[HACKERS] Synchronous Log Shipping Replication

Hi,

In PGCon 2008, I proposed synchronous log shipping replication.
Sorry for late posting, but I'd like to start the discussion
about its implementation from now.
http://www.pgcon.org/2008/schedule/track/Horizontal%20Scaling/76.en.html

First of all, I'm not planning to put the prototype which I demoed
in PGCon into core directly.

- Portability issues (using message queue, multi-threaded ...)
- Have too much dependency on Heartbeat

Yes, since the prototype is useful reference of implementation,
I plan to open it ASAP. But, I'm sorry - it still takes a month
to open it.

Pavan re-designed the sync replication based on the prototype
and I posted that design doc on wiki. Please check it if you
are interested in it.
http://wiki.postgresql.org/wiki/NTT%27s_Development_Projects

This design is too huge. In order to enhance the extensibility
of postgres, I'd like to divide the sync replication into
minimum hooks and some plugins and to develop it, respectively.
Plugins for the sync replication plan to be available at the
time of 8.4 release.

In my design, WAL sending is achieved as follow by WALSender.
WALSender is a new process which I introduce.

1) On COMMIT, backend requests WALSender to send WAL.
2) WALSender reads WAL from walbuffers and send it to slave.
3) WALSender waits for the response from slave and replies
backend.

I propose two hooks for WAL sending.

WAL-writing hook
----------------
This hook is for backend to communicate with WALSender.
WAL-writing hook intercepts write system call in XLogWrite.
That is, backend requests WAL sending whenever write is called.

WAL-writing hook is available also for other uses e.g.
Software RAID (writes WAL into two files for durability).

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.

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.

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.

Which idea should we adopt?

Comments welcome.

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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