MySQL dual master

Question

For my current project we are thinking of setting up a dual master replication topology for a geographically separated setup; one db on the us east coast and the other db in

Answer 1

Setting up mysql as dual master does actually work fine in the right scenario done correctly. But I am not sure it fits very well in your scenario.

First of all, dual master setup in mysql is really a ring-setup. Server A is defined as master of B, while B is at the same time defined as the master of A, so both servers act as both master and slave. The replication works by shipping a binary log containing the sql statements which the slave inserts when it sees fit, which is usually right away. But if you're hammering it with local insertions, it will take a while to catch up. The slave insertions are sequential by the way, so you won't get any benefit of multiple cores etc.

The primary use of dual master mysql is to have redundancy on the server level with automatic fail-over (often using hearbeat on linux). Excluding mysql-cluster (for various reasons), this is the only usable automatic failover for mysql. The setup for basic dual master is easily found on google. The heartbeat stuff is a bit more work. But this is not really what you were asking about, since this really behaves as a single database server.

If you want the dual master setup because you always want to write to a local database (write to both of them at the same time), you'll need to write your application with this in mind. You can never have auto-incrementing values in the database, and when you have unique values, you must make sure that the two locations never write the same value. For example location A could write odd unique numbers and location B could write even unique numbers. The reason is that you're not guaranteed that the servers are in sync at any given time, so if you've inserted a unique row in A, and then an overlapping unique row in B before the second server catches up, you'll have a broken system. And if something first breaks, the entire system stops.

To sum it up: it's possible, but you'll need to tip-toe very carefully if you're building business software on top of this.

Answer 2

Just a note on the technical aspects of your plan: You have to know that MySQL does not officially support multi-master replication (only MySQL Cluster provides support for synchronous replication).

But there is at least one "hack" that makes multi-master-replication possible even with a normal MySQL replication setup. Please see Patrick Galbraith's "MySQL Multi-Master Replication" for a possible solution. I don't have any experience with this setup, so I don't dare to judge on how feasible this approach would be.

Answer 3

There are several things to consider when replicating databases geographically. If you are doing this for performance reasons, be sure your replication model supports your data being "eventually consistent" as it can take time to bring the replication current in both, or many, locations. If your throughput or response times between locations is not good, active replication may not be the best option.

Answer 4

Because of the one-to-many architecture of MySQL replication, you have to have a replication ring with multiple masters: that is, each replicates from the next in a loop. For two, they replicate off each other. This has been supported from as far back as v3.23.

In a previous place I worked, we did it with v3.23 with quite a number of customers as a way of providing exactly what you're asking. We used SSH tunnels over the Internet to do the replication. It took us some time to get it reliable and several times we had to do a binary copy of one database to another (fortunately, none of them were over 2Gb nor needed 24-hour access). Also the replication in v3 was not nearly as stable as in v4 but even in v5, it will just stop if it detects any sort of error.

To accomodate the inevitable replication lag, we re-structured the application so that it didn't rely on AUTOINCREMENT fields (and removed that attribute from the tables). This was reasonably straightforward due to the data-access layer we had developed; instead of it using mysql_insert_id() for new objects, it created the new ID first and inserted it along with the rest of the row. We also implemented site IDs that we stored in the top half of the ID, because they were BIGINTs. This also meant we didn't have to change the application when we had a client who wanted the database in three locations. :-)

It wasn't 100% robust. InnoDB was just gaining some visibility so we couldn't easily use transactions, although we considered it. So there were race conditions occasionally when two objects tried to be created with the same ID. This meant one failed and we tried to report that in the app. But it was still a significant part of someone's job to watch over the replication and fix things when it broke. Importantly, to fix it before we got too far out of sync, because in a few cases the databases were being used in both sites and would quickly become difficult to re-integrate if we had to rebuild one.

It was a good exercise to be a part of, but I wouldn't do it again. Not in MySQL.