Thursday, July 03, 2008

Update on SimpleDBM's Data Dictionary

I have chosen to implement option 3 as described in a previous post. For each table, a special container is created, which is used to store not only the table structure but also structures of all associated indexes. Special containers are created for indexes as well but these contain pointers to the table structures. (The reason we need the index definition containers is that during recovery an index update may be performed prior to a table update.)

A memory cache is maintained of the entire data dictionary. When a new table is created, its structure is immediately saved into the memory cache. The structure is also persisted using a PostCommitAction - which is a type of redo only log that is executed once the transaction commits.

At system restart, the storage system is scanned for containers that have a specific naming convention and are in a well known location. This way, SimpleDBM is able to initialize the memory cache at startup. Note that this initialization occurs post recovery. The list of open containers is consulted to ensure that only those definitions are loaded that are relevant.

There is a problem that during recovery, any redo/undo operation on a table or index will require access to the table/index definition. Since the data dictionary memory cache is not available at this stage - it is populated after recovery, the solution is to retrieve the definitions on demand.

The table/index creation process uses a standalone transaction. During this time the container IDs are exclusively locked. This ensures that by the time any subsequent transaction performs a data manipulation operation against the table, the data dictionary memory cache has been updated to contain the table definition.

At present tables cannot be dropped, as this has not yet been implemented.

Along with the Data Dictionary implementation, there is also a high level Database API that is available. This hasn't been released yet as it is still being tested. I am hoping that a fully tested version will be available towards the later part of this year.

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