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

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CI mirrored tables

Problem statement

As part of the database decomposition work, which had the goal of splitting the single database GitLab is using, into two databases: main and ci, came the big challenge of removing all joins between the main and the ci tables. That is because PostgreSQL doesn't support joins between tables that belong to different databases. However, some core application models in the main database are queried very often by the CI side. For example:

• Namespace, in the namespaces table.
• Project, in the projects table.

Not being able to do joins on these tables brings a great challenge. The team chose to perform logical replication of those tables from the main database to the CI database, in the new tables:

• ci_namespace_mirrors, as a mirror of the namespaces table
• ci_project_mirrors, as a mirror of the projects table

This logical replication means two things:

1. The main database tables can be queried and joined to the namespaces and projects tables.
2. The ci database tables can be joined with the ci_namespace_mirrors and ci_project_mirrors tables.

graph LR

    subgraph "Main database (tables)"
        A[namespaces] -->|updates| B[namespaces_sync_events]
        A -->|deletes| C[loose_foreign_keys_deleted_records]
        D[projects] -->|deletes| C
        D -->|updates| E[projects_sync_events]
    end

    B --> F
    C --> G
    E --> H

   subgraph "Sidekiq worker jobs"
     F[Namespaces::ProcessSyncEventsWorker]
     G[LooseForeignKeys::CleanupWorker]
     H[Projects::ProcessSyncEventsWorker]
   end

    F -->|do update| I
    G -->|delete records| I
    G -->|delete records| J
    H -->|do update| J

    subgraph "CI database (tables)"
        I[ci_namespace_mirrors]
        J[ci_project_mirrors]
    end

This replication was restricted only to a few attributes that are needed from each model:

• From Namespace we replicate traversal_ids.
• From Project we replicate only the namespace_id, which represents the group which the project belongs to.

Keeping the CI mirrored tables in sync with the source tables

We must care about two type 3 events to keep the source and the target tables in sync:

1. Creation of new namespaces or projects.
2. Updating the namespaces or projects.
3. Deleting namespaces/projects.

graph LR
    subgraph CI["CI Tables"]
        E[other CI tables]
        F{queries with joins allowed}
        G[ci_project_mirrors]
        H[ci_namespace_mirrors]

        E---F
        F---G
        F---H
    end

    Main["Main Tables"]---L["⛔ ← Joins are not allowed → ⛔"]
    L---CI

    subgraph Main["Main Tables"]
        A[other main tables]
        B{queries with joins allowed}
        C[projects]
        D[namespaces]

        A---B
        B---C
        B---D
    end

Create and update

Syncing the data of newly created or updated namespaces or projects happens in this order:

1. On the main database: Any INSERT or UPDATE on the namespaces or projects tables adds an entry to the tables namespaces_sync_events, and projects_sync_events. These tables also exist on the main database. These entries are added by triggers on both of the tables.
2. On the model level: After a commit happens on either of the source models Namespace or Project, it schedules the corresponding Sidekiq jobs Namespaces::ProcessSyncEventsWorker or Projects::ProcessSyncEventsWorker to run.
3. These workers then:
   1. Read the entries from the tables (namespaces/project)_sync_events from the main database, to check which namespaces or projects to sync.
   2. Copy the data for any updated records into the target tables ci_namespace_mirrors, ci_project_mirrors.

Delete

When any of namespaces or projects are deleted, the target records on the mirrored CI tables are deleted using the loose foreign keys (LFK) mechanism.

By having these items in the config/gitlab_loose_foreign_keys.yml, the LFK mechanism was already working as expected. It deleted any records on the CI mirrored tables that mapped to deleted namespaces or projects in the main database.

ci_namespace_mirrors:
  - table: namespaces
    column: namespace_id
    on_delete: async_delete
ci_project_mirrors:
  - table: projects
    column: project_id
    on_delete: async_delete

Consistency Checking

To make sure that both syncing mechanisms work as expected, we deploy two extra worker jobs, triggered by cron jobs every few minutes:

1. Database::CiNamespaceMirrorsConsistencyCheckWorker
2. Database::CiProjectMirrorsConsistencyCheckWorker

These jobs:

1. Scan both of the source tables on the main database, using a cursor.
2. Compare the items in the namespaces and projects with the target tables on the ci database.
3. Report the items that are not in sync to Kibana and Prometheus.
4. Corrects any discrepancies.