Re: [RFC] CREATE QUEUE (log-only table) for londiste/pgQ ccompatibility

On 10/16/2012 11:18 AM, Simon Riggs wrote:
> On 16 October 2012 09:56, Hannu Krosing <hannu(at)2ndquadrant(dot)com> wrote:
>> Hallo postgresql and replication hackers
>>
>> This mail is an additional RFC which proposes a simple way to extend the
>> new logical replication feature so it can cover most usages of
>> skytools/pgq/londiste
>>
>> While the current work for BDR/LCR (bi-directional replication/logical
>> replication) using WAL is theoretically enought to cover _replication_ offered by
>> Londiste it falls short in one important way - there is currently no support for pure
>> queueing, that is for "streams" of data which does not need to be stored in the source
>> database.
>>
>> Fortunately there is a simple solution - do not store it in the source
>> database :)
>>
>> The only thing needed for adding this is to have a table type which
>>
>> a) generates a INSERT record in WAL
>>
>> and
>>
>> b) does not actually store the data in a local file
>>
>> If implemented in userspace it would be a VIEW (or table) with a
>> before/instead
>> trigger which logs the inserted data and then cancels the insert.
>>
>> I'm sure this thing could be implemented, but I leave the tech discussion to
>> those who are currently deep in WAL generation/reconstruction .
>>
>> If we implement logged only tables / queues we would not only enable a more
>> performant pgQ replacement for implementing full Londiste / skytools
>> functionality
>> but would also become a very strong player to be used as persistent basis
>> for message queueing solutions like ActiveMQ, StorMQ, any Advanced Message
>> Queuing Protocol (AMQP) and so on.
>
> Hmm, I was assuming that we'd be able to do that by just writing extra
> WAL directly. But now you've made me think about it, that would be
> very ugly.
>
> Doing it this was, as you suggest, would allow us to write WAL records
> for queuing/replication to specific queue ids. It also allows us to
> have privileges assigned. So this looks like a good idea and might
> even be possible for 9.3.
>
> I've got a feeling we may want the word QUEUE again in the future, so
> I think we should call this a MESSAGE QUEUE.
>
> CREATE MESSAGE QUEUE foo;
> DROP MESSAGE QUEUE foo;
I would like this to be very similar to a table, so it would be

CREATE MESSAGE QUEUE(fieldname type, ...) foo;

perhaps even allowing defaults and constraints. again, this
depends on how complecxt the implementation would be.

for the receiving side it would look like a table with only inserts,
and in this case there could even be a possibility to use it as
a remote log table.

>
> GRANT INSERT ON MESSAGE QUEUE foo TO ...;
> REVOKE INSERT ON MESSAGE QUEUE foo TO ...;
>
> Rules wouldn't. DELETE and UPDATE wouldn't work, nor would SELECT.
>
> Things for next release: Triggers, SELECT sees a stream of changes,
> CHECK clauses to constrain what can be written.
>
> One question: would we require the INSERT statement to parse against a
> tupledesc, or would it be just a single blob of TEXT or can we send
> any payload? I'd suggest just a single blob of TEXT, since that can be
> XML or JSON etc easily enough.
>