Introducing Cacher

“If you wish to make apple pie from scratch, you must first create the universe”, and if you wish to build an app using the interpreted language framework of your choice, you usually have to install a whole universe of dependencies. If your build environment is persistent, you probably don’t feel the pain of waiting for the universe to download once the initial pip install is complete. Subsequent invocations of pip will see the local modules you previously downloaded and avoid re-downloading and installing them. For “clean room” build environments, this pain is felt each time we build.

The build performance improvements of persistent build environments bring with them some important caveats. If you don’t perform a “clean room” build there’s always a chance some stray bits or accidental config changes have crept into the build environment, leading to inconsistent build output. To deal with this, many of us use tools like Habitat Builder or Travis CI that do create the universe each time they bake an apple pie, in Builder’s case starting with an empty chroot environment for each build.

While universe-creation ensures consistent results (this is a very good thing in Builder), it’s sometimes annoyingly slow for local Studio-based development. A Habitat Studio build must perform a brand-new pip install download of every Python module our project depends on, on every build. This can add many seconds or even minutes to each build. Frustration with these long waits led to the creation of cacher, a package that speeds up local Studio-based development.

cacher make use of a Habitat package’s ability to define an environment variable and “push” that variable to any package that depends on it. For more details on how that works see Christopher Maier’s blog post here.

The actual for cacher relatively short:

pkg_maintainer="Blake Irvin 

In this case, we are taking advantage of the fact that pip, the Python dependency manager that smartB (my employer) uses, respects the XDG_CACHE_HOME environment variable. The directory /hab/cache/artifacts is loopback-mounted into the Studio Docker container, which means that we’ll cache our pip modules in the same persistent location that Habitat uses to cache .hart artifacts. We use similar techniques for both NPM and Go.

Here’s some performance improvement examples from my (old, slow) MacBook building our (relatively large) Python API, with examples both before and after adding bixu/cacher to our pkg_build_deps:

Without cacher:
  api: Build time: 7m59s
With cacher, first run:
  api: Build time: 9m31s
With cacher, second run:
  api: Build time: 6m56s
With cacher, third run:
  api: Build time: 6m4s

Build performance improves in this case by ~25%.

Right now, cacher supports only the dependency managers discussed above, but any dependency manager whose behavior can be configured using environment variables could be supported in future. Pull requests are most welcome!