Multiple Databases

To allow GitLab to scale further we decomposed the GitLab application database into multiple databases. The two databases are main and ci. GitLab supports being run with either one database or two databases. On GitLab.com we are using two separate databases.

For the purpose of building the Cells architecture, we are decomposing the databases further, to introduce another database gitlab_main_clusterwide.

GitLab Schema

For properly discovering allowed patterns between different databases the GitLab application implements the database dictionary.

The database dictionary provides a virtual classification of tables into a gitlab_schema which conceptually is similar to PostgreSQL Schema. We decided as part of using database schemas to better isolated CI decomposed features that we cannot use PostgreSQL schema due to complex migration procedures. Instead we implemented the concept of application-level classification. Each table of GitLab needs to have a gitlab_schema assigned:

More schemas to be introduced with additional decomposed databases

The usage of schema enforces the base class to be used:

• ApplicationRecord for gitlab_main/gitlab_main_cell.
• MainClusterwide::ApplicationRecord for gitlab_main_clusterwide.
• Ci::ApplicationRecord for gitlab_ci
• Geo::TrackingBase for gitlab_geo
• Gitlab::Database::SharedModel for gitlab_shared
• PackageMetadata::ApplicationRecord for gitlab_pm

Choose either the gitlab_main_cell or gitlab_main_clusterwide schema

Depending on the use case, your feature may be cell-local or clusterwide and hence the tables used for the feature should also use the appropriate schema.

When you choose the appropriate schema for tables, consider the following guidelines as part of the Cells architecture:

• Default to gitlab_main_cell: We expect most tables to be assigned to the gitlab_main_cell schema by default. Choose this schema if the data in the table is related to projects or namespaces.
• Consult with the Tenant Scale group: If you believe that the gitlab_main_clusterwide schema is more suitable for a table, seek approval from the Tenant Scale group This is crucial because it has scaling implications and may require reconsideration of the schema choice.

To understand how existing tables are classified, you can use this dashboard.

After a schema has been assigned, the merge request pipeline might fail due to one or more of the following reasons, which can be rectified by following the linked guidelines:

• Cross-database joins
• Cross-database transactions
• Cross-database foreign keys

Defining a sharding key for all cell-local tables

All tables with the following gitlab_schema are considered "cell-local":

• gitlab_main_cell
• gitlab_ci

All newly created cell-local tables are required to have a sharding_key defined in the corresponding db/docs/ file for that table.

The purpose of the sharding key is documented in the Organization isolation blueprint, but in short this column is used to provide a standard way of determining which Organization owns a particular row in the database. The column will be used in the future to enforce constraints on data not cross Organization boundaries. It will also be used in the future to provide a uniform way to migrate data between Cells.

The actual name of the foreign key can be anything but it must reference a row in projects or groups. The chosen sharding_key column must be non-nullable.

Setting multiple sharding_key, with nullable columns are also allowed, provided that the table has a check constraint that correctly ensures at least one of the keys must be non-nullable for a row in the table. See NOT NULL constraints for multiple columns for instructions on creating these constraints.

The following are examples of valid sharding keys:

• The table entries belong to a project only:

  sharding_key:
    project_id: projects

• The table entries belong to a project and the foreign key is target_project_id:

  sharding_key:
    target_project_id: projects

• The table entries belong to a namespace/group only:

  sharding_key:
    namespace_id: namespaces

• The table entries belong to a namespace/group only and the foreign key is group_id:

  sharding_key:
    group_id: namespaces

• The table entries belong to a namespace or a project:

  sharding_key:
    project_id: projects
    namespace_id: namespaces

The sharding key must be immutable

The choice of a sharding_key should always be immutable. Therefore, if your feature requires a user experience which allows data to be moved between projects or groups/namespaces, then you may need to redesign the move feature to create new rows. An example of this can be seen in the move an issue feature. This feature does not actually change the project_id column for an existing issues row but instead creates a new issues row and creates a link in the database from the original issues row. If there is a particularly challenging existing feature that needs to allow moving data you will need to reach out to the Tenant Scale team early on to discuss options for how to manage the sharding key.

Using the same sharding key for projects and namespaces

Developers may also choose to use namespace_id only for tables that can belong to a project where the feature used by the table is being developed following the Consolidating Groups and Projects blueprint. In that case the namespace_id would need to be the ID of the ProjectNamespace and not the group that the namespace belongs to.

