Merge Trains (PREMIUM)

• Introduced in GitLab Premium 12.0.
• Squash and merge support introduced in GitLab Premium 12.6.

When pipelines for merged results are enabled, the pipeline jobs run as if the changes from your source branch have already been merged into the target branch.

However, the target branch may be changing rapidly. When you're ready to merge, if you haven't run the pipeline in a while, the target branch may have already changed. Merging now could introduce breaking changes.

Merge trains can prevent this from happening. A merge train is a queued list of merge requests, each waiting to be merged into the target branch.

Many merge requests can be added to the train. Each merge request runs its own merged results pipeline, which includes the changes from all of the other merge requests in front of it on the train. All the pipelines run in parallel, to save time.

If the pipeline for a merge request fails, the breaking changes are not merged, and the target branch is unaffected. The merge request is removed from the train, and all pipelines behind it restart.

If the pipeline for the merge request at the front of the train completes successfully, the changes are merged into the target branch, and the other pipelines continue to run.

To add a merge request to a merge train, you need permissions to push to the target branch.

NOTE: Note: Each merge train can run a maximum of twenty pipelines in parallel. If more than twenty merge requests are added to the merge train, the merge requests will be queued until a slot in the merge train is free. There is no limit to the number of merge requests that can be queued.

Merge train example

Three merge requests (A, B and C) are added to a merge train in order, which creates three merged results pipelines that run in parallel:

1. The first pipeline runs on the changes from A combined with the target branch.
2. The second pipeline runs on the changes from A and B combined with the target branch.
3. The third pipeline runs on the changes from A, B, and C combined with the target branch.

If the pipeline for B fails, it is removed from the train. The pipeline for C restarts with the A and C changes, but without the B changes.

If A then completes successfully, it merges into the target branch, and C continues to run. If more merge requests are added to the train, they will now include the A changes that are included in the target branch, and the C changes that are from the merge request already in the train.

Read more about how merge trains keep your master green.

Watch this video for a demonstration on how parallel execution of Merge Trains can prevent commits from breaking the default branch.

Prerequisites

To enable merge trains:

• You must have maintainer permissions.
• You must be using GitLab Runner 11.9 or later.
• In GitLab 12.0 and later, you need Redis 3.2 or later.

Enable merge trains

To enable merge trains for your project:

1. If you are on a self-managed GitLab instance, ensure the feature flag is set correctly.
2. Configure your CI/CD configuration file so that the pipeline or individual jobs run for merge requests.
3. Visit your project's Settings > General and expand Merge requests.
4. Check Enable merge trains and pipelines for merged results.
5. Click Save changes.

CAUTION: Caution: If you select the check box but don't configure your CI/CD to use pipelines for merge requests, your merge requests may become stuck in an unresolved state or your pipelines may be dropped.

Start a merge train

To start a merge train:

1. Visit a merge request.
2. Click the Start merge train button.

Other merge requests can now be added to the train.

Add a merge request to a merge train

To add a merge request to a merge train:

1. Visit a merge request.
2. Click the Add to merge train button.

If pipelines are already running for the merge request, you cannot add the merge request to the train. Instead, you can schedule to add the merge request to a merge train when the latest pipeline succeeds.

Remove a merge request from a merge train

1. Visit a merge request.
2. Click the Remove from merge train button.

If you want to add the merge request to a merge train again later, you can.

View a merge request's current position on the merge train

After a merge request has been added to the merge train, the merge request's current position is displayed under the pipeline widget:

Immediately merge a merge request with a merge train

If you have a high-priority merge request (for example, a critical patch) that must be merged urgently, you can bypass the merge train by using the Merge Immediately option. This is the fastest option to get the change merged into the target branch.

CAUTION: Caution: Each time you merge a merge request immediately, the current merge train is recreated and all pipelines restart.

Troubleshooting

Merge request dropped from the merge train immediately

If a merge request is not mergeable (for example, it's WIP, there is a merge conflict, etc.), your merge request will be dropped from the merge train automatically.

In these cases, the reason for dropping the merge request is in the system notes.

To check the reason:

1. Open the merge request that was dropped from the merge train.
2. Open the Discussion tab.
3. Find a system note that includes either:
   • The text ... removed this merge request from the merge train because ...
   • ... aborted this merge request from the merge train because ... The reason is given in the text after the because ... phrase.

Merge When Pipeline Succeeds cannot be chosen

Merge When Pipeline Succeeds is currently unavailable when Merge Trains are enabled.

See the related issue for more information.

Merge Train Pipeline cannot be retried

A Merge Train pipeline cannot be retried because the merge request is dropped from the merge train upon failure. For this reason, the retry button does not appear next to the pipeline icon.

In the case of pipeline failure, you should re-enqueue the merge request to the merge train, which will then initiate a new pipeline.

Unable to add to merge train with message "The pipeline for this merge request failed."

Sometimes the Start/Add to Merge Train button is not available and the merge request says, "The pipeline for this merge request failed. Please retry the job or push a new commit to fix the failure."

This issue occurs when Pipelines must succeed is enabled in Settings > General > Merge requests. This option requires that you run a new successful pipeline before you can re-add a merge request to a merge train.

Merge trains ensure that each pipeline has succeeded before a merge happens, so you can clear the Pipelines must succeed check box and keep Enable merge trains and pipelines for merged results (merge trains) enabled.

If you want to keep the Pipelines must succeed option enabled along with Merge Trains, you can create a new pipeline for merged results when this error occurs by going to the Pipelines tab and clicking Run pipeline. Then click Start/Add to merge train when pipeline succeeds.

See the related issue for more information.

Merge Trains feature flag (PREMIUM ONLY)

To enable and disable the Merge Trains feature, use the :disable_merge_trains feature flag.

To check if the feature flag is enabled on your GitLab instance, ask an administrator to execute the following commands:

> sudo gitlab-rails console                         # Login to Rails console of GitLab instance.
> Feature.enabled?(:disable_merge_trains)           # Check if it's disabled or not.
> Feature.enable(:disable_merge_trains)             # Disable Merge Trains.
> Feature.disable(:disable_merge_trains)            # Enable Merge Trains.

When you disable this feature, all existing merge trains are cancelled and the Start/Add to Merge Train button no longer appears in merge requests.