Tuesday, July 21, 2009

PBMS is transactional!

The PBMS engine now has built in support for transaction so that if you reference or dereference BLOBs during a transaction the changes you made will be committed or rolled back with the transaction. Up until now PBMS had depended on the host engine to handle transactions and inform it what needed to be done in the event of a rollback.

I have implemented transaction support by adding a circular transaction log and transaction cache. The circular transaction log is dynamic and can grow and shrink as required. The transaction records are all small (30 byte) fixed length records so the log doesn’t need to be that large to be able to handle a lot of transactions. A head and tail pointer are maintained to indicate where the head and tail of the circular log is. If the circular log becomes full then the records just overflow and are written to the end of the log. Once the transaction reader has caught up after an overflow has occurred it simply resizes the circular log to include the overflow and everything continues as normal with a larger circular log. The circular log will be reduced to its normal size once free space becomes available at the end of the log.

If anyone has any use for something like this them selves it has been written as a standalone module so it would not be to difficult to pull it out and build it into something else.


Paul McCullagh said...

Hi Barry,

This is great progress!

What is now involved in making an engine capable of streaming blobs?

Do you have a demo for download?

Barry Leslie said...

Hi Paul,

It is still a work in progress but I hope to have something finalized in he next couple of weeks after which I will make a demo version available.

I still want to do a bit more work on the engine level backup.