Update: Fixed typo


Slow builds are perhaps the most irritating thing for a developer. Having worked on two projects now with +30 minute continuous integration builds, it’s something of a personal bugbear for me. Before we look at some ways to speed up the build, I thought I’d start with my definition of what a build is…

What is a build?

Quite simply a build is the process of taking source code and producing some artifacts. The artifacts could be a war file, some test output or even a full blown desktop application. A project might (and probably should) have several different builds for different purposes – one for developers, QA, production etc.

Speeding up the build

If you have a build which you want to go faster, there are two approaches you can take

  1. Speed up the creation of the artifacts themselves
  1. Reduce the number of artifacts being created

    I’m not really going to talk much about the first point – that is typically either trivial or very hard. If you have to compile 10,000 source files there is a relatively little you can do to speed that up. If you have some tests which are IO bound, you can speed them up by mocking out the IO if possible.

    The biggest wins come when you can reduce what the build is doing, either by reducing the number of artifacts it is trying to produce, or reducing the scope of the artifacts being produced.

    Option 1: Reduce the work

    You might consider splitting the build on horizontal lines, in order to create fewer artifacts. For example a typically developer build will compile and run the unit tests, and might produce a deployed application from exploratory testing. You could split this single build into three separate builds – one to compile the code, one to test it, the other to create the deployed app. This is only useful if not all the artifacts are required – in many circumstances they are. One technique that has worked in my experience is the idea of a tiered build – we had two continuous integration builds, one designed for fast developer feedback which ran just the unit tests, and a longer QA build which runs all the regression and functional tests. This is something I’ll go into at a later date.

    Option 2: Divide and conquer

    The other approach is to split things along vertical lines. In the case of the developer build, keep the build compiling, testing and deploying the code, but just have it compile, test and deploy less code, by splitting the code along functional boundaries. This will work as long as the developer is working completely within said functional boundary – subdivide your code base too finely and you’ll introduce too much complexity in terms of integrating your artifacts. Subdivide too broadly and you’ll end up compiling and testing code needlessly.

    Attempting to break up a code base after work has already started is not a simple job. It will involve disruption to development, and your development effort will need to be refocused to work as much as possible within your new components. Working out where to break code up before you start work is likewise not easy – it might not be clear initially where the divisions can be made, and you might find out later that you didn’t need to break up the code, in which case all you’ve done is add complexity to your build process. In either case, whilst the individual component builds will end up being simpler, and the builds for each component shorter, builds at the integration level will become more complex.