Introducing Buster

December 28, 2012
Hello brother

Hello brother

Introducing Buster

Buster is a utility for hitting a list of URLs periodically.

At work, we hit certain sites with a special URL parameter that causes the full page cache to be rebuilt every 5 minutes. Previously, this was done in a cron task. That’s fine but I felt this could be a good learning experience and would end up being more configurable. By the way, if you do want to use curl/cron to accomplish this task, be sure to redirect output to /dev/null or you’ll end up with a multi gigabyte spool file from cron!

Getting Buster

Either clone from my GitHub or download it from HackageDB. It will install an executable called buster

Usage

Run buster config.yml

See the README and examples directory for what a config should look like. Buster supports optional automatic config file reloading or reloading via a HUP signal.

Reflections

At this stage in my Haskell career, if I embark on a new projects, its because I want to learn some new things along the way. Buster itself isn’t that compelling of a utility, but the learning experience was.

YAML

Since I work in a Ruby shop, I thought it would be a good idea to have the configuration be in YAML. That way, coworkers could pretend they are using a normal Ruby executable without noticing Haskell’s slow infiltration into day to day operations.

The yaml package is interesting because it uses the mighty aeson parsing combinators. That was kind of weird but it made writing the config parser simple:

instance FromJSON Config where
  parseJSON (Object v) = Config <$> v .:? "verbose" .!= False
                                <*> v .:? "monitor" .!= False
                                <*> v .:  "urls"
                                <*> v .:? "log_file"
  parseJSON _          = fail "Expecting Object"

instance FromJSON UrlConfig where
  parseJSON (Object v) = UrlConfig <$> v .: "url"
                                   <*> v .: "interval"
                                   <*> v .:? "method" .!= "GET"
  parseJSON _          = fail "Expecting Object"

Config Reload with hinotify

hinotify is a pretty thin abstraction over linux’ inotify feature for monitoring files. I’d used it before in Ruby but I was pleased with being able to monitor the config without resorting to polling.

Exceptions Runing My Life

I started writing a section here and then realized there was too much in my head to confine it to a section in an introductory post. Keep an eye out for a separate post on exceptions in Haskell and why I hate trifling with them. Long story short: the errors library and the EitherT monad transformer helped a great deal. I tried my best to intercept exception-throwing interfaces (though who knows since the goddamn type doesn’t communicate exceptional behavior) at low levels and normalize them to an Either interface. I have decided that if there are going to be errors in my code, the compiler should not allow my program to compile without confronting them in some manner.

Testing with Temporary Files

Keep an eye out for a small post about this. I wanted to be able to test the parsing of actual config files without having to worry about explicit cleanup. The temporary package is really nice for tasks like this.