# Continuous Delivery For Julia Packages

TL;DR; Every Pull Request should increment the version number in the Project.toml, and then you should register the release immediately after merging. Why do this? Because people are making PRs to your repo because they want that change. Don’t make them wait for you to tag a release. Also for all the normal advantages of continuous delivery.

When Julia-1.0 and Pkg3 came out over a year ago, and the more recent (but still many months ago) change over from REQUIRE to Project.toml for the General Registry, all started taking SemVer seriously. And that means we can continuously release packages, without worrying about breaking anyone’s code.

The majority of packages are specifying compat bounds (and in my opinion all should), using the SemVer based specifiers in the Project.toml. (Rather than just lower bounds, which is what people used to do in the REQUIRE days.) This means that if you tag a new release as breaking, no existing package will see it, until they are ready – which they show by updating their compat bounds.

Note that in Julia’s extension of SemVer, incrementing leading non-zero marks a breaking change. So even packages still on v0.x.y are still safe to depend on with compat bounds.

So continuous delivery is safe.

## Required for Continuous Delivery

Continuous delivery requires a few things up front.

### Automated Tests:

Julia has a pretty solid Test standard library, and a bunch of packages like ReferenceTests.jl. If you are not already doing this then get on that before all other things. You don’t need to go all in on test driven development if that isn’t your thing. But at very least get some basic test coverage. Because Julia is not a static language, the compiler will not catch basic errors like typos, you need to run the code to get that. (Or run some static analysis, but you would trigger that in your tests.)

### Continuous Integration:

Something needs to run your tests, on every PR. Everyone really should be using PkgTemplates.jl, to create new packages. It will setup the config for TravisCI, AppVeyor, and CirrusCI, plus setup Documentation and other good things. All you need to do is enable the repos on your account with your favorite CI service. Existing packages can find and copy the configs easy enough.

Also something needs to deploy your docs after you make a release. Documenter.jl has great instructions for setting up TravisCI for that. Further PkgTemplates, as I mentioned, will set most of that up for you, barring the deploy keys.

### Work broken into small chunks

This is the standard way to work in open source. You get 1 new feature or bug fix per PR. It gets reviewed, then merged. If you are not using PRs like this, on your projects, you’ll need to switch to it. It is a good way to work, even on personal projects.

## So on to how to do this:

When someone (including yourself) makes a PR to your repo, you have them include in the PR the appropriate change to the version number, for if it is patch, a new feature, or a new breaking feature. (Or if you are still in 0.x.y just for if it is nonbreaking or breaking)

Then immediately after merging the PR, tag a release. Hopefully, soon after that release merged into General, and then TagBot will make a matching github release with automatic release notes listing changes (or you could do that manually).

Compares to deploying a application, releasing a library is pretty simple, no need to build and deploy installers etc. For proper continous deployment, one would have the release be automatically tagged when the PR is merged. Currently, that is fairly hard to setup, and I am not aware of anyone doing so. Til then manually triggering Registrator will have to do.

## What if I don’t want to release right now? -DEV versions.

There are not many good reasons to hold off on a release after merging in a PR, but there are some. One reason is if you already have a release pending waiting to be merged into the General Registry; thenm you really shouldn’t open up another one without a good reason (like backporting on a different branch). Another potential reason (kind of the complement of that) is if you know that more PRs are coming in soon, especially if they are breaking (since that would mean your packages users would have more versions to work through in their own packages Compat sections.)

### Add the -DEV suffix to your version.

In this case the best thing to do is to change the version number to what it would be, but add a -DEV suffix, e.g. 2.0.0-DEV. Rather than leaving it at 1.7.2 which would be a lie, since it has had a breaking change. Also better than changing it to 2.0.0 and not making a release since that gets confusing since you need to workout which versions in the Project.toml are released and it is just a mess to think about.

### How to change the version number if it is already -DEV.

If the current version is already 2.0.0-DEV and the PR adds a breaking feature, you don’t want to be changing it to 3.0.0-DEV and never release 2.0.0. Correct is in that case to leave it as is. If however, it is at 2.0.1-DEV, then correct is to change it to 3.0.0-DEV, as this indicates the change that made it -DEV was a patch, and so adding breaking change is not permitted. But it is permitted to roll patch changes into major releases.

This pattern holds: If there are nonzero version digits to the right of the approprate position for he change being made in this PR, then you need to increment (and thus zero those), otherwise you don’t. So if the current version is x.y.z-DEV:

• So for patch level changes, you do not need to change the version number
• for minor level changes: if z != 0, then you need to increment y (and zero z)
• for major level changes: if y !=0 or z != 0, then you need to increment x (and zero y, and z) The reason for this pattern is that making these changes to the dev version always zeros the lesser digits, so you know that those digits are already zero-ed and it is still -DEV, that such a change has already been made and so your PR’s new change will get rolled into that one.

### Compat and -DEV versions:

A nice feature for testing. While you can’t register packages with a -DEV (or other prerelease markers), you can still install them via pkg> dev MyPackage, into a project. And julia will treat 2.0.0-DEV as compatible with 2.0.0 in the [compat] section of the Project.toml. This is a bit odd, since v"2.0.0-DEV" < v"2.0.0, but practically it is quite useful.

## What if I mistakenly make a breaking release.

If you happen to push a change that was breaking, then it isn’t the end of the world. Since you are releasing more often, people will see it sooner, and thus open an issue sooner. The correct thing to do in these circumstances is git revert the PR the introduced the change, then tag a new patch release, then either workout how to reintegrate that change in a nonbreaking way, or re-apply the PR and tag a breaking change. Tagging the patch release means that people can update and they will see what is effectively the same code as before the PR. The package manager will see the fixed (reverted) code as the better (higher) semver than the mistakenly broken one.

## Why do this?

In enterprise continuous delivery is a well-established Agile practice that you can read a bunch about. So I am not going to talk about that here, you can google for it. In open-source there are additional advantages: you avoid annoying your contributors.

There are few things as frustrating as the following tale. Someone opens an issue on one of my projects. I determine that it is caused by an unstream bug in a library I am using. I fix that bug, and have my patch merged. I think all is done and that it will no doubt solved for the end-user shortly. A month later someone else opens the same issue on my project. Because no release has been made on the library I fixed. I have to go back over to the library, open issues nagging them to release. Maybe even send a maintainer an direct message. Often the reason relates to them thinking another core maintainer wants to hold off (which might be true, for better or worse.) By sticking to a policy of continous delivery, this can be avoided. It is clear to everyone that when a PR gets made, a release is assumed to be shortly after.

Your contributors are making PRs because they want the feature/bugfix. You have already indicated you are happy with it by merging, why hold off releasing it to the world?