Mario Splivalo wrote:
On Wed, 2005-10-26 at 12:09 -0400, Jan Wieck wrote:
You must have missed the FAQ and other side notes about replication in
the MySQL manual. Essentially MySQL replication is nothing but a query
duplicating system, with the added sugar of taking care of now() and
some other non-deterministic things, but not all of them.
Non-deterministic user defined procedures, functions and triggers will
simply blow MySQL's sophisticated replication apart.
That is just not true. I haven't tried MySQL 4 and above, but in 3.5.x
replication wos working as expected, and the load that replication posed
was insignificant. The only TRUE problem was that replication was
unidirectional. That SAME problem has Slony, and other 'replication
systems' available for postgres.
I think you're missing the "non-deterministic" bit. Mysql replication is
based on shipping statements AFAICT
http://dev.mysql.com/doc/refman/5.0/en/binary-log.html
"The binary log contains all statements which updated data or
potentially could have updated it (for example, a DELETE which matched
no rows). Statements are stored in the form of “events” that describe
the modifications."
So - if your statement contains something non-deterministic that isn't
catered for in Mysql's code then it will break.
At it's simplest - if I write a function my_random() and then do:
UPDATE foo SET a=1 WHERE b < my_random();
IF my_random() returns different results on different machines, then the
replication will be broken. See the manual entry below:
http://dev.mysql.com/doc/refman/5.0/en/replication-features.html
That's not to say the system is worthless - it works fine for many
people. But it does have limitations.
--
Richard Huxton
Archonet Ltd
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