On the Evilness of Feature Branching - Why do Teams use Feature Branches


In part 1 of this series - a Tale of Two Teams - I introduced two quite different teams. One novice team practising trunk-based development, the other experienced but being used by GitFlow. Now I would like to explore why teams use feature branches. What are their reasons? What problems are they trying to solve with long-running branches?


I like the way that question is asked. It gets people in the state of mind of wondering why someone else doesn’t see the world like they/we do. And they really don’t, but it’s gonna make sense in their own context.

– Wouter Lagerweij (@wouterla), Aug 16, 2021

Some definitions

Before we move on, let me first clarify two definitions. I am sure you know these concepts. But, as it often goes in our industry, people like to redefine practices and concepts to make sure they align with their way of working. Therefore, I am going to reiterate these definitions to make sure we are all aligned on the definitions.

  • What is mainline? and
  • What is feature branching?

Mainline is the line of development which is the reference from which the builds of your system are created that feed into your deployment pipeline.

Jez Humble

For Git, this is the main branch. For Mercurial, this is the default branch. For CVS and SubVersion, this is trunk. Hence, why we speak about trunk-based development.

Feature Branching is a practice where people do not merge their code into mainline until the feature they are working on is “done” (but not yet “done done”).

Jez Humble

Without pretence, in 2021, Done should at least mean running satisfactorily in production.

However, this is still not a standard practice. Many times, Done still means developer-complete, i.e. the feature is implemented. Hopefully, the feature is guarded by unit tests and automated acceptance tests. At best, it has been validated by the product manager on the branch.

But there is still a lot of work to be done before the feature gets into production. Here is a minimal list of activities to move from developer-complete to live traffic:

  • execution of all the unit tests and static code analysis;
  • creation of a new release candidate;
  • deployment of the release candidate to a staging environment;
  • execution of the smoke tests;
  • execution of all the automated acceptance tests;
  • then manual exploratory testing can start;
  • eventually, in parallel, execution of the security and performance tests;
  • if all testing was successful, deployment into production can start;
  • again execution of the smoke tests;
  • and finally, it can be released to the end-user.

When all of the above has been executed, we call the feature Done Done.

The implementation of a feature starts with the creation of a branch. The feature grows on that branch. Upon completion of the feature, it goes through a manual gating process called a Code Review, also known as a Pull Request. When the gating process is successful, the feature branch gets merged back into mainline. This aligns with GitHub Flow’s definition of a feature branch. It is, nevertheless, easy to slip out of merging after 24 hours.

Sometimes, depending on the organisation, a feature branch contains more than one feature. It might hold an epic of features.

To be clear, when speaking about feature branching we truly mean long-running branches, i.e. branches that last for longer than a day.

Reasons for Feature Branching

To understand the reasons teams put forward to use feature branches, I asked proponents of feature branching for their reasons.

It allows us to work in isolation, therefore we are more productive

When I first heard this argument, I have to admit this sounded fair to me. But, then I started to think this through. Wait a minute, this is a local optimisation.

We are optimising for individual developer productivity. But designing IT systems and software products is rarely an individual activity. Most of the time it is a team activity.

Developing in isolation can help an individual go faster. But it does not help a team go faster. Merge time and rework cannot be estimated and will vary wildly, and the team can only go as fast as the slowest merge.

Steve Smith

As long as we have not merged back into mainline we cannot know how much work is still left to do. Because of potential merge conflicts, and because of potential rework caused by merge conflicts at merge time. Integrating changes into mainline becomes a time-consuming and rather unpredictable activity. This makes the whole IT delivery process very unpredictable which increases the lead time, and the time to market for your product.

There is, however, more. Though individual productivity is seen as leading, it is already accepted that what has to be delivered is not individual. Why else are teams obsessed with code reviews and pull requests? But the code review comes too late in the process, introduces waiting and is too slow. And this, again, increases lead time and time market.

But, the goal of any organisation is to make money by sustainably minimising the lead time to create a positive business impact.

Lead Time is the clock wall time between us, as a team or as an organisation, having an idea, that idea gets designed and implemented and finally gets in the hands of the users and used by real users.

Business impact is anything that:

  • creates money, this is turn-over;
  • saves money, this is cost-savings;
  • and protects money, this is being ahead of your competition.

We want to reduce the lead time because we want to accelerate feedback. We want to know as fast as possible if the thing we have just implemented and deployed into production is actually being used and how it is being used. Based on this information, we can make new decisions. We can run new experiments to find new unmet needs of our customers and find new ways to delight our customers. Which is an immense competitive advantage.

