MySQL: Writing to slave node

Question

Lets say I have a datbase of Cars. I have Makes and Models (FK to Makes). I plan on having users track their cars. each Car has a FK to Model. Now, I have a lot of users, and I want to split up my database to distribute load. The Makes and Models tables don't change so much, but they need to be shared across shards. My thought is to use MySQL replication from a master DB of makes and models to each slave database. My question is: Can I safely write to the slave databases assuming I don't write to those tables on the master?

And while on the subject, is there anyway to guarantee one slave database has the latest data? For example, someone just added the 'Taurus' make, and then wants to add their car. Can I ensure that the slave database they are using has the latest master data?

Answer 1

Yes, in general you can safely write to a table on the slaves that is not being written on the master. If you do things like insert auto_increment rows on the slaves and on the master, independently, you will of course have problems. You should configure that table to be excluded from replication entirely, really.

For checking whether you have the latest data, SHOW SLAVE STATUS includes a field Seconds_Behind_Master that tells you whether the slave is up to date. Obviously you want it to be zero. To be certain that inserted and replicated data is present, of course, you need to wait a second and then see that Seconds_Behind_Master is zero.

Answer 2

This was a good solution I gleaned while searching

I included the main point as avilable here:

http://erlycoder.com/43/mysql-master-slave-and-master-master-replication-step-by-step-configuration-instructions-

MySQL master-master replication and autoincrement indexes

If you are using master-slave replication, than most likely you will design your application the way to write to master and read from slave or several slaves. But when you are using master-master replication you are going to read and write to any of master servers. So, in this case the problem with autoincremental indexes will raise. When both servers will have to add a record (different one each server simultaneously) to the same table. Each one will assign them the same index and will try to replicate to the salve, this will create a collision. Simple trick will allow to avoid such collisions on MySQL server.

On the Master 1/Slave 2 add to /etc/my.cnf:

auto_increment_increment= 2
auto_increment_offset   = 1

On the Master 2/Slave 1 add to /etc/my.cnf:

auto_increment_increment= 2
auto_increment_offset   = 2