What are linked processes in Elixir?
Overview
In Elixir, code runs inside processes. Processes are secluded from each other, run side-by-side to one another, and interact via message passing. Processes are not only the reason for concurrency in Elixir, but they also provide the means for building shared and fault-tolerant programs.
Elixir’s processes should not be mixed with operating system processes. Processes in Elixir are notably lightweight in terms of memory and CPU (even compared to threads as used in many other programming languages). Because of this, it is not unusual to have tens or even hundreds of thousands of processes running concurrently.
Definition
The processes in Elixir can be mutually linked, and the processes linked with their other process (parent process) are automatically started by their parent process. These types of processes are called linked processes.
Process links have an essential role when developing parallel and fault-tolerant applications. They support instant shutting down of parts of the system or even the whole system when required. This prevents the application from operating in a poor state.
Problem
Linked processes in Elixir are the solution to a problem that is defined below.
In the above illustration the, Process has just crashed. Assume that the Process remains running, heedless of the crash. It presumes to obtain a result from the Process and report back to the Process. Nonetheless, this will never occur, so this system is left in a poor state.
Solution
Thanks to the process linking, we can avoid the above problem of the running system being in a poor state.
When two processes are linked, they form a unique connection. As soon as a process exits, it will notify the others. All processes in a chain of linked processes will eventually be notified when a crash occurs. By default, linked processes will eliminate and clean up the memory they use, limiting other potential problems down the line.
In the above illustration, the crash of Process will trigger a chain reaction, and Process will terminate, followed by Process.
Pragmatic example of linked processes
defmodule Countdown dodef timer doreceive do:start ->send(self(), {:tick,3}){:tick, 0} ->exit(:done){:tick, value} ->Process.sleep(1000)IO.puts(value)send(self(), {:tick, value - 1})endtimer()enddef watcher dotimer_pid = spawn_link(&timer/0)send(timer_pid, :start)receive domsg ->IO.puts("I getting this message: #{inspect(msg)}")after4000 ->IO.puts("I'm not getting any messages from this timer")endendendCountdown.watcher()
Explanation
-
Line 17: We link the
watcher()function to thetimer()function . -
Line 18: When
timer()exits, thewatcher()function also dies due to the process linking. That’s why we are not getting any output from thewatcher()function. -
Output: We get the message
** (EXIT from #PID<0.94.0>) :donefor the termination of thewatcher()function.