By using feature branching we are doing quite the opposite! We are going to delay feedback by increasing lead time and time to market.

Therefore instead of optimising for developer productivity, we should optimise for team productivity and introduce team flow by adopting a team-oriented branching strategy, like trunk-based development.

If it hurts, do it more often. Bring the pain forward.

Dave Farley

If a refactoring goes nowhere, we can just delete it

Again, when I first heard this argument, I thought: fair enough, this sounds evident.

If we accept the “throwing the refactoring away”-argument, we can still argue we do not want to let the refactoring grow too big. If it grows too big, throwing it away would be a waste of the work. As a result, the refactoring will never be thrown away because of the sunk cost fallacy.

Conversely, if we would keep the refactoring small, why bother the overhead of creating a feature branch?

Key is: small steps are better.

However, what I think they are trying to say here is: “We have this problem for which we do not know the solution right away. So we are just trying something, by committing code into a remote branch, hoping that somewhere we will get to a solution. If in the end, we do not get to a solution, we can just delete our work by deleting the remote branch.

If we do not know the solution right away, why do we not spike out some ideas?

A spike solution is a very simple program to explore potential solutions. Build the spike to only address the problem under examination and ignore all other concerns. Most spikes are not good enough to keep, so expect to throw it away.

– Don Wells, extremeprogramming.org

The purpose of a Spike is to write some throwaway code to test out an idea.

Spikes are sharp, they focus on one problem, and short, they are a small experiment.

The output of a spike is not production code.

The output of a spike is knowledge, not code.

During a spike, you should never commit code into a version control system. From my humble experience, the minute code lands in version control, it is labelled production code. From then on, it becomes extremely hard to throw away that code. Again, because of the sunk cost fallacy. I have to admit, this is hardcore spiking. This only works when keeping the spike under 24 hours. This is maybe not always possible. Sometimes spikes require more investigation, reading, meetings. There will be confusion and sometimes a need for sharing. To save us from landing spike code into production code, we should commit into a “spikes” branch that is automatically deleted after 72 hours.

We need to make a distinction, here, between a feature branch and a temporary shared space. However, a spike that takes longer than 3 days, is not a spike.

After a couple of hours, we should know if our idea worked out or not. If it did not work out, no worries, we just throw away that code and try another idea. If it did work out, fantastic, we have just found the solution to our problem. Again we throw away that code and start over again. But this time, we implement the solution in small incremental steps using the knowledge we have just created during that spike.

It allows us to control the quality of what goes into production

Only features that went successfully through a quality gating process get merged into mainline. As such, these features get deployed into production. This is how teams, again and again, try to control the quality of what goes into production.

What is quality? Handling this would require a whole article on its own. Therefore, I will limit the discussion to …

It’s highly contextual, once we get beyond obvious problems like systems crashing. I go with value to someone who matters. I ask them what value means to them.

– Fiona Charles (@FionaCCharles), Oct 20, 2021

Frequently, the quality gating is a manual code review through a pull request. This manual step introduces unexpected side effects.

When the pull request is ready, engineers wait for the code review to happen. Because the review does not start immediately, engineers start new work. This is already a violation of the lean principle: Stop Starting, Start Finishing. It creates more work in progress. Work in progress hinders the flow of work through the value stream. It blocks delivery of features. Therefore it also delays feedback and delays finding new ways to delight the customer.

Other engineers should start reviewing the pull request as soon as possible. But that does not happen either as they are too busy completing their feature. As a result, pull requests pile up. Again, this creates more work in progress. More feedback is delayed. Less unmet needs of the customer are uncovered.

Because feature branches involve a full feature or worse an epic of features, pull requests are commonly big. Reviewing these pull requests takes a considerable amount of time. As a consequence, pull requests are reviewed hastily in a shallow way. So, what are then the quality benefits of code reviews when they are done superficially? What is the added value for the time spent and the delays introduced? Asking the question is to answer it.

Also, engineers reviewing the pull request have very little context about the feature being reviewed. This is caused by this focus on individual productivity: one engineer, one feature. For that reason, engineers limit code reviews to purely technical reviews. They do not review the functionality. Again, this has a limited quality benefit.

When comments or issues arise from a review, engineers must get back into the context of the pull request. This context switching again takes time and introduces delays. These code review issues also initiate rework happening far too late in the process.

Not to speak about the time wasted in the regular ping-pongs between the reviewee and reviewer.

