Proposal: Generic WAL logical messages

Hello,

I would like to introduce concept of generic WAL logical messages.

These messages just create WAL record with arbitrary data as specified
by user. For standard WAL reply, these are basically noop, but in
logical decoding they are be decoded and the special callback of the
output plugin is be called for them.

These messages can be both transactional (decoded on commit) or
non-transactional (decoded immediately). Each message has prefix to
differentiate between individual plugins. The prefix has to be
registered exactly once (in similar manner as security label providers)
to avoid conflicts between plugins.

There are three main use-cases for these:
a) reliable communication between two nodes in multi-master setup - WAL
already handles correctly the recovery etc so we don't have to invent
new complex code to handle crashes in middle of operations

b) out of order messaging in logical replication - if you want to send
something to other node immediately without having to wait for commit
for example to acquire remote lock, this can be used for that

c) "queue tables" - combination of this feature (there is SQL interface)
and before triggers can be used to create tables for which all inserts
only go to the WAL so they can behave like queue without having to store
the data in the table itself (kind of the opposite of unlogged tables)

Initial implementation of this proposal is attached.

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