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diff --git a/doc/development/database/avoiding_downtime_in_migrations.md b/doc/development/database/avoiding_downtime_in_migrations.md new file mode 100644 index 00000000000..ad2768397e6 --- /dev/null +++ b/doc/development/database/avoiding_downtime_in_migrations.md @@ -0,0 +1,491 @@ +--- +stage: Enablement +group: Database +info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments +--- + +# Avoiding downtime in migrations + +When working with a database certain operations may require downtime. Since we +cannot have downtime in migrations we need to use a set of steps to get the +same end result without downtime. This guide describes various operations that +may appear to need downtime, their impact, and how to perform them without +requiring downtime. + +## Dropping Columns + +Removing columns is tricky because running GitLab processes may still be using +the columns. To work around this safely, you will need three steps in three releases: + +1. Ignoring the column (release M) +1. Dropping the column (release M+1) +1. Removing the ignore rule (release M+2) + +The reason we spread this out across three releases is that dropping a column is +a destructive operation that can't be rolled back easily. + +Following this procedure helps us to make sure there are no deployments to GitLab.com +and upgrade processes for self-managed installations that lump together any of these steps. + +### Step 1: Ignoring the column (release M) + +The first step is to ignore the column in the application code. This is +necessary because Rails caches the columns and re-uses this cache in various +places. This can be done by defining the columns to ignore. For example, to ignore +`updated_at` in the User model you'd use the following: + +```ruby +class User < ApplicationRecord + include IgnorableColumns + ignore_column :updated_at, remove_with: '12.7', remove_after: '2020-01-22' +end +``` + +Multiple columns can be ignored, too: + +```ruby +ignore_columns %i[updated_at created_at], remove_with: '12.7', remove_after: '2020-01-22' +``` + +If the model exists in CE and EE, the column has to be ignored in the CE model. If the +model only exists in EE, then it has to be added there. + +We require indication of when it is safe to remove the column ignore with: + +- `remove_with`: set to a GitLab release typically two releases (M+2) after adding the + column ignore. +- `remove_after`: set to a date after which we consider it safe to remove the column + ignore, typically after the M+1 release date, during the M+2 development cycle. + +This information allows us to reason better about column ignores and makes sure we +don't remove column ignores too early for both regular releases and deployments to GitLab.com. For +example, this avoids a situation where we deploy a bulk of changes that include both changes +to ignore the column and subsequently remove the column ignore (which would result in a downtime). + +In this example, the change to ignore the column went into release 12.5. + +### Step 2: Dropping the column (release M+1) + +Continuing our example, dropping the column goes into a _post-deployment_ migration in release 12.6: + +```ruby + remove_column :user, :updated_at +``` + +### Step 3: Removing the ignore rule (release M+2) + +With the next release, in this example 12.7, we set up another merge request to remove the ignore rule. +This removes the `ignore_column` line and - if not needed anymore - also the inclusion of `IgnoreableColumns`. + +This should only get merged with the release indicated with `remove_with` and once +the `remove_after` date has passed. + +## Renaming Columns + +Renaming columns the normal way requires downtime as an application may continue +using the old column name during/after a database migration. To rename a column +without requiring downtime we need two migrations: a regular migration, and a +post-deployment migration. Both these migration can go in the same release. + +### Step 1: Add The Regular Migration + +First we need to create the regular migration. This migration should use +`Gitlab::Database::MigrationHelpers#rename_column_concurrently` to perform the +renaming. For example + +```ruby +# A regular migration in db/migrate +class RenameUsersUpdatedAtToUpdatedAtTimestamp < Gitlab::Database::Migration[1.0] + disable_ddl_transaction! + + def up + rename_column_concurrently :users, :updated_at, :updated_at_timestamp + end + + def down + undo_rename_column_concurrently :users, :updated_at, :updated_at_timestamp + end +end +``` + +This will take care of renaming the column, ensuring data stays in sync, and +copying over indexes and foreign keys. + +If a column contains one or more indexes that don't contain the name of the +original column, the previously described procedure will fail. In that case, +you'll first need to rename these indexes. + +### Step 2: Add A Post-Deployment Migration + +The renaming procedure requires some cleaning up in a post-deployment migration. +We can perform this cleanup using +`Gitlab::Database::MigrationHelpers#cleanup_concurrent_column_rename`: + +```ruby +# A post-deployment migration in db/post_migrate +class CleanupUsersUpdatedAtRename < Gitlab::Database::Migration[1.0] + disable_ddl_transaction! + + def