Lastly, I want to make this vital observation. Regularly one does not know what the purpose is of a code review, what kind of things should come out of a code review, except “it is better when more eyes have seen the code”. Thereby, the real question “when is something good or not” remains unanswered.

Because these reviews have hardly any value, doing the review becomes less and less important. Hence, they are regularly delayed and thus it takes even more time to get passed the review. Consequently, it introduces the necessary irritations and frustrations inside the team.

This whole quality control via pull requests is at best a quality theatre with little quality benefits that introduce lots of delays and rework. We might as well stop performing code reviews for the sake of code reviews. It would at least have the benefit to stop blocking the flow of work.

As opposed to trying to control quality through manual merges we should control quality through the adoption of Continuous Integration and, by extension, Continuous Delivery.

The objective is to eliminate unfit release candidates as early in the process as we can … You are effectively prevented from releasing into production builds that are not thoroughly tested and found to be fit for their intended purpose.

Dave Farley and Jez Humble, Continuous Delivery: Reliable Software Releases through Build, Test, and Deployment Automation

We aim to eliminate bad quality release candidates as early as possible. Only changes that successfully went through all stages of the deployment pipeline, and that have been thoroughly tested, get into production.

How is that meeting quality? It starts with the execution of unit tests which are the expression of how engineers have understood functionality. It is followed by the execution of the automated acceptance tests which are the expression of how the three amigos understood the functionality. It continues with the manual exploratory testing which uncovers more unknowns that feed into improving unit tests and automated acceptance tests. If any of these tests fail, it blocks the release candidate because it does not meet the value that matters to someone, the user.

It allows us to keep mainline stable

There is this commonly accepted belief that branches are required to keep mainline stable. However, the opposite is more frequently true. More often than not, branches will be the cause of an unstable mainline.

Why is that?

Because branches delay integration, they create a bigger inventory of un-integrated code. The bigger the inventory the more changes are pulled during the merge. Merging back the branch into mainline becomes harder and more difficult. Resulting in demanding, time-consuming merges that repeatedly will result in an unstable mainline.

Because branches also delay communication with the team, integrating the branch becomes exponentially more difficult with the lifetime of the branch. When keeping a branch open for 8 hours, 5 days or 30 days, the team can only adapt to the changes on the branch, at best, within 8 hours, 5 days or 30 days. That leaves a lot of time for the team to accumulate more and more code which makes the integration of the branch harder and harder. On the other hand, when committing directly into mainline, the whole team sees the changes within minutes, allowing the team to accommodate immediately with the new changes.

I guess the reason for this commonly accepted belief of we need branches to keep mainline stable is:

  • Teams have trouble accepting they can actually commit intermediate changes into mainline when a feature is not yet finalised. For many teams, this is inconceivable.
  • Teams often lack the skills to keep an IT system always working on mainline while having a whole team committing continuously into mainline.
  • Teams commonly miss the practices to apply changes to a codebase without blocking the rest of the team.
  • Branching is a commonly accepted practice in the industry. Few question this.
  • Lastly, teams have usually always worked this way. They simply cannot imagine another way of working.

All of the above often creates a negative feedback loop. Because teams don’t know how to keep IT systems always working on mainline, because teams are often not good at applying changes without blocking team members, and because teams are used to work this way, they continue this mode of operation. As a result, teams are stuck in a local optimum instead of adopting a continuous improvement mindset.

It allows us to control which features get into production

Or, more precisely, we can prevent that unfinished features get into production. Because incomplete functionality sits on the feature branch. The feature cannot be deployed into production as long as the branch has not been merged back into mainline.

Feature Branching is a poor man’s modular architecture, instead of building systems with the ability to easy swap in and out features at runtime or deploy-time they couple themselves to the source control providing this mechanism through manual merging.

Dan Bodart

In fact, we are now using the Version Control System to turn features off and on through manual merging. This is nothing else than a poor man’s modular architecture.

Not to mention the risks introduced by turning features off and on using manual merging. Manual merging can have unintended side effects. It is impossible to test features turned off or on in different configurations. This way of working is downright dangerous. It is a sure way to hell. Or like Ron Jeffries puts it “Matches for children”.

Instead of using our Version Control System as a manual toggling mechanism we should design our systems in such a way that we can turn features off and on at deploy time or at runtime time.

This is how GitHub does it

This is my favourite argument from proponents of feature branching.

Literally every GitHub repository that accepts pull requests uses short lived branches. It’s insane to say it “doesn’t work” when it’s the pre-eminent method of software collaboration today.

