Re: PostgreSQL + Replicator developer meeting 10/28 - Mailing list pgsql-hackers

On Tue, 2008-10-28 at 15:18 -0700, Joshua Drake wrote:
> On Tue, 28 Oct 2008 19:46:42 +0200
> Hannu Krosing <hannu@2ndQuadrant.com> wrote:
> 
> > >  The current topics are:
> > >
> > >     * New MCP architecture
> > 
> > What's new ? 
> > 
> > I have some doubts about the current architecture based on my reading
> > of replicator wiki, but would like to learn about the "new"
> > architecture before spending too much time in studying the "old" one.
> 
> The two obvious problems with the existing MCP architecture is:
> 
>  1. Single point of failure

For async replication there is always SPoF, at least the master until
first slave has aquired log is a SPoF, or do you plan that both Master
and "MCP|Slave" to keep the log and be able to step in for each other if
the other fails?

>  2. Portability

Portability to where ? Other DBMS's ? Other PG versions ?


for me there was also two more problems:

3. separate "replication log", which at least seems to be able to get
out of sync with main DB. 

Why don't you just use a DB table, WAL-logged and all

4. Also, again from reading Replicator FAQ, it seems that there is a
window of corruption/data loss when rotating the Replicators transaction
log. I think that doing it with copy/truncate either needs locking the
logfile (== bad performance, during copy/truncate) or is just a
data-eating failure waiting to happen.

pgQ has a solution to that by rotating queue tables, and postgreSQL core
also does effectively "rotate" WAL log segments. To be robust _and_
effective, Replicator should also rotate the logfile itself.

> The new architecture is set to remove both of those. The short version
> is the MCP will be moved into the backend. Thus:
> 
> Master->MCP|Slave ->Slave1
>                   ->Slave2
>                   ->Slave3
> 
> The process being, Master sends data to MCP|Slave, MCP|Slave writes it
> to disk (optionally restores it)

Will this first send be sync or async ? Or have you planned to have it
be configurable among several robustness vs. performance levels, similar
to the planned integrated WAL-shipping.

if async, will it also use MVCC for keeping log on Master (l.ike Slony
and pgQ do), just to be at least as reliable as postgreSQL core itself
and not require a full resync at server crash.

> and then forwards it to 1,2,3 who then
> receive the data, write it to disk and then restore it.
> 
> If master dies, you can promote to any of the slaves and the left over
> slaves will connect to the promoted slave and begin receiving updates.
> 
> If the MCP|Slave dies a new Slave can begin the MCP|Slave process.
> 
> Alvaro or Alexey can speak more technically about implementation than I
> can.

Alvaro - I guess you already have discussed most of it, but basically
you need to solve all the same problems that WAL-shipping based Hot
Standby is solving and Slony/pgQ/Londiste have solved.

Hopefully you get it more robust than Slony when making changes under
high load :)

Will there be an helper application for setting up and configuring
changes in replication. or will it all be done using added SQL
commands ?

How will DDL be handled ( i understood that you don't yet have DDL
replication) ?

Will Slave tables be kind-of-read-only like Slony slaves ? Or even
_really_ read only like Simon's Hot Standby ?

> > 
> > >     * DDL Replication
> > 
> > Is it there alread, or is it just  a planned feature ?
> 
> Planned feature.

Did the plans got any clearer during this meeting ?
> >  * How hard would it be to extract DDL replication part and use it as
> >    basis for DDL after trigger support for use in trigger based
> >    replication/auditing like Slony ann pgQ/Londiste ?
> 
> Hmm I am not sure. We are pretty deep into the core and only use
> triggers for GRANT/REVOKE/CREATE ROLE .

By the way, why did you choose "pretty deep into the core" approach
instead of triggers ?

I mean, you probably end up duplicating (or missing) lots of
postgreSQL-s internal goodness instead of just using what is already
available ?

> Sincerely,
> 
> Joshua D. Drake

Thanks for the update.

I hope something useful will come out of this too, though I hoped that
it already had some advantages over trigger-based replication, like
ability to replicate DDL .

-----------------------
Hannu Krosing