Listed below are recipes for advanced online DDL usage:

• Duplicate migration detection
• Duplicate migration indication
• Near instant REVERTs

Duplicate migration detection #

Two migrations sharing the same context and DDL are considered duplicate, and only one will run to completion.

Consider this DDL:

alter table `customer` add column `status` int unsigned not null;

You can only run this change successfully once. Once it it applied, the column status exists; any attempt to run the migration again yields an error.

Sometimes it is desirable to be able to retry a migration. For example, if you apply a migration on a sharded keyspace, where one or more of the shards can be down. In such scenario some shards receive and apply the DDL, while other shards do not, and are not aware of its existence. Attempting to re-apply the same DDL will generate errors on the shards that have received and applied it on the first attempt.

vtctldclient ApplySchema accepts a --migration_context flag. By default, Vitess auto-generates a unique context per execution of vtctldclient ApplySchema. You may supply your own value, which can be an arbitrary text (limited to 1024 characters). You may search for migrations with a particular context via SHOW VITESS_MIGRATIONS LIKE '<context-value>'. Also, any SHOW VITESS_MIGRATIONS ... command outputs the context value in the migration_context column.

When Vitess meets a migration which has exact same DDL and exact same (non-empty) context as some older migration, it considers it as a duplicate. The new migration does get a UUID of its own, and is tracked as a new migration. But if the previous migration (or, if there are multiple past duplicate migrations with same DDL and context, any one of those) is complete, then the new migration is also implicitly assumed to be complete.

Thus, the new migration does not get to execute if an identical previous migration was successful.

Usage:

$ vtctldclient ApplySchema -- --migration_context="1111-2222" --ddl_strategy='vitess' --sql "alter table customer add column status int unsigned not null" commerce

$ vtctldclient ApplySchema -- --migration_context="1111-2222" --ddl_strategy='vitess' --sql "alter table customer add column status int unsigned not null" commerce

In the above, the two calls are identical. Specifically, they share the exact same --migration_context value of 1111-2222, and the exact same --sql.

Notes:

• As mentioned, the new migration will have its own UUID.
• The new migration does get to be scheduled.
• It will only be marked as complete if one previous identical migration (same DDL, same non-empty context) is likewise complete.
• If, for example, there's a single previous identical migration which is failed, the new migration gets to be executed.
• Continuing the above example: if the new migration is successful, the previous migration remains in failed state.
• The decision whether to run the migration or to mark it as implicitly complete only takes place when the migration is scheduled to run.

The context may also be set via VTGate:

mysql> set @@ddl_strategy='vitess --';
mysql> set @@migration_context='1111-2222';
mysql> alter table customer add column status int unsigned not null;

By default, the migration context for an Online DDL issued via VTGate is the value of @@session_uuid. If @@migration_context is non-empty, then its value is used.

Duplicate migration indication #

You may go one step beyond duplicate migration detection by explicitly supplying migration UUIDs such that duplicate migrations are never submitted in the first place.

Consider the following example, note --uuid_list flag:

$ vtctldclient ApplySchema -- --uuid_list "73380089_7764_11ec_a656_0a43f95f28a3" --ddl_strategy='vitess' --sql "alter table customer add column status int unsigned not null" commerce

$ vtctldclient ApplySchema -- --uuid_list "73380089_7764_11ec_a656_0a43f95f28a3" --ddl_strategy='vitess' --sql "alter table customer add column status int unsigned not null" commerce

Normally Vitess generates a UUID for each migration, thus having a new, unique ID per migration. With -uuid_list, you can force Vitess into using your own supplied UUID. There cannot be two migrations with the same UUID. Therefore, any subsequent submission of a migration with an already existing UUID is implicitly discarded. The 2nd call does return the migration UUID, but is internally discarded.

This feature is useful for external management systems (e.g. schema deployment tools) which may want to control the identity of migrations. The external systems are thus able to own the UUID and can re-submit at will.

Notes:

• --uuid_list accepts zero or more comma separated UUID values
• If empty, Vitess calculates UUID for the migrations
• Number of supplied UUIDs must match the number of DDL statements in --sql
• Each UUID must be in RFC 4122 format, with underscored instead of dashes. Examples of valid UUIDs: 73380089_7764_11ec_a656_0a43f95f28a3 and 28dc5ebc_78e6_11ec_accf_ab29e6ca1002.
• If multiple UUIDs are given, they must all be different from one another.
• It is the caller's responsibility to ensure the UUIDs are indeed unique. If the user submits an ApplySchema with an already existing --uuid_list=<UUID> value, Vitess takes no steps to validate whether the DDL is identical to the already existing submission.

Gated cut-over #

Some migrations only make sense to run together; or, rather, it's desirable that they complete at the same time. This can be true:

• For multiple table changes in a single shard, and/or:
• For a table change across multiple shards.

