Migration Style Guide
When writing migrations for GitLab, you have to take into account that these are run by hundreds of thousands of organizations of all sizes, some with many years of data in their database.
In addition, having to take a server offline for an upgrade small or big is a big burden for most organizations. For this reason, it is important that your migrations are written carefully, can be applied online, and adhere to the style guide below.
Migrations are not allowed to require GitLab installations to be taken
offline ever. Migrations always must be written in such a way to avoid
downtime. In the past we had a process for defining migrations that allowed for
downtime by setting a DOWNTIME
constant. You may see this when looking at
older migrations. This process was in place for 4 years without ever being
used and as such we've learned we can always figure out how to write a migration
differently to avoid downtime.
When writing your migrations, also consider that databases might have stale data or inconsistencies and guard for that. Try to make as few assumptions as possible about the state of the database.
Don't depend on GitLab-specific code since it can change in future versions. If needed copy-paste GitLab code into the migration to make it forward compatible.
Choose an appropriate migration type
The first step before adding a new migration should be to decide which type is most appropriate.
There are currently three kinds of migrations you can create, depending on the kind of work it needs to perform and how long it takes to complete:
-
Regular schema migrations. These are traditional Rails migrations in
db/migrate
that run before new application code is deployed (for GitLab.com before Canary is deployed). This means that they should be relatively fast, no more than a few minutes, so as not to unnecessarily delay a deployment.One exception is a migration that takes longer but is absolutely critical for the application to operate correctly. For example, you might have indices that enforce unique tuples, or that are needed for query performance in critical parts of the application. In cases where the migration would be unacceptably slow, however, a better option might be to guard the feature with a feature flag and perform a post-deployment migration instead. The feature can then be turned on after the migration finishes.
Migrations used to add new models are also part of these regular schema migrations. The only differences are the Rails command used to generate the migrations and the additional generated files, one for the model and one for the model's spec.
-
Post-deployment migrations. These are Rails migrations in
db/post_migrate
and are run independently from the GitLab.com deployments. Pending post migrations are executed on a daily basis at the discretion of release manager through the post-deploy migration pipeline. These migrations can be used for schema changes that aren't critical for the application to operate, or data migrations that take at most a few minutes. Common examples for schema changes that should run post-deploy include:- Clean-ups, like removing unused columns.
- Adding non-critical indices on high-traffic tables.
- Adding non-critical indices that take a long time to create.
These migrations should not be used for schema changes that are critical for the application to operate. Making such schema changes in a post-deployment migration have caused issues in the past, for example this issue. Changes that should always be a regular schema migration and not be executed in a post-deployment migration include:
- Creating a new table, example:
create_table
. - Adding a new column to an existing table, example:
add_column
.
NOTE: Post-deployment migration is often abbreviated as PDM.
-
Batched background migrations. These aren't regular Rails migrations, but application code that is executed via Sidekiq jobs, although a post-deployment migration is used to schedule them. Use them only for data migrations that exceed the timing guidelines for post-deploy migrations. Batched background migrations should not change the schema.
Use the following diagram to guide your decision, but keep in mind that it is just a tool, and the final outcome will always be dependent on the specific changes being made:
graph LR
A{Schema<br/>changed?}
A -->|Yes| C{Critical to<br/>speed or<br/>behavior?}
A -->|No| D{Is it fast?}
C -->|Yes| H{Is it fast?}
C -->|No| F[Post-deploy migration]
H -->|Yes| E[Regular migration]
H -->|No| I[Post-deploy migration<br/>+ feature flag]
D -->|Yes| F[Post-deploy migration]
D -->|No| G[Background migration]
Also refer to Migration type to use when choosing which migration type to use when adding a database index.
How long a migration should take
In general, all migrations for a single deploy shouldn't take longer than 1 hour for GitLab.com. The following guidelines are not hard rules, they were estimated to keep migration duration to a minimum.
NOTE: Keep in mind that all durations should be measured against GitLab.com.
NOTE: The result of a database migration pipeline includes the timing information for migrations.
Migration Type | Recommended Duration | Notes |
---|---|---|
Regular migrations | <= 3 minutes |
A valid exception are changes without which application functionality or performance would be severely degraded and which cannot be delayed. |
Post-deployment migrations | <= 10 minutes |
A valid exception are schema changes, since they must not happen in background migrations. |
Background migrations | > 10 minutes |
Since these are suitable for larger tables, it's not possible to set a precise timing guideline, however, any single query must stay below 1 second execution time with cold caches. |
Decide which database to target
GitLab connects to two different Postgres databases: main
and ci
. This split can affect migrations
as they may run on either or both of these databases.
Read Migrations for Multiple databases to understand if or how a migration you add should account for this.
Create a regular schema migration
To create a migration you can use the following Rails generator:
bundle exec rails g migration migration_name_here
This generates the migration file in db/migrate
.
Regular schema migrations to add new models
To create a new model you can use the following Rails generator:
bundle exec rails g model model_name_here
This will generate:
- the migration file in
db/migrate
- the model file in
app/models
- the spec file in
spec/models
Schema Changes
Changes to the schema should be committed to db/structure.sql
. This
file is automatically generated by Rails when you run
bundle exec rails db:migrate
, so you typically should not
edit this file by hand. If your migration is adding a column to a
table, that column is added at the bottom. Do not reorder
columns manually for existing tables as this causes confusion to
other people using db/structure.sql
generated by Rails.
NOTE:
Creating an index asynchronously requires two merge requests.
When done, commit the schema change in the merge request
that adds the index with add_concurrent_index
.
When your local database in your GDK is diverging from the schema from
main
it might be hard to cleanly commit the schema changes to
Git. In that case you can use the scripts/regenerate-schema
script to
regenerate a clean db/structure.sql
for the migrations you're
adding. This script applies all migrations found in db/migrate
or db/post_migrate
, so if there are any migrations you don't want to
commit to the schema, rename or remove them. If your branch is not
targeting the default Git branch, you can set the TARGET
environment variable.
# Regenerate schema against `main`
scripts/regenerate-schema
# Regenerate schema against `12-9-stable-ee`
TARGET=12-9-stable-ee scripts/regenerate-schema
The scripts/regenerate-schema
script can create additional differences.
