Understanding Elixir Agent

This and the other “Deck” posts are a repurposing of flashcard study decks to Q&A blog posts.

What is an Elixir Agent?

They are a means of spawning an Elixir Process but with added the intention of keeping track of state.

When should you used an Agent vs GenServer?

This is a matter of the developers personal preference. `Agent` requires less code to write, but is slightly less efficient than a GenServer.

Agents are said to separate the client and server API's, explain this through an example Server:

# Compute in the agent/server 
def get_something(agent) do 
  Agent.get(agent, fn state -> do_something_expensive(state) end)

# Compute in the agent/client 
def get_something(agent) do 
  Agent.get(agent, & &1) |> do_something_expensive() 

In the first get_something function above the agent process is calling the do_something_expensive/1 function. In the second get_something function it is client process (the process that called get_something/1) that computes do_something_expensive/1. In the case of a long running process it is generally preferred to handle the expensive function in the client so the Agent is not blocked. If the function were do_something_inexpensive where the process would not be blocked by heavy compute, then it could make more sense to handle it within the agent process. Code example from https://hexdocs.pm/elixir/1.12/Agent.html

Why do we not want clients to be able to pass callback functions to the Agent?

If the client were to pass in a function that does a lot of processing the `Agent` would be blocked until the request is completed. Keeping the client and server API's separate alleviates this concern.

Are Agents usually supervised or unsupervised?

Supervised. An `Agent` is usually started under a `Supervisor`.