German Velasco

Software Developer

An introduction to concurrency in Elixir

Concurrency is a first-class citizen in the Elixir platform. The concurrency model is that of isolated, independent processes that share no memory and communicate via asynchronous message passing. In other words, a process can run concurrently without concern of others, and processes communicate with each other only via send-and-forget messages.

It’s worth noting that these processes are not OS processes, they are BEAM processes. They are lightweight, and you can have thousands of them running concurrently.

Creating processes

In elixir we create new processes via spawn/1, spawn/3, spawn_link/1 and spawn_link/3, each of which takes a function (anonymous or in a module) that will run in the new process. The spawn functions create a new process completely isolated from the one creating it. The spawn_link functions, on the other hand, create a link between the creating process and the created process. We’ll talk more about links further down below.

You can think of spawning a process as telling a process to run a function in isolation. That function may call other functions (like an http endpoint), or it may call itself infinitely in a loop (usually done to preserve state), or it may run the function and exit.

Let’s open up iex and take a look at it. Note that the console is running in a process itself. To see it’s pid, type self().

iex(1)> self()
#PID<0.80.0>

Now, say we run the following in iex

iex(2)> spawn(fn -> 1 + 1 end)

A new process is spawned, it runs the anonymous function fn -> 1 + 1 end and exits normally. In iex we see the pid being returned but we do not see the result of 1 + 1 because the processes are isolated, and the result is being returned in the spawned process.

But what if we want the spawned process to tell us the result? They need to communicate via messages.

Communication between processes

Elixir has simple primitives for message passing. They are send/2 and receive/1.

We send a message to a process, and each process receives messages in their mailbox.

Let’s now try to send the result of 1 + 1 in the example above to the creating process.

Open up iex and let’s do the following,

iex(1)> creator_pid = self()
iex(2)> spawn(fn ->
...(2)>   result = 1 + 1
...(2)>   send(creator_pid, {:result, result})
...(2)> end)

The code above spawns a new process with a function that calculates the result of 1 + 1 and sends the result in a tuple to the creator pid (the iex process in this case).

In order to receive the message, we use receive/1 in iex.

iex(3)> receive do
...(3)>   {:result, result} -> IO.inspect(result)
...(3)> end
2

With that, we should see the number 2 in our console. Communication between messages achieved!

Linking processes

Elixir processes are isolated, and that’s great. But what do we do when two processes are conceptually related to each other? What happens if we don’t want process x to live if process y dies? We link them.

Links are an elixir primitive for detecting a process crash. These links are bi-directional, meaning that when either process crashes, the other receives a notification that the process has crashed. If this message is not handled, the receiving process will also crash.

This allows us to propagate errors and take down groups of processes that should only exist together. Now if we don’t want process x to exist without y, we can link them and if one of them dies, it will also take down the other. We can do this via Process.link/1, spawn_link/1, and spawn_link/3. Let’s take a look at spawn_link/1 in iex.

We will spawn a linked process, and in the function we will raise an error so that we cause the spawned process to crash. The link should propagate the error to the creator process (the iex process) and we should see the iex process go down. We will also see iex restart itself, but that restarting mechanism is performed by a supervisor. It is not an automatic response of a process that crashes.

In iex,

iex(1)> spawn_link(fn ->
...(1)>   raise "what are you doing!"
...(1)> end)

And you should see something like this,

15:32:54.067 [error] Process #PID<0.109.0> raised an exception
** (RuntimeError) what are you doing!
  :erlang.apply/2
** (EXIT from #PID<0.80.0>) and exception was raised:
  ** (RuntimeError) what are you doing!
    :erlang.apply/2

where process 80 was the process running the iex console. Note that your pids might be different from those in my example.

Stopping error propagation

By default when a process receives an exit signal from another process (and the signal is not :normal) the recipient is also taken down. That is why linked processes go down when one exits with an error.

But what if we want to stop the propagation of errors? That is, what if we want a process to be notified of the failure of another process but not be taken down with it? For that we can trap exits.

Trapping exits

Trap exit is a flag that can be set in a process. We can do so with Process.flag(:trap_exit, true).

When we set this flag in our process, it means that instead of receiving an exit signal from its linked process, it will receive a message in its mailbox that the process has died. This message is a tuple with of this form, {:EXIT, from_pid, reason}.

So if process y dies, process x now gets a message in its mailbox, and it can choose how it wants to handle such a message.

Let’s take a look at it in iex,

iex(1)> Process.flag(:trap_exit, true)

iex(2)> spawn_link(fn ->
...(2)>   raise "what are you doing!"
...(2)> end)

At this point we should see that the spawned process crashed but the iex process was not taken down like last time. Let’s take a look at all the messages in the console with the neat helper flush/0,

iex(3)> flush()
{:EXIT, #PID<0.101.0>,
 {%RuntimeError{message: "what are you doing!"},
  [{:erlang, :apply, 2, []}]}}

A note on supervisors

Supervisors are elixir processes that can restart other processes when they fail. We saw this in action when the iex console died, and it was restarted automatically. And though supervisors are out of the scope of this post, the foundations of supervisors are all in here. A supervisor is, in essence, a process that is linked to other processes, it traps exit signals, and restarts the processes that crash.

Further resources

I found the book Elixir in Action extremely helpful in understanding these concepts. I also recommend reading learn you some erlang. It is great read.