258 lines
11 KiB
Markdown
258 lines
11 KiB
Markdown
---
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stage: Verify
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group: Pipeline Execution
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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---
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# Merge trains **(PREMIUM)**
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> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/9186) in GitLab 12.0.
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> - [Squash and merge](../../user/project/merge_requests/squash_and_merge.md) support [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/13001) in GitLab 12.6.
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For more information about why you might want to use merge trains, read [How starting merge trains improve efficiency for DevOps](https://about.gitlab.com/blog/2020/01/30/all-aboard-merge-trains/).
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When [merged results pipelines](merged_results_pipelines.md) are
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enabled, the pipeline jobs run as if the changes from your source branch have already
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been merged into the target branch.
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However, the target branch may be changing rapidly. When you're ready to merge,
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if you haven't run the pipeline in a while, the target branch may have already changed.
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Merging now could introduce breaking changes.
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*Merge trains* can prevent this from happening. A merge train is a queued list of merge
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requests, each waiting to be merged into the target branch.
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Many merge requests can be added to the train. Each merge request runs its own merged results pipeline,
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which includes the changes from all of the other merge requests in *front* of it on the train.
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All the pipelines run in parallel, to save time. The author of the internal merged result commit is always the user that initiated the merge.
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If the pipeline for a merge request fails, the breaking changes are not merged, and the target
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branch is unaffected. The merge request is removed from the train, and all pipelines behind it restart.
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If the pipeline for the merge request at the front of the train completes successfully,
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the changes are merged into the target branch, and the other pipelines continue to
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run.
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To add a merge request to a merge train, you need [permissions](../../user/permissions.md) to merge or push to the
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target branch.
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Each merge train can run a maximum of **twenty** pipelines in parallel.
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If more than twenty merge requests are added to the merge train, the merge requests
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are queued until a slot in the merge train is free. There is no limit to the
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number of merge requests that can be queued.
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## Merge train example
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Three merge requests (`A`, `B` and `C`) are added to a merge train in order, which
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creates three merged results pipelines that run in parallel:
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1. The first pipeline runs on the changes from `A` combined with the target branch.
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1. The second pipeline runs on the changes from `A` and `B` combined with the target branch.
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1. The third pipeline runs on the changes from `A`, `B`, and `C` combined with the target branch.
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If the pipeline for `B` fails, it is removed from the train. The pipeline for
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`C` restarts with the `A` and `C` changes, but without the `B` changes.
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If `A` then completes successfully, it merges into the target branch, and `C` continues
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to run. If more merge requests are added to the train, they now include the `A`
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changes that are included in the target branch, and the `C` changes that are from
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the merge request already in the train.
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<i class="fa fa-youtube-play youtube" aria-hidden="true"></i>
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Watch this video for a demonstration on
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[how parallel execution of merge trains can prevent commits from breaking the default branch](https://www.youtube.com/watch?v=D4qCqXgZkHQ).
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## Prerequisites
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To enable merge trains:
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- You must have the Maintainer role.
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- You must be using [GitLab Runner](https://gitlab.com/gitlab-org/gitlab-runner) 11.9 or later.
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- Your repository must be a GitLab repository, not an
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[external repository](../ci_cd_for_external_repos/index.md).
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Merge trains do not work with [Semi-linear history merge requests](../../user/project/merge_requests/methods/index.md#merge-commit-with-semi-linear-history)
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or [fast-forward merge requests](../../user/project/merge_requests/methods/index.md#fast-forward-merge).
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## Enable merge trains
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To enable merge trains for your project:
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1. If you are on a self-managed GitLab instance, ensure the [feature flag](#merge-trains-feature-flag) is set correctly.
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1. [Configure your CI/CD configuration file](merge_request_pipelines.md#prerequisites)
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so that the pipeline or individual jobs run for merge requests.
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1. On the top bar, select **Main menu > Projects** and find your project.
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1. On the left sidebar, select **Settings > Merge requests**.
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1. In the **Merge method** section, verify that **Merge commit** is selected.
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1. In the **Merge options** section, select **Enable merged results pipelines** (if not already selected) and **Enable merge trains**.
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1. Select **Save changes**.
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In GitLab 13.5 and earlier, there is only one checkbox, named
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**Enable merge trains and pipelines for merged results**.
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WARNING:
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If you select the check box but don't configure your CI/CD to use
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merge request pipelines, your merge requests may become stuck in an
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unresolved state or your pipelines may be dropped.
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## Start a merge train
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To start a merge train:
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1. Visit a merge request.
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1. Select **Start merge train**.
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Other merge requests can now be added to the train.
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## Add a merge request to a merge train
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To add a merge request to a merge train:
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1. Visit a merge request.
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1. Select **Add to merge train**.
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If pipelines are already running for the merge request, you cannot add the merge request
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to the train. Instead, you can schedule to add the merge request to a merge train **when the latest
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pipeline succeeds**.
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## Remove a merge request from a merge train
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1. Visit a merge request.
