Hey! Watch it!...or how to monitor files in Elixir

File watchers and Elixir

I’m working on a feature that reloads a config file if the file is updated. The easiest way I could figure out was to watch the file, and reload the code. Thinking in JavaScript, I’d expect something like:

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// fake
Dir.watch('configs/', function(changedFile){
  console.log(`${changedFile} was updated`);
})

But if you’ve been working with Elixir for a bit, you’ll know there’s gonna be a whole bunch of message passing to get this working. In the end, we wind up with something like:

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# Starting the process
:fs.start_link(:my_watcher, Path.absname("config"))
# Subscribing `self` to receive events.
:fs.subscribe(:my_watcher)

receive do
  {_watcher_process, {:fs, :file_event}, {changedFile, _type}} ->
   IO.puts("#{changedFile} was updated")
end

Getting Started and the FS library

I picked the fs library since it’s what Phoenix uses. In this post, we’ll be using master in a mix project. Follow along!

A new project!

Create a new mix project with mix new watch_it, and add fs to your dependencies in mix.exs.

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#...
defp deps do
  [{:fs, github: "synrc/fs"}]
end
#...

And start the fs application with:

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def application do
  [applications: [:logger, :fs]]
end

Then install everything with mix deps.get

And kick the tires in iex -S mix (this will compile a bunch).

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:fs.start_link(:my_watcher, Path.absname("config"))
:fs.subscribe(:my_watcher)
flush
# {#PID<0.129.0>, {:fs, :file_event},
# {'/Users/StevenNunez/code/watch_it/config/fake_config.exs', [:created]}}

I don’t know about you, but I’m excited. What happened? :fs.start_link(:my_watcher, Path.absname("config")) created a process that watches a directory for changes. We give the process a reference of :my_watcher. We’ll use it later. The next line is where the magic is. :fs.subscribe(:my_watcher) finds the :my_watcher process, then notifies self whenever a file changes.

We can add a bit of pattern matching to make this nicer. (Make sure you’re exiting iex -S mix between examples)

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:fs.start_link(:my_watcher, Path.absname("config"))
:fs.subscribe(:my_watcher)
receive do
  {_watcher_process, {:fs, :file_event}, {changedFile, _type}} ->
   IO.puts("#{changedFile} was updated")
end

Conclusion

I plan on using this library a lot! Using this with separate process responding to changes to the file system is super powerful. Phoenix uses it for it’s live reloading. Maybe we can replace some JavaScript build tools ;-)