Re: replication from Oracle to PostgreSQL?

On 08/11/2011 03:58 PM, Joshua D. Drake wrote:
> I would imagine it would be pretty simple to fork the triggers that do
> replication in Londiste or Slony (as long as everything else was
> PostgreSQL) to be able to replicate from Oracle to PostgreSQL.

That's the approach taken by some of our in-house tools. "simple"
wouldn't be the right word to describe the process that went into
building them though. You have to get the basics working, then worry
about non-deterministic statements (random, timestamps, etc.),
sequencing, all of the stuff any statement based replication can be
faced with--except now there's the quirks of two databases to worry about.

One of the reasons Slony and Londiste statement extraction works as well
as it does is because new functions were added to PostgreSQL 8.3 for
them, to make it easier to get the sort of snapshot information needed
to grab consistent copies of data. That problem comes back again when
the origin database isn't a PostgreSQL one too.

There are a couple of ways to solve this problem, but all of them are
complicated enough that they're only sitting in commercial/consultant
projects right now (in addition to ones mentioned here already, I'm sure
Continuent has some applicable stuff too). I think Kevin's customer may
be in for a surprise at how much it will cost to duplicate this feature
set from scratch--I'd be surprised to see that total come in under what
licensing an already built package would sell for. Companies who aren't
willing to throw some money toward buying/customizing at least some
PostgreSQL software, if it allows escaping from Oracle licensing, should
reconsider their strategy. Not everything you'll want to make a
conversion easy is going to be free.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.us