Using non-replicated read/write databases on a MySQL slave server I have a MySQL slave server that has two databases being replicated from a master. What I'd like to do is add a third database that I will exclude from replication and use as a read/write database. Basically this is a database to store local session variables, so I won't have to run MySQL on the application server in this region.

So my questions are:


Can I do this?
If I can, is it a good idea, or should I keep the slave strictly to read only databases (beyond practical considerations)?


Any answer much appreciated, technical explanations and links to more reading would be great. I've found it difficult to Google much on this topic.
You can safely write to a schema on the slave if that schema does not exist on the master or if you correctly use replication filters to prevent its replication to the slave.  

"Correctly" is the important word in that sentence.  Refer to this page:

https://dev.mysql.com/doc/refman/5.7/en/replication-options-slave.html

Check the --replicate-do-db and other filter settings, and note that the behavior changes based on whether binlog events are formatted as rows or as statements.  When statement-based replication is being used, replication filters only apply to the database in use as the current database.  When row-based replication is used, the filters are applied regardless of current database.

Be sure you get this right, otherwise, you could have a situation like this:

replicate-ignore-db=monkey
binlog_format=STATEMENT


Then you do this:

USE zebra;
TRUNCATE monkey.tablex;


Guess what?  tablex just got truncated on both master and slave.  However, if you had this instead:

replicate-ignore-db=monkey
binlog_format=ROW


and did the same TRUNCATE on the master, monkey.tablex would not be truncated on the slave.

Note that this all applies according the binlog_format in use at the time the event is logged, and since binlog_format can be changed on a per-session basis, you can get some surprises unless you know exactly what's going on.

Having said all that, if the schema you're writing to does not even exist on the master, then you have nothing to worry about, other than the fact that you can't have read_only or super_read_only set on the slave if you plan on writing to it, so you can't count on it preventing write activity.