Introduction to Merge Queues

Understand what merge queues are and how to use them.

Development workflows can often be complex, with multiple developers collaborating on a single codebase. This leads to multiple pull requests that need to be merged in a specific order to ensure code stability and compatibility. In this context, merge queues are an essential tool.

A merge queue is a mechanism to manage and control the order of merging pull requests into the main branch of your repository. It helps streamline the development process, reduce merge conflicts, and maintain a healthy and stable codebase.

In this guide, we will explore the concept of merge queues, their importance in continuous integration and development workflows, and how Mergify’s Merge Queue can simplify and improve your development process.

Watch this video below or read this page to discover why a merge queue is a key component of your development workflow.

Why Do You Need a Merge Queue?

Section titled Why Do You Need a Merge Queue?

In modern software development, teams often work in parallel on multiple features or bug fixes. This parallel work, although necessary for swift development, poses a risk when changes are merged into the main branch.

Even with sophisticated CI/CD pipelines validating the correctness of each change, a problem arises when multiple changes are ready to merge simultaneously. While each change may be correct and compatible with the main branch at the time it was last updated, merging one could introduce incompatibilities with others that are also ready to merge.

This is the risk of merging pull requests that are not up-to-date with the main branch: they may have been validated against an outdated version of the codebase.

To mitigate this risk, engineers often find themselves in a race to update and merge their pull requests before others do, leading to what we can call a “merge war”. This race not only distracts from productive development work but also puts unnecessary pressure on engineers and creates an inefficient working environment.

A merge queue addresses this issue by automatically ordering pull requests ready to be merged, updating each one against the main branch, re-running validations, and merging them only if they pass. This ensures that each change is always up-to-date and validated against the real state of the main branch at the time it’s merged.

With a merge queue, there is no longer a need for engineers to participate in a “merge war”. The queue takes care of update and merge processes, allowing engineers to focus on their core development tasks, while maintaining a steady, efficient, and error-free integration flow.

Understanding The Problem: A Real-world Example

Section titled Understanding The Problem: A Real-world Example

Let’s consider a situation with a having a pull request ready to merge.

The developer made their changes on their feature branches. They’ve pushed their changes, and their pull request have successfully passed all CI checks. The CI pipeline has validated that their changes are compatible with the main branch, and everything seems ready to merge.

g cluster_pr PR #1 ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w me main m2:e->me:w pr11 m2:e->pr11:w ci Continuous Integration m2->ci Success pr12 pr11->pr12 pr12->ci Success

Step 2: Main Branch Gets a New Commit

Section titled Step 2: Main Branch Gets a New Commit

Meanwhile, a second developer merges another change into the main branch. This change doesn’t conflict with the previous pull request at the code level, so GitHub still marks them as safe to merge.

g cluster_pr PR #1 ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w m2bis m2:e->m2bis:w pr11 m2->pr11:w me main m2bis:e->me:w ci Continuous Integration m2bis->ci Success pr12 pr11->pr12 pr12->ci Success

What’s not seen here, is that the new commit on the main branch have caused a change in the behavior of the code that makes the changes in our developer’s PR incompatible.

This could be anything: a new linter check has been added, a functional test has been added and won’t pass with this new code, a file or a component has been renamed, etc.

Step 3: Merging the PR Would Fail the CI

Section titled Step 3: Merging the PR Would Fail the CI

If the pull request gets merged now, it will create a failure in the main branch because its changes are not compatible with the latest commit from the main branch. The CI system couldn’t know about this, because it validated the developer’s pull request against the previous main branch commit, and not the latest.

g cluster_pr PR #1 ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w m2bis m2:e->m2bis:w pr11 m2->pr11:w me main mmerge mmerge:e->me:w ci Continuous Integration mmerge->ci Failure m2bis:e->mmerge pr12 pr11->pr12 pr12:e->mmerge

Despite the CI check, the main branch now has a failure because the CI could not anticipate the effect of the latest changes on the changes in the developer’s pull request.

This scenario breaks the main branch. This could prevent any deployment of the code and will make any pull request created from this latest commit to not pass the CI, blocking the entire development team until the situation is manually addressed by an engineer.

This example demonstrates the risk of merging pull requests that are not up-to-date with the main branch. It’s a common scenario in active development teams, and it’s exactly the kind of problem a merge queue is designed to prevent.

Solving the Problem Using a Merge Queue

Section titled Solving the Problem Using a Merge Queue

A merge queue provides an effective solution to the problem described above. Let’s walk through the same example but this time using a merge queue.

After the developer’s pull request have passed their initial CI checks, they’re added to the merge queue rather than being merged immediately.