Define a desired_sharding_key to automatically backfill a sharding_key

We need to backfill a sharding_key to hundreds of tables that do not have one. This process will involve creating a merge request like https://gitlab.com/gitlab-org/gitlab/-/merge_requests/136800 to add the new column, backfill the data from a related table in the database, and then create subsequent merge requests to add indexes, foreign keys and not-null constraints.

In order to minimize the amount of repetitive effort for developers we've introduced a concise declarative way to describe how to backfill the sharding_key for this specific table. This content will later be used in automation to create all the necessary merge requests.

An example of the desired_sharding_key was added in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/139336 and it looks like:

--- # db/docs/security_findings.yml
table_name: security_findings
classes:
- Security::Finding

...

desired_sharding_key:
  project_id:
    references: projects
    backfill_via:
      parent:
        foreign_key: scanner_id
        table: vulnerability_scanners
        sharding_key: project_id
        belongs_to: scanner

To understand best how this YAML data will be used you can map it onto the merge request we created manually in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/136800. The idea will be to automatically create this. The content of the YAML specifies the parent table and its sharding_key to backfill from in the batched background migration. It also specifies a belongs_to relation which will be added to the model to automatically populate the sharding_key in the before_save.

Define a desired_sharding_key when the parent table also has one

By default, a desired_sharding_key configuration will validate that the chosen sharding_key exists on the parent table. However, if the parent table also has a desired_sharding_key configuration and is itself waiting to be backfilled, you need to include the awaiting_backfill_on_parent field. For example:

desired_sharding_key:
  project_id:
    references: projects
    backfill_via:
      parent:
        foreign_key: package_file_id
        table: packages_package_files
        sharding_key: project_id
        belongs_to: package_file
    awaiting_backfill_on_parent: true

There are likely edge cases where this desired_sharding_key structure is not suitable for backfilling a sharding_key. In such cases the team owning the table will need to create the necessary merge requests to add the sharding_key manually.

Exempting certain tables from having sharding keys

Certain tables can be exempted from having sharding keys by adding

exempt_from_sharding: true

to the table's database dictionary file. This can be used for:

• JiHu specific tables, since they do not have any data on the .com database. !145905
• tables that are marked to be dropped soon, like operations_feature_flag_scopes. !147541
• tables that mandatorily need to be present per cell to support a cell's operations, have unique data per cell, but cannot have a sharding key defined. For example, zoekt_nodes.

When tables are exempted from sharding key requirements, they also do not show up in our progress dashboard.

The impact of gitlab_schema

The usage of gitlab_schema has a significant impact on the application. The gitlab_schema primary purpose is to introduce a barrier between different data access patterns.

This is used as a primary source of classification for:

• Discovering cross-joins across tables from different schemas
• Discovering cross-database transactions across tables from different schemas

The special purpose of gitlab_shared

gitlab_shared is a special case that describes tables or views that, by design, contain data across all decomposed databases. This classification describes application-defined tables (like loose_foreign_keys_deleted_records).

Be careful to use gitlab_shared as it requires special handling while accessing data. Since gitlab_shared shares not only structure but also data, the application needs to be written in a way that traverses all data from all databases in sequential manner.

Gitlab::Database::EachDatabase.each_model_connection([MySharedModel]) do |connection, connection_name|
  MySharedModel.select_all_data...
end

As such, migrations modifying data of gitlab_shared tables are expected to run across all decomposed databases.

The special purpose of gitlab_internal

gitlab_internal describes Rails-defined tables (like schema_migrations or ar_internal_metadata), as well as internal PostgreSQL tables (for example, pg_attribute). Its primary purpose is to support other databases, like Geo, that might be missing some of those application-defined gitlab_shared tables (like loose_foreign_keys_deleted_records), but are valid Rails databases.

The special purpose of gitlab_pm

gitlab_pm stores package metadata describing public repositories. This data is used for the License Compliance and Dependency Scanning product categories and is maintained by the Composition Analysis Group. It is an alias for gitlab_main intended to make it easier to route to a different database in the future.

Migrations

Read Migrations for Multiple Databases.

CI/CD Database

Configure single database

By default, GDK is configured to run with multiple databases.

