----- Original Message -----
> From: "Marcus Bointon" <marcus@stripped>
> Writing to both masters is not improving your redundancy - it's
> reducing it! If your replication fails then you've got incomplete
> data on BOTH servers, not just one.
Say what? If that's the case, you haven't set up your replication correctly. There are
quite a few reasons to not use both masters concurrently, but this is not one of them.
> You really don't want to do it.
> This is why MMM sets the passive master to read-only. The increment
No, it does that because it sets up an active/passive master pair and wants to prevent
accidental writes on the passive master.
You can perfectly use an active-active dual-master setup, but your application must be
built around it. Most applications aren't.
> As has been said many times, master-master replication is not a
> scaling solution.
Depends. It can be a very useful scaling solution if it fits your environment.
A very summary view of dual active master:
* it is NOT a WRITE-scaling solution (both masters still must write all data), but it *is*
a valid option in a read-heavy, low-write environment.
* a minor problem for dual-active is clashing autoincrements on inserts - this is easily
solved using offsetting.
* the MAJOR problem, however, is that due to the asynchronous nature of replication, it is
possible to have conflicting updates. There is no solution for this in the standard
version of MySQL, although a number of vendors do have solutions.
Imagine a dual-active scenario, with a client connected to each host. Client a updates
record 123 on host A. At approximately the same moment, client b deletes that same record
123 on host B. Both statements succeed (asynchronous, remember) and then get sent to the
other side. Host A receives a delete statement for record 123 and executes it normally.
Host B, however, receives an update statement for record 123 and fails with a "no such
record" error. Your databases are now inconsistent, and replication grinds to a halt.
THAT is the problem with writing to both masters at once.
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