debian-mirror-gitlab/doc/development/database/loose_foreign_keys.md

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Loose foreign keys

Problem statement

In relational databases (including PostgreSQL), foreign keys provide a way to link two database tables together, and ensure data-consistency between them. In GitLab, foreign keys are vital part of the database design process. Most of our database tables have foreign keys.

With the ongoing database decomposition work, linked records might be present on two different database servers. Ensuring data consistency between two databases is not possible with standard PostgreSQL foreign keys. PostgreSQL does not support foreign keys operating within a single database server, defining a link between two database tables in two different database servers over the network.

Example:

• Database "Main": projects table
• Database "CI": ci_pipelines table

A project can have many pipelines. When a project is deleted, the associated ci_pipeline (via the project_id column) records must be also deleted.

With a multi-database setup, this cannot be achieved with foreign keys.

Asynchronous approach

Our preferred approach to this problem is eventual consistency. With the loose foreign keys feature, we can configure delayed association cleanup without negatively affecting the application performance.

How it works

In the previous example, a record in the projects table can have multiple ci_pipeline records. To keep the cleanup process separate from the actual parent record deletion, we can:

1. Create a DELETE trigger on the projects table. Record the deletions in a separate table (deleted_records).
2. A job checks the deleted_records table every minute or two.
3. For each record in the table, delete the associated ci_pipelines records using the project_id column.

NOTE: For this procedure to work, we must register which tables to clean up asynchronously.

Example migration and configuration

Configure the loose foreign key

Loose foreign keys are defined in a YAML file. The configuration requires the following information:

• Parent table name (projects)
• Child table name (ci_pipelines)
• The data cleanup method (async_delete or async_nullify)

The YAML file is located at config/gitlab_loose_foreign_keys.yml. The file groups foreign key definitions by the name of the child table. The child table can have multiple loose foreign key definitions, therefore we store them as an array.

Example definition:

ci_pipelines:
  - table: projects
    column: project_id
    on_delete: async_delete

If the ci_pipelines key is already present in the YAML file, then a new entry can be added to the array:

ci_pipelines:
  - table: projects
    column: project_id
    on_delete: async_delete
  - table: another_table
    column: another_id
    on_delete: :async_nullify

Track record changes

To know about deletions in the projects table, configure a DELETE trigger using a post-deployment migration. The trigger needs to be configured only once. If the model already has at least one loose_foreign_key definition, then this step can be skipped:

class TrackProjectRecordChanges < Gitlab::Database::Migration[1.0]
  include Gitlab::Database::MigrationHelpers::LooseForeignKeyHelpers

  enable_lock_retries!

  def up
    track_record_deletions(:projects)
  end

  def down
    untrack_record_deletions(:projects)
  end
end

Remove the foreign key

If there is an existing foreign key, then it can be removed from the database. As of GitLab 14.5, the following foreign key describes the link between the projects and ci_pipelines tables:

ALTER TABLE ONLY ci_pipelines
ADD CONSTRAINT fk_86635dbd80
FOREIGN KEY (project_id)
REFERENCES projects(id)
ON DELETE CASCADE;

The migration must run after the DELETE trigger is installed and the loose foreign key definition is deployed. As such, it must be a post-deployment migration dated after the migration for the trigger. If the foreign key is deleted earlier, there is a good chance of introducing data inconsistency which needs manual cleanup:

class RemoveProjectsCiPipelineFk < Gitlab::Database::Migration[1.0]
  disable_ddl_transaction!

  def up
    with_lock_retries do
      remove_foreign_key_if_exists(:ci_pipelines, :projects, name: "fk_86635dbd80")
    end
  end

  def down
    add_concurrent_foreign_key(:ci_pipelines, :projects, name: "fk_86635dbd80", column: :project_id, target_column: :id, on_delete: "cascade")
  end
end

At this point, the setup phase is concluded. The deleted projects records should be automatically picked up by the scheduled cleanup worker job.

Testing

The "it has loose foreign keys" shared example can be used to test the presence of the ON DELETE trigger and the loose foreign key definitions.

Simply add to the model test file:

it_behaves_like 'it has loose foreign keys' do
  let(:factory_name) { :project }
end

After removing a foreign key, use the "cleanup by a loose foreign key" shared example to test a child record's deletion or nullification via the added loose foreign key:

it_behaves_like 'cleanup by a loose foreign key' do
  let!(:model) { create(:ci_pipeline, user: create(:user)) }
  let!(:parent) { model.user }
end

Caveats of loose foreign keys

Record creation

The feature provides an efficient way of cleaning up associated records after the parent record is deleted. Without foreign keys, it's the application's responsibility to validate if the parent record exists when a new associated record is created.