WARNING: Switching back-and-forth between single and multiple databases in the same development instance is discouraged. Any data in the ci database will not be accessible in single database mode. For single database, you should use a separate development instance.

To configure GDK to use a single database:

1. On the GDK root directory, run:

   gdk config set gitlab.rails.databases.ci.enabled false

2. Reconfigure GDK:

   gdk reconfigure

To switch back to using multiple databases, set gitlab.rails.databases.ci.enabled to true and run gdk reconfigure.

Removing joins between ci and non ci tables

Queries that join across databases raise an error. Introduced in GitLab 14.3, for new queries only. Pre-existing queries do not raise an error.

Because GitLab can be run with multiple separate databases, referencing ci tables with non ci tables in a single query is not possible. Therefore, using any kind of JOIN in SQL queries will not work.

Suggestions for removing cross-database joins

The following sections are some real examples that were identified as joining across databases, along with possible suggestions on how to fix them.

Remove the code

The simplest solution we've seen several times now has been an existing scope that is unused. This is the easiest example to fix. So the first step is to investigate if the code is unused and then remove it. These are some real examples:

• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67162
• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66714
• https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66503

There may be more examples where the code is used, but we can evaluate if we need it or if the feature should behave this way. Before complicating things by adding new columns and tables, consider if you can simplify the solution and still meet the requirements. One case being evaluated involves changing how certain UsageData is calculated to remove a join query in https://gitlab.com/gitlab-org/gitlab/-/issues/336170. This is a good candidate to evaluate, because UsageData is not critical to users and it may be possible to get a similarly useful metric with a simpler approach. Alternatively we may find that nobody is using these metrics, so we can remove them.

Use preload instead of includes

The includes and preload methods in Rails are both ways to avoid an N+1 query. The includes method in Rails uses a heuristic approach to determine if it needs to join to the table, or if it can load all of the records in a separate query. This method assumes it needs to join if it thinks you need to query the columns from the other table, but sometimes this method gets it wrong and executes a join even when not needed. In this case using preload to explicitly load the data in a separate query allows you to avoid the join, while still avoiding the N+1 query.

You can see a real example of this solution being used in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67655.

Remove a redundant join

Sometimes there are cases where a query is doing excess (or redundant) joins.

A common example occurs where a query is joining from A to C, via some table with both foreign keys, B. When you only care about counting how many rows there are in C and if there are foreign keys and NOT NULL constraints on the foreign keys in B, then it might be enough to count those rows. For example, in MR 71811, it was previously doing project.runners.count, which would produce a query like:

select count(*) from projects
inner join ci_runner_projects on ci_runner_projects.project_id = projects.id
where ci_runner_projects.runner_id IN (1, 2, 3)

This was changed to avoid the cross-join by changing the code to project.runner_projects.count. It produces the same response with the following query:

select count(*) from ci_runner_projects
where ci_runner_projects.runner_id IN (1, 2, 3)

Another common redundant join is joining all the way to another table, then filtering by primary key when you could have instead filtered on a foreign key. See an example in MR 71614. The previous code was joins(scan: :build).where(ci_builds: { id: build_ids }), which generated a query like:

select ...
inner join security_scans
inner join ci_builds on security_scans.build_id = ci_builds.id
where ci_builds.id IN (1, 2, 3)

However, as security_scans already has a foreign key build_id, the code can be changed to joins(:scan).where(security_scans: { build_id: build_ids }), which produces the same response with the following query:

select ...
inner join security_scans
where security_scans.build_id IN (1, 2, 3)

Both of these examples of removing redundant joins remove the cross-joins, but they have the added benefit of producing simpler and faster queries.

Limited pluck followed by a find

Using pluck or pick to get an array of ids is not advisable unless the returned array is guaranteed to be bounded in size. Usually this is a good pattern where you know the result will be at most 1, or in cases where you have a list of in memory ids (or usernames) that need to be mapped to another list of equal size. It would not be suitable when mapping a list of ids in a one-to-many relationship as the result will be unbounded. We can then use the returned ids to obtain the related record:

allowed_user_id = board_user_finder
  .where(user_id: params['assignee_id'])
  .pick(:user_id)

User.find_by(id: allowed_user_id)

You can see an example where this was used in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126856

Sometimes it might seem easy to convert a join into a pluck but often this results in loading an unbounded amount of ids into memory and then re-serializing those in a following query back to Postgres. These cases do not scale and we recommend attempting one of the other options. It might seem like a good idea to just apply some limit to the plucked data to have bounded memory but this introduces unpredictable results for users and often is most problematic for our largest customers (including ourselves), and as such we advise against it.

