Question

I am developing a Codeigniter (2.0.2) Application, which will utilise a Master database for all write operations (INSERT/UPDATE/DELETE) and a read replica for all read operations (SELECT).

Now I know I can access two different database objects within the code to route the individual requests to the specific database server, but i’m thinking there has a better way, automated way. I’ll be using MySQL and Active Record, and also want to build in Memcache checking – although it won’t be used immediately, I’d like the option there for the future, built in at this stage.

I’m thinking if its possible to add a hook/library of some kind to intercept the $this->db->query so that the following happens:

1) SQL Query received

2) Check if SELECT query

2a) If SELECT, see if Memcache is active, if so encode SQL and check Memcache for response.

2b) If no memcache response, or Memcache is not active, execute query as normal through READ MySQL server.

3) Query was NOT select, so execute query as normal through the WRITE MySQL server.

4) Return response.

I’m sure that looking at this, it should be quite simple to do, but no matter how I look at it i’m just not seeing a potential answer – but there’s got to be one! Can anyone help/assist?

In addition, I also want the ability to be able to log all write SQL commands for troubleshooting, presumably the best way is to introduce 3a) Write SQL command to plain text file … into the above scheme. I don’t believe MySQL actually logs the non-SELECT queries in anyway … does it?

Answer 1

That type of behavior is a little bit beyond the normal scope of CI. Unfortunately, your best bet is to manually extend the database drivers, specifically override the function simple_query or _execute (simple_query is a wrapper around _execute which simply ensures initialization). That is really the only place where you can guarantee that you can catch all of the queries and branch the logic accordingly. (You may also want to override close as that is the cleanup script)

(Personally, I would have a the SELECT DB load a secondary DB into itself and just call $write_db->simple_query conditionally, that seems like it would be the least trouble).