Declarative migrations

Vitess’s managed schema changes offer declarative online schema migrations:

  • The user may indicate a desired table schema and Vitess will make it so, whether the table exists or not, or
  • The user may indicate a table should not exist, and Vitess will make it so.

Declarative DDLs are expressed via:

  • Complete CREATE TABLE statement (make the table in desired state)
  • DROP TABLE statement (make the table go)

Altering tables in declarative DDL is done by issuing CREATE TABLE statements with the desired state. ALTER statements are not allowed.

Declarative DDLs have the property of being idempotent. For example, a user may submit the same CREATE TABLE statement twice, one after another. If the 1st is successful, then the 2nd is a noop, and considered as implicitly successful. Likewise, two DROP TABLE DDLs for same statement will each ensure the table does not exist. If the 1st is successful, then the 2nd has nothing to do and is implicitly successful.

Usage #

Add -declarative to any of the online DDL strategies. Example:

mysql> set @@ddl_strategy='online -declarative';

-- The following migration creates a new table:
mysql> create table decl_table(id int primary key);
| uuid                                 |
| b06475e5_8a74_11eb_badd_f875a4d24e90 |

-- The next migration will implicitly ALTER the table decl_table into desired state:
mysql> create table decl_table(id int primary key, ts timestamp not null);
| uuid                                 |
| b7d6e6fb_8a74_11eb_badd_f875a4d24e90 |

-- Next migration does not change table structure, hence is a noop and implicitly successful:
mysql> create table decl_table(id int primary key, ts timestamp not null);
| uuid                                 |
| 110574b1_8a75_11eb_badd_f875a4d24e90 |

Consider migration b7d6e6fb_8a74_11eb_badd_f875a4d24e90 above, which results in an ALTER. A look into the migration shows:

mysql> show vitess_migrations like 'b7d6e6fb_8a74_11eb_badd_f875a4d24e90'\G
*************************** 1. row ***************************
                 id: 19
     migration_uuid: b7d6e6fb_8a74_11eb_badd_f875a4d24e90
           keyspace: commerce
              shard: 0
       mysql_schema: vt_commerce
        mysql_table: decl_table
migration_statement: create table decl_table (
	id int primary key,
	ts timestamp not null
           strategy: online
            options: -declarative
    added_timestamp: 2021-03-21 20:39:08
requested_timestamp: 2021-03-21 20:39:07
    ready_timestamp: 2021-03-21 20:39:10
  started_timestamp: 2021-03-21 20:39:10
 liveness_timestamp: 2021-03-21 20:39:13
completed_timestamp: 2021-03-21 20:39:13
  cleanup_timestamp: NULL
   migration_status: complete
          artifacts: _b7d6e6fb_8a74_11eb_badd_f875a4d24e90_20210321203910_vrepl,
            retries: 0
             tablet: zone1-0000000100
     tablet_failure: 0
           progress: 100
  migration_context: vtgate:38368dbe-8a60-11eb-badd-f875a4d24e90
         ddl_action: alter
        eta_seconds: 0

Note how while the migration statement is create, the migration’s ddl_action ends up being alter, and message indicates the alter options.

You may add -declarative even if you otherwise supply flags to your favorite strategy. For example, the following is valid:

set @@ddl_strategy='gh-ost -declarative -max-load=Threads_running=100';

Vitess notes down the -declarative flag and does not pass it to gh-ost, pt-osc or VReplication.

Implementation details #

The user submits a declarative DDL. Tables schedule the migration to execute, but at time of execution, may modify the migration on the fly and end up running a different migration.

Consider the following types of migrations:

  • A REVERT has no special behavior, it acts as a normal revert.
  • ALTER is rejected (migration will fail)
  • DROP: silently mark as successful if the table does not exist. Otherwise treat the DDL as a normal DROP.
  • CREATE: either,
    • The table does not exist: proceed as normal CREATE
    • The table exists: evaluate the SQL diff between the existing table schema and the proposed schema. Either:
      • There is no diff (exact same schema): silently mark as successful
      • There is a diff: rewrite the DDL as an actual ALTER, run using relevant strategy.

Declarative DDLs are revertible. Note:

  • A declarative migration which ends up being an ALTER is only revertible if executed with online strategy.
  • A declarative migration which ends up being a noop (and implicitly successful), implies a noop revert.

Declarative migrations