Re: Synchronous Log Shipping Replication - Mailing list pgsql-hackers

Markus Wanner wrote:
> > Hook for WALSender
> > ------------------
> > This hook is for introducing WALSender. There are the following
> > three ideas of how to introduce WALSender. A required hook
> > differs by which idea is adopted.
> > 
> > a) Use WALWriter as WALSender
> > 
> >    This idea needs WALWriter hook which intercepts WALWriter
> >    literally. WALWriter stops the local WAL write and focuses on
> >    WAL sending. This idea is very simple, but I don't think of
> >    the use of WALWriter hook other than WAL sending.

The problem with this approach is that you are not sending WAL to the
disk _while_ you are, in parallel, sending WAL to the slave;  I think
this is useful for performance reasons in synrchonous replication.

> > b) Use new background process as WALSender
> > 
> >    This idea needs background-process hook which enables users
> >    to define new background processes. I think the design of this
> >    hook resembles that of rmgr hook proposed by Simon. I define
> >    the table like RmgrTable. It's for registering some functions
> >    (e.g. main function and exit...) for operating a background
> >    process. Postmaster calls the function from the table suitably,
> >    and manages a start and end of background process. ISTM that
> >    there are many uses in this hook, e.g. performance monitoring
> >    process like statspack.

I think starting/stopping a process for each WAL send is too much
overhead.

> > c) Use one backend as WALSender
> > 
> >    In this idea, slave calls the user-defined function which
> >    takes charge of WAL sending via SQL e.g. "SELECT pg_walsender()".
> >    Compared with other ideas, it's easy to implement WALSender
> >    because postmater handles the establishment and authentication
> >    of connection. But, this SQL causes a long transaction which
> >    prevents vacuum. So, this idea needs idle-state hook which
> >    executes plugin before transaction starts. I don't think of
> >    the use of this hook other than WAL sending either.
> 
> The above cited wiki page sounds like you've already decided for b).

I assumed that there would be a background process like bgwriter that
would be notified during a commit and send the appropriate WAL files to
the slave.

> I'm unclear on what you want hooks for. If additional processes get 
> integrated into Postgres, those certainly need to get integrated very 
> much like we integrated other auxiliary processes. I wouldn't call that 
> 'hooking', but YMMV.

Yea, I am unclear how this is going to work using simple hooks.

It sounds like Fujii-san is basically saying they can only get the hooks
done for 8.4, not the actual solution.  But, as I said above, I am
unclear how a hook solution would even work long-term;  I am afraid it
would be thrown away once an integrated solution was developed.

--  Bruce Momjian  <bruce@momjian.us>        http://momjian.us EnterpriseDB
http://enterprisedb.com
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