TIL_09_June_2017 - Process & Message passing mechanism (Elixir)

[vinhnglx]

I. Process in Elixir

• In BEAM, everything runs in a process.

• BEAM process shouldn't be confused with an OS process.

• Using spawn/1 to create a new process

spawn(fn -> say_hello.("Vincent") end)

• After the process is created, spawn immediately returns. The result from this process will be returning soon.

• Two processes can't share any memory.

• Remember, processes are completely independent and isolated.

Example:

iex > say_hello = fn(your_name) -> 
  Process.sleep(4000)
  "Hello #{your_name}" 
end

iex > Enum.map(1..5, fn(x) -> say_hello.(x) end)
["Hello 1", "Hello 2", "Hello 3", "Hello 4", "Hello 5"] # 20 seconds

iex > query = fn(your_name) ->
  spawn(fn -> IO.puts(say_hello.(your_name)) end)
end

iex > query.("vincent")
"Hello vincent" # 4 seconds

iex > Enum.map(1..5, fn(x) -> query.(x) end)
Hello 1
Hello 2
Hello 3
Hello 4
Hello 5
# 4 seconds

[vinhnglx]

II. Message passing mechanism

Processes are completely independent and isolated so how can they communicate? Processes will communicate via messages.

So, in general, the progress will be, for example, we have two processes A and B

• When process A wants process B to do something, it sends asynchronous message to B.
• This message will be stored in the mailbox of the receiver. The mailbox is a FIFO queue limited only by the available memory.
• Using Kernel.send/2 to send the message. We also need to know process identifier PID before send.
• To pull the message from the mailbox, use the receive construct.
• The receive construct works as follows:
  1. Take the first message from the mailbox
  2. Try to match it against any of the provided patterns, going from top to bottom.
  3. If a pattern matches the message, run the corresponding code.
  4. If no pattern matches put the message back to the mailbox at the same position it originally occupied, and next message is processed.
  5. If there are no more messages in the queue, wait for new one to arrive. When a new message arrives, start from step 1.
  6. If no message arrives in the given amount of time and the after clause is specified then run the code from after block.

Example:

  def receive_msg do
    receive do
      {:message, msg} -> IO.puts "Receive message from #{msg}"
    end
  end

iex(1)> pid = spawn(Practices, :receive_msg, [])
#PID<0.119.0>
iex(2)> send pid, {:message, "ding"}
Receive message from ding
{:message, "ding"}

[vinhnglx]

Can refer #10 for the example