De-normalize some foreign key to the table

De-normalization refers to adding redundant precomputed (duplicated) data to a table to simplify certain queries or to improve performance. In this case, it can be useful when you are doing a join that involves three tables, where you are joining through some intermediate table.

Generally when modeling a database schema, a "normalized" structure is preferred because of the following reasons:

• Duplicate data uses extra storage.
• Duplicate data needs to be kept in sync.

Sometimes normalized data is less performant so de-normalization has been a common technique GitLab has used to improve the performance of database queries for a while. The above problems are mitigated when the following conditions are met:

1. There isn't much data (for example, it's just an integer column).
2. The data does not update often (for example, the project_id column is almost never updated for most tables).

One example we found was the security_scans table. This table has a foreign key security_scans.build_id which allows you to join to the build. Therefore you could join to the project like so:

select projects.* from security_scans
inner join ci_builds on security_scans.build_id = ci_builds.id
inner join projects on ci_builds.project_id = projects.id

The problem with this query is that ci_builds is in a different database from the other two tables.

The solution in this case is to add the project_id column to security_scans. This doesn't use much extra storage, and due to the way these features work, it's never updated (a build never moves projects).

This simplified the query to:

select projects.* from security_scans
inner join projects on security_scans.project_id = projects.id

This also improves performance because you don't need to join through an extra table.

You can see this approach implemented in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66963 . This MR also de-normalizes pipeline_id to fix a similar query.

De-normalize into an extra table

Sometimes the previous de-normalization (adding an extra column) doesn't work for your specific case. This may be due to the fact that your data is not 1:1, or because the table you're adding to is already too wide (for example, the projects table shouldn't have more columns added).

In this case you may decide to just store the extra data in a separate table.

One example where this approach is being used was to implement the Project.with_code_coverage scope. This scope was essentially used to narrow down a list of projects to only those that have at one point in time used code coverage features. This query (simplified) was:

select projects.* from projects
inner join ci_daily_build_group_report_results on ci_daily_build_group_report_results.project_id = projects.id
where ((data->'coverage') is not null)
and ci_daily_build_group_report_results.default_branch = true
group by projects.id

This work is still in progress but the current plan is to introduce a new table called projects_with_ci_feature_usage which has 2 columns project_id and ci_feature. This table would be written to the first time a project creates a ci_daily_build_group_report_results for code coverage. Therefore the new query would be:

select projects.* from projects
inner join projects_with_ci_feature_usage on projects_with_ci_feature_usage.project_id = projects.id
where projects_with_ci_feature_usage.ci_feature = 'code_coverage'

The above example uses as a text column for simplicity but we should probably use an enum to save space.

The downside of this new design is that this may need to be updated (removed if the ci_daily_build_group_report_results is deleted). Depending on your domain, however, this may not be necessary because deletes are edge cases or impossible, or because the user impact of seeing the project on the list page may not be problematic. It's also possible to implement the logic to delete these rows if or whenever necessary in your domain.

Finally, this de-normalization and new query also improves performance because it does less joins and needs less filtering.

Use disable_joins for has_one or has_many through: relations

Sometimes a join query is caused by using has_one ... through: or has_many ... through: across tables that span the different databases. These joins sometimes can be solved by adding disable_joins:true. This is a Rails feature which we backported. We also extended the feature to allow a lambda syntax for enabling disable_joins with a feature flag. If you use this feature we encourage using a feature flag as it mitigates risk if there is some serious performance regression.

You can see an example where this was used in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66709/diffs.

With any change to DB queries it is important to analyze and compare the SQL before and after the change. disable_joins can introduce very poorly performing code depending on the actual logic of the has_many or has_one relationship. The key thing to look for is whether any of the intermediate result sets used to construct the final result set have an unbounded amount of data loaded. The best way to tell is by looking at the SQL generated and confirming that each one is limited in some way. You can tell by either a LIMIT 1 clause or by WHERE clause that is limiting based on a unique column. Any unbounded intermediate dataset could lead to loading too many IDs into memory.