If this happens, use a manual procedure where <migration ID>
is the DATETIME
part of the migration file.
# Rebase against master
git rebase master
# Rollback changes
VERSION=<migration ID> bundle exec rails db:rollback:main
# Checkout db/structure.sql from master
git checkout origin/master db/structure.sql
# Migrate changes
VERSION=<migration ID> bundle exec rails db:migrate:main
After a table has been created, it should be added to the database dictionary, following the steps mentioned in the database dictionary guide.
Avoiding downtime
The document "Avoiding downtime in migrations" specifies various database operations, such as:
- dropping and renaming columns
- changing column constraints and types
- adding and dropping indexes, tables, and foreign keys
- migrating
integer
primary keys tobigint
and explains how to perform them without requiring downtime.
Reversibility
Your migration must be reversible. This is very important, as it should be possible to downgrade in case of a vulnerability or bugs.
Note: On GitLab production environments, if a problem occurs, a roll-forward strategy is used instead of rolling back migrations using db:rollback
.
On self-managed instances we advise users to restore the backup which was created before the upgrade process started.
The down
method is used primarily in the development environment, for example, when a developer wants to ensure
their local copy of structure.sql
file and database are in a consistent state when switching between commits or branches.
In your migration, add a comment describing how the reversibility of the migration was tested.
Some migrations cannot be reversed. For example, some data migrations can't be
reversed because we lose information about the state of the database before the migration.
You should still create a down
method with a comment, explaining why
the changes performed by the up
method can't be reversed, so that the
migration itself can be reversed, even if the changes performed during the migration
can't be reversed:
def down
# no-op
# comment explaining why changes performed by `up` cannot be reversed.
end
Migrations like this are inherently risky and additional actions are required when preparing the migration for review.
Atomicity and transaction
By default, migrations are a single transaction: it's opened at the beginning of the migration, and committed after all steps are processed.
Running migrations in a single transaction makes sure that if one of the steps fails, none of the steps are executed, leaving the database in a valid state. Therefore, either:
- Put all migrations in one single-transaction migration.
- If necessary, put most actions in one migration and create a separate migration for the steps that cannot be done in a single transaction.
For example, if you create an empty table and need to build an index for it,
you should use a regular single-transaction migration and the default
rails schema statement: add_index
.
This operation is a blocking operation, but it doesn't cause problems because the table is not yet used,
and therefore it does not have any records yet.
NOTE: Subtransactions are disallowed in general. Use multiple, separate transactions if needed as described in Heavy operations in a single transaction.
Heavy operations in a single transaction
When using a single-transaction migration, a transaction holds a database connection for the duration of the migration, so you must make sure the actions in the migration do not take too much time. In general, transactions must execute quickly. To that end, observe the maximum query time limit for each query run in the migration.
If your single-transaction migration takes long to finish, you have several options. In all cases, remember to select the appropriate migration type depending on how long a migration takes
-
Split the migration into multiple single-transaction migrations.
-
Use multiple transactions by using
disable_ddl_transaction!
. -
Keep using a single-transaction migration after adjusting statement and lock timeout settings. If your heavy workload must use the guarantees of a transaction, you should check your migration can execute without hitting the timeout limits. The same advice applies to both single-transaction migrations and individual transactions.
- Statement timeout: the statement timeout is configured to be
15s
for GitLab.com's production database but creating an index often takes more than 15 seconds. When you use the existing helpers includingadd_concurrent_index
, they automatically turn off the statement timeout as needed. In rare cases, you might need to set the timeout limit yourself by usingdisable_statement_timeout
.
- Statement timeout: the statement timeout is configured to be
NOTE:
To run migrations, we directly connect to the primary database, bypassing PgBouncer
to control settings like statement_timeout
and lock_wait_timeout
.
Temporarily turn off the statement timeout limit
The migration helper disable_statement_timeout
enables you to
temporarily set the statement timeout to 0
per transaction or per connection.
-
You use the per-connection option when your statement does not support running inside an explicit transaction, like
CREATE INDEX CONCURRENTLY
. -
If your statement does support an explicit transaction block, like
ALTER TABLE ... VALIDATE CONSTRAINT
, the per-transaction option should be used.
Using disable_statement_timeout
is rarely needed, because
the most migration helpers already use them internally when needed.
For example, creating an index usually takes more than 15 seconds,
which is the default statement timeout configured for GitLab.com's production database.
The helper add_concurrent_index
creates an index inside the block
passed to disable_statement_timeout
to disable the statement timeout per connection.
If you are writing raw SQL statements in a migration,
you may need to manually use disable_statement_timeout
.
Consult the database reviewers and maintainers when you do.
Disable transaction-wrapped migration
You can opt out of running your migration as a single transaction by using
disable_ddl_transaction!
, an ActiveRecord method.
The method might be called in other database systems, with different results.
At GitLab we exclusively use PostgreSQL.
You should always read disable_ddl_transaction!
as meaning:
"Do not execute this migration in a single PostgreSQL transaction. I'll open PostgreSQL transaction(s) only when and if I need them."
NOTE:
Even if you don't use an explicit PostgreSQL transaction .transaction
(or BEGIN; COMMIT;
),
every SQL statement is still executed as a transaction.
See the PostgreSQL documentation on transactions.
NOTE:
In GitLab, we've sometimes referred to
the migrations that used disable_ddl_transaction!
as non-transactional migrations.
It just meant the migrations were not executed as single transactions.
When should you use disable_ddl_transaction!
? In most cases,
the existing RuboCop rules or migration helpers can detect if you should be
using disable_ddl_transaction!
.
Skip disable_ddl_transaction!
if you are unsure whether to use it or not in your migration,
and let the RuboCop rules and database reviews guide you.
Use disable_ddl_transaction!
when PostgreSQL requires an operation to be executed outside an explicit transaction.
- The most prominent example of such operation is the command
CREATE INDEX CONCURRENTLY
. PostgreSQL allows the blocking version (CREATE INDEX
) to be run inside a transaction. UnlikeCREATE INDEX
,CREATE INDEX CONCURRENTLY
must be performed outside a transaction. Therefore, even though a migration may run just one statementCREATE INDEX CONCURRENTLY
, you should disabledisable_ddl_transaction!
