Nickolay wrote:
one important addition: the message cannot be removed from queue table
until it is transmitted, so DELETE is not an option :)
Hi All,
I have a trivial task. There is a table with messages queue, let's say
"msg_queue".
There are a few processes and each of them is taking one message from
this table at a time to transmit into communication channel.
I've done it my way, but I have postgresql's messages about deadlocks
and a lot of warnings.
I my program, every process is doing approx the following procedure:
SELECT ... FROM msg_queue WHERE busy = false ORDER BY ... LIMIT 1;
if a message was found:
BEGIN;
SELECT id FROM msg_queue WHERE id = ... AND busy = false FOR SHARE;
UPDATE msg_queue SET busy = true, channel_id = ... WHERE id = ... AND
busy = false;
COMMIT;
I do understand that this way is stupid, but I have not came with
anything else yet.
Could somebody share ideas how to do this so the same message 100%
WOULD NOT be transmitted over two or more channels.
Sorry for the newbie question!
Best regards, Nick.
how about this:
andy=# create table msg (id integer, busy boolean, message text);
CREATE TABLE
andy=# insert into msg values (1, false, 'message one');
INSERT 0 1
andy=# insert into msg values (2, false, 'message two');
INSERT 0 1
CREATE OR REPLACE FUNCTION public.getmsg() RETURNS integer LANGUAGE plpgsql
AS $function$
declare
rec record;
begin
for rec in select id from msg where busy = false order by id loop
update msg set busy = true where id = rec.id and busy = false;
if found then
return rec.id;
end if;
end loop;
return -1;
end;
$function$
It returns -1 if no message found. Not 100% sure, but a quick two session test seemed to work.
-Andy
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