An example where you may see very poor performance is the following hypothetical code:

class Project
  has_many :pipelines
  has_many :builds, through: :pipelines
end

class Pipeline
  has_many :builds
end

class Build
  belongs_to :pipeline
end

def some_action
  @builds = Project.find(5).builds.order(created_at: :desc).limit(10)
end

In the above case some_action will generate a query like:

select * from builds
inner join pipelines on builds.pipeline_id = pipelines.id
where pipelines.project_id = 5
order by builds.created_at desc
limit 10

However, if you changed the relation to be:

class Project
  has_many :pipelines
  has_many :builds, through: :pipelines, disable_joins: true
end

Then you would get the following 2 queries:

select id from pipelines where project_id = 5;

select * from builds where pipeline_id in (...)
order by created_at desc
limit 10;

Because the first query does not limit by any unique column or have a LIMIT clause, it can load an unlimited number of pipeline IDs into memory, which are then sent in the following query. This can lead to very poor performance in the Rails application and the database. In cases like this, you might need to re-write the query or look at other patterns described above for removing cross-joins.

How to validate you have correctly removed a cross-join

RSpec is configured to automatically validate all SQL queries do not join across databases. If this validation is disabled in spec/support/database/cross-join-allowlist.yml then you can still validate an isolated code block using with_cross_joins_prevented.

You can use this method like so:

it 'does not join across databases' do
  with_cross_joins_prevented do
    ::Ci::Build.joins(:project).to_a
  end
end

This will raise an exception if the query joins across the two databases. The previous example is fixed by removing the join, like so:

it 'does not join across databases' do
  with_cross_joins_prevented do
    ::Ci::Build.preload(:project).to_a
  end
end

You can see a real example of using this method for fixing a cross-join in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67655.

Allowlist for existing cross-joins

The easiest way of identifying a cross-join is via failing pipelines.

As an example, in !130038 we moved the notification_settings table to the gitlab_main_cell schema, by marking it as such in the db/docs/notification_settings.yml file.

The pipeline failed with the following error:

Database::PreventCrossJoins::CrossJoinAcrossUnsupportedTablesError:

Unsupported cross-join across 'users, notification_settings' querying 'gitlab_main_clusterwide, gitlab_main_cell' discovered when executing query 'SELECT "users".* FROM "users" WHERE "users"."id" IN (SELECT "notification_settings"."user_id" FROM ((SELECT "notification_settings"."user_id" FROM "notification_settings" WHERE "notification_settings"."source_id" = 119 AND "notification_settings"."source_type" = 'Project' AND (("notification_settings"."level" = 3 AND EXISTS (SELECT true FROM "notification_settings" "notification_settings_2" WHERE "notification_settings_2"."user_id" = "notification_settings"."user_id" AND "notification_settings_2"."source_id" IS NULL AND "notification_settings_2"."source_type" IS NULL AND "notification_settings_2"."level" = 2)) OR "notification_settings"."level" = 2))) notification_settings)'

To make the pipeline green, this cross-join query must be allow-listed.

A cross-join across databases can be explicitly allowed by wrapping the code in the ::Gitlab::Database.allow_cross_joins_across_databases helper method. Alternative way is to mark a given relation as relation.allow_cross_joins_across_databases.

This method should only be used:

• For existing code.
• If the code is required to help migrate away from a cross-join. For example, in a migration that backfills data for future use to remove a cross-join.

The allow_cross_joins_across_databases helper method can be used as follows:

# Scope the block executing a object from database
::Gitlab::Database.allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/336590') do
  subject.perform(1, 4)
end

# Mark a relation as allowed to cross-join databases
def find_diff_head_pipeline
  all_pipelines
    .allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/336891')
    .for_sha_or_source_sha(diff_head_sha)
    .first
end

In model associations or scopes, this can be used as in the following example:

class Group < Namespace
 has_many :users, -> {
    allow_cross_joins_across_databases(url: "https://gitlab.com/gitlab-org/gitlab/-/issues/422405")
  }, through: :group_members
end

WARNING: Overriding an association can have unintended consequences and may even lead to data loss, as we noticed in issue 424307. Do not override existing ActiveRecord associations to mark a cross-join as allowed, as in the example below.

class Group < Namespace
  has_many :users, through: :group_members

  # DO NOT override an association like this.
  def users
    super.allow_cross_joins_across_databases(url: "https://gitlab.com/gitlab-org/gitlab/-/issues/422405")
  end
end

The url parameter should point to an issue with a milestone for when we intend to fix the cross-join. If the cross-join is being used in a migration, we do not need to fix the code. See https://gitlab.com/gitlab-org/gitlab/-/issues/340017 for more details.

