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Don’t Feature Branch

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Don’t Feature Branch

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I recently attended the Devoxx conference. One of the speakers was talking on a topic close to my heart, Continuous Delivery. His presentation was essentially a tools demonstration, but one of the significant themes of his presentation was the use of feature-branching as a means of achieving CD. He said that the use of feature-branching was a debatable point within the sphere of CD and CI, we’ll I’d like to join the debate.

In this speaker’s presentation he demonstrated the use of an “integration branch” on which builds were continuously built and tested. First I’d like to say that I am not an opponent of distributed version control systems (DVCS), but there are some ways in which you can use them that compromise continuous integration.

So here is a diagram of what I understood the speaker to be describing, with one proviso, I am not certain at which point the speaker was recommending branching the “integration branch” from “head”.

In this digram there are four branches of the code. Head, the Integration branch and two feature branches. The speaker made the important point that the whole point of the the integration branch is to maintain continuous integration, so although feature branches 1 and 2 are maintained as separate branches, he recommended frequent merges back to the Integration branch. Without this any notion of CI is impossible.

So the Integration branch is a common, consistent representation of all changes. This is great, as long as each of these merges happens with a frequency of more than once per day this precisely matches my mental model of what CI is all about. In addition, providing that all of the subsequent deployment pipeline stages are also run against each change in the integration branch and releases are made from that branch this matches my definition of a Continuous Delivery style deployment pipeline too. The first problem is that if all of these criteria are met, then the head branch is redundant – the integration branch is the real head, so why bother with head at all? Actually I keep the integration branch and call it head!

There is another interpretation of this that depends on when the integration branch is merged to head, and this is what I think the speaker intended. Let’s assume that the idea here is to allow the decision of which features can be merged into the production release, from head, late in the process. In this case the integration branch, still running CI on the basis of fine-grained commits, is evaluating a common shared picture of all changes on all branches. The problem is that if a selection is made at the point at which integration is merged back to head then head is not what was evaluated, so either you would need to re-run every single test against the new ‘truth’ on head or take the risk that your changes will be safe (with no guarantees at all).

If you run the tests and they fail, what now? You have broken the feedback cycle of CI and may be seeing problems that were introduced at any point in the life of the branches and so may be very complex to diagnose or fix. This is the very problem that CI was designed to eliminate.

Through the virtues of CI on the integration branch, at every successful merge into that branch, you will know that features represented by feature branches 1 and 2 work successfully together. What you can’t know for certain is that either of them will work in isolation – you haven’t tested that case. So if you decide to merge only one of them back to head, you are about to release a previously untested scenario. Depending on your project, and your the nature of your specific changes, you may get away with this, but that is just luck. This is a risk that genuine CI and CD can eliminate, so why not do that instead and reduce the need to depend on luck?

Further, as I see it the whole and only point of branching is to isolate changes between branches, this is the polar opposite of the intent of CI, which depends upon evaluating every change, as frequently as practical, against the shared common picture of what ‘current’ means in the system as a whole. So if the feature branches are consistently merging with the integration branch, or any other shared picture of the current state of the system – like head, then it isn’t really a “feature branch” since it isn’t isolated and separate.

Let’s examine an alternative interpretation, that in this case I am certain that the speaker at the conference didn’t intend. The alternative is that the feature branches are real branches. This means that they are kept isolated, so that people working on them can concentrate on those changes and only those changes without worrying about what is going on elsewhere. This picture represents that case – just to be clear, this is a terrible idea if you mean to benefit from CI!

In this case feature branch 1 is not merged with the integration branch, or any other shared picture, until the feature is complete. The problem is that when feature branch 2 is merged it had no view of what was happening on feature branch 1 and so the merge problem it faces could be nothing at all or represent days or even weeks of effort. There is no way to tell. The people working independently on these branches cannot possibly predict the impact of the work elsewhere because they have no view of it. This is entirely unrelated to the quality of merge tools, the merge problems can be entirely functional, nothing to do with the syntactic content of the programming language constructs. No merge tool can predict that the features that I write and features that you write will work nicely together, and if we are working in isolation we won’t discover that they don’t until we come to the point of merge and discover that we have evolved fundamentally different, incompatible, interpretations. This horrible anti-pattern is what CI was invented to fix. For those of us that lived through projects that suffered, all to common, periods of merge-hell before we adopted CI never want to go back to it.

So I am left with two conclusions. One, for me the definition of CI is that you must have a single shared picture of the state of the system and every change is evaluated against that single shared picture. The corollary of this is that there is no point having a separate integration branch, rather release from head. My second conclusion is that either these things aren’t feature branches and so CI (and CD) can succeed, or they are feature branches and CI is impossible.

One more thought, feature-branching is a term that is, these days, closely associated with DVCS systems and their use, but I think it is the wrong term. For the reasons that I have outlined above these are not real branches, or they are incompatible with CI (one or the other). The only use I can see for a badly mis-named idea of “feature branching” is that if you maintain a separate branch in you DVCS, but compromise the isolation of that branch to facilitate CI, then you do have an association between all of the commits that represent the set of changes that are associated with particular feature. Not something that I can see an immense amount of value in to be honest, but I can imagine that it may be interesting occasionally. If that is the real value then I think it would benefit from a different name. This is much less like a branch and more like a change-set or more accurately in configuration management terms a collection of change-sets.

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