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1. Select **Remove from merge train**.
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If you want to add the merge request to a merge train again later, you can.
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## View a merge request's current position on the merge train
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After a merge request has been added to the merge train, the merge request's
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current position is displayed under the pipeline widget:
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## Immediately merge a merge request with a merge train
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If you have a high-priority merge request (for example, a critical patch) that must
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be merged urgently, you can bypass the merge train by using the **Merge Immediately** option.
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This is the fastest option to get the change merged into the target branch.
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WARNING:
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Each time you merge a merge request immediately, the current merge train is recreated,
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all pipelines restart, and [redundant pipelines are cancelled](#automatic-pipeline-cancellation).
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### Automatic pipeline cancellation
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> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/12996) in GitLab 12.3.
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GitLab CI/CD can detect the presence of redundant pipelines, and cancels them
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to conserve CI resources.
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When a user merges a merge request immediately in an ongoing merge
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train, the train is reconstructed, because it recreates the expected
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post-merge commit and pipeline. In this case, the merge train may already
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have pipelines running against the previous expected post-merge commit.
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These pipelines are considered redundant and are automatically
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canceled.
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## Troubleshooting
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### Merge request dropped from the merge train immediately
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If a merge request is not mergeable (for example, it's a draft merge request or it has a merge
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conflict), the merge train drops your merge request automatically.
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In these cases, the reason for dropping the merge request is in the **system notes**.
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To check the reason:
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1. Open the merge request that was dropped from the merge train.
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1. Select the **Discussion** tab.
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1. Find a system note that includes either:
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- **... removed this merge request from the merge train because ...**
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- **... aborted this merge request from the merge train because ...**
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The reason is given in the text after the **because ...** phrase.
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### Merge When Pipeline Succeeds cannot be chosen
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[Merge When Pipeline Succeeds](../../user/project/merge_requests/merge_when_pipeline_succeeds.md)
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is currently unavailable when merge trains are enabled.
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See [the related issue](https://gitlab.com/gitlab-org/gitlab/-/issues/12267)
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for more information.
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### Merge train pipeline cannot be retried
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When a merge train pipeline fails, the merge request is dropped from the train and the pipeline can't be retried.
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Merge train pipelines run on the merged result of the changes in the merge request and
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the changes from other merge requests already on the train. If the merge request is dropped from the train,
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the merged result is out of date and the pipeline can't be retried.
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Instead, you should [add the merge request to the train](#add-a-merge-request-to-a-merge-train)
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again, which triggers a new pipeline.
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If a job only fails intermittently, you can try using the [`retry`](../yaml/index.md#retry)
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keyword in the `.gitlab-ci.yml` file to have the job retried before the pipeline completes.
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If it succeeds after a retry, the merge request is not removed from the merge train.
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### Unable to add to merge train with message "The pipeline for this merge request failed."
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Sometimes the **Start/Add to merge train** button is not available and the merge request says,
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"The pipeline for this merge request failed. Please retry the job or push a new commit to fix the failure."
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This issue occurs when [**Pipelines must succeed**](../../user/project/merge_requests/merge_when_pipeline_succeeds.md#require-a-successful-pipeline-for-merge)
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is enabled in **Settings > General > Merge requests**. This option requires that you
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run a new successful pipeline before you can re-add a merge request to a merge train.
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Merge trains ensure that each pipeline has succeeded before a merge happens, so
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you can:
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- Clear the **Pipelines must succeed** checkbox.
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- Select the **Enable merged results pipelines** and **Enable merge trains** checkboxes.
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In GitLab 13.5 and earlier, there is only one checkbox, named
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**Enable merge trains and pipelines for merged results**.
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If you want to keep the **Pipelines must succeed** option selected along with merge
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trains, create a new merged results pipeline when this error occurs:
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1. On the **Pipelines** tab, select **Run pipeline**.
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1. Select **Start/Add to merge train when pipeline succeeds**.
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See [the related issue](https://gitlab.com/gitlab-org/gitlab/-/issues/35135)
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for more information.
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### Merge trains feature flag **(PREMIUM SELF)**
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In [GitLab 13.6 and later](https://gitlab.com/gitlab-org/gitlab/-/issues/244831),
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you can [enable or disable merge trains in the project settings](#enable-merge-trains).
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In GitLab 13.5 and earlier, merge trains are automatically enabled when
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[merged results pipelines](merged_results_pipelines.md) are enabled.
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To use merged results pipelines without using merge trains, you can enable a
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[feature flag](../../user/feature_flags.md) that blocks the merge trains feature.
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[GitLab administrators with access to the GitLab Rails console](../../administration/feature_flags.md)
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can enable the feature flag to disable merge trains:
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```ruby
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Feature.enable(:disable_merge_trains)
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```
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After you enable this feature flag, all existing merge trains are cancelled and
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the **Start/Add to merge train** button no longer appears in merge requests.
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To disable the feature flag, and enable merge trains again:
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```ruby
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Feature.disable(:disable_merge_trains)
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```
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