Removing cross-database transactions

When dealing with multiple databases, it's important to pay close attention to data modification that affects more than one database. Introduced GitLab 14.4, an automated check prevents cross-database modifications.

When at least two different databases are modified during a transaction initiated on any database server, the application triggers a cross-database modification error (only in test environment).

Example:

# Open transaction on Main DB
ApplicationRecord.transaction do
  ci_build.update!(updated_at: Time.current) # UPDATE on CI DB
  ci_build.project.update!(updated_at: Time.current) # UPDATE on Main DB
end
# raises error: Cross-database data modification of 'main, ci' were detected within
# a transaction modifying the 'ci_build, projects' tables

The code example above updates the timestamp for two records within a transaction. With the ongoing work on the CI database decomposition, we cannot ensure the schematics of a database transaction. If the second update query fails, the first update query will not be rolled back because the ci_build record is located on a different database server. For more information, look at the transaction guidelines page.

Fixing cross-database transactions

A transaction across databases can be explicitly allowed by wrapping the code in the Gitlab::Database::QueryAnalyzers::PreventCrossDatabaseModification.temporary_ignore_tables_in_transaction helper method.

For cross-database transactions in Rails callbacks, the cross_database_ignore_tables method can be used.

These methods should only be used for existing code.

The temporary_ignore_tables_in_transaction helper method can be used as follows:

class GroupMember < Member
   def update_two_factor_requirement
     return unless user

     # To mark and ignore cross-database transactions involving members and users/user_details/user_preferences
     Gitlab::Database::QueryAnalyzers::PreventCrossDatabaseModification.temporary_ignore_tables_in_transaction(
       %w[users user_details user_preferences], url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/424288'
     ) do
       user.update_two_factor_requirement
     end
   end
end

The cross_database_ignore_tables method can be used as follows:

class Namespace < ApplicationRecord
  include CrossDatabaseIgnoredTables

  # To mark and ignore cross-database transactions involving namespaces and routes/redirect_routes happening within Rails callbacks.
  cross_database_ignore_tables %w[routes redirect_routes], url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/424277'
end

Removing the transaction block

Without an open transaction, the cross-database modification check cannot raise an error. By making this change, we sacrifice consistency. In case of an application failure after the first UPDATE query, the second UPDATE query will never execute.

The same code without the transaction block:

ci_build.update!(updated_at: Time.current) # CI DB
ci_build.project.update!(updated_at: Time.current) # Main DB

Asynchronous processing

If we need more guarantee that an operation finishes the work consistently we can execute it within a background job. A background job is scheduled asynchronously and retried several times in case of an error. There is still a very small chance of introducing inconsistency.

Example:

current_time = Time.current

MyAsyncConsistencyJob.perform_async(cu_build.id)

ci_build.update!(updated_at: current_time)
ci_build.project.update!(updated_at: current_time)

The MyAsyncConsistencyJob would also attempt to update the timestamp if they differ.

Aiming for perfect consistency

At this point, we don't have the tooling (we might not even need it) to ensure similar consistency characteristics as we had with one database. If you think that the code you're working on requires these properties, then you can disable the cross-database modification check in your tests by wrapping the offending test code with a block and create a follow-up issue.

allow_cross_database_modification_within_transaction(url: 'gitlab issue URL') do
  ApplicationRecord.transaction do
    ci_build.update!(updated_at: Time.current) # UPDATE on CI DB
    ci_build.project.update!(updated_at: Time.current) # UPDATE on Main DB
  end
end

Don't hesitate to reach out to the Pods group for advice.

Avoid dependent: :nullify and dependent: :destroy across databases

There may be cases where we want to use dependent: :nullify or dependent: :destroy across databases. This is technically possible, but it's problematic because these hooks run in the context of an outer transaction from the call to #destroy, which creates a cross-database transaction and we are trying to avoid that. Cross-database transactions caused this way could lead to confusing outcomes when we switch to decomposed, because now you have some queries happening outside the transaction and they may be partially applied while the outer transaction fails, which could lead to surprising bugs.

