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The Shadow Pager is a design for a replacement to pager.c -- the objective is to extend SQLite with the ability to run read and write transactions concurrently. In particular, this design provides the following:

- Read transactions are never blocked by any other transactions

- Write transactions are never blocked by Read transactions

So, the only blocking that takes place is that Write transactions block other Write transactions.

The cost of this new freedom for read transactions is data duplication. Write transactions always write to free disk pages, so the written data cannot interfere with read transactions. The space taken by the old version(s) of the data cannot be reclaimed until the read transaction(s) begun before the new versions were committed complete. So, the amount of data duplication is application dependent, but is expected to be small in practice.

Note that since the prior version of all written data remains in the database, there is no need for a journal file. So in the best case, with quick read transactions and therefore low data duplication, the shadow pager will use the same disk space as the present journaling SQLite implementation.

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Attachments:

• shadow.pdf 55188 bytes added by anonymous on 2004-Mar-31 03:50:15 UTC.