Agent Obsession

One of the most valuable learning experiences of 2016 for me was getting to attend James Edward Gray’s SimAlchemy training at the start of ElixirConf. It was very timely information for me as I was right in the middle of writing my first process heavy application in Elixir.

Towards the end of the talk he went though a number tips and common anti-patterns and one hit me hard: GenServer Envy. This is when your use of either a Task or Agent goes beyond their suggested use cases and you start treating them like GenServers.

I had a special case of GenServer Envy that I’m calling Agent Obsession. You might have it too if:

• The responsibility for interacting with your Agent process is spread across your application.
• You don’t actually know the api for GenServer (quick, how many arguments does handle_cast take?).
• You don’t know the use case described in the documents for Agent.
• You know that an Agent is best used for “a simple wrapper arounds state” but can’t articulate what a complicated wrapper around state would entail.

There’s a line of thinking among Elixir devs that if you aren’t sure if your process should be backed by an Agent or a GenServer just start with an Agent and refactor to a GenServer when it gets painful.

I think this approach can be fine for experienced Elixir developers, but for folks new to Elixir I’d suggest the doing the opposite. When in doubt use GenServer, if after some time you feel it’s too verbose refactor into an Agent. This will lead you to a better understanding of how to build highly concurrent fault-tolerant applications which is the coolest part of using Elixir anyway.