The user may submit multiple migrations such that non auto-completes. The user can gather information as to whether all migrations are in a good position to complete, termed "ready to complete". The user may then invoke a COMPLETE command such that all migrations complete closely (but not atomically) to one another.

Consider the following:

$ vtctldclient ApplySchema --ddl_strategy='vitess --postpone-completion --allow-concurrent' --sql "alter table customer add column country int not null default 0; alter table order add column shipping_country int not null default 0" commerce
29231906_776f_11ec_a656_0a43f95f28a3
3cc4ae0e_776f_11ec_a656_0a43f95f28a3

The combination of --postpone-completion --allow-concurrent means migration start sequentially, but at some point all (two in our example) end up running concurrently.

A show vitess_migrations like '29231906_776f_11ec_a656_0a43f95f28a3' presents the column ready_to_complete, with values 0 (not ready) or 1 (ready).

When all migrations for the relevant UUIDs show 1 for ready_to_complete, the user can then either:

$ vtctldclient ApplySchema --sql "alter vitess_migration complete all" commerce

Assuming these are the only migrations awaiting completion, or, explicitly issue a complete for each of the migrations:

$ vtctldclient ApplySchema --sql "alter vitess_migration '29231906_776f_11ec_a656_0a43f95f28a3' complete all" commerce
$ vtctldclient ApplySchema --sql "alter vitess_migration '3cc4ae0e_776f_11ec_a656_0a43f95f28a3' complete all" commerce

Near instant REVERTs #

A REVERT operation is available for CREATE, DROP and ALTER statements. Both CREATE and DROP reverts are near instantaneous by design. An ALTER's revert is fast, but not instantaneous. While it does not need to copy any table data, hence not proportional to table size and migration runtime, it does need to apply any changes made to the table since migration completion. This means tracking and applying binary log events, and the operation runtime is generally proportional to the time elapsed since migration completion.

It is possible to preemptively initiate an "open-ended" revert, such that a new workflow prepares the grounds for a revert, but requires a user interaction to actually cut over.

The use case and workflow is as follows:

• A long migration runs. The user suspects there might be a risk to the schema change
• As soon as the migration completes, the user issues an open ended revert, preparing the ground to undoing the schema change
• The open ended revert keeps track of binary log changes via VReplication
• If the original migration turns out to be fine, the user cancels the revert
• If the original migration has negative impact, the user completes the revert (thus undoing the schema change)

Consider the following example. We run a 5 hour long migration to drop an index:

$ vtctldclient ApplySchema --ddl_strategy='vitess' --sql "alter table customer drop index joined_timestamp_idx" commerce
29231906_776f_11ec_a656_0a43f95f28a3

As soon as the migration completes, we run:

$ vtctldclient ApplySchema --ddl_strategy='vitess --postpone-completion --allow-concurrent' --sql "revert vitess_migration '29231906_776f_11ec_a656_0a43f95f28a3'" commerce
3cc4ae0e_776f_11ec_a656_0a43f95f28a3

The above begins a REVERT migration that is open-ended (does not complete), via --postpone-completion. We also request the migration to run concurrently via --allow-concurrent such that it does not block any further "normal" migrations on other tables. Note 3cc4ae0e_776f_11ec_a656_0a43f95f28a3 is the UUID for the reverted migration.

Finally, if we are satisfied that the drop index migration went well, we issue:

$ vtctldclient ApplySchema --sql "alter vitess_migration '3cc4ae0e_776f_11ec_a656_0a43f95f28a3' cancel" commerce

That is, we cancel the REVERT operation.

Or, should we have not dropped the index? If our migration seems to have been wrong, we run:

$ vtctldclient ApplySchema --sql "alter vitess_migration '3cc4ae0e_776f_11ec_a656_0a43f95f28a3' complete" commerce

Which means we want to apply the revert. Since the revert is already running in the background, it is likely that binary log processing is up to date, and cut-over is near instantaneous.

Inter-dependent migrations #

It is possible to submit inter-dependent migrations within the same ApplySchema command, and have them complete in the correct order, even if they run concurrently. Examples for inter-dependent migrations:

• Creating two new views, one of which reads from the other.
• Adding a column to a table, and creating a new view that reads from that column.
• Adding a column to a table, altering an existing view that reads from that table, to now read the new column.

In the above examples there has to be a strict ordering to the migrations. You cannot just create a view that reads from a yet non-existent column.

vitess offers the --in-order-completion DDL strategy flag. It is the responsibility of the user to supply the migrations in a valid ordering, and it is vitess's responsibility to complete the migrations in that same order.

Note that there can be scenarios with impossible ordering. Those hardly make sense in production, in the first place, and it is the user's responsibility to supply a sequence that works. When in doubt, it's advisable to submit migrations in stages: only apply one migration to completion, and then apply another.

An example for in-order submission:

$ vtctldclient ApplySchema --ddl_strategy='vitess --allow-concurrent --in-order-completion' --sql "create table t1 (id int primary key); create view v1 as select id from t1;" commerce