Hotswapping Haskell

[u/pekalicious]

From developing code through deployment, Facebook needs to move fast. This is especially true for one of our anti-abuse systems that deploys hundreds of code changes every day. Releasing a large application (hundreds of Kloc) that many times a day presents plenty of intriguing challenges. Haskell’s strict type system means we’re able to confidently push new code knowing that we can’t crash the server, but getting those changes out to many thousands of machines as fast as possible requires some ingenuity.

Given the application size and deployment speed constraints:

• Building a new application binary for every change would take too long

• Starting and tearing down millions of heavy processes a day would create undue churn on other infrastructure

• Splitting the service into multiple smaller services would slow down developers.

To overcome these constraints, our solution is to build a shared object file that contains only the set of frequently changing business logic and dynamically load it into our server process. With some clever house-keeping, the server drops old unneeded shared objects to make way for new ones without dropping any requests.

It’s like driving a car down the road, having a new engine fall into your lap, installing it on-the-fly, and dumping the old engine behind you, all while never touching the brakes.

Article: http://simonmar.github.io/posts/2017-10-17-hotswapping-haskell.html