Concurrent Programming Erlang logo

Concurrent Programming



Definitions

  • Process - A concurrent activity. A complete virtual machine. The system may have many concurrent processes executing at the same time.
  • Message - A method of communication between processes.
  • Timeout - Mechanism for waiting for a given time period.
  • Registered Process - Process which has been registered under a name.
  • Client/Server Model - Standard model used in building concurrent systems.

back to top


Creating a New Process

Before:

Single process

Code in Pid1

Pid2 = spawn(Mod, Func, Args)
After
Two processes

Pid2 is process identifier of the new process - this is known only to process Pid1.

back to top


Simple Message Passing

Simple message with self()

self() - returns the Process Identity (Pid) of the process executing this function.

From and Msg become bound when the message is received. Messages can carry data.

Message with data

  • Messages can carry data and be selectively unpacked.
  • The variables A and D become bound when receiving the message.
  • If A is bound before receiving a message then only data from this process is accepted.

back to top


An Echo process


-module(echo).
-export([go/0, loop/0]).
 
go() ->
	Pid2 = spawn(echo, loop, []),
	Pid2 ! {self(), hello},
	receive 
		{Pid2, Msg} ->
			io:format("P1 ~w~n",[Msg])
	end,
	Pid2 ! stop.

loop() ->
	receive
		{From, Msg} -> 
			From ! {self(), Msg},
			loop();
		stop ->
			true
	end.

back to top


Selective Message Reception

Two sender and one reader using two receive statements

The message foo is received - then the message bar - irrespective of the order in which they were sent.

back to top


Selection of any message

Two sender and one reader using one receive statement

The first message to arrive at the process C will be processed - the variable Msg in the process C will be bound to one of the atoms foo or bar depending on which arrives first.

back to top


A Telephony Example

A and B sends to Call

ringing_a(A, B) -> 
	receive
		{A, on_hook} ->
			A ! {stop_tone, ring},
			B ! terminate,
			idle(A);
		{B, answered} ->
			A ! {stop_tone, ring},
			switch ! {connect, A, B},
			conversation_a(A, B)
	end.
This is the code in the process `Call. A and B are local bound variables in the process Call.

back to top


Pids can be sent in messages

Pass messages

  • A sends a message to B containing the Pid of A.
  • B sends a transfer message to C.
  • C replies directly to A.

back to top


Registered Processes

register(Alias, Pid) Registers the process Pid with the name Alias.

start() ->
	Pid = spawn(num_anal, server, [])
	register(analyser, Pid).

analyse(Seq) ->
	analyser ! {self(),{analyse,Seq}},
	receive
		{analysis_result,R} ->
			R
	end.
Any process can send a message to a registered process.

back to top


Client Server Model

Cliends/server

Protocol

Protocol

Server code

-module(myserver).

server(Data) ->
	receive
		{From,{request,X}} ->
			{R, Data1} = fn(X, Data),
			From ! {myserver,{reply, R}},
			server(Data1)
	end.

Interface Library

-export([request/1]).

request(Req) ->
	myserver ! {self(),{request,Req}},
	receive
		{myserver,{reply,Rep}} ->
			Rep
	end.

back to top


Timeouts

Action after timeout

If the message foo is received from A within the time Time perform Actions1 otherwise perform Actions2.

Uses of Timeouts

sleep(T)- process suspends for T ms.

sleep(T) ->
	receive
	after
		T ->
			true
	end.
suspend() - process suspends indefinitely.
suspend() ->
	receive
	after
		infinity ->
			true
	end.
alarm(T, What) - The message What is sent to the current process iin T miliseconds from now

set_alarm(T, What) ->
	spawn(timer, set, [self(),T,What]).

set(Pid, T, Alarm) ->
	receive
	after
		T ->
			Pid ! Alarm
	end.
receive
	Msg ->
		... ;
end
flush() - flushes the message buffer


flush() ->
	receive 
		Any ->
			flush()
	after 
		0 ->
			true
	end.
A value of 0 in the timeout means check the message buffer first and if it is empty execute the following code.

back to top


Powered by Erlang Web