As an aside, don't expect to turn on mysql replication and have
applications magically start syncing between sites.
I have yet to find an easy transparent mysql replication layer that you
can plug underneath existing applications without things dying horribly.
The only way I've managed to get something like this to work on in house
applications is active/slave and then I had to rewrite all the database
calls so that all the reads came from the local slave and the writes
went to the master for the remote sites.
You could in theory use MySQL clustering but my head exploded when I
tried to implement that without code changes. Its much easier to
designate one box as the "all writes happen here" and use the slaves as
read slaves.
Its also nice to turn on replication to have a slave for backups so you
can do mysql dumps on large tables without impacting performance on the
main host.
Wayne Merricks
The Voice Asia
On 03/03/14 16:28, Fred Gleason wrote:
On Mar 3, 2014, at 10:46 49, Cowboy <c...@cwf1.com> wrote:
If you were so inclined, you could check and compare the timestamps
on the files in /var/lib/mysql/databasename/tablename.MYI between the
two ( or three, or... ) machines and know which is the most recent
No, No, NO - you do *not* want to do this! MySQL makes extensive use of
internal file caching as a technique for boosting performance. The only time
that the timestamps on the actual files can be taken as indicative of the true
state of things is after doing a FLUSH TABLES WITH READ LOCK call (perhaps —
InnoDB tables can get you in trouble here) or ideally if the mysqld(8) daemon
is shut down.
A setup where two MySQL instances update each other is known as ‘active-active
replication’. It can be done, but it requires careful application design to
ensure that updates to a particular table are wholly confined to one or the
other of the instances (a technique known as ‘data sharding’). The consequence
of violating this rule can be data collisions in things like column
auto-increment values, which in turn will halt replication and require manual
intervention to recover from the inconsistent state.
Cheers!
|-------------------------------------------------------------------------|
| Frederick F. Gleason, Jr. | Chief Developer |
| | Paravel Systems |
|-------------------------------------------------------------------------|
| Like the ski resort of girls looking for husbands and husbands looking |
| for girls, the situation is not as symmetrical as it might seem. |
| -- Alan McKay |
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