A Complete Guide to Trunk-Based Development

Trunk-based development is a software development model that hinges on a single source-of-truth: the trunk. Developers frequently integrate code changes into this central codebase, often multiple times per day. The goal is to shorten the time and divergence of branches from the main code, which simplifies the integration process and reduces code conflicts.

This concept isn’t new. It’s a method refined and tested over the years by both small startups and large tech companies. As the sector has expanded, so has the focus on trunk-based development, elevating it from a niche strategy to a favored industry approach.

Understanding Trunk Based Development

Trunk-based development is about two key ideas: keeping a single source-of-truth and making frequent commits and integrations.

Principle of a Single Source-of-Truth

In trunk-based development, there’s one mainline: the trunk. It’s the single source-of-truth. Everyone works from the same place. No matter how big the team or how complex the project, all developers pull and commit their code changes to this trunk. Instead of working on isolated feature branches, developers work on the same shared main branch. This way, we avoid the code conflicts that often come up when merging separate, long-lived branches.

Frequent Commits and Integrations

Trunk-based development encourages developers to commit and integrate their changes into the trunk often. This practice makes sure developers keep their work in sync, keeping all changes in one snapshot of the project. We’re continually feeding progress back into the main codebase, which makes it easier to spot integration issues early. As the saying goes, the devil is in the details, and in trunk-based development, these details are key to functionality and stability.

How Does It Compare to Other Development Models?

Trunk-based development differs from other development models in how it manages the codebase. A quick comparison helps highlight these differences:

• Gitflow: Gitflow, often used for larger projects, maintains two main branches with an infinite lifetime: master and develop. Features, hotfixes, and releases are managed in separate branches and then merged into ‘develop’ or ‘master’ when ready. This is different from trunk-based development, where all work happens on the trunk.
• Feature Branching: In feature branching, every new feature gets a new branch, and merges happen when the feature is complete. This model often leads to longer-lived branches, which can result in challenging integration problems. Trunk-based development differs here too, by emphasizing frequent commits to the main branch and only using short-lived branches (if at all) for pull requests (PRs) if reviews are desired before each merge.

While people have different opinions about which model is best, the usefulness of a model ultimately depends on the specific situations it’s used in. The focus should always be on what works best for the project and what gets the best results.

How Trunk-Based Development Works

Trunk-based development works well because of its strategies: developing from the trunk, using short-term feature branches, integrating continuously, and releasing software from the trunk. Adding these steps to your regular workflow is essential for mastering trunk-based development.

Developing From the Trunk

Developers start their work from the trunk–the main branch of the code repository. Everyone in the team draws from and contributes to this central code pool. This unified approach connects everyone to the same goal. Developers check out and commit code to the trunk, ensuring their work aligns with the overall project’s direction. This process encourages transparency, keeps everyone in sync and allows for immediate detection of any code integration problems.

Short-Lived Feature Branches

Unlike many development models where branches may exist for weeks or even months, in trunk-based development, branches are temporary. Developers create short-term branches for developing specific features or fixes and merge these back into the trunk typically within a day. After finishing the feature or fix, the developer merges the branch back into the trunk and removes the branch. This method minimizes the risk of merge conflicts and keeps the codebase tidy and manageable.

Continuous Integration and Testing

Continuous integration (CI) is crucial to trunk-based development. As developers regularly commit code changes to the trunk, there’s a constant need for CI to ensure these changes don’t break the build or conflict with other contributions. Automated testing comes into play here, processing each commit to check its viability and maintain the consistency of the codebase. This process helps teams to detect errors early in the cycle, rather than at the end when they could be more difficult (and costly) to fix. By combining development from the trunk, short-lived feature branches, and continuous integration, trunk-based development promotes quick feedback, early error detection, and high-quality software.