. It's also the reason why the use of the helperadd_concurrent_index
requiresdisable_ddl_transaction!
CREATE INDEX CONCURRENTLY
is more of the exception than the rule.
Use disable_ddl_transaction!
when you need to run multiple transactions in a migration for any reason.
Most of the time you would be using multiple transactions to avoid running one slow transaction.
- For example, when you insert, update, or delete (DML) a large amount of data, you should perform them in batches. Should you need to group operations for each batch, you can explicitly open a transaction block when processing a batch. Consider using a batched background migration for any reasonably large workload.
Use disable_ddl_transaction!
when migration helpers require them.
Various migration helpers need to run with disable_ddl_transaction!
because they require a precise control on when and how to open transactions.
- A foreign key can be added inside a transaction, unlike
CREATE INDEX CONCURRENTLY
. However, PostgreSQL does not provide an option similar toCREATE INDEX CONCURRENTLY
. The helperadd_concurrent_foreign_key
instead opens its own transactions to lock the source and target table in a manner that minimizes locking while adding and validating the foreign key. - As advised earlier, skip
disable_ddl_transaction!
if you are unsure and see if any RuboCop check is violated.
Use disable_ddl_transaction!
when your migration does not actually touch PostgreSQL databases
or does touch multiple PostgreSQL databases.
- For example, your migration might target a Redis server. As a rule, you cannot interact with an external service inside a PostgreSQL transaction.
- A transaction is used for a single database connection.
If your migrations are targeting multiple databases, such as both
ci
andmain
database, follow Migrations for multiple databases.
Naming conventions
Names for database objects (such as tables, indexes, and views) must be lowercase. Lowercase names ensure that queries with unquoted names don't cause errors.
We keep column names consistent with ActiveRecord's schema conventions.
Custom index and constraint names should follow the constraint naming convention guidelines.
Truncate long index names
PostgreSQL limits the length of identifiers, like column or index names. Column names are not usually a problem, but index names tend to be longer. Some methods for shortening a name that's too long:
- Prefix it with
i_
instead ofindex_
. - Skip redundant prefixes. For example,
index_vulnerability_findings_remediations_on_vulnerability_remediation_id
becomesindex_vulnerability_findings_remediations_on_remediation_id
. - Instead of columns, specify the purpose of the index, such as
index_users_for_unconfirmation_notification
.
Migration timestamp age
The timestamp portion of a migration filename determines the order in which migrations are run. It's important to maintain a rough correlation between:
- When a migration is added to the GitLab codebase.
- The timestamp of the migration itself.
A new migration's timestamp should never be before the previous required upgrade stop. Migrations are occasionally squashed, and if a migration is added whose timestamp falls before the previous required stop, a problem like what happened in issue 408304 can occur.
For example, if we are currently developing against GitLab 16.0, the previous required stop is 15.11. 15.11 was released on April 23rd, 2023. Therefore, the minimum acceptable timestamp would be 20230424000000.
Best practice
While the above should be considered a hard rule, it is a best practice to try to keep migration timestamps to within three weeks of the date it is anticipated that the migration will be merged upstream, regardless of how much time has elapsed since the last required stop.
To update a migration timestamp:
-
Migrate down the migration for the
ci
andmain
databases:rake db:migrate:down:main VERSION=<timestamp> rake db:migrate:down:ci VERSION=<timestamp>
-
Delete the migration file.
-
Recreate the migration following the migration style guide.
Migration helpers and versioning
Various helper methods are available for many common patterns in database migrations. Those
helpers can be found in Gitlab::Database::MigrationHelpers
and related modules.
In order to allow changing a helper's behavior over time, we implement a versioning scheme for migration helpers. This allows us to maintain the behavior of a helper for already existing migrations but change the behavior for any new migrations.
For that purpose, all database migrations should inherit from Gitlab::Database::Migration
,
which is a "versioned" class. For new migrations, the latest version should be used (which
can be looked up in Gitlab::Database::Migration::MIGRATION_CLASSES
) to use the latest version
of migration helpers.
In this example, we use version 2.1 of the migration class:
class TestMigration < Gitlab::Database::Migration[2.1]
def change
end
end
Do not include Gitlab::Database::MigrationHelpers
directly into a
migration. Instead, use the latest version of Gitlab::Database::Migration
, which exposes the latest
version of migration helpers automatically.
Retry mechanism when acquiring database locks
When changing the database schema, we use helper methods to invoke DDL (Data Definition Language) statements. In some cases, these DDL statements require a specific database lock.
Example:
def change
remove_column :users, :full_name, :string
end
Executing this migration requires an exclusive lock on the users
table. When the table
is concurrently accessed and modified by other processes, acquiring the lock may take
a while. The lock request is waiting in a queue and it may also block other queries
on the users
table once it has been enqueued.
More information about PostgreSQL locks: Explicit Locking
For stability reasons, GitLab.com has a short statement_timeout
set. When the migration is invoked, any database query has
a fixed time to execute. In a worst-case scenario, the request sits in the
lock queue, blocking other queries for the duration of the configured statement timeout,
then failing with canceling statement due to statement timeout
error.
This problem could cause failed application upgrade processes and even application stability issues, since the table may be inaccessible for a short period of time.
To increase the reliability and stability of database migrations, the GitLab codebase
offers a method to retry the operations with different lock_timeout
settings
and wait time between the attempts. Multiple shorter attempts to acquire the necessary
lock allow the database to process other statements.
Lock retries are controlled by two different helpers:
-
enable_lock_retries!
: enabled by default for alltransactional
migrations. -
with_lock_retries
: enabled manually for a block withinnon-transactional
migrations.
Transactional migrations
Regular migrations execute the full migration in a transaction. lock-retry mechanism is enabled by default (unless disable_ddl_transaction!
).
This leads to the lock timeout being controlled for the migration. Also, it can lead to retrying the full migration if the lock could not be granted within the timeout.
