I’m not sure I get the example. Why would you not just use Phoenix Channels?
smpallen99
peerreynders
We’re talking about barebones Elixir here. The topic started about Agents - but they really are best understood in the context of the concurrency primitives they are based on and the (more commonly used) counterpart GenServer.
The primary objective is Agents (and it’s relatives); the chat application was merely an example that was asked for in this context (admittedly far from a great one for Agents).
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smpallen99
smpallen99
I’m going to clarify how I use Agents based on some of the other posts that came after mine. I’ve been hearing a lot of noise about Agents being misused or that they provide not value. At least that’s my take away from various source I can’t remember.
I use Agents mostly as a key/value storage solution. In most cases I could have used ETS to accomplish the same. And from what I’ve read, the performance would probably be better. However, I probably use mostly out of laziness. I just find it hard to remember Erlang’s (IMHO cryptic) syntax. I did find that using an Agent came in handy once when I had to extend an existing Agent based module for a port reuse strategy.
In this case, I had a general Agent handling long lived state (but not requiring vm restart persistence) in a pure Elixir application (no database). I needed to later add a port reuse strategy. This means, reading the data store, checking to see if a port was available for reuse. So this required reads and updates that that were concurrency safe.
I could have moved the whole design over to a GenServer, but instead, I just wrapped the code in an Agent.get_and_update. That was a few years ago. The implementation has been reused on another project since then. I have not has an issue with the design on hundreds of the products sold.
Personally, I would not use an Agent for logic any more complicated then that. If you need behaviour, I much prefer using either a GenServer or a GenFSM.
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peerreynders
My personal issue with Agents is that they depend on “functions as the message payload” - I’m all for “functions being first class citizens” within a process but I get a bit queasy when functions start moving between processes especially as those processes could be on two different nodes.
- By accepting functions in messages the process is essentially giving up the autonomy to manage its own state - it’s completely at the mercy of whatever decides to send it a function, potentially risking its state that other processes are depending on.
- If the sent function references any application modules - those dependencies better be well in sync on the nodes that the message/function is travelling between.
I feel that the functions that manipulate process state should be collocated with the process (and the state), so I’d go with a GenServer even if it is a bit more work.
As “cool” as it is to be able to execute functions in an atomic fashion on a state that is potentially “conceptually shared” between multiple processes, I’d much rather have those functions be an integral part of the process. I guess that makes me a process-autonomy advocate.
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smpallen99
Some interesting points @peerreynders. To be honest, all my Elixir projects over the last 3 years have all been single node solutions. Busy ones, but all single node, mainly enterprise telephony relates. I have played around with multi node, but nothing commercially. With that said, I have not given much thought to the distribution aspect of Agents.
Also, when I design a module with an Agent, I typically add the api to the module. So the consumers are calling an API with data. In only one instance did I expose an interface where the consumer passes their own fn. And I wasn’t very comfortable with the design.
I am, however, staring to see the pain points around using Agents, especially for the inexperienced. Its been an informative discussion for me.
As for Agent vs GenServer, I guess I save a few lines of code. Basically its 1 function / API call for Agent vs 2 for GenServer. And not even that if I used something like exactor which I’ve pretty well abandoned these days.
I’m almost convinced to abandon Agents in favour of GenServers 
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