Elastic migration to backfill epics

What does this MR do and why?

Context

Currently all epic searches are doing a Basic Search and we want to allow Advanced Search to be used when Elasticsearch is available for faster and better searching.

To achieve this, we need the following:

• Create the index in Elasticsearch: Create Epic Elasticsearch index (!121635 - merged)
• Make sure epics are created/updated/deleted when needed: Keep epics index up to date in Elasticsearch (!123526 - merged)
  • Enable the feature flag and remove from code
• Backfill all epics 👈 This MR
• Perform advanced search using the new epic index

Note: The first three MRs should be merged in the same milestone.

Details

Adds an Elastic migration to backfill Epics.

• If elastic namespace limiting is disabled: indexes all epics
• If elastic namespace limiting is enabled: indexes epics belonging to groups that are indexed

NOTE: Before this migration is merged, elastic_index_epics feature flag has to be fully enabled and removed from code.

Estimated run time

For the total number of epic records in Gitlab.com, this migration will require 11 migration runs. However, namespace limiting is enabled so there will be less epics -> this migration will finish in less than 11 migration runs.

Logs

MigrationWorker: migration[BackfillEpics] executing migrate method
[Elastic::Migration: 20230614090600] Indexing epics starting from id = 0
[Elastic::Migration: 20230614090600] Executing iteration 1 with last epic id: 48
[Elastic::Migration: 20230614090600] Setting migration_state to {\"max_processed_id\":48}
[Elastic::Migration: 20230614090600] Migration completed? max_processed_id(48); maximum_epic_id(48)
Elastic::MigrationWorker","message":"MigrationWorker: migration[BackfillEpics] updating with completed: true

If the BATCH_SIZE is 10 and ITERATIONS_PER_RUN is 2 as an example:

MigrationWorker: migration[BackfillEpics] executing migrate method
BackfillEpics","message":"[Elastic::Migration: 20230614090600] Indexing epics starting from id = 0
[Elastic::Migration: 20230614090600] Executing iteration 1 with last epic id: 10
[Elastic::Migration: 20230614090600] Executing iteration 2 with last epic id: 20
[Elastic::Migration: 20230614090600] Setting migration_state to {\"max_processed_id\":20}
[Elastic::Migration: 20230614090600] Migration completed? max_processed_id(20); maximum_epic_id(48)
MigrationWorker: migration[BackfillEpics] updating with completed: false
MigrationWorker: migration[BackfillEpics] kicking off next migration batch
MigrationWorker: migration[BackfillEpics] executing migrate method
[Elastic::Migration: 20230614090600] Indexing epics starting from id = 20
[Elastic::Migration: 20230614090600] Executing iteration 1 with last epic id: 30
[Elastic::Migration: 20230614090600] Executing iteration 2 with last epic id: 40
[Elastic::Migration: 20230614090600] Setting migration_state to {\"max_processed_id\":40}
[Elastic::Migration: 20230614090600] Migration completed? max_processed_id(40); maximum_epic_id(48)
MigrationWorker: migration[BackfillEpics] updating with completed: false
MigrationWorker: migration[BackfillEpics] kicking off next migration batch
MigrationWorker: migration[BackfillEpics] executing migrate method
[Elastic::Migration: 20230614090600] Indexing epics starting from id = 40
[Elastic::Migration: 20230614090600] Executing iteration 1 with last epic id: 48
[Elastic::Migration: 20230614090600] Setting migration_state to {\"max_processed_id\":48}
[Elastic::Migration: 20230614090600] Migration completed? max_processed_id(48); maximum_epic_id(48)
MigrationWorker: migration[BackfillEpics] updating with completed: true

How to set up and validate locally

1. Disable elastic index limiting
2. Execute the migration worker a few times: Elastic::MigrationWorker.new.perform
3. Check that Epic.count records are enqueued: Elastic::ProcessBookkeepingService.queue_size
4. Enable elastic index limiting and add a group containing epics
5. Delete the migration record from elasticsearch: curl -X "DELETE" "http://localhost:9200/gitlab-development-migrations/_doc/20230614090600"
6. Execute the migration worker a few times: Elastic::MigrationWorker.new.perform
7. Check that the number of epics in the limited group is equal to the number of records enqueued: Elastic::ProcessBookkeepingService.queue_size

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #250699 (closed)