Occasionally a migration may need to acquire multiple locks on different objects. To prevent catalog bloat, ask for all those locks explicitly before performing any DDL. A better strategy is to split the migration, so that we only need to acquire one lock at the time.
Multiple changes on the same table
With the lock-retry methodology enabled, all operations wrap into a single transaction. When you have the lock, you should do as much as possible inside the transaction rather than trying to get another lock later. Be careful about running long database statements within the block. The acquired locks are kept until the transaction (block) finishes and depending on the lock type, it might block other database operations.
def up
add_column :users, :full_name, :string
add_column :users, :bio, :string
end
def down
remove_column :users, :full_name
remove_column :users, :bio
end
Changing default value for a column
Changing column defaults can cause application downtime if a multi-release process is not followed. See avoiding downtime in migrations for changing column defaults for details.
def up
change_column_default :merge_requests, :lock_version, from: nil, to: 0
end
def down
change_column_default :merge_requests, :lock_version, from: 0, to: nil
end
Creating a new table when we have two foreign keys
Only one foreign key should be created per transaction. This is because the addition of a foreign key constraint requires a SHARE ROW EXCLUSIVE
lock on the referenced table, and locking multiple tables in the same transaction should be avoided.
For this, we need three migrations:
- Creating the table without foreign keys (with the indices).
- Add foreign key to the first table.
- Add foreign key to the second table.
Creating the table:
def up
create_table :imports do |t|
t.bigint :project_id, null: false
t.bigint :user_id, null: false
t.string :jid, limit: 255
t.index :project_id
t.index :user_id
end
end
def down
drop_table :imports
end
Adding foreign key to projects
:
We can use the add_concurrent_foreign_key
method in this case, as this helper method
has the lock retries built into it.
disable_ddl_transaction!
def up
add_concurrent_foreign_key :imports, :projects, column: :project_id, on_delete: :cascade
end
def down
with_lock_retries do
remove_foreign_key :imports, column: :project_id
end
end
Adding foreign key to users
:
disable_ddl_transaction!
def up
add_concurrent_foreign_key :imports, :users, column: :user_id, on_delete: :cascade
end
def down
with_lock_retries do
remove_foreign_key :imports, column: :user_id
end
end
Usage with non-transactional migrations
Only when we disable transactional migrations using disable_ddl_transaction!
, we can use
the with_lock_retries
helper to guard an individual sequence of steps. It opens a transaction
to execute the given block.
A custom RuboCop rule ensures that only allowed methods can be placed within the lock retries block.
disable_ddl_transaction!
def up
with_lock_retries do
add_column(:users, :name, :text, if_not_exists: true)
end
add_text_limit :users, :name, 255 # Includes constraint validation (full table scan)
end
The RuboCop rule generally allows standard Rails migration methods, listed below. This example causes a RuboCop offense:
disable_ddl_transaction!
def up
with_lock_retries do
add_concurrent_index :users, :name
end
end
When to use the helper method
You can only use the with_lock_retries
helper method when the execution is not already inside
an open transaction (using PostgreSQL subtransactions is discouraged). It can be used with
standard Rails migration helper methods. Calling more than one migration
helper is not a problem if they're executed on the same table.
Using the with_lock_retries
helper method is advised when a database
migration involves one of the high-traffic tables.
Example changes:
-
add_foreign_key
/remove_foreign_key
-
add_column
/remove_column
change_column_default
-
create_table
/drop_table
The with_lock_retries
method cannot be used within the change
method, you must manually define the up
and down
methods to make the migration reversible.
How the helper method works
- Iterate 50 times.
- For each iteration, set a pre-configured
lock_timeout
. - Try to execute the given block. (
remove_column
). - If
LockWaitTimeout
error is raised, sleep for the pre-configuredsleep_time
and retry the block. - If no error is raised, the current iteration has successfully executed the block.
For more information check the Gitlab::Database::WithLockRetries
class. The with_lock_retries
helper method is implemented in the Gitlab::Database::MigrationHelpers
module.
In a worst-case scenario, the method:
- Executes the block for a maximum of 50 times over 40 minutes.
- Most of the time is spent in a pre-configured sleep period after each iteration.
- After the 50th retry, the block is executed without
lock_timeout
, just like a standard migration invocation. - If a lock cannot be acquired, the migration fails with
statement timeout
error.
The migration might fail if there is a very long running transaction (40+ minutes)
accessing the users
table.
Lock-retry methodology at the SQL level
In this section, we provide a simplified SQL example that demonstrates the use of lock_timeout
.
You can follow along by running the given snippets in multiple psql
sessions.
When altering a table to add a column,
AccessExclusiveLock
, which conflicts with most lock types, is required on the table.
If the target table is a very busy one, the transaction adding the column
may fail to acquire AccessExclusiveLock
in a timely fashion.
Suppose a transaction is attempting to insert a row into a table:
-- Transaction 1
BEGIN;
INSERT INTO my_notes (id) VALUES (1);
At this point Transaction 1 acquired RowExclusiveLock
on my_notes
.
Transaction 1 could still execute more statements prior to committing or aborting.
There could be other similar, concurrent transactions that touch my_notes
.
Suppose a transactional migration is attempting to add a column to the table without using any lock retry helper:
-- Transaction 2
BEGIN;
ALTER TABLE my_notes ADD COLUMN title text;
Transaction 2 is now blocked because it cannot acquire
AccessExclusiveLock
on my_notes
table
as Transaction 1 is still executing and holding the RowExclusiveLock
on my_notes
.
A more pernicious effect is blocking the transactions that would
normally not conflict with Transaction 1 because Transaction 2
is queueing to acquire AccessExclusiveLock
.
In a normal situation, if another transaction attempted to read from and write
to the same table my_notes
at the same time as Transaction 1,
the transaction would go through
since the locks needed for reading and writing would not
conflict with RowExclusiveLock
held by Transaction 1.
However, when the request to acquire AccessExclusiveLock
is queued,
the subsequent requests for conflicting locks on the table would block although
they could be executed concurrently alongside Transaction 1.
If we used with_lock_retries
, Transaction 2 would instead quickly
timeout after failing to acquire the lock within the specified time period
and allow other transactions to proceed:
-- Transaction 2 (version with with lock timeout)
BEGIN;
SET LOCAL lock_timeout to '100ms'; -- added by the lock retry helper.
