Readers and Writers - with message passing

Remember the readers and writers problem ?

• Permit multiple readers, but only 1 writer - reader has priority
• Three process types : reader, writer and dbmanager

Process READER
loop
 	SEND(dbmanager, startread) ;
	RECEIVE(dbmanager, msg) ; {wait for ok}
	{read_database}
	SEND(dbmanager, endread) ;
	{use data}
end

Process WRITER
loop
	{generate data}
	SEND(dbmanager, startwrite) ;
	RECEIVE(dbmanager, msg) ; {wait for ok}
	{write database}
	SEND(dbmanager, endwrite) ;
	{use data}
end

What does dbmanager do ?

Process DBMANAGER
rc = 0 : integer {reader count}
writing = false : boolean ;
msg : message ;
rq, wq : queue {reader, writer queue}
src : processid ;

loop
 src := RECEIVEANY(msg) ;
 CASE msg OF

startread : IF NOT writing
	    THEN send(src,ok); rc=rc+1 END
	    ELSE addq(src,rq) {writer busy, Q reader}

endread :   rc := rc -1 ;
	    IF rc=0 & notempty(wq)
	    THEN {First writer on Q can write}
	     src := removeq(wq); SEND(src,ok); writing=TRUE ;
	    END 

startwrite : IF rc=0 & NOT writing
	     THEN send(src,ok); writing := TRUE  END
	     ELSE addq(src,wq) {database in use}

endwrite : writing := FALSE ;
 	   IF empty(rq) & notempty(wq)
	   THEN src := removeq(wq) ; send(src,ok); writing := TRUE ;
	   END
	   ELSE WHILE notempty(rq)
	        DO {all queued readers continue}
		src := removeq(rq) ;
		send(src,ok) ; rc=rc + 1 ;
		END
	   END
 END{case} END {loop}