Baron Schwartz wrote: > If table meta-data is stored in tables, and InnoDB is the default > storage engine, is there any reason DDL should not be transactional?
I don't see why metadata must be stored in a table. Sure, given a flexible API, it *could* be, but storing the metadata in a storage engine's tables is just about persistence. We could just as easily persist the metadata in a file using proto buffer's Message.write(CodedOutputStream *out) methods. I think that something that is more important than transactional safety of DDL commands is having those commands be performed online, with no (or little) locking out of readers while the write operations are being performed. What enables online operations for DDL commands has to do with the way that metadata (I've started referring to this as catalog data) is managed in the kernel. If the catalog itself allows for versioning of catalog objects (e.g. a schema table definition), then a storage engine can rebuild or alter a schema table based on a newly supplied definition behind the scenes. When the rebuild is done, the catalog is notified of the update, locks the pointer to a catalog object and swaps the pointer with a newly updated one. Readers of the old version continue to see the old definition and may work on that definition and storage area. New readers will see the new definition and once no more readers of the old definition are around, the catalog can notify the storage engine that it can free up the resources that were dependent on the old definition. Of course, it's more complex than this, but this is the basic idea of online DDL operations... Proto buffers plays a central role in all of this. The generated classes from the .proto files will be passed to and from the storage engine to the catalog, effectively enabling online communication of changes in the schema table. </stream of thought> -jay _______________________________________________ Mailing list: https://launchpad.net/~drizzle-discuss Post to : [email protected] Unsubscribe : https://launchpad.net/~drizzle-discuss More help : https://help.launchpad.net/ListHelp