Releasing Software From the Trunk

Unlike other development models that might use separate branches for releases, trunk-based development goes straight to the source. The trunk–being the single source of truth–is the basis for all production releases. Since every commit could be potentially releasable, teams can quickly respond to market changes and reduce the “idea-to-production” cycle time. This approach allows for more flexible and frequent deployments, supporting the agile mindset that is central to trunk-based development.

4 Benefits of Trunk Based Development

Faster Release Cycles

With trunk-based development, the trunk is always ready for deployment. A continuous integration (CI) system regularly checks changes made to the trunk, keeping the code in shape for production at all times. The result? Faster cycles of product launches. Each commit could be the next release, enabling teams to react promptly to market shifts and customer needs, and reducing the “time-to-market” for all features. Shorter cycles make your software more competitive.

Improved Code Quality

Trunk-based development puts a premium on high-quality codebase. Regularly merging changes into the trunk prevents overly complicated code because the “work in progress” must stay small to ensure smooth integration. Combined with automated testing, short-lived branches keep close to the trunk, making sure all team members’ work meshes well, resulting in a codebase that’s easier to maintain and navigate.

Reduced Merge Conflicts

Long-lived feature branches are infamous for causing complicated merge conflicts. This issue usually arises because the branches separate too much from the trunk over time. But trunk-based development promotes frequent commits and merges, practically wiping out large, unexpected merge conflicts.

By merging small changes into the trunk regularly, developers can quickly fix any conflicts, allowing the team to spend less time sorting out code and more time building useful features.

Increased Productivity

With a strategy that avoids complicated merges, promotes quality code, and supports quicker releases, your team’s productivity can skyrocket. Developers can concentrate on making new features or fixing problems instead of struggling with complicated merges or navigating unwieldy feature branches.

Best Practices for Implementing Trunk-Based Development

To make the most of trunk-based development, adopt good practices. These will help integrate changes smoothly, maintain code quality, and help your team adapt to the process.

Cultivating a Culture of Continuous Integration (CI)

Regularly merging changes into the ‘trunk’, and validating these changes with an automated build, catches integration problems early. It’s important to create a strong CI culture. Encourage your team to commit changes often, use automated builds, and understand the importance of these habits for codebase quality and consistency.

Automating Testing Processes

Automated testing is vital to trunk-based development’s continuous integration practice. It speeds up the validation process and reduces human error from manual testing. Establish a robust testing framework that checks the trunk after each commit. This framework should include unit tests for individual code parts and integration tests to ensure different components work together. The aim is to maintain the quality of your central codebase by identifying and fixing issues promptly.

Ensuring Good Code Review Practices

Good reviews spot coding errors, enforce coding standards, share knowledge among team members, and encourage a shared codebase understanding. Make code reviews part of your team’s workflow and ensure they’re a positive learning experience, not a judgment process. Your developers should feel motivated to write readable, high-quality code that meets the project’s style and standards.

Managing Feature Toggles Effectively

Feature toggles, or feature flags, let teams switch features on or off. This gives you more control over your application. In trunk-based development, they’re crucial for integrating code for incomplete features into the trunk without exposing those features to all users. When using feature toggles, set clear policies for retiring them. Unused toggles can accumulate and complicate the codebase.

Mastering Software Development with Trunk-Based Development

Trunk-based development supports efficient work habits, high-quality code, and a smooth release process. With trunk-based development, you can easily implement a solid continuous integration culture and use feature toggles to your advantage.

Yes, there will be challenges, but a systematic approach can help you overcome them. No matter the size of your team or the complexity of your project, using trunk-based development can enhance teamwork, speed up time-to-market, and improve your coding practices.

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Feature Management & Experimentation Can Help Your Trunk-Based Development

Feature Management and Experimentation by Harness gives you the confidence to move fast without breaking things. Set up feature flags and safely deploy to production, controlling who sees which features and when. Connect every flag to contextual data, so you can know if your features are making things better or worse and act without hesitation. Effortlessly conduct feature experiments like A/B tests without slowing down. 

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