I mean this is what an Agent is right? It can’t handle_info.
benwilson512
Author of Craft GraphQL APIs in Elixir with Absinthe
dimitarvp
True. Though Agent is even more of a confusing name. 
But again, I don’t object to any of that. It takes a few tries and you learn.
1
D4no0
I see nothing difficult about GenServer, in general once you get going. The serialization of messages is a thing that you can shoot yourself in the leg, this is a reason I steer away from using more complicated things with GenServer.
Nonetheless it beats stone age concurrency from languages like java, where you get undefined behavior once you introduce concurrency in 95% of cases.
1
dimitarvp
That’s what I am saying as well – takes a little getting used to and I didn’t mind it as mentioned above. I am pointing out that it can be difficult to some novices is all.
Personally it took me half an afternoon tinkering in iex and I got it all the way to 90%, with the only exceptions being some intricacies about what does use GenServer do exactly, how do you define child_spec, what’s the difference between an ID and a name – and I got those just fine the second time I did a self-training session back in 2016.
Pfff, obviously. That’s why we are all here.
1
D4no0
It seems that you were lucky enough to not get memory leaks in production when using huge binaries 
. The best time to read about how GenServer works is when the prod server is on fire.
2
dimitarvp
Indeed I was, but that also comes with me depriving myself of sleep in order to learn proactively before prod is on fire. So don’t bet that I was in a better position.
Back to original topic, I think OP got good answers.
1
Aetherus
Suppose you have such a class in C#:
class Foo
{
int Bar { get; set; } = 0;
int Baz { get; set; } = 0;
int Add(int bar, int baz) {
this.Bar = bar;
this.Baz = baz;
return this.Bar + this.Baz;
}
}
and an instance of that class
var foo = new Foo();
what will happen when one thread calls foo.Add(1, 2); while another thread calls foo.Add(3, 4) at the same time? You can’t guarantee the first thread gets 3 and the second thread gets 7. That’s because an object in OOP language gives you an illusion of isolation, but actually it still has to do whatever other threads ask it to do immediately.
On the other hand, when you have a GenServer module like this:
defmodule Foo do
use GenServer
def start_link(_) do
GenServer.start_link(__MODULE__, {0, 0}, name: __MODULE__)
end
def add(foo, bar) do
GenServer.call(__MODULE__, {:add, foo, bar})
end
@impl true
def init(state) do
{:ok, state}
end
@impl true
def handle_call({:add, foo, bar}, _, _) do
{:reply, foo + bar, {foo, bar}}
end
end
When two different processes call Foo.add(a, b), you can guarantee that each process gets the correct result, only that it can be a little bit slow because the calculations and state settings are done sequentially.
A process is like a man working in an office. He has a mailbox to help him queue up his jobs to do. He picks a job and handles it whenever he sees fit. If you (another process) care about the result of the job, you need to wait for that man to finish the job and send the result back to you in a mail message. If you lose your patience, it’s up to you to decide what to do next.
4
D4no0
At the end of the day this becomes a problem when you have side-effects or global state, because if we talk about from the concept of a thread, it cannot run code concurrently (abstractions over threads are out of question). The biggest problem in those languages becomes synchronization of resources (that erlang VM thought about out of the box).
3
derek-zhou
Although you can make such analogy to get you going, it is still better to understand what an Erlang process / GenServer is on it’s own to move up to the next level. If you latch onto the GenServer-is-Object understanding much longer, eventually you will shoot yourself in the foot. A GenServer is much heavier weight than an Object in a typical OO language; so if you try to literately translate an OO program into Elixir, you will be disappointed by the performance.
9
sodapopcan
That’s right you don’t “object”!
…I’m sorry. And I’m not even a dad!
@derek-zhou has the most succinct advice for newbies who would otherwise just go ahead and repurpose module+process as an object anyway. Of course, there is always this package.
2