Rails and MySQL go hand in hand. ActiveRecord is perfectly capable of using a number of different databases but MySQL is by far the most popular choice for production deployments. And therein lies a dirty secret: when it comes to performance and 'scalability' of the framework, the Ruby MySQL gem is a serious offender. The presence of the GIL means that concurrency is already somewhat of a myth in the Ruby VM, but the architecture of the driver makes the problem even worse. Let's take a look under the hood.Dissecting Ruby MySQL drivers
The native mysql gem many of us use in production was designed to expose a blocking API: you issue a SQL query, and the library blocks until the server returns a response. So far so good, but unfortunately it also introduces a nasty side effect. Because it blocks inside of the native code (inside mysql_real_query() C function), the entire Ruby VM is frozen while we wait for the response. So, if you query happens to have taken several seconds, it means that no other block, fiber, or thread will be executed by the Ruby VM. Ever wondered why your threaded Mongrel server never really flexed its threaded muscle? Well, now you know.
Fortunately, the little known mysqlplus gem addresses the immediate problem. Instead of using a single blocking call, it forwards the query to the server, and then starts polling for the response. For the curious, there are also two implementations, one in pure Ruby with a select loop, and a native (C) one which uses rb_thread_select. The benefit? Well, now you can have multiple threads execute database queries without blocking the entire VM! In fact, with a little extra work, we can even get some concurrency out of ActiveRecord.http://www.igvita.com/2010/04/15/non-blocking-activerecord-rails/?utm_source=feedburner&utm_campaign=Feed%3A+igvita+%28igvita.com%29&utm_content=feed
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