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<h2 class="title" style="clear: both"><a id="bdb-concepts"></a>Berkeley DB Concepts</h2>
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<p>
If you are a SQLite user who is migrating to the BDB SQL interface,
then there are a few Berkeley DB-specific concepts you might want
to know about.
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Environments. The directory that is created
alongside your database file, and which ends with
the "-journal" suffix, is actually a Berkeley DB
environment directory.
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The Locking Subsystem
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The Berkeley DB library implements locking in a different
way to SQLite. SQLite implements locking at a database
level - any operation will take a lock on the entire
database. Berkeley DB implements a scheme called page
level locking. The database divides data into relatively
small blocks. Each block corresponds to a page in
database terms. Each block can contain multiple pieces
of user information. Berkeley DB takes locks on
individual pages. This allows for greater concurrency
in applications, but means that applications are more
likely to encounter deadlocks.
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See:
<a class="xref" href="lockingnotes.html" title="Chapter 2. Locking Notes">Locking Notes</a>
for more information.
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The Journal Subsystem
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The BDB SQL interface implements write ahead logging (WAL),
it stores journal files differently to the SQLite
WAL implementation. BDB SQL interface rolls over journal files
when they get to a certain size (default 10MB). It is
possible for the to be multiple journal files active
at one time with BDB SQL interface.
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