ALTER TABLE my_notes ADD COLUMN title text;
The lock retry helper would repeatedly try the same transaction at different time intervals until it succeeded.
SET LOCAL
scopes the parameter (lock_timeout
) change to
the transaction.
Removing indexes
If the table is not empty when removing an index, make sure to use the method
remove_concurrent_index
instead of the regular remove_index
method.
The remove_concurrent_index
method drops indexes concurrently, so no locking is required,
and there is no need for downtime. To use this method, you must disable single-transaction mode
by calling the method disable_ddl_transaction!
in the body of your migration
class like so:
class MyMigration < Gitlab::Database::Migration[2.1]
disable_ddl_transaction!
INDEX_NAME = 'index_name'
def up
remove_concurrent_index :table_name, :column_name, name: INDEX_NAME
end
end
You can verify that the index is not being used with Grafana:
sum by (type)(rate(pg_stat_user_indexes_idx_scan{env="gprd", indexrelname="INSERT INDEX NAME HERE"}[30d]))
It is not necessary to check if the index exists prior to
removing it, however it is required to specify the name of the
index that is being removed. This can be done either by passing the name
as an option to the appropriate form of remove_index
or remove_concurrent_index
,
or by using the remove_concurrent_index_by_name
method. Explicitly
specifying the name is important to ensure the correct index is removed.
For a small table (such as an empty one or one with less than 1,000
records),
it is recommended to use remove_index
in a single-transaction migration,
combining it with other operations that don't require disable_ddl_transaction!
.
Disabling an index
Disabling an index is not a safe operation.
Adding indexes
Before adding an index, consider if one is necessary. The Adding Database indexes guide contains more details to help you decide if an index is necessary and provides best practices for adding indexes.
Testing for existence of indexes
If a migration requires conditional logic based on the absence or presence of an index, you must test for existence of that index using its name. This helps avoids problems with how Rails compares index definitions, which can lead to unexpected results.
For more details, review the Adding Database Indexes guide.
Adding foreign-key constraints
When adding a foreign-key constraint to either an existing or a new column also remember to add an index on the column.
This is required for all foreign-keys, for example, to support efficient cascading deleting: when a lot of rows in a table get deleted, the referenced records need to be deleted too. The database has to look for corresponding records in the referenced table. Without an index, this results in a sequential scan on the table, which can take a long time.
Here's an example where we add a new column with a foreign key
constraint. Note it includes index: true
to create an index for it.
class Migration < Gitlab::Database::Migration[2.1]
def change
add_reference :model, :other_model, index: true, foreign_key: { on_delete: :cascade }
end
end
When adding a foreign-key constraint to an existing column in a non-empty table,
we have to employ add_concurrent_foreign_key
and add_concurrent_index
instead of add_reference
.
If you have a new or empty table that doesn't reference a
high-traffic table,
we recommend that you use add_reference
in a single-transaction migration. You can
combine it with other operations that don't require disable_ddl_transaction!
.
You can read more about adding foreign key constraints to an existing column.
NOT NULL
constraints
See the style guide on NOT NULL
constraints for more information.
Adding Columns With Default Values
With PostgreSQL 11 being the minimum version in GitLab, adding columns with default values has become much easier and
the standard add_column
helper should be used in all cases.
Before PostgreSQL 11, adding a column with a default was problematic as it would have caused a full table rewrite.
Removing the column default for non-nullable columns
If you have added a non-nullable column, and used the default value to populate existing data, you need to keep that default value around until at least after the application code is updated. You cannot remove the default value in the same migration, as the migrations run before the model code is updated and models will have an old schema cache, meaning they won't know about this column and won't be able to set it. In this case it's recommended to:
- Add the column with default value in a standard migration.
- Remove the default in a post-deployment migration.
The post-deployment migration happens after the application restarts, ensuring the new column has been discovered.
Changing the column default
One might think that changing a default column with change_column_default
is an
expensive and disruptive operation for larger tables, but in reality it's not.
Take the following migration as an example:
class DefaultRequestAccessGroups < Gitlab::Database::Migration[2.1]
def change
change_column_default(:namespaces, :request_access_enabled, from: false, to: true)
end
end
Migration above changes the default column value of one of our largest
tables: namespaces
. This can be translated to:
ALTER TABLE namespaces
ALTER COLUMN request_access_enabled
SET DEFAULT false
In this particular case, the default value exists and we're just changing the metadata for
request_access_enabled
column, which does not imply a rewrite of all the existing records
in the namespaces
table. Only when creating a new column with a default, all the records are going be rewritten.
NOTE: A faster ALTER TABLE ADD COLUMN with a non-null default was introduced on PostgreSQL 11.0, removing the need of rewriting the table when a new column with a default value is added.
For the reasons mentioned above, it's safe to use change_column_default
in a single-transaction migration
without requiring disable_ddl_transaction!
.
Updating an existing column
To update an existing column to a particular value, you can use
update_column_in_batches
. This splits the updates into batches, so we
don't update too many rows at in a single statement.
This updates the column foo
in the projects
table to 10, where some_column
is 'hello'
:
update_column_in_batches(:projects, :foo, 10) do |table, query|
query.where(table[:some_column].eq('hello'))
end
If a computed update is needed, the value can be wrapped in Arel.sql
, so Arel
treats it as an SQL literal. It's also a required deprecation for Rails 6.
The below example is the same as the one above, but
the value is set to the product of the bar
and baz
columns:
update_value = Arel.sql('bar * baz')
update_column_in_batches(:projects, :foo, update_value) do |table, query|
query.where(table[:some_column].eq('hello'))
end
In the case of update_column_in_batches
, it may be acceptable
to run on a large table, as long as it is only updating a small subset of the
rows in the table, but do not ignore that without validating on the GitLab.com
staging environment - or asking someone else to do so for you - beforehand.
Removing a foreign key constraint
When removing a foreign key constraint, we need to acquire a lock on both tables
that are related to the foreign key. For tables with heavy write patterns, it's a good
idea to use with_lock_retries
, otherwise you might fail to acquire a lock in time.
