Managing a merge queue

You can increase development velocity with a merge queue for pull requests in your repository.

Who can use this feature?

People with admin permissions can manage merge queues for pull requests targeting selected branches of a repository.

Pull request merge queues are available in any organization-owned repository on GitHub Enterprise Server.

In this article

About merge queues

A merge queue helps increase velocity by automating pull request merges into a busy branch and ensuring the branch is never broken by incompatible changes.

The merge queue provides the same benefits as the Require branches to be up to date before merging branch protection, but does not require a pull request author to update their pull request branch and wait for status checks to finish before trying to merge.

Using a merge queue is particularly useful on branches that have a relatively high number of pull requests merging each day from many different users.

Once a pull request has passed all required branch protection checks, a user with write access to the repository can add the pull request to the queue. The merge queue will ensure the pull request's changes pass all required status checks when applied to the latest version of the target branch and any pull requests already in the queue.

A merge queue may use GitHub Actions or your own CI provider to run required checks on pull requests in a merge queue. For more information, see "GitHub Actions documentation."

For more information on merging a pull request using a merge queue, see Merging a pull request with a merge queue.

Configuring continuous integration (CI) workflows for merge queues

Note

  • A merge queue cannot be enabled with branch protection rules that use wildcard characters (*) in the branch name pattern.
  • A merge queue will wait for required checks to be reported before it can proceed with merging. You must update your CI configuration to trigger and report on merge group events when requiring a merge queue.
  • Merge queue and pull requests checks are coupled and configured under branch protection rules or rulesets. For more information, see Managing a merge queue.

Triggering merge group checks with GitHub Actions

You must use the merge_group event to trigger your GitHub Actions workflow when a pull request is added to a merge queue.

Note

If your repository uses GitHub Actions to perform required checks or if you require workflows via organization rulesets on pull requests in your repository, you need to update the workflows to include the merge_group event as an additional trigger. Otherwise, status checks will not be triggered when you add a pull request to a merge queue. The merge will fail as the required status check will not be reported. The merge_group event is separate from the pull_request and push events.

A workflow that reports a check which is required by the target branch's protections would look like this:

on:
  pull_request:
  merge_group:

For more information on the merge_group event, see Events that trigger workflows.

Triggering merge group checks with third-party CI providers

With third-party CI providers, you will need to update your CI configuration to run when a branch that begins with the special prefix gh-readonly-queue/{base_branch} is pushed to. These are the temporary branches that are created on your behalf by a merge queue and contain a different sha from the pull request.

Managing a merge queue

Repository administrators can require a merge queue by enabling the branch protection setting "Require merge queue" in the protection rules for the base branch. For more information, see Managing a branch protection rule.

Once you have enabled the "Require merge queue" setting, you can also access the following settings:

  • Merge method: Select which method to use when merging queued pull requests: merge, rebase, or squash.

  • Build concurrency: The maximum number of merge_group webhooks to dispatch (between 1 and 100), throttling the total amount of concurrent CI builds. This affects the velocity of merges that a merge queue can complete.

  • Only merge non-failing pull requests: This setting determines how a merge queue forms groups of pull requests to be merged.

    Enabled?Description
    YesAll pull requests must satisfy required checks to be merged.
    NoPull requests that have failed required checks can be added to a group as long as the last pull request in the group has passed required checks. If the last pull request in the group has passed required checks, this means that the checks have passed for the combined set of changes in the merge group. Leaving this checkbox unselected can be useful if you have intermittent test failures, but don't want false negatives to hold up the queue.

  • Status check timeout: Choose how long the queue should wait for a response from CI before assuming that checks have failed.

  • Merge limits: Select the minimum and maximum number of pull requests to merge into the base branch at the same time (between 1 and 100), and a timeout after which the queue should stop waiting for more entries and merge with fewer than the minimum number.

    Note

    Merge limits do not combine merge_group builds. Merge limits only affect merges to the base branch once one or more merge_group has satisfied build checks.

    Merge LimitUse Case
    Maximum pull requests to mergeYou can specify a maximum group size, which is useful if merges to your base branch trigger a deployment, and you want to make sure you’re not deploying too many changes at once.
    Minimum pull requests to mergeYou can specify a minimum group size, which is useful if merges to your base branch trigger a lengthy CI build or deploy process, and you don’t want to hold up the following entries in the queue.
    Wait timeYou can specify a timeout for reaching the minimum group size, which allows smaller groups to merge if there are no more PRs queued within your specified time limit.

