## Summary Roll out [the feature](https://gitlab.com/gitlab-org/gitlab/-/issues/417973) currently behind the `async_write_diffs_cache_on_mr_create` feature flag. The flag defers the merge request diffs highlight cache write in `MergeRequests::AfterCreateService` to the new `MergeRequests::WriteDiffsCacheWorker`, moving the CPU-bound syntax highlighting (~11s at P99 on large merge requests) off the new-MR preparation critical path. Introduced in gitlab-org/gitlab!243117. - DRI: @marc_shaw - Team Slack channel: `#g_code_review` > [!note] > Process and guidance live in the docs — this issue is just the commands and a place to track the rollout. > "Rolling out" means incrementally enabling the flag on GitLab.com to validate stability — it is not the same as releasing the feature, which happens when the flag is removed. > [Feature flag controls](https://docs.gitlab.com/development/feature_flags/controls/) · [Feature flag lifecycle](https://handbook.gitlab.com/handbook/product-development/how-we-work/product-development-flow/feature-flag-lifecycle/#feature-flag-lifecycle) ## What could go wrong? The cache is **display-only**, so the blast radius is small: - If the worker is delayed or fails, the first diff render hits a cold cache and warms it on demand (`Gitlab::Diff::HighlightCache#write_if_empty`); subsequent renders are warm. This means a slightly slower first render, never missing or incorrect data. - The cache key is content-addressed on the immutable merge request diff, so a concurrent worker write and on-render write are idempotent and cannot corrupt each other. - Nothing on the merge request readiness, mergeability, pipeline, approval, or webhook paths reads this cache. Dashboards to watch on <https://dashboards.gitlab.net>: - `MergeRequests::WriteDiffsCacheWorker` Sidekiq execution/queue latency (urgency `:low`, boundary `:cpu`) — watch for Sidekiq CPU saturation during merge-request-creation bursts (for example, large imports). - `MergeRequests::AfterCreateService` / `NewMergeRequestWorker` duration — expect the creation path to get faster as the write moves off it. ## Rollout Run all production `/chatops` in [`#production`](https://gitlab.slack.com/archives/C101F3796) and cross-post the results to `#g_code_review`. Background: [incremental rollout process](https://docs.gitlab.com/development/feature_flags/controls/#process), [feature actors](https://docs.gitlab.com/development/feature_flags/#feature-actors). The flag actor is the merge request target project. **Non-production** ``` /chatops gitlab run feature set async_write_diffs_cache_on_mr_create 50 --actors --dev --pre --staging --staging-ref /chatops gitlab run feature set async_write_diffs_cache_on_mr_create true --dev --pre --staging --staging-ref ``` **Production** — percentage rollout (wait ≥15 min between steps, watch dashboards): ``` /chatops gitlab run feature set async_write_diffs_cache_on_mr_create <percentage> --actors ``` Or target specific actors instead: ``` /chatops gitlab run feature set --project=gitlab-org/gitlab,gitlab-org/gitlab-foss async_write_diffs_cache_on_mr_create true /chatops gitlab run feature set --group=gitlab-org,gitlab-com async_write_diffs_cache_on_mr_create true /chatops gitlab run feature set --user=marc_shaw async_write_diffs_cache_on_mr_create true ``` ## Before global rollout Confirm the relevant gotchas before going to 100% — see [enabling a feature for GitLab.com](https://docs.gitlab.com/development/feature_flags/controls/#enabling-a-feature-for-gitlabcom): - [Docs + version history](https://docs.gitlab.com/development/documentation/feature_flags/) updated - [Breaking changes](https://docs.gitlab.com/development/documentation/release_notes/#deprecations-removals-and-breaking-changes) announced, if any - [Change management issue](https://handbook.gitlab.com/handbook/engineering/infrastructure-platforms/change-management/#feature-flags-and-the-change-management-process) opened, if required - [External API consumers](https://docs.gitlab.com/development/feature_flags/#do-not-use-feature-flags-in-external-api-consumers) handled with a fail-open mechanism, if applicable ## Cleanup Remove the flag once [deemed stable](https://handbook.gitlab.com/handbook/product-development/how-we-work/product-development-flow/feature-flag-lifecycle/#feature-flag-lifecycle) — see [cleaning up](https://docs.gitlab.com/development/feature_flags/controls/#cleaning-up). Track it here, or open a follow-up [Feature Flag Cleanup issue](https://gitlab.com/gitlab-org/gitlab/-/issues/new?description_template=Feature%20Flag%20Cleanup). Remove the flag and its YAML definition from the codebase, then: ``` /chatops gitlab run release check https://gitlab.com/gitlab-org/gitlab/-/merge_requests/243117 <milestone> /chatops gitlab run feature delete async_write_diffs_cache_on_mr_create --dev --pre --staging --staging-ref --production ``` ## Rollback ``` /chatops gitlab run feature set async_write_diffs_cache_on_mr_create false # production /chatops gitlab run feature set async_write_diffs_cache_on_mr_create false --dev --pre --staging --staging-ref # non-production /chatops gitlab run feature delete async_write_diffs_cache_on_mr_create --dev --pre --staging --staging-ref --production # remove entirely ```