You might also run into deadlocks when acquiring a lock, because ordinarily
the application writes in parent,child
order. However, removing a foreign
key acquires the lock in child,parent
order. To resolve this, you can
explicitly acquire the lock in parent,child
, for example:
disable_ddl_transaction!
def up
with_lock_retries do
execute('lock table ci_pipelines, ci_builds in access exclusive mode')
remove_foreign_key :ci_builds, to_table: :ci_pipelines, column: :pipeline_id, on_delete: :cascade, name: 'the_fk_name'
end
end
def down
add_concurrent_foreign_key :ci_builds, :ci_pipelines, column: :pipeline_id, on_delete: :cascade, name: 'the_fk_name'
end
Dropping a database table
NOTE: After a table has been dropped, it should be added to the database dictionary, following the steps in the database dictionary guide.
Dropping a database table is uncommon, and the drop_table
method
provided by Rails is generally considered safe. Before dropping the table,
consider the following:
If your table has foreign keys on a high-traffic table (like projects
), then
the DROP TABLE
statement is likely to stall concurrent traffic until it fails with statement timeout error.
Table has no records (feature was never in use) and no foreign keys:
- Use the
drop_table
method in your migration.
def change
drop_table :my_table
end
Table has records but no foreign keys:
- Remove the application code related to the table, such as models, controllers and services.
- In a post-deployment migration, use
drop_table
.
This can all be in a single migration if you're sure the code is not used. If you want to reduce risk slightly, consider putting the migrations into a second merge request after the application changes are merged. This approach provides an opportunity to roll back.
def up
drop_table :my_table
end
def down
# create_table ...
end
Table has foreign keys:
- Remove the application code related to the table, such as models, controllers, and services.
- In a post-deployment migration, remove the foreign keys using the
with_lock_retries
helper method. In another subsequent post-deployment migration, usedrop_table
.
This can all be in a single migration if you're sure the code is not used. If you want to reduce risk slightly, consider putting the migrations into a second merge request after the application changes are merged. This approach provides an opportunity to roll back.
Removing the foreign key on the projects
table using a non-transactional migration:
# first migration file
class RemovingForeignKeyMigrationClass < Gitlab::Database::Migration[2.1]
disable_ddl_transaction!
def up
with_lock_retries do
remove_foreign_key :my_table, :projects
end
end
def down
add_concurrent_foreign_key :my_table, :projects, column: COLUMN_NAME
end
end
Dropping the table:
# second migration file
class DroppingTableMigrationClass < Gitlab::Database::Migration[2.1]
def up
drop_table :my_table
end
def down
# create_table with the same schema but without the removed foreign key ...
end
end
Dropping a sequence
- Introduced in GitLab 15.1.
Dropping a sequence is uncommon, but you can use the drop_sequence
method provided by the database team.
Under the hood, it works like this:
Remove a sequence:
- Remove the default value if the sequence is actually used.
- Execute
DROP SEQUENCE
.
Re-add a sequence:
- Create the sequence, with the possibility of specifying the current value.
- Change the default value of the column.
A Rails migration example:
class DropSequenceTest < Gitlab::Database::Migration[2.1]
def up
drop_sequence(:ci_pipelines_config, :pipeline_id, :ci_pipelines_config_pipeline_id_seq)
end
def down
default_value = Ci::Pipeline.maximum(:id) + 10_000
add_sequence(:ci_pipelines_config, :pipeline_id, :ci_pipelines_config_pipeline_id_seq, default_value)
end
end
NOTE:
add_sequence
should be avoided for columns with foreign keys.
Adding sequence to these columns is only allowed in the down method (restore previous schema state).
Truncate a table
- Introduced in GitLab 15.11.
Truncating a table is uncommon, but you can use the truncate_tables!
method provided by the database team.
Under the hood, it works like this:
- Finds the
gitlab_schema
for the tables to be truncated. - If the
gitlab_schema
for the tables is included in the connection'sgitlab_schema
s, it then executes theTRUNCATE
statement. - If the
gitlab_schema
for the tables is not included in the connection'sgitlab_schema
s, it does nothing.
Swapping primary key
- Introduced in GitLab 15.5.
Swapping the primary key is required to partition a table as the partition key must be included in the primary key.
You can use the swap_primary_key
method provided by the database team.
Under the hood, it works like this:
- Drop the primary key constraint.
- Add the primary key using the index defined beforehand.
class SwapPrimaryKey < Gitlab::Database::Migration[2.1]
disable_ddl_transaction!
TABLE_NAME = :table_name
PRIMARY_KEY = :table_name_pkey
OLD_INDEX_NAME = :old_index_name
NEW_INDEX_NAME = :new_index_name
def up
swap_primary_key(TABLE_NAME, PRIMARY_KEY, NEW_INDEX_NAME)
end
def down
add_concurrent_index(TABLE_NAME, :id, unique: true, name: OLD_INDEX_NAME)
add_concurrent_index(TABLE_NAME, [:id, :partition_id], unique: true, name: NEW_INDEX_NAME)
unswap_primary_key(TABLE_NAME, PRIMARY_KEY, OLD_INDEX_NAME)
end
end
NOTE: Make sure to introduce the new index beforehand in a separate migration in order to swap the primary key.
Integer column type
By default, an integer column can hold up to a 4-byte (32-bit) number. That is a max value of 2,147,483,647. Be aware of this when creating a column that holds file sizes in byte units. If you are tracking file size in bytes, this restricts the maximum file size to just over 2GB.
To allow an integer column to hold up to an 8-byte (64-bit) number, explicitly
set the limit to 8-bytes. This allows the column to hold a value up to
9,223,372,036,854,775,807
.
Rails migration example:
add_column(:projects, :foo, :integer, default: 10, limit: 8)
Strings and the Text data type
See the text data type style guide for more information.
Timestamp column type
By default, Rails uses the timestamp
data type that stores timestamp data
without time zone information. The timestamp
data type is used by calling
either the add_timestamps
or the timestamps
method.
Also, Rails converts the :datetime
data type to the timestamp
one.
