Pull Request Lifecycle

The merge queue in Mergify is a powerful tool for maintaining the integrity of your codebase, ensuring that every Pull Request (PR) is up-to-date and passes all necessary checks before being merged. But once a PR is added to a merge queue, what exactly happens? Understanding the lifecycle of a PR in the merge queue is essential for managing your PRs effectively and ensuring a smooth, efficient workflow.

In this guide, we will walk you through the journey of a PR once it enters the merge queue, from the moment it’s added until it’s either successfully merged or removed from the queue. We’ll also explain how to monitor and manage your merge queue effectively, and share some best practices for handling common scenarios such as requeueing and dequeueing PRs.

A pull request can be added to the merge queue either manually or through a pull request rule using Mergify’s workflow automation. Here’s an overview of the process:

1. Manual addition: A pull request can be manually added to the merge queue using the @mergifyio queue command.

2. Automated addition: A pull request can also be automatically added to the merge queue based on the rules defined in the pull_request_rules section of your Mergify configuration file.

3. Meeting queue conditions: Before a pull request can be added to the merge queue, it must meet certain conditions, specified under queue_conditions in your queue_rules. These conditions might include requirements such as passing all checks, having a certain number of approved reviews, or having specific labels.

4. Entering the queue: Once a pull request meets the specified queue conditions, it is added to the end of the merge queue. However, priority rules can be used to alter its position in the queue.

Please note that, by default, only one pull request is processed at a time to ensure that each pull request is tested against the most recent state of the base branch. You can test multiple pull requests at once by using batches or in advance by using parallel checks.

The lifecycle of a pull request in the merge queue begins as soon as it enters the queue.

1. Position in the queue: The pull request assumes its position in the queue based on the order in which it entered. As mentioned in the previous section, the order may be influenced by any defined priority rules.

2. Waiting for its turn: The pull request will wait in the queue until it is its turn to be processed. The processing of pull requests in the queue is sequential, which means that only one pull request is processed at a time by default.

3. Validating the pull request: When a pull request is next in line, it enters the validation phase. During this phase, the Mergify engine will update it with the base branch. But before the actual merge happens, it will check that the merge_conditions specified in the configuration file are satisfied. This ensures that only pull requests that meet the necessary conditions are merged.

4. Merge success or failure:

   • If the validation is successful, meaning all the merge_conditions are met, the pull request leaves the queue and is merged into the base branch.

   • If the merge is not successful, the pull request is removed from the queue. This could be due to various reasons, such as conflicts with the base branch or failure of some checks.

Remember that while a pull request is in the queue, its status and position can be monitored, and if necessary, it can be manually requeued or dequeued — which we will explore in the following sections.

When a pull request enters the merge queue, Mergify performs a series of validation checks to ensure that the pull request is ready to be merged. These checks are based on the merge_conditions specified in the Mergify configuration file. The checks can include verifying that required reviews are complete, certain labels are applied, the build is successful, or any custom conditions that you’ve specified.

Before validating any of the merge_conditions, Mergify will update the pull request with the latest changes from the base branch before attempting to merge. This is done to ensure that the pull request is always up-to-date with the base branch and that the merging will not introduce any conflicts. Mergify uses the update action to achieve this by default, merging the base branch in the pull request. You can change this behavior using the update_method option.

If the pull request cannot be updated due to conflicts that can’t be resolved automatically, it will be removed from the queue, and you will be notified to manually resolve the conflicts and requeue the pull request.

In case any of the queue_conditions stop being true during the update, Mergify will also stop the merge process. For example, if a new commit is pushed to the base branch that makes a test on the pull request fail, the pull request would be removed from the queue. In such situations, the issues need to be addressed and the pull request has to be requeued once all checks pass again.

This process ensures that your codebase is kept clean and that only fully validated and up-to-date pull requests make it to your main branch.

A pull request can be removed or “dequeued” from the merge queue in a couple of ways:

1. Non-compliance with queue_conditions: If a pull request no longer meets the queue_conditions specified in the Mergify configuration file, it will automatically be removed from the queue. For instance, if one of the queue_conditions is the presence of a specific label, and that label gets removed from the pull request, Mergify will automatically remove the pull request from the queue.

2. Use of the dequeue command: A pull request can be manually removed from the queue by issuing the @mergifyio dequeue command. This is particularly useful in situations where you need to stop a pull request from being merged, such as when a critical issue is discovered.

Please note, a pull request removed from the queue with the dequeue command will not rejoin the queue automatically — it will need to be manually re-added to the queue, either by re-applying the queue command or by re-satisfying the queue_conditions.

These methods provide granular control over your merge queue, ensuring that only the appropriate pull requests get processed and merged.

Even with all the automated checks and balances in place, it’s possible for a pull request to fail the validation checks. This could be due to a wide range of issues, such as coding errors, conflicts with the base branch, failing CI tests, or not meeting certain merge_conditions. When this happens, the pull request is removed from the queue, and Mergify informs the pull request author and reviewers about the failure.

Here’s how to handle such situations:

1. Review the failure reasons: Mergify will provide a detailed report outlining why the pull request failed the checks. You should start by reviewing these details to understand what went wrong.

2. Update the pull request: Once you’ve identified the issue, the next step is to fix it. This might involve updating the code, resolving merge conflicts, or making other necessary changes to the pull request.

3. Re-run the checks: After updating the pull request, the validation checks need to be re-run. If you’re using a continuous integration (CI) system, it will likely automatically run the checks again once the pull request is updated.

4. Requeue the pull request: Once all the issues have been resolved and the validation checks pass, the pull request needs to be requeued. You can do this by issuing the @mergifyio queue command.

By carefully addressing each failed check and ensuring all the issues are fixed, you can maintain the integrity of your codebase while ensuring that every pull request is ready to be merged when it’s time.

At times, you might find it necessary to requeue a pull request. This could happen when a pull request is removed from the queue because it failed to meet the merge_conditions of the queue it was in. This typically happens when a check failed because a flaky test; the PR is removed from the queue whereas it might have been mergeable.

In that case, the PR might be re-added to the merge queue to try again by triggering the @mergifyio requeue command, which will reset its status to a neutral state so Mergify is happy to retry it.

To requeue a pull request, you can use the @mergifyio requeue command in the pull request conversation. Once issued, the command will place the pull request back in the merge queue according to the queue’s queue_conditions.

On the other hand, if you manually dequeued a pull request using the @mergifyio dequeue command, you will need to use the @mergifyio queue command to put back the PR in a queue matching its queue_conditions.

The merge queue tests pull requests against the latest version of the base branch. Therefore, if a new commit is pushed to the base branch, Mergify resets the process and starts over with the updated base branch.

To keep track of the status and progress of your pull requests in the merge queue, Mergify provides multiple tools and indicators. Here’s how you can use them:

1. Check the Mergify dashboard: The Mergify dashboard provides a comprehensive view of your queues. You can see the number of pull requests in each queue and their position. You can access it by navigating to your repository on the Mergify dashboard and clicking on the “Queues” tab.

2. Check the Mergify checks tab on the PR: On each pull request page, the “checks” tab will show the status of the Mergify checks. If the pull request is in the queue, you will see a “Mergify — Queued” status.

Monitoring your pull requests’ status helps you understand their position in the queue and estimate when they might be merged, making the entire process more transparent and manageable.