Re: [HACKERS] Proposal for a cascaded master-slave replication system

Hans-Jürgen Schönig wrote:

> Jan,
>
> First of all we really appreciate that this is going to be an Open
> Source project.
> There is something I wanted to add from a marketing point of view: I
> have done many public talks in the 2 years or so. There is one question
> people keep asking me: "How about the pgreplication project?". In every
> training course, at any conference people keep asking for synchronous
> replication. We have offered this people some async solutions which are
> already out there but nobody seems to be interested in having it (my
> person impression). People keep asking for a sync approach via email but
> nobody seems to care about an async approach. This does not mean that
> async is bad but we can see a strong demand for synchronous replication.
>
> Meanwhile we seem to be in a situation where PostgreSQL is rather
> competing against Oracle than against MySQL. In our case there are more
> people asking for Oracle -> Pg migration than for MySQL -> Pg. MySQL
> does not seem to be the great enemy because most people know that it is
> an inferior product anyway. What I want to point out is that some people
> want an alternative Oracle's Real Application Cluster. They want load
> balancing and hot failover. Even data centers asking for replication did
> not want to have an async approach in the past.

Hans-Jürgen,

we are well aware of the high demand for multi-master replication
addressing load balancing and clustering. We have that need ourself as
well and I plan to work on a follow-up project as soon as Slony-I is
released. But as of now, we see a higher priority for a reliable master
slave system that includes the cascading and backup features described
in my concept. There are a couple of different similar product out
there, I know. But show me one of them where you can failover without
becoming the single point of failure? We've just recently seen ... or
better "where not able to see anything any more" how failures tend to
ripple through systems - half of the US East Coast was dark. So where is
the replication system where a slave becomes the "master", and not a
standalone server. Show me one that has a clear concept of failback, one
that has hot-join as a primary design goal. These are the features that
I expect if something is labeled "Enterprise Level".

As far as my ideas for multi-master go, it will be a synchronous
solution using group communication. My idea is "group commit" instead of
2-Phase ... and an early stage test hack has replicated some update 3
weeks ago. The big challange will be to integrate the two systems so
that a node can start as an asynchronous Slony-I slave, catch up ... and
switch over to synchronous multimaster without stopping the cluster. I
have no clue yet how to do that, but I refuse to think smaller.

Jan

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