For non-trivial objects that need to clean up data outside the database (for example, object storage), we recommend the setting dependent: :restrict_with_error. Such objects should be removed explicitly ahead of time. Using dependent: :restrict_with_error ensures that we forbid destroying the parent object if something is not cleaned up.

If all you need to do is clean up the child records themselves from PostgreSQL, consider using loose foreign keys.

Foreign keys that cross databases

There are many places where we use foreign keys that reference across the two databases. This is not possible to do with two separate PostgreSQL databases, so we need to replicate the behavior we get from PostgreSQL in a performant way. We can't, and shouldn't, try to replicate the data guarantees given by PostgreSQL which prevent creating invalid references, but we still need a way to replace cascading deletes so we don't end up with orphaned data or records that point to nowhere, which might lead to bugs. As such we created "loose foreign keys" which is an asynchronous process of cleaning up orphaned records.

Allowlist for existing cross-database foreign keys

The easiest way of identifying a cross-database foreign key is via failing pipelines.

As an example, in !130038 we moved the notification_settings table to the gitlab_main_cell schema, by marking it in the db/docs/notification_settings.yml file.

notification_settings.user_id is a column that points to users, but the users table belongs to a different database, thus this is now treated as a cross-database foreign key.

We have a spec to capture such cases of cross-database foreign keys in no_cross_db_foreign_keys_spec.rb, which would fail if such a cross-database foreign key is encountered.

To make the pipeline green, this cross-database foreign key must be allow-listed.

To do this, explicitly allow the existing cross-database foreign key to exist by adding it as an exception in the same spec (as in this example). This way, the spec will not fail.

Later, this foreign key can be converted to a loose foreign key, like we did in !130080.

Testing for multiple databases

In our testing CI pipelines, we test GitLab by default with multiple databases set up, using both main and ci databases. But in merge requests, for example when we modify some database-related code or add the label ~"pipeline:run-single-db" to the MR, we additionally run our tests in two other database modes: single-db and single-db-ci-connection.

To handle situations where our tests need to run in specific database modes, we have some RSpec helpers to limit the modes where tests can run, and skip them on any other modes.

Testing for multiple databases, including main_clusterwide

By default, we do not setup the main_clusterwide connection in CI pipelines. However, if you add the label ~"pipeline:run-clusterwide-db", the pipelines will run with 3 connections, main, ci and main_clusterwide.

NOTE: This setup is not completely ready yet, and running pipelines in the setup may fail some jobs. As of July 2023, this is only used by group::tenant scale to test out changes while building Cells.

Locking writes on the tables that don't belong to the database schemas

When the CI database is promoted and the two databases are fully split, as an extra safeguard against creating a split brain situation, run the Rake task gitlab:db:lock_writes. This command locks writes on:

• The gitlab_main tables on the CI Database.
• The gitlab_ci tables on the Main Database.

This Rake task adds triggers to all the tables, to prevent any INSERT, UPDATE, DELETE, or TRUNCATE statements from running against the tables that need to be locked.

If this task was run against a GitLab setup that uses only a single database for both gitlab_main and gitlab_ci tables, then no tables will be locked.

To undo the operation, run the opposite Rake task: gitlab:db:unlock_writes.

Monitoring

The status of the table locks is checked using the Database::MonitorLockedTablesWorker. It will lock tables if needed.

The result of this script is available in Kibana. If the counts are not 0, there are some tables that should have been locked but are not. The fields json.extra.database_monitor_locked_tables_worker.results.ci.tables_need_locks and json.extra.database_monitor_locked_tables_worker.results.main.tables_need_locks should contain a list of tables that have the wrong state.

The logging is monitored using a Elasticsearch Watcher. The watcher is called table_locks_needed and the source code is in the GitLab Runbook repository. The alerts are sent to #g_tenant-scale Slack channel.

Automation

There are two processes that automatically lock tables:

• Database migrations. See Gitlab::Database::MigrationHelpers::AutomaticLockWritesOnTables
• The Database::MonitorLockedTablesWorker locks tables if needed. This can be disabled by the lock_tables_in_monitoring feature flag.

