Re: Applying logical replication changes by more than one process

On 21.03.2016 15:10, Petr Jelinek wrote:
> Hi,
>
> On 19/03/16 11:46, Konstantin Knizhnik wrote:
>> Hi,
>>
>> I am trying to use logical replication mechanism in implementation of
>> PostgreSQL multimaster and faced with one conceptual problem.
>> Originally logical replication was intended to support asynchronous
>> replication. In this case applying changes by single process should not
>> be a bottleneck.
>> But if we are using distributed transaction manager to provide global
>> consistency, then applying transaction by one worker  leads to very bad
>> performance and what is worser: cause unintended serialization of
>> transactions, which is not taken in account by distributed deadlock
>> detection algorithm and so can cause
>> undetected deadlocks.
>>
>> So I have implemented pool of background workers which can apply
>> transactions concurrently.
>> It works and shows acceptable performance. But now I am thinking about
>> HA and tracking origin LSNs which are needed to correctly specify slot
>> position in case of recovery. And there is a problem: as far as I
>> understand to correctly record origin LSN in WAL and advance slot
>> position it is necessary to setup session
>> using replorigin_session_setup. It is not so convenient in case of using
>> pool of background workers, because we have to setup session for each
>> commit.
>> But the main problem is that for each slot session can be associated
>> only with one process:
>>
>>          else if (curstate->acquired_by != 0)
>>          {
>>              ereport(ERROR,
>>                      (errcode(ERRCODE_OBJECT_IN_USE),
>>               errmsg("replication identifier %d is already active for
>> PID %d",
>>                      curstate->roident, curstate->acquired_by)));
>>          }
>>
>> Which once again means that there can be only one process applying 
>> changes.
>>
>
> That's not true, all it means is that you can do 
> replorigin_session_setup for same origin only in one process but you 
> don't need to have it setup for session to update it, the 
> replorigin_advance() works just fine.

But RecordTransactionCommit is using replorigin_session_advance, not 
replorigin_advance.
And replorigin_session_advance requires that session was setup:

void
replorigin_session_advance(XLogRecPtr remote_commit, XLogRecPtr 
local_commit)
{    Assert(session_replication_state != NULL);
}

"session_replication_state" is private variable which is set by 
replorigin_session_setup.
But attempt to call replorigin_session_setup from multiple process cause 
above error.

-- 
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company