I work on a few high traffic websites that all share the same database and that are all heavily database driven. Our SQL server is max-ed out and, although we have already implemented many changes that have helped but the server is still working too hard.

We employ some caching in our website but the type of queries we use negate using SQL dependency caching.

We tried SQL replication to try and kind of load balance but that didn't prove very successful because the replication process is quite demanding on the servers too and it needed to be done frequently as it is important that data is up to date.

We do use a Varnish web caching server (Linux based) to take a bit of the load off both the web and database server but as a lot of the sites are customised based on the user we can only do so much.

Anyway, the reason for this question... Varnish gave me an idea for a possible application that might help in this situation.

Just like Varnish sits between a web browser and the web server and caches response from the web server, I was wondering about the possibility of creating something that sits between the web server and the database server.

Imagine that all SQL queries go through this SQL caching server. If it's a first time query then it will get recorded, and the result requested from the SQL server and stored locally on the cache server. If it's a repeat request within a set time then the result gets retrieved from the local copy without the query being sent to the SQL server. The caching server could also take advantage of SQL dependency caching notifications.

This seems like a good idea in theory. There's still the same amount of data moving back and forward from the web server, but the SQL server is relieved of the work of processing the repeat queries.

I wonder about how difficult it would be to build a service that sort of emulates requests and responses from SQL server, whether SQL server's own caching is doing enough of this already that this wouldn't be a benefit, or even if someone has done this before and I haven't found it? I would welcome any feedback or any references to any relevant projects.