Manually lock tables

If you need to manually lock a table, use a database migration. Create a regular migration and add the code for locking the table. For example, set a write lock on shards table in CI database:

class EnableWriteLocksOnShards < Gitlab::Database::Migration[2.2]
  def up
    # On main database, the migration should be skipped
    # We can't use restrict_gitlab_migration in DDL migrations
    return if Gitlab::Database.db_config_name(connection) != 'ci'

    Gitlab::Database::LockWritesManager.new(
      table_name: 'shards',
      connection: connection,
      database_name: :ci,
      with_retries: false
    ).lock_writes
  end

  def down
    # no-op
  end
end

Truncating tables

When the databases main and ci are fully split, we can free up disk space by truncating tables. This results in a smaller data set: For example, the data in users table on CI database is no longer read and also no longer updated. So this data can be removed by truncating the tables.

For this purpose, GitLab provides two Rake tasks, one for each database:

• gitlab:db:truncate_legacy_tables:main will truncate the CI tables in Main database.
• gitlab:db:truncate_legacy_tables:ci will truncate the Main tables in CI database.

NOTE: These tasks can only be run when the tables in the database are locked for writes.

WARNING: The examples in this section use DRY_RUN=true. This ensures no data is actually truncated. GitLab highly recommends to have a backup available before you run any of these tasks without DRY_RUN=true.

These tasks have the option to see what they do without actually changing the data:

$ sudo DRY_RUN=true gitlab-rake gitlab:db:truncate_legacy_tables:main
I, [2023-07-14T17:08:06.665151 #92505]  INFO -- : DRY RUN:
I, [2023-07-14T17:08:06.761586 #92505]  INFO -- : Truncating legacy tables for the database main
I, [2023-07-14T17:08:06.761709 #92505]  INFO -- : SELECT set_config('lock_writes.ci_build_needs', 'false', false)
I, [2023-07-14T17:08:06.765272 #92505]  INFO -- : SELECT set_config('lock_writes.ci_build_pending_states', 'false', false)
I, [2023-07-14T17:08:06.768220 #92505]  INFO -- : SELECT set_config('lock_writes.ci_build_report_results', 'false', false)
[...]
I, [2023-07-14T17:08:06.957294 #92505]  INFO -- : TRUNCATE TABLE ci_build_needs, ci_build_pending_states, ci_build_report_results, ci_build_trace_chunks, ci_build_trace_metadata, ci_builds, ci_builds_metadata, ci_builds_runner_session, ci_cost_settings, ci_daily_build_group_report_results, ci_deleted_objects, ci_freeze_periods, ci_group_variables, ci_instance_variables, ci_job_artifact_states, ci_job_artifacts, ci_job_token_project_scope_links, ci_job_variables, ci_minutes_additional_packs, ci_namespace_mirrors, ci_namespace_monthly_usages, ci_partitions, ci_pending_builds, ci_pipeline_artifacts, ci_pipeline_chat_data, ci_pipeline_messages, ci_pipeline_metadata, ci_pipeline_schedule_variables, ci_pipeline_schedules, ci_pipeline_variables, ci_pipelines, ci_pipelines_config, ci_platform_metrics, ci_project_mirrors, ci_project_monthly_usages, ci_refs, ci_resource_groups, ci_resources, ci_runner_machines, ci_runner_namespaces, ci_runner_projects, ci_runner_versions, ci_runners, ci_running_builds, ci_secure_file_states, ci_secure_files, ci_sources_pipelines, ci_sources_projects, ci_stages, ci_subscriptions_projects, ci_trigger_requests, ci_triggers, ci_unit_test_failures, ci_unit_tests, ci_variables, external_pull_requests, p_ci_builds, p_ci_builds_metadata, p_ci_job_annotations, p_ci_runner_machine_builds, taggings, tags RESTRICT

The tasks will first find out the tables that need to be truncated. Truncation will happen in stages because we need to limit the amount of data removed in one database transaction. The tables are processed in a specific order depending on the definition of the foreign keys. The number of tables processed in one stage can be changed by adding a number when invoking the task. The default value is 5:

sudo DRY_RUN=true gitlab-rake gitlab:db:truncate_legacy_tables:main\[10\]

It is also possible to limit the number of tables to be truncated by setting the UNTIL_TABLE variable. For example in this case, the process will stop when ci_unit_test_failures has been truncated:

sudo DRY_RUN=true UNTIL_TABLE=ci_unit_test_failures gitlab-rake gitlab:db:truncate_legacy_tables:main