A bad example: record creation with the given ID (project_id comes from user input). In this example, nothing prevents us from passing a random project ID:

Ci::Pipeline.create!(project_id: params[:project_id])

A good example: record creation with extra check:

project = Project.find(params[:project_id])
Ci::Pipeline.create!(project_id: project.id)

Association lookup

Consider the following HTTP request:

GET /projects/5/pipelines/100

The controller action ignores the project_id parameter and finds the pipeline using the ID:

  def show
  # bad, avoid it
  pipeline = Ci::Pipeline.find(params[:id]) # 100
end

This endpoint still works when the parent Project model is deleted. This can be considered a a data leak which should not happen under normal circumstances:

def show
  # good
  project = Project.find(params[:project_id])
  pipeline = project.pipelines.find(params[:pipeline_id]) # 100
end

NOTE: This example is unlikely in GitLab, because we usually look up the parent models to perform permission checks.

A note on dependent: :destroy and dependent: :nullify

We considered using these Rails features as an alternative to foreign keys but there are several problems which include:

1. These run on a different connection in the context of a transaction which we do not allow.
2. These can lead to severe performance degradation as we load all records from PostgreSQL, loop over them in Ruby, and call individual DELETE queries.
3. These can miss data as they only cover the case when the destroy method is called directly on the model. There are other cases including delete_all and cascading deletes from another parent table that could mean these are missed.

Risks of loose foreign keys and possible mitigations

In general, the loose foreign keys architecture is eventually consistent and the cleanup latency might lead to problems visible to GitLab users or operators. We consider the tradeoff as acceptable, but there might be cases where the problems are too frequent or too severe, and we must implement a mitigation strategy. A general mitigation strategy might be to have an "urgent" queue for cleanup of records that have higher impact with a delayed cleanup.

Below are some more specific examples of problems that might occur and how we might mitigate them. In all the listed cases we might still consider the problem described to be low risk and low impact, and in that case we would choose to not implement any mitigation.

The record should be deleted but it shows up in a view

This hypothetical example might happen with a foreign key like:

ALTER TABLE ONLY vulnerability_occurrence_pipelines
    ADD CONSTRAINT fk_rails_6421e35d7d FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE;

In this example we expect to delete all associated vulnerability_occurrence_pipelines records whenever we delete the ci_pipelines record associated with them. In this case you might end up with some vulnerability page in GitLab which shows an occurrence of a vulnerability. However, when you try to click a link to the pipeline, you get a 404, because the pipeline is deleted. Then, when you navigate back you might find the occurrence has disappeared too.

Mitigation

When rendering the vulnerability occurrences on the vulnerability page we could try to load the corresponding pipeline and choose to skip displaying that occurrence if pipeline is not found.

The deleted parent record is needed to render a view and causes a 500 error

This hypothetical example might happen with a foreign key like:

ALTER TABLE ONLY vulnerability_occurrence_pipelines
    ADD CONSTRAINT fk_rails_6421e35d7d FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE;

In this example we expect to delete all associated vulnerability_occurrence_pipelines records whenever we delete the ci_pipelines record associated with them. In this case you might end up with a vulnerability page in GitLab which shows an "occurrence" of a vulnerability. However, when rendering the occurrence we try to load, for example, occurrence.pipeline.created_at, which causes a 500 for the user.

Mitigation

When rendering the vulnerability occurrences on the vulnerability page we could try to load the corresponding pipeline and choose to skip displaying that occurrence if pipeline is not found.

The deleted parent record is accessed in a Sidekiq worker and causes a failed job

This hypothetical example might happen with a foreign key like:

ALTER TABLE ONLY vulnerability_occurrence_pipelines
    ADD CONSTRAINT fk_rails_6421e35d7d FOREIGN KEY (pipeline_id) REFERENCES ci_pipelines(id) ON DELETE CASCADE;

In this example we expect to delete all associated vulnerability_occurrence_pipelines records whenever we delete the ci_pipelines record associated with them. In this case you might end up with a Sidekiq worker that is responsible for processing a vulnerability and looping over all occurrences causing a Sidekiq job to fail if it executes occurrence.pipeline.created_at.

Mitigation

When looping through the vulnerability occurrences in the Sidekiq worker, we could try to load the corresponding pipeline and choose to skip processing that occurrence if pipeline is not found.