PostgreSQL - implementing a reliable queue

Question

I am trying to implement a reliable queue with multiple writers and a multiple readers using postgres database. How to avoid missing rows when a queue reader scans a table t

Answer 1

Considering multiple readers, it is necessary to have control on which records each reader had received already.

Also, it's been said the order is a condition to send records to a reader as well. So, if some further transaction had committed before a earlier one, we have to "stop" and just send records again when it had committed, to maintain the order of records sent to the reader.

That said, check the implementation:

-- lets create our queue table 
drop table if exists queue_records cascade;
create table if not exists queue_records 
(
  cod serial primary key,
  date_posted timestamp default timeofday()::timestamp,
  message text
);


-- lets create a table to save "checkpoints" per reader_id
drop table if exists queue_reader_checkpoint cascade;
create table if not exists queue_reader_checkpoint 
(
  reader_id text primary key,
  last_checkpoint numeric
);



CREATE OR REPLACE FUNCTION get_queue_records(pREADER_ID text)
RETURNS SETOF queue_records AS
$BODY$
DECLARE
    vLAST_CHECKPOINT    numeric;
    vCHECKPOINT_EXISTS  integer;
    vRECORD         queue_records%rowtype;
BEGIN

    -- let's get the last record sent to the reader 
    SELECT  last_checkpoint
    INTO    vLAST_CHECKPOINT
    FROM    queue_reader_checkpoint
    WHERE   reader_id = pREADER_ID;

    -- if vLAST_CHECKPOINT is null (this is the very first time of reader_id), 
    -- sets it to the last cod from queue. It means that reader will get records from now on.
    if (vLAST_CHECKPOINT is null) then
        -- sets the flag indicating the reader does not have any checkpoint recorded
        vCHECKPOINT_EXISTS = 0;
        -- gets the very last commited record
        SELECT  MAX(cod)
        INTO    vLAST_CHECKPOINT
        FROM    queue_records;
    else
        -- sets the flag indicating the reader already have a checkpoint recorded
        vCHECKPOINT_EXISTS = 1; 
    end if;

    -- now let's get the records from the queue one-by-one 
    FOR vRECORD IN 
            SELECT  *
            FROM    queue_records
            WHERE   COD > vLAST_CHECKPOINT 
            ORDER   BY COD
    LOOP

        -- if next record IS EQUALS to (vLAST_CHECKPOINT+1), the record is in the expected order
        if (vRECORD.COD = (vLAST_CHECKPOINT+1)) then

            -- let's save the last record read
            vLAST_CHECKPOINT = vRECORD.COD;

            -- and return it
            RETURN NEXT vRECORD;

        -- but, if it is not, then is out of order
        else
            -- the reason is some transaction did not commit yet, but there's another further transaction that alread did.
            -- so we must stop sending records to the reader. And probably next time he calls, the transaction will have committed already;
            exit;
        end if;
    END LOOP;


    -- now we have to persist the last record read to be retrieved on next call
    if (vCHECKPOINT_EXISTS = 0) then
        INSERT INTO queue_reader_checkpoint (reader_id, last_checkpoint) values (pREADER_ID, vLAST_CHECKPOINT);
    else        
        UPDATE queue_reader_checkpoint SET last_checkpoint = vLAST_CHECKPOINT where reader_id = pREADER_ID;
    end if; 
end;
$BODY$ LANGUAGE plpgsql VOLATILE;