up + cleanup_concurrent_column_rename :users, :updated_at, :updated_at_timestamp + end + + def down + undo_cleanup_concurrent_column_rename :users, :updated_at, :updated_at_timestamp + end +end +``` + +If you're renaming a [large table](https://gitlab.com/gitlab-org/gitlab/-/blob/master/rubocop/rubocop-migrations.yml#L3), please carefully consider the state when the first migration has run but the second cleanup migration hasn't been run yet. +With [Canary](https://gitlab.com/gitlab-com/gl-infra/readiness/-/tree/master/library/canary/) it is possible that the system runs in this state for a significant amount of time. + +## Changing Column Constraints + +Adding or removing a `NOT NULL` clause (or another constraint) can typically be +done without requiring downtime. However, this does require that any application +changes are deployed _first_. Thus, changing the constraints of a column should +happen in a post-deployment migration. + +Avoid using `change_column` as it produces an inefficient query because it re-defines +the whole column type. + +You can check the following guides for each specific use case: + +- [Adding foreign-key constraints](../migration_style_guide.md#adding-foreign-key-constraints) +- [Adding `NOT NULL` constraints](not_null_constraints.md) +- [Adding limits to text columns](strings_and_the_text_data_type.md) + +## Changing Column Types + +Changing the type of a column can be done using +`Gitlab::Database::MigrationHelpers#change_column_type_concurrently`. This +method works similarly to `rename_column_concurrently`. For example, let's say +we want to change the type of `users.username` from `string` to `text`. + +### Step 1: Create A Regular Migration + +A regular migration is used to create a new column with a temporary name along +with setting up some triggers to keep data in sync. Such a migration would look +as follows: + +```ruby +# A regular migration in db/migrate +class ChangeUsersUsernameStringToText < Gitlab::Database::Migration[1.0] + disable_ddl_transaction! + + def up + change_column_type_concurrently :users, :username, :text + end + + def down + undo_change_column_type_concurrently :users, :username + end +end +``` + +### Step 2: Create A Post Deployment Migration + +Next we need to clean up our changes using a post-deployment migration: + +```ruby +# A post-deployment migration in db/post_migrate +class ChangeUsersUsernameStringToTextCleanup < Gitlab::Database::Migration[1.0] + disable_ddl_transaction! + + def up + cleanup_concurrent_column_type_change :users, :username + end + + def down + undo_cleanup_concurrent_column_type_change :users, :username, :string + end +end +``` + +And that's it, we're done! + +### Casting data to a new type + +Some type changes require casting data to a new type. For example when changing from `text` to `jsonb`. +In this case, use the `type_cast_function` option. +Make sure there is no bad data and the cast will always succeed. You can also provide a custom function that handles +casting errors. + +Example migration: + +```ruby + def up + change_column_type_concurrently :users, :settings, :jsonb, type_cast_function: 'jsonb' + end +``` + +## Changing The Schema For Large Tables + +While `change_column_type_concurrently` and `rename_column_concurrently` can be +used for changing the schema of a table without downtime, it doesn't work very +well for large tables. Because all of the work happens in sequence the migration +can take a very long time to complete, preventing a deployment from proceeding. +They can also produce a lot of pressure on the database due to it rapidly +updating many rows in sequence. + +To reduce database pressure you should instead use a background migration +when migrating a column in a large table (for example, `issues`). This will +spread the work / load over a longer time period, without slowing down deployments. + +For more information, see [the documentation on cleaning up background +migrations](background_migrations.md#cleaning-up). + +## Adding Indexes + +Adding indexes does not require downtime when `add_concurrent_index` +is used. + +See also [Migration Style Guide](../migration_style_guide.md#adding-indexes) +for more information. + +## Dropping Indexes + +Dropping an index does not require downtime. + +## Adding Tables + +This operation is safe as there's no code using the table just yet. + +## Dropping Tables + +Dropping tables can be done safely using a post-deployment migration, but only +if the application no longer uses the table. + +## Renaming Tables + +Renaming tables requires downtime as an application may continue +using the old table name during/after a database migration. + +If the table and the ActiveRecord model is not in use yet, removing the old +table and creating a new one is the preferred way to "rename" the table. + +Renaming a table is possible without downtime by following our multi-release +[rename table process](rename_database_tables.md#rename-table-without-downtime). + +## Adding Foreign Keys + +Adding foreign keys usually works in 3 steps: + +1. Start a transaction +1. Run `ALTER TABLE` to add the constraint(s) +1. Check all existing data + +Because `ALTER TABLE` typically acquires an exclusive lock until the end of a +transaction this means this approach