Saturday 6 June 2009

Why the version hasn’t changed

Changes like this are possible in Pyrrho because of the distinctive way that Pyrrho is designed. Nearly all DBMSs store data structures like indexes in permanent storage within the files making up the database. Pyrrho by contrast places in permanent storage only the data required for durability of each committed transaction. This approach minimises disk activity during normal operations, to about one-seventieth of the disk activity of rival systems.
The server’s data structures are private to the server, and are initialised when the server starts up, and the server brings its state up to date by re-reading the database file, which is just the transaction record. This is a time-consuming process for a large database, equivalent to a cold-start with resynchronisation step for other products.
The format of this data is as far as possible version and platform-independent – there is not even any assumption for what “double precision” means, or how many bits make up a long integer. This means that every aspect of the organisation of data structures can evolve without breaking backward compatibility. Database files created by version 0.1 can still be used in every version of Pyrrho, so all Pyrrho data bases can always be managed by the latest version of the server. New features, such as the recently introduced concept of provenance, or URI-based data types, introduce new or modified data formats which would not be understood by earlier versions, but they really should not be in use. Upgrading to the latest version is always recommended, and is free of charge.

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