Postponed migrations
Postponed migrations await final cut-over until the user manually issues a completion statement.
Normally, migrations are executed by Vitess and completed automatically. For example, an ALTER on a large table can take hours or more to complete. Vitess automatically instates the new schema in place whenever it is satisfied that the ALTER is complete. Or, a DROP statement could wait in queue while other statements are running, only to actually execute hours later.
Some engineers wish to have more control over the cut-over time. With postponed migrations, it is possible to:
- Invoke a migration that postpones completion
- Manually
COMPLETEa migration
This lets an engineer observe the change of schema at a point when they're comfortably at their console and prepared to take action should any issue occur.
Postpone completion #
Add -postpone-completion to any* (see supported migrations) of the online DDL strategies. Example:
mysql> set @@ddl_strategy='online -postpone-completion';
-- The migration is tracked, but the table won't get created
mysql> create table mytable(id int primary key);
+--------------------------------------+
| uuid |
+--------------------------------------+
| a1dac193_4b86_11ec_a827_0a43f95f28a3 |
+--------------------------------------+
Migrations executed with -postpone-completion are visible on show vitess_migrations just as normal. They will present either as queued, ready or running, at the scheduler's discretion. But they will not actually make changes to affected tables. The column postpone_completion indicates that the migration will not auto-complete:
mysql> show vitess_migrations like 'a1dac193_4b86_11ec_a827_0a43f95f28a3' \G
id: 1
migration_uuid: a1dac193_4b86_11ec_a827_0a43f95f28a3
keyspace: commerce
shard: 0
mysql_schema: vt_commerce
mysql_table: my_table
migration_statement: create table my_table (
id int primary key
)
strategy: online
options: --postpone-completion --allow-zero-in-date
added_timestamp: 2021-11-22 11:23:35
requested_timestamp: 0000-00-00 00:00:00
ready_timestamp: NULL
started_timestamp: NULL
liveness_timestamp: NULL
completed_timestamp: NULL
cleanup_timestamp: NULL
migration_status: queued
log_path:
artifacts:
retries: 0
tablet: zone1-0000000100
tablet_failure: 0
progress: 0
migration_context: vtgate:a1d8c5e0-4b86-11ec-a827-0a43f95f28a3
ddl_action: create
message:
eta_seconds: -1
rows_copied: 0
table_rows: 0
added_unique_keys: 0
removed_unique_keys: 0
log_file:
retain_artifacts_seconds: 86400
postpone_completion: 1
-- The migration is tracked, will start running when scheduler chooses, but will not cut-over
-- to replace the table with the new schema
mysql> alter table another_table add column ts timestamp not null;
+--------------------------------------+
| uuid |
+--------------------------------------+
| b7d6e6fb_8a74_11eb_badd_f875a4d24e90 |
+--------------------------------------+
*************************** 1. row ***************************
id: 3
migration_uuid: 3091ef2a_4b87_11ec_a827_0a43f95f28a3
...
strategy: online
options: --postpone-completion
added_timestamp: 2021-11-22 11:27:34
requested_timestamp: 0000-00-00 00:00:00
ready_timestamp: 2021-11-22 11:27:35
started_timestamp: 2021-11-22 11:27:35
liveness_timestamp: 2021-11-22 11:27:39
completed_timestamp: NULL
cleanup_timestamp: NULL
migration_status: running
...
postpone_completion: 1
Completing a migration #
Completing a postponed migration is achieved by:
mysql> alter vitess_migration 'b7d6e6fb_8a74_11eb_badd_f875a4d24e90' complete;
This command instructs Vitess that the migration should not kept waiting any further.
ALTER may not be done copying all necessary data)After issuing the command, value of postpone_completion turns to 0:
mysql> show vitess_migrations like '3091ef2a_4b87_11ec_a827_0a43f95f28a3' \G
*************************** 1. row ***************************
id: 3
migration_uuid: 3091ef2a_4b87_11ec_a827_0a43f95f28a3
...
strategy: online
options: --postpone-completion
added_timestamp: 2021-11-22 11:27:34
requested_timestamp: 0000-00-00 00:00:00
ready_timestamp: 2021-11-22 11:27:35
started_timestamp: 2021-11-22 11:27:35
liveness_timestamp: 2021-11-22 11:29:32
completed_timestamp: NULL
cleanup_timestamp: NULL
migration_status: running
...
postpone_completion: 0
In the above the migration is still running. The scheduler has not determined yet that it is ready to cut-over. Continuing the example, two seconds later:
mysql> show vitess_migrations like '3091ef2a_4b87_11ec_a827_0a43f95f28a3' \G
*************************** 1. row ***************************
id: 3
migration_uuid: 3091ef2a_4b87_11ec_a827_0a43f95f28a3
...
strategy: online
options: --postpone-completion
added_timestamp: 2021-11-22 11:27:34
requested_timestamp: 0000-00-00 00:00:00
ready_timestamp: 2021-11-22 11:27:35
started_timestamp: 2021-11-22 11:27:35
liveness_timestamp: 2021-11-22 11:29:32
completed_timestamp: 2021-11-22 11:29:33
cleanup_timestamp: NULL
migration_status: complete
...
postpone_completion: 0
Supported migrations #
Postponed completion is supported for:
CREATEandDROPfor all online strategiesALTERmigrations inonlinestrategyALTERmigrations ingh-oststrategyREVERTof any of the above, as well as further cascadingREVERToperations
Postponed completion is not supported in:
directstrategypt-oscforALTERmigrations
declarative migrations will remain queued when -postpone-migration is specified, until alter vitess_migration ... complete is issued. This is true whether the declarative migration implies an eventual CREATE, DROP or ALTER.
Implementation details #
The two strong cases for postponed migrations are DROP and log ALTERs. Both carry an amount of risk to production above other migrations.
Postponed ALTER migrations (in online and gh-ost strategies) are actually executed, and begin copying table data as well as track ongoing changes. But as they reach the point where cut-over is agreeable, they stall, and keep waiting until the user issues the alter vitess_migration ... complete statement. Assuming the user runs the statement when all data has already been copied, it is typically a matter of seconds until the migration completes and the new schema is instated.
For CREATE and DROP statements, there's no such backfill process as with ALTER, and the migrations are simply not scheduled, until the user issues the complete statement. Once the statement is issued, the migrations still need to be scheduled, and may be possibly delayed by an existing queue of migrations.