– James Nugent (@jen20), Jul 13 2021

In my humble opinion, this sounds like Cargo Culting the Open Source world.

A cargo cult is an indigenist millenarian belief system in which adherents perform rituals which they believe will cause a more technologically advanced society to deliver goods.

Wikipedia

Many IT teams adopt Open Source rituals, like branching and pull requests, thinking they will achieve higher technological maturity, unseen quality levels and profound team coolness.

Obviously, none of this is true. Teams will not ever achieve technological maturity when they are hiding their problems behind branches. They will not reach better quality with the “many eyes” principle as the many security incidents of the past years, from SSL to LDAP, have made clear.

Let me repeat this …

What works for the open-source community, where a core team maintains a system and accepts contributions from the outside world does not necessarily work very well for a co-located team in a corporate environment.

– Me, On the Evilness of Feature Branching - A Tale of Two Teams

The Open Source development is a model with few people having plenty of knowledge (seniors) and many people having little knowledge (juniors). Therefore, pull requests for accepting outside contributions is a good fit. But it is also fragile and slow. If you apply this model in a commercial organisation, organise teams the same way, do not share knowledge inside and outside the team, and there are too many juniors for too few seniors, then feature branching could be a work-around to fix for that. But, it means they will move slowly, they will not be flexible and it will create dull and frustrating work for team members.

Conclusion

It is a safety blanket. This is obvious to me and the members of the XP and Continuous Delivery community. But are teams aware of that safety blanket?

To this, Wouter Lagerwije replied to me:

They are aware of the problems they would see if they switch to trunk-based development. They do not want to confront those problems. And moreover they do not know how to confront those problems.

They see those problems as a result of inherent complexity of software development. While we see them as accidental complexity. In other words: they do not think there is another way to avoid those problems, and see feature branches as a healthy compromise. While we see that the compromise is keeping them from progressing from that local optimum they are in.

By adopting feature branches, teams and organisations seek security. They add more processes to fix for missing technical excellence, lack of trust and lack of self-confidence.

They hope to avoid breaking things in front of stakeholders, i.e. first of all the users, but also C-level and investors. They try to avoid looking bad and avoid damaging their reputation.

This lack of technical excellence and this fear of breaking things, creates this uncontrollable fear of getting bad code into the codebase and getting bad code into production. But, bad code is not necessarily a problem even if it gets into production, as long as we can detect it and fix it fast.

However, branching will only bring a false sense of security and a false sense of control. Because again … it delays feedback. It introduces batch work. Because of the increased batch sizes, it increases changesets and thus increases the amount of code released into the wild. In turn, this increases risks. In the end, what they are trying to avoid, i.e. taking risks, breaking things, is in effect reinforced by adopting more processes through using feature branching.

In all honesty, it is this false sense of security that makes feature branching so evil. Feature branches tend to hide the real problems teams have. Whereas, when teams commit immediately into mainline, it will uncover their problems and allow teams to literally do something about it.

To conclude, teams do not have the necessary practices in place allowing them to move away from feature branches. This is the exact reason teams are reaching that safety blanket.

Sincerely, breaking with this way of working requires quite some expertise and experience. Changing this is not an overnight task. It requires a tremendous change in approach and mindset. It demands the adoption of a reasonable amount of organisational and technological changes. But, the problem is, organisations can only take a certain amount of changes at the same time. How can they pace that many changes? How can they organise and sequence this? Many organisations are overwhelmed and see the number of changes as insurmountable. Although it is enormously liberating once organisations have learned the required practices, allowing them to move away from branching and enabling the fast flow of work through their value stream by adopting trunk-based development.

Acknowledgment

Steve Smith, Wouter Lagerweij and Dietrich Moerman for reviewing and providing essential feedback on the article.

All the magnificent people present at my SoCraTes 2021 session Why do teams use Feature Branching?.

The many people who replied to my LinkedIn question Why do teams use Feature Branching?.

Lagavulin, Port Charlotte, Glenfarclas and Black Bottle for keeping me company late at night.

The Series

The On the Evilness of Feature Branching series:

  1. A Tale of Two Teams
  2. Why Do Teams Use Feature Branches?
  3. Why Are Feature Branches a Problem?
  4. How Can We Avoid Feature Branching Problems?
  5. Questions I Regularly Get Asked about Trunk-Based Development
  6. What Are The Benefits of Trunk-Based Development?
  7. Where is the Evilness of Feature Branching?