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<title>Berkeley DB Reference Guide: Terminology</title>
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></h3></td>
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<h1 align=center>Terminology</h1>
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<p>Here are some definitions that will be helpful in understanding
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<p><dt>Thread of control<dd>Berkeley DB is indifferent to the type or style of threads being used by the
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application; or, for that matter, if threads are being used at all --
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because Berkeley DB supports multiprocess access. In the Berkeley DB documentation,
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any time we refer to a <i>thread of control</i>, it can be read as
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a true thread (one of many in an application's address space) or a
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<p><dt>Free-threaded<dd>A Berkeley DB handle that can be used by multiple threads simultaneously
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without any application-level synchronization is called
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<p><dt>Transaction<dd>A <i>transaction</i> is a one or more operations on one or more
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databases that should be treated as a single unit of work. For example,
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changes to a set of databases, in which either all of the changes must be
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applied to the database(s) or none of them should. Applications specify
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when each transaction starts, what database operations are included in
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<p><dt>Transaction abort/commit<dd>Every transaction ends by <i>committing</i> or <i>aborting</i>.
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If a transaction commits, Berkeley DB guarantees that any database changes
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included in the transaction will never be lost, even after system or
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application failure. If a transaction aborts, or is uncommitted when
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the system or application fails, then the changes involved will never
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appear in the database.
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<p><dt>System or application failure<dd><i>System or application failure</i> is the phrase we use to
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describe something bad happening near your data. It can be an
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application dumping core, being interrupted by a signal, the disk
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filling up, or the entire system crashing. In any case, for whatever
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reason, the application can no longer make forward progress, and its
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databases are left in an unknown state.
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<p><dt>Recovery<dd><i>Recovery</i> is what makes the database consistent after a system
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or application failure. The recovery process includes review of log
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files and databases to ensure that the changes from each committed
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transaction appear in the database, and that no changes from an
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unfinished (or aborted) transaction do. Whenever system or application
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failure occurs, applications must usually run recovery.
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<p><dt>Deadlock<dd><i>Deadlock</i>, in its simplest form, happens when one thread of
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control owns resource A, but needs resource B; while another thread of
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control owns resource B, but needs resource A. Neither thread of
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control can make progress, and so one has to give up and release all
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its resources, at which time the remaining thread of control can make
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