Following on from my post yesterday on the various states of a Storage Engine, I said I’d have a go with the Cursor object too. A Cursor is used by the Drizzle kernel to get and set data in a table. There can be more than one cursor open at once, and more than one per thread. If your engine cannot cope with this, it is its responsibility to figure it out and return the appropriate errors.

Let’s look at a really simple operation, inserting a couple of rows and then reading them back via a full table scan.

Now, this graph is slightly incomplete as there is no doEndTableScan() call. But you can see in which order things are meant to happen. In this case, “store_lock()” means that store_lock() has been called, so when coming back from doInsertRecord() we do not call store_lock() again, rather, we’re just in a state where it has already been executed.

For MySQL handler, think ::write_row() for doInsertRecord() and ::rnd_init() for doStartTableScan().

This diagram was again auto-generated from my test engine.