c0a8b007
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_schema | mysql_table |            migration_uuid            | strategy |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+
| test-0000000101 |   -40 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:37:33 |                     | running          |
| test-0000000401 | c0-   | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:37:33 |                     | running          |
| test-0000000201 | 40-80 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:37:33 |                     | running          |
| test-0000000301 | 80-c0 | vt_commerce  | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | gh-ost   | 2020-09-09 06:37:33 |                     | running          |
+-----------------+-------+--------------+-------------+--------------------------------------+----------+---------------------+---------------------+------------------+

Auto retry after failure

Vitess keeps track of:

  • which vttablet initiated the migration
  • how many times a migration has been retried
  • whether a migration failed due to a vttablet failure (as is the case in a failover scenario)

Vitess will auto-retry a failed migration when:

  • The migration failed due to a vttablet failure, and
  • it has not been retried (this is a temporary restriction)

The migration will be transitioned into queued state, as if the user requested a retry operation. Note that this takes place on a per-shard basis.

The primary use case is a primary failure and failover. The newly promoted tablet will be able to retry the migration that broke during the previous primary failure. To clarify, the migration will start anew, as at this time there is no mechanism to resume a broken migration.

gh-ost and pt-online-schema-change

The user must pick one of these migration tools. The tools differ in features, operation, load, and more.

Using 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.
  • 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 -ddl_strategy "gh-ost --allow-nullable-unique-key --chunk-size 200" ApplySchema ...

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.

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.

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 -ddl_strategy "pt-osc --alter-foreign-keys-method auto --chunk-size 200" ApplySchema ...

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.

Throttling

Schema migrations use the tablet throttler, which is a cooperative throttler service based on replication lag. The tablet throttler automatically detects topology REPLICA tablets and adapts to changes in the topology. See Tablet throttler.

NOTE that at this time the tablet throttler is an experimental feature and is opt in. Enable it with vttablet's -enable-lag-throttler flag. If the tablet throttler is disabled, schema migrations will not throttle on replication lag.

Table cleanup

Both gh-ost and pt-online-schema-change leave artifacts behind. Whether successful or failed, either the original table or the ghost table are left still populated at the end of the migration. Vitess explicitly configures both tools to not drop those tables. The reason is that in MySQL, a DROP TABLE operation can be dangerous in production as it commonly locks the buffer pool for a substantial period.

Artifact tables are identifiable via SELECT artifacts FROM _vt.schema_migrations in a VExec command, see below.

Vitess automatically cleans up those tables as soon as a migration completes (either successful or failed). You will normally not need to do anything.

VExec commands for greater control and visibility

vtctlclient OnlineDDL command should provide with most needs. However, Vitess gives the user greater control through the VExec command and via SQL queries.

For schema migrations, Vitess allows operations on the virtual table _vt.schema_migrations. Queries on this virtual table scatter to the underlying tablets and gather or manipulate data on their own, private backend tables (which incidentally are called by the same name). VExec only allows specific types of queries on that table.

  • SELECT: you may SELECT any column, or SELECT *. vtctlclient OnlineDDL show commands only present with a subset of columns, and so running VExec SELECT provides greater visibility. Some columns that are not shown are:
    • log_path: tablet server and path where migration logs are.
    • artifacts: tables created by the migration. This can be used to determine which tables need cleanup.
    • alter: the exact alter statement used by the migration
    • options: any options passed by the user (e.g. --max-load=Threads_running=200)
    • Various timestamps indicating the migration progress Aggregate functions do not work as expected and should be avoided. LIMIT and OFFSET are not supported.
  • UPDATE: you may directly update the status of a migration. You may only change status into cancel or retry, which Vitess interprets similarly to a vtctlclient OnlineDDL cancel/retry command. However, you get greater control as you may filter on a specific shard.
  • DELETE: unsupported
  • INSERT: unsupported, used internally only to advertise new migration requests to the tablets.

The syntax to run VExec queries is:

vtctlclient VExec <keyspace>.<migration_id> "<sql query>"

Examples:


$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "select * from _vt.schema_migrations"

$ vtctlclient VExec commerce.91b5c953-e1e2-11ea-a097-f875a4d24e90 "update _vt.schema_migrations set migration_status='retry'

$ vtctlclient VExec commerce.91b5c953-e1e2-11ea-a097-f875a4d24e90 "update _vt.schema_migrations set migration_status='retry' where shard='40-80'
$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "select shard, mysql_table, migration_uuid, started_timestamp, completed_timestamp, migration_status from _vt.schema_migrations"
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_table |            migration_uuid            |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
| test-0000000301 | 80-c0 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000101 |   -40 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000201 | 40-80 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 08:31:47 |                     | failed           |
| test-0000000401 | c0-   | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "update _vt.schema_migrations set migration_status='retry' where migration_uuid='2201058f_f266_11ea_bab4_0242c0a8b007' and shard='40-80'"
+-----------------+--------------+
|     Tablet      | RowsAffected |
+-----------------+--------------+
| test-0000000201 |            1 |
+-----------------+--------------+

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "select shard, mysql_table, migration_uuid, started_timestamp, completed_timestamp, migration_status from _vt.schema_migrations"
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_table |            migration_uuid            |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
| test-0000000301 | 80-c0 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000201 | 40-80 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 08:34:59 |                     | running          |
| test-0000000101 |   -40 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000401 | c0-   | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "update _vt.schema_migrations set migration_status='cancel' where migration_uuid='2201058f_f266_11ea_bab4_0242c0a8b007' and shard='40-80'"
+-----------------+--------------+
|     Tablet      | RowsAffected |
+-----------------+--------------+
| test-0000000201 |            1 |
+-----------------+--------------+

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "select shard, mysql_table, migration_uuid, started_timestamp, completed_timestamp, migration_status from _vt.schema_migrations"
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
|     Tablet      | shard | mysql_table |            migration_uuid            |  started_timestamp  | completed_timestamp | migration_status |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+
| test-0000000401 | c0-   | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000101 |   -40 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
| test-0000000201 | 40-80 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 08:34:59 |                     | failed           |
| test-0000000301 | 80-c0 | demo        | 2201058f_f266_11ea_bab4_0242c0a8b007 | 2020-09-09 06:37:33 |                     | failed           |
+-----------------+-------+-------------+--------------------------------------+---------------------+---------------------+------------------+

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "update _vt.schema_migrations set migration_status='cancel' where migration_uuid='2201058f_f266_11ea_bab4_0242c0a8b007' and shard='40-80'"
<no result>

$ vtctlclient VExec commerce.2201058f_f266_11ea_bab4_0242c0a8b007 "select shard, log_path from _vt.schema_migrations"
+-----------------+-------+-----------------------------------------------------------------------------+
|     Tablet      | shard |                                   log_path                                  |
+-----------------+-------+-----------------------------------------------------------------------------+
| test-0000000201 | 40-80 | 11ac2af6e63e:/tmp/online-ddl-2201058f_f266_11ea_bab4_0242c0a8b007-657478384 |
| test-0000000101 |   -40 | e779a82d35d7:/tmp/online-ddl-2201058f_f266_11ea_bab4_0242c0a8b007-901629215 |
| test-0000000401 | c0-   | 5aad1249ab91:/tmp/online-ddl-2201058f_f266_11ea_bab4_0242c0a8b007-039568897 |
| test-0000000301 | 80-c0 | 5e7c662679d3:/tmp/online-ddl-2201058f_f266_11ea_bab4_0242c0a8b007-532703073 |
+-----------------+-------+-----------------------------------------------------------------------------+