Example:
# timestamps
create_table :users do |t|
t.timestamps
end
# add_timestamps
def up
add_timestamps :users
end
# :datetime
def up
add_column :users, :last_sign_in, :datetime
end
Instead of using these methods, one should use the following methods to store timestamps with time zones:
add_timestamps_with_timezone
timestamps_with_timezone
datetime_with_timezone
This ensures all timestamps have a time zone specified. This, in turn, means existing timestamps don't suddenly use a different time zone when the system's time zone changes. It also makes it very clear which time zone was used in the first place.
Storing JSON in database
The Rails 5 natively supports JSONB
(binary JSON) column type.
Example migration adding this column:
class AddOptionsToBuildMetadata < Gitlab::Database::Migration[2.1]
def change
add_column :ci_builds_metadata, :config_options, :jsonb
end
end
By default hash keys will be strings. Optionally you can add a custom data type to provide different access to keys.
class BuildMetadata
attribute :config_options, :ind_jsonb # for indifferent accesss or :sym_jsonb if you need symbols only as keys.
end
When using a JSONB
column, use the JsonSchemaValidator to keep control of the data being inserted over time.
class BuildMetadata
validates :config_options, json_schema: { filename: 'build_metadata_config_option' }
end
Additionally, you can expose the keys in a JSONB
column as
ActiveRecord attributes. Do this when you need complex validations,
or ActiveRecord change tracking. This feature is provided by the
jsonb_accessor
gem,
and does not replace JsonSchemaValidator
.
module Organizations
class OrganizationSetting < ApplicationRecord
belongs_to :organization
validates :settings, json_schema: { filename: "organization_settings" }
jsonb_accessor :settings,
restricted_visibility_levels: [:integer, { array: true }]
validates_each :restricted_visibility_levels do |record, attr, value|
value&.each do |level|
unless Gitlab::VisibilityLevel.options.value?(level)
record.errors.add(attr, format(_("'%{level}' is not a valid visibility level"), level: level))
end
end
end
end
end
You can now use restricted_visibility_levels
as an ActiveRecord attribute:
> s = Organizations::OrganizationSetting.find(1)
=> #<Organizations::OrganizationSetting:0x0000000148d67628>
> s.settings
=> {"restricted_visibility_levels"=>[20]}
> s.restricted_visibility_levels
=> [20]
> s.restricted_visibility_levels = [0]
=> [0]
> s.changes
=> {"settings"=>[{"restricted_visibility_levels"=>[20]}, {"restricted_visibility_levels"=>[0]}], "restricted_visibility_levels"=>[[20], [0]]}
Encrypted attributes
Do not store attr_encrypted
attributes as :text
in the database; use
:binary
instead. This uses the bytea
type in PostgreSQL and makes storage more
efficient:
class AddSecretToSomething < Gitlab::Database::Migration[2.1]
def change
add_column :something, :encrypted_secret, :binary
add_column :something, :encrypted_secret_iv, :binary
end
end
When storing encrypted attributes in a binary column, we need to provide the
encode: false
and encode_iv: false
options to attr_encrypted
:
class Something < ApplicationRecord
attr_encrypted :secret,
mode: :per_attribute_iv,
key: Settings.attr_encrypted_db_key_base_32,
algorithm: 'aes-256-gcm',
encode: false,
encode_iv: false
end
Testing
See the Testing Rails migrations style guide.
Data migration
Prefer Arel and plain SQL over usual ActiveRecord syntax. In case of
using plain SQL, you need to quote all input manually with quote_string
helper.
Example with Arel:
users = Arel::Table.new(:users)
users.group(users[:user_id]).having(users[:id].count.gt(5))
#update other tables with these results
Example with plain SQL and quote_string
helper:
select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(id) > 1").each do |tag|
tag_name = quote_string(tag["name"])
duplicate_ids = select_all("SELECT id FROM tags WHERE name = '#{tag_name}'").map{|tag| tag["id"]}
origin_tag_id = duplicate_ids.first
duplicate_ids.delete origin_tag_id
execute("UPDATE taggings SET tag_id = #{origin_tag_id} WHERE tag_id IN(#{duplicate_ids.join(",")})")
execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})")
end
If you need more complex logic, you can define and use models local to a migration. For example:
class MyMigration < Gitlab::Database::Migration[2.1]
class Project < MigrationRecord
self.table_name = 'projects'
end
def up
# Reset the column information of all the models that update the database
# to ensure the Active Record's knowledge of the table structure is current
Project.reset_column_information
# ... ...
end
end
When doing so be sure to explicitly set the model's table name, so it's not derived from the class name or namespace.
Be aware of the limitations when using models in migrations.
Modifying existing data
In most circumstances, prefer migrating data in batches when modifying data in the database.
We introduced a new helper each_batch_range
which facilitates the process of iterating over a collection in a performant way. The default size of the batch is defined in the BATCH_SIZE
constant.
See the following example to get an idea.
Purging data in batch:
include ::Gitlab::Database::DynamicModelHelpers
disable_ddl_transaction!
def up
each_batch_range('ci_pending_builds', scope: ->(table) { table.ref_protected }, of: BATCH_SIZE) do |min, max|
execute <<~SQL
DELETE FROM ci_pending_builds
USING ci_builds
WHERE ci_builds.id = ci_pending_builds.build_id
AND ci_builds.status != 'pending'
AND ci_builds.type = 'Ci::Build'
AND ci_pending_builds.id BETWEEN #{min} AND #{max}
SQL
end
end
- The first argument is the table being modified:
'ci_pending_builds'
. - The second argument calls a lambda which fetches the relevant dataset selected (the default is set to
.all
):scope: ->(table) { table.ref_protected }
. - The third argument is the batch size (the default is set in the
BATCH_SIZE
constant):of: BATCH_SIZE
.
Here is an example MR illustrating how to use our new helper.
Using application code in migrations (discouraged)
The use of application code (including models) in migrations is generally discouraged. This is because the migrations stick around for a long time and the application code it depends on may change and break the migration in future. In the past some background migrations needed to use application code in order to avoid copying hundreds of lines of code spread across multiple files into the migration. In these rare cases it's critical to ensure the migration has good tests so that anyone refactoring the code in future will learn if they break the migration. Using application code is also discouraged for batched background migrations , the model needs to be declared in the migration.