would require downtime. + +GitLab allows you to work around this by using +`Gitlab::Database::MigrationHelpers#add_concurrent_foreign_key`. This method +ensures that no downtime is needed. + +## Removing Foreign Keys + +This operation does not require downtime. + +## Migrating `integer` primary keys to `bigint` + +To [prevent the overflow risk](https://gitlab.com/groups/gitlab-org/-/epics/4785) for some tables +with `integer` primary key (PK), we have to migrate their PK to `bigint`. The process to do this +without downtime and causing too much load on the database is described below. + +### Initialize the conversion and start migrating existing data (release N) + +To start the process, add a regular migration to create the new `bigint` columns. Use the provided +`initialize_conversion_of_integer_to_bigint` helper. The helper also creates a database trigger +to keep in sync both columns for any new records ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/migrate/20210608072312_initialize_conversion_of_ci_stages_to_bigint.rb)): + +```ruby +class InitializeConversionOfCiStagesToBigint < ActiveRecord::Migration[6.1] + include Gitlab::Database::MigrationHelpers + + TABLE = :ci_stages + COLUMNS = %i(id) + + def up + initialize_conversion_of_integer_to_bigint(TABLE, COLUMNS) + end + + def down + revert_initialize_conversion_of_integer_to_bigint(TABLE, COLUMNS) + end +end +``` + +Ignore the new `bigint` columns: + +```ruby +module Ci + class Stage < Ci::ApplicationRecord + include IgnorableColumns + ignore_column :id_convert_to_bigint, remove_with: '14.2', remove_after: '2021-08-22' + end +``` + +To migrate existing data, we introduced new type of _batched background migrations_. +Unlike the classic background migrations, built on top of Sidekiq, batched background migrations +don't have to enqueue and schedule all the background jobs at the beginning. +They also have other advantages, like automatic tuning of the batch size, better progress visibility, +and collecting metrics. To start the process, use the provided `backfill_conversion_of_integer_to_bigint` +helper ([example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/migrate/20210608072346_backfill_ci_stages_for_bigint_conversion.rb)): + +```ruby +class BackfillCiStagesForBigintConversion < ActiveRecord::Migration[6.1] + include Gitlab::Database::MigrationHelpers + + TABLE = :ci_stages + COLUMNS = %i(id) + + def up + backfill_conversion_of_integer_to_bigint(TABLE, COLUMNS) + end + + def down + revert_backfill_conversion_of_integer_to_bigint(TABLE, COLUMNS) + end +end +``` + +### Monitor the background migration + +Check how the migration is performing while it's running. Multiple ways to do this are described below. + +#### High-level status of batched background migrations + +See how to [check the status of batched background migrations](../../update/index.md#checking-for-background-migrations-before-upgrading). + +#### Query the database + +We can query the related database tables directly. Requires access to read-only replica. +Example queries: + +```sql +-- Get details for batched background migration for given table +SELECT * FROM batched_background_migrations WHERE table_name = 'namespaces'\gx + +-- Get count of batched background migration jobs by status for given table +SELECT + batched_background_migrations.id, batched_background_migration_jobs.status, COUNT(*) +FROM + batched_background_migrations + JOIN batched_background_migration_jobs ON batched_background_migrations.id = batched_background_migration_jobs.batched_background_migration_id +WHERE + table_name = 'namespaces' +GROUP BY + batched_background_migrations.id, batched_background_migration_jobs.status; + +-- Batched background migration progress for given table (based on estimated total number of tuples) +SELECT + m.table_name, + LEAST(100 * sum(j.batch_size) / pg_class.reltuples, 100) AS percentage_complete +FROM + batched_background_migrations m + JOIN batched_background_migration_jobs j ON j.batched_background_migration_id = m.id + JOIN pg_class ON pg_class.relname = m.table_name +WHERE + j.status = 3 AND m.table_name = 'namespaces' +GROUP BY m.id, pg_class.reltuples; +``` + +#### Sidekiq logs + +We can also use the Sidekiq logs to monitor the worker that executes the batched background +migrations: + +1. Sign in to [Kibana](https://log.gprd.gitlab.net) with a `@gitlab.com` email address. +1. Change the index pattern to `pubsub-sidekiq-inf-gprd*`. +1. Add filter for `json.queue: cronjob:database_batched_background_migration`. + +#### PostgreSQL slow queries log + +Slow queries log keeps track of low queries that took above 1 second to execute. To see them +for batched background migration: + +1. Sign in to [Kibana](https://log.gprd.gitlab.net) with a `@gitlab.com` email address. +1. Change the index pattern to `pubsub-postgres-inf-gprd*`. +1. Add filter for `json.endpoint_id.keyword: Database::BatchedBackgroundMigrationWorker`. +1. Optional. To see only updates, add a filter for `json.command_tag.keyword: UPDATE`. +1. Optional. To see only failed statements, add a filter for `json.error_severity.keyword: ERROR`. +1. Optional. Add a filter by table name. + +#### Grafana dashboards + +To monitor the health of the database, use these additional metrics: + +- [PostgreSQL