Unit Testing Redis Lua Scripts

Coverage Increase your test coverage! (Photo credit: John Proctor)

At Gitter, we love Redis: it's an indispensable tool for us. Redis Lua scripts allow us to execute complex logic in a fast, consitent, atomic manner. Unfortunately, some of our scripts have, over time, grown pretty complex, increasing their cyclomatic complexity and making thorough testing difficult.

For this reason, I wanted to create a Lua environment which matches the environment available when running the scripts in Redis. This allows us to:

  • Write unit tests
  • Use Lua debugging tools, like ZeroBrane to debug scripts outside of the Redis environment
  • Use functionality not available in Redis while debugging, such as print. Obviously you'll need to remove these statements before running the script in Redis.

When running your tests in Lua, they're not atomic, so be aware of this.

Caveats

My knowledge of Lua is limited. In fact, before writing this test harness, every Lua script I've ever written has run inside a Redis EVAL. There are almost certainly better ways of much of what I'm presenting here, so please let me know if you have feedback.

However, it's probably the case that many Redis users have the same level of experience of Lua as I do, which is why I thought this post might be useful to others.

Dependencies

Firstly you'll need to install some dependencies.

  • lua 5.1 (Redis currently uses lua 5.1)
  • Some lua modules:
    • luacov
    • busted
    • redis-lua

If you're working on a Mac and have HomeBrew, you can install everything using:

  • $ brew install lua51
  • $ wget -O http://luarocks.org/releases/luarocks-2.2.0.tar.gz
  • $ tar -xvzf luarocks-2.2.0.tar.gz
  • $ cd luarocks-2.2.0
  • $ ./configure --lua-version=5.1 --versioned-rocks-dir
  • $ make build
  • $ make install
  • $ for i in luacov busted redis-lua; do luarocks-5.1 install $i; done

Note: Luarocks is available in HomeBrew, but I had some trouble getting it to work with lua 5.1 (the HomeBrew package installs it for 5.2), hence the custom build.

The Redis Script to Test

For this example, we'll use an extremely simple script from this article at RedisGreen.

All this script does is increment one key and set a hash value.

Building a Test Harness

This code originally came from this blog article: Make LUA debugging easier in Redis.

The harness exposes the KEYS, ARGV and redis global variables available to our Redis Lua scripts. If you use any of the additional libraries available to Redis scripts (documented here), you'll need to expose them as global variables too.

It exports a single function call_redis_script(script, keys, argv) which we'll be using in our tests. It sets the global variables KEYS and ARGV and then executes the Redis Lua script, returning the result, for us to perform assertions on.

Writing Tests

For the tests, we'll use Busted, a popular, easy-to-use Lua testing framework.

The unit test is fairly self-explanatory:

To run the tests, run

$ busted -c test-file-incr-and-stor.lua.

This will produce the following output:

Coverage

The -c flag tells Busted to record coverage information. You can convert this into a report using luacov.

Luacov will produce a report including all your modules, so it's best to give it a regular expression to limit the report to your redis scripts.

Be aware too that each each time you run your tests the coverage stats are added to cumulatively, so you'll need to delete the stats after each run.

This command will generate the report, print it and then delete the stats file:

$ luacov '^incr\\-and\\-stor.lua' && cat luacov.report.out && rm luacov.stats.out

This will produce the following output:

Woot! 100% code coverage! Okay, obviously obtaining 100% code coverage for this script is trivial, but for more complex scripts, this technique can be extremely useful.

By using these scripts and increasing our test coverage, we've been able to debug quicker and increase quality. Hopefully you'll find them useful too. If you do, or if you have any comments, come chat to us in gitterHQ/gitter.

Andrew Newdigate

Andrew Newdigate

Andrew is a cofounder and the CTO of Gitter. He's from Cape Town in South Africa and lives in London in the UK with his wife and two young kids.

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