Re: [PATCH 06/16] Add support for a generic wal reading facility dubbed XLogReader

On Thursday, June 14, 2012 11:19:00 PM Heikki Linnakangas wrote:
> On 13.06.2012 14:28, Andres Freund wrote:
> > Features:
> > - streaming reading/writing
> > - filtering
> > - reassembly of records
> > 
> > Reusing the ReadRecord infrastructure in situations where the code that
> > wants to do so is not tightly integrated into xlog.c is rather hard and
> > would require changes to rather integral parts of the recovery code
> > which doesn't seem to be a good idea.
> 
> It would be nice refactor ReadRecord and its subroutines out of xlog.c.
> That file has grown over the years to be really huge, and separating the
> code to read WAL sounds like it should be a pretty natural split. I
> don't want to duplicate all the WAL reading code, so we really should
> find a way to reuse that. I'd suggest rewriting ReadRecord into a thin
> wrapper that just calls the new xlogreader code.
> 
> > Missing:
> > - "compressing" the stream when removing uninteresting records
> > - writing out correct CRCs
> > - validating CRCs
> > - separating reader/writer
> 
> - comments.
> 
> At a quick glance, I couldn't figure out how this works. There seems to
> be some callback functions? If you want to read an xlog stream using
> this facility, what do you do? Can this be used for writing WAL, as well
> as reading? If so, what do you need the write support for?
Oh, btw, the callbacks and parameters are somewhat documented in the 
xlogreader.h header in the XLogReaderState struct.
Still needs improvement though.

Andres
-- 
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services