Tuple Statistics](https://dashboards.gitlab.net/d/000000167/postgresql-tuple-statistics?orgId=1&refresh=1m): if you see high rate of updates for the tables being actively converted, or increasing percentage of dead tuples for this table, it might mean that autovacuum cannot keep up. +- [PostgreSQL Overview](https://dashboards.gitlab.net/d/000000144/postgresql-overview?orgId=1): if you see high system usage or transactions per second (TPS) on the primary database server, it might mean that the migration is causing problems. + +### Prometheus metrics + +Number of [metrics](https://gitlab.com/gitlab-org/gitlab/-/blob/294a92484ce4611f660439aa48eee4dfec2230b5/lib/gitlab/database/background_migration/batched_migration_wrapper.rb#L90-128) +for each batched background migration are published to Prometheus. These metrics can be searched for and +visualized in Thanos ([see an example](https://thanos-query.ops.gitlab.net/graph?g0.expr=sum%20(rate(batched_migration_job_updated_tuples_total%7Benv%3D%22gprd%22%7D%5B5m%5D))%20by%20(migration_id)%20&g0.tab=0&g0.stacked=0&g0.range_input=3d&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D&g0.end_input=2021-06-13%2012%3A18%3A24&g0.moment_input=2021-06-13%2012%3A18%3A24)). + +### Swap the columns (release N + 1) + +After the background is completed and the new `bigint` columns are populated for all records, we can +swap the columns. Swapping is done with post-deployment migration. The exact process depends on the +table being converted, but in general it's done in the following steps: + +1. Using the provided `ensure_batched_background_migration_is_finished` helper, make sure the batched +migration has finished ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L13-18)). +If the migration has not completed, the subsequent steps fail anyway. By checking in advance we +aim to have more helpful error message. +1. Create indexes using the `bigint` columns that match the existing indexes using the `integer` +column ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L28-34)). +1. Create foreign keys (FK) using the `bigint` columns that match the existing FKs using the +`integer` column. Do this both for FK referencing other tables, and FKs that reference the table +that is being migrated ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L36-43)). +1. Inside a transaction, swap the columns: + 1. Lock the tables involved. To reduce the chance of hitting a deadlock, we recommended to do this in parent to child order ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L47)). + 1. Rename the columns to swap names ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L49-54)) + 1. Reset the trigger function ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L56-57)). + 1. Swap the defaults ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L59-62)). + 1. Swap the PK constraint (if any) ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L64-68)). + 1. Remove old indexes and rename new ones ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L70-72)). + 1. Remove old FKs (if still present) and rename new ones ([see an example](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb#L74)). + +See example [merge request](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66088), and [migration](https://gitlab.com/gitlab-org/gitlab/-/blob/41fbe34a4725a4e357a83fda66afb382828767b2/db/post_migrate/20210707210916_finalize_ci_stages_bigint_conversion.rb). + +### Remove the trigger and old `integer` columns (release N + 2) + +Using post-deployment migration and the provided `cleanup_conversion_of_integer_to_bigint` helper, +drop the database trigger and the old `integer` columns ([see an example](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69714)). + +### Remove ignore rules (release N + 3) + +In the next release after the columns were dropped, remove the ignore rules as we do not need them +anymore ([see an example](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/71161)). + +## Data migrations + +Data migrations can be tricky. The usual approach to migrate data is to take a 3 +step approach: + +1. Migrate the initial batch of data +1. Deploy the application code +1. Migrate any remaining data + +Usually this works, but not always. For example, if a field's format is to be +changed from JSON to something else we have a bit of a problem. If we were to +change existing data before deploying application code we'll most likely run +into errors. On the other hand, if we were to migrate after deploying the +application code we could run into the same problems. + +If you merely need to correct some invalid data, then a post-deployment +migration is usually enough. If you need to change the format of data (for example, from +JSON to something else) it's typically best to add a new column for the new data +format, and have the application use that. In such a case the procedure would +be: + +1. Add a new column in the new format +1. Copy over existing data to this new column +1. Deploy the application code +1. In a post-deployment migration, copy over any remaining data + +In general there is no one-size-fits-all solution, therefore it's best to +discuss these kind of migrations in a merge request to make sure they are +implemented in the best way possible. |