Usually you can avoid using application code (specifically models) in a
migration by defining a class that inherits from MigrationRecord
(see
examples below).
If using are using a model (including defined in the migration), you should
first
clear the column cache
using reset_column_information
.
If using a model that leverages single table inheritance (STI), there are special considerations.
This avoids problems where a column that you are using was altered and cached in a previous migration.
my_column
to the users table
Example: Add a column It is important not to leave out the User.reset_column_information
command, to ensure that the old schema is dropped from the cache and ActiveRecord loads the updated schema information.
class AddAndSeedMyColumn < Gitlab::Database::Migration[2.1]
class User < MigrationRecord
self.table_name = 'users'
end
def up
User.count # Any ActiveRecord calls on the model that caches the column information.
add_column :users, :my_column, :integer, default: 1
User.reset_column_information # The old schema is dropped from the cache.
User.find_each do |user|
user.my_column = 42 if some_condition # ActiveRecord sees the correct schema here.
user.save!
end
end
end
The underlying table is modified and then accessed by using ActiveRecord.
This also needs to be used if the table is modified in a previous, different migration,
if both migrations are run in the same db:migrate
process.
This results in the following. Note the inclusion of my_column
:
== 20200705232821 AddAndSeedMyColumn: migrating ==============================
D, [2020-07-06T00:37:12.483876 #130101] DEBUG -- : (0.2ms) BEGIN
D, [2020-07-06T00:37:12.521660 #130101] DEBUG -- : (0.4ms) SELECT COUNT(*) FROM "user"
-- add_column(:users, :my_column, :integer, {:default=>1})
D, [2020-07-06T00:37:12.523309 #130101] DEBUG -- : (0.8ms) ALTER TABLE "users" ADD "my_column" integer DEFAULT 1
-> 0.0016s
D, [2020-07-06T00:37:12.650641 #130101] DEBUG -- : AddAndSeedMyColumn::User Load (0.7ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1 [["LIMIT", 1000]]
D, [2020-07-18T00:41:26.851769 #459802] DEBUG -- : AddAndSeedMyColumn::User Update (1.1ms) UPDATE "users" SET "my_column" = $1, "updated_at" = $2 WHERE "users"."id" = $3 [["my_column", 42], ["updated_at", "2020-07-17 23:41:26.849044"], ["id", 1]]
D, [2020-07-06T00:37:12.653648 #130101] DEBUG -- : ↳ config/initializers/config_initializers_active_record_locking.rb:13:in `_update_row'
== 20200705232821 AddAndSeedMyColumn: migrated (0.1706s) =====================
If you skip clearing the schema cache (User.reset_column_information
), the column is not
used by ActiveRecord and the intended changes are not made, leading to the result below,
where my_column
is missing from the query.
== 20200705232821 AddAndSeedMyColumn: migrating ==============================
D, [2020-07-06T00:37:12.483876 #130101] DEBUG -- : (0.2ms) BEGIN
D, [2020-07-06T00:37:12.521660 #130101] DEBUG -- : (0.4ms) SELECT COUNT(*) FROM "user"
-- add_column(:users, :my_column, :integer, {:default=>1})
D, [2020-07-06T00:37:12.523309 #130101] DEBUG -- : (0.8ms) ALTER TABLE "users" ADD "my_column" integer DEFAULT 1
-> 0.0016s
D, [2020-07-06T00:37:12.650641 #130101] DEBUG -- : AddAndSeedMyColumn::User Load (0.7ms) SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT $1 [["LIMIT", 1000]]
D, [2020-07-06T00:37:12.653459 #130101] DEBUG -- : AddAndSeedMyColumn::User Update (0.5ms) UPDATE "users" SET "updated_at" = $1 WHERE "users"."id" = $2 [["updated_at", "2020-07-05 23:37:12.652297"], ["id", 1]]
D, [2020-07-06T00:37:12.653648 #130101] DEBUG -- : ↳ config/initializers/config_initializers_active_record_locking.rb:13:in `_update_row'
== 20200705232821 AddAndSeedMyColumn: migrated (0.1706s) =====================
High traffic tables
Here's a list of current high-traffic tables.
Determining what tables are high-traffic can be difficult. Self-managed instances might use different features of GitLab with different usage patterns, thus making assumptions based on GitLab.com not enough.
To identify a high-traffic table for GitLab.com the following measures are considered. The metrics linked here are GitLab-internal only:
- Read operations
- Number of records
- Size is greater than 10 GB
Any table which has some high read operation compared to current high-traffic tables might be a good candidate.
As a general rule, we discourage adding columns to high-traffic tables that are purely for analytics or reporting of GitLab.com. This can have negative performance impacts for all self-managed instances without providing direct feature value to them.
Milestone
Beginning in GitLab 16.6, all new migrations must specify a milestone, using the following syntax:
class AddFooToBar < Gitlab::Database::Migration[2.2]
milestone '16.6'
def change
# Your migration here
end
end
Adding the correct milestone to a migration enables us to logically partition migrations into their corresponding GitLab minor versions. This:
- Simplifies the upgrade process.
- Alleviates potential migration ordering issues that arise when we rely solely on the migration's timestamp for ordering.
Autovacuum wraparound protection
This is a special autovacuum
run mode for PostgreSQL and it requires a ShareUpdateExclusiveLock
on the
table that it is vacuuming. For larger tables
this could take hours and the lock can conflict with most DDL migrations that
try to modify the table at the same time. Because the migrations will not be
able to acquire the lock in time, they will fail and block the deployments.
The post-deploy migration (PDM) pipeline can check and halt its execution if it
detects a wraparound prevention vacuum process on one of the tables. For this to
happen we need to use the complete table name in the migration name. For example
add_foreign_key_between_ci_builds_and_ci_job_artifacts
will check for vacuum
on ci_builds
and ci_job_artifacts
before executing the migrations.
If the migration doesn't have conflicting locks, the vacuum check can be skipped
by not using the complete table name, for example create_async_index_on_job_artifacts
.