How merge queues work

As pull requests are added to the merge queue, the merge queue ensures that they are merged in a first-in-first-out order where the required checks are always satisfied.

A merge queue creates temporary branches with a special prefix to validate pull request changes. When a pull request is added to the merge queue, the changes in the pull request are grouped into a merge_group with the latest version of the base_branch as well as changes from pull requests ahead of it in the queue. GitHub Enterprise Server will merge all these changes into the base_branch once the checks required by the branch protections of base_branch pass.

For information about merge methods, see About pull request merges.

Successful CI

When multiple pull requests are added to the merge queue and when the temporary merge_group branches have successful CI results, they are both merged. In the following scenario, two pull requests are successfully added to the queue and merged to the target branch.

  1. User adds pull request #1 to the merge queue.
  2. The merge queue creates a temporary branch with the prefix of main/pr-1 that contains code changes from the target branch and pull request #1. A merge_group webhook event of type checks_requested is dispatched and the merge queue will await a response from your CI provider.
  3. User adds pull request #2 to the merge queue.
  4. The merge queue creates a temporary branch with the prefix of main/pr-2 that contains code changes from the target branch, pull request #1, and pull request #2, and dispatches webhooks.
  5. When the GitHub Enterprise Server API receives successful CI responses for merge_group branches main/pr-1 and main/pr-2, the temporary branch main/pr-2 will be merged in to the target branch. The target branch now contains both changes from pull request #1 and #2.

Failing CI

After grouping a pull request with the latest version of the target branch and changes ahead of it in the queue, if there are failed required status checks or conflicts with the base branch, the pull request will be removed from the queue. The pull request timeline will display the reason why the pull request was removed from the queue.

The following scenario outlines what happens when a CI reports a failing status about one pull request.

  1. User adds pull request #1 to the merge queue.
  2. The merge queue creates a temporary branch with the prefix of main/pr-1 that contains code changes from the target branch and pull request #1. A merge_group webhook event of type checks_requested is dispatched and the merge queue will await a response from your CI provider.
  3. User adds pull request #2 to the merge queue.
  4. The merge queue creates a temporary branch with the prefix of main/pr-2 that contains code changes from the target branch, pull request #1, and pull request #2, and dispatches webhooks.
  5. When the GitHub Enterprise Server API receives a failing status for main/pr-1, the merge queue automatically removes pull request #1 from the merge queue.
  6. The merge queue recreates the temporary branch with the prefix of main/pr-2 to only contain changes from the target branch and pull request #2.
  7. When the GitHub Enterprise Server API receives successful CI responses for merge_group branch main/pr-2, the temporary branch main/pr-2 will be merged in to the target branch without pull request #1 included.

There are a number of reasons a pull request can be removed from a merge queue:

  • Configured CI service is reporting test failures for a merge group
  • Timed out awaiting a successful CI result based off the configured timeout setting
  • User requesting a removal via the API or merge queue interface
  • Branch protection failure that could not automatically be resolved

Jumping to the top of the queue

When adding a pull request to a merge queue, there is an option to move your pull request to the top of the queue.

Note

Be aware that jumping to the top of a merge queue will cause a full rebuild of all in-progress pull requests, as the reordering of the queue introduces a break in the commit graph. Heavily utilizing this feature can slow down the velocity of merges for your target branch.

The following scenario outlines what happens when a user jumps the queue.

  1. User adds pull request #1 to the merge queue.
  2. The merge queue creates a temporary branch with the prefix of main/pr-1 that contains code changes from the target branch and pull request #1. A merge_group webhook event of type checks_requested is dispatched and the merge queue will await a response from your CI provider.
  3. User adds pull request #2 to the merge queue.
  4. The merge queue creates a temporary branch with the prefix of main/pr-2 that contains code changes from the target branch, pull request #1, and pull request #2, and dispatches webhooks.
  5. User adds pull request #3 to the merge queue with the jump option which introduces a break in the commit graph.
  6. The merge queue creates a temporary branch with the prefix of main/pr-3 that contains code changes from the target branch and pull request #3, and dispatches webhooks.
  7. The merge queue recreates the temporary branches with the prefix of main/pr-1 and main/pr-2 that contain the changes from pull request #3, and dispatches webhooks.

Further reading