Here’s where the magic happens. The merge queue will now update the pull request to include changes from the latest commits from the main branch, creating a new temporary merge commit.

g cluster_pr PR #1 pr11 pr12 pr11->pr12 prm Merge main into PR ci Continuous Integration prm->ci Failure pr12->prm ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w m2:ne->pr11:w m2bis m2:e->m2bis:w me main m2bis:ne->prm:sw m2bis:e->me:w m2bis->ci Success

It then runs CI checks on this new merge commit. This ensures that the changes in the pull request are compatible with the current state of the main branch, not its state when the PR was originally created.

In our example above, this would catch the incompatibility problem and prevents the pull request to get merged and to break the main branch. The PR is therefore not merged and is returned to the developer for revision.

By always testing against the latest state of the main branch, a merge queue helps to prevent broken main branches, maintaining the integrity of your codebase and ensuring that your team can always deliver working software.

When multiple pull requests are mergeable, they are scheduled to be merged sequentially, and are updated on top of each other. The pull request branch update is only done when the pull request is ready to be merged.

That means that when a first pull request has been merged, and the second one is outdated like this:

g cluster_pr1 PR #1 cluster_pr2 PR #2 ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w mpr1 Merge PR#1 into main m2->mpr1 pr11 m2:e->pr11:w pr21 m2:n->pr21:w me main mpr1:e->me:w pr12 pr11->pr12 pr12:e->mpr1:w pr22 pr21->pr22

A merge queue will make sure the pull request #2 is updated with the latest tip of the base branch before being merged:

g cluster_pr2 PR #2 cluster_pr1 PR #1 ms m0 ms:e->m0:w m1 m0:e->m1:w m2 m1:e->m2:w mpr1 Merge PR#1 into main m2->mpr1 pr11 m2:e->pr11:w pr21 m2:n->pr21:w mpr2 Merge PR#2 into main mpr1->mpr2 pr23 Merge main into PR#2 mpr1:e->pr23:w me main mpr2:e->me:w pr12 pr11->pr12 pr12:e->mpr1:w pr22 pr21->pr22 pr22->pr23 pr23:e->mpr2:w

Determining when to use a merge queue largely depends on your team’s development practices, the complexity of your project, and the rate of incoming pull requests. Here are some common scenarios where a merge queue can be beneficial:

  • High Volume of Pull Requests: if your project is receiving a large number of pull requests, it can be challenging to keep track of which PRs are ready for merging and which ones need further checks or updates. A merge queue automates this process, ensuring that only validated and up-to-date PRs are merged.

  • Complex Projects: for complex projects with multiple dependencies, a small change can potentially have significant effects. Running updated CI checks on each PR in a merge queue ensures that all changes are compatible with the

  • Agile Teams: teams that practice Agile methodologies such as continuous integration and continuous deployment (CI/CD) can benefit significantly from a merge queue. By automating the testing and merging process, a merge queue helps maintain the pace of continuous delivery while ensuring code quality.

  • Teams with Varying Time Zones: if your team members are distributed across various time zones, coordinating PR reviews and merges can be tricky. With a merge queue, PRs can be merged in an orderly manner as they pass checks, regardless of who is currently online.

  • Prioritizing Important Pull Requests: sometimes, you have important pull requests that should be merged before others, irrespective of the order in which they were created. With a merge queue, you can prioritize these important PRs to be merged as soon as they pass their CI checks.

In summary, a merge queue can be a valuable tool for any team looking to streamline their development process, improve code quality, and maintain a healthy main branch. With Mergify’s merge queues, you can automate this entire process, making your team more productive and your project more robust.

Benefits of Using Merge Queues

Section titled Benefits of Using Merge Queues

Merge queues offer a multitude of benefits that can greatly enhance the efficiency and quality of your software development process. Here are some key advantages:

  • Ensures Codebase Stability: by merging only up-to-date and CI-checked PRs, merge queues significantly reduce the risk of introducing bugs or conflicts into your codebase, thus enhancing its stability.

  • Saves Time and Reduces Manual Labor: merge queues automate the process of testing and merging PRs, saving developers from the tedious task of constantly updating and checking their PRs. This allows them to focus more on writing quality code and less on administrative tasks.

  • Optimizes Resource Usage: merge queues minimize the use of CI resources by avoiding unnecessary checks on PRs that are not up-to-date with the main branch. Usage of advanced options such as batches.

  • Enhances Collaboration: merge queues promote a more collaborative environment where developers don’t need to compete to get their PRs merged. Instead, they can confidently work knowing that their PRs will be handled fairly and efficiently.

  • Streamlines Workflow: by automatically handling PR updates and merges, a merge queue streamlines the software development workflow. This makes the process smoother, faster, and less prone to human error.

In essence, merge queues can significantly boost your team’s productivity and the quality of your product, making them a worthwhile addition to your development toolbox.