This and the other “Deck” posts are a repurposing of flashcard study decks to Q&A blog posts. Google was not showing love to this content as a set of flashcards and I didn’t want to delete them entirely, I hope you find it useful.
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)
end
# Compute in the agent/client
def get_something(agent) do
Agent.get(agent, & &1) |> do_something_expensive()
end
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.