Concurrent Programming

• Definitions
• Creating a new process
• Simple message passing
• An Echo Process
• Selective Message Reception
• Selection of Any Message
• A Telephony Example
• Pids can be sent in messages
• Registered Processes
• The Client Server Model
• Timeouts

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.

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Creating a New Process

Code in Pid1

Pid2 = spawn(Mod, Func, Args)
After

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

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Simple Message Passing

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.

• 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.

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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.

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Selective Message Reception

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

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Selection of any message

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.

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A Telephony Example

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.

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Pids can be sent in messages

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

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Registered Processes

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

analyse(Seq) ->
	analyser ! {self(),{analyse,Seq}},
	receive
		{analysis_result,R} ->
			R
	end.

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Client Server Model

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.

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Timeouts

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

Uses of Timeouts

sleep(T) ->
	receive
	after
		T ->
			true
	end.

suspend() ->
	receive
	after
		infinity ->
			true
	end.

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

set(Pid, T, Alarm) ->
	receive
	after
		T ->
			Pid ! Alarm
	end.
receive
	Msg ->
		... ;
end

flush() ->
	receive 
		Any ->
			flush()
	after 
		0 ->
			true
	end.

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