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<p>
The Lock subsystem is useful outside the context of Berkeley
DB. It can be used to manage concurrent access to any
collection of either ephemeral or persistent objects. That is,
the lock region can persist across invocations of an
application, so it can be used to provide long-term locking
(for example, conference room scheduling).
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In order to use the locking subsystem in such a general way,
the applications must adhere to a convention for identifying
objects and lockers. Consider a conference room scheduling
problem, in which there are three conference rooms scheduled
in half-hour intervals. The scheduling application must then
select a way to identify each conference room/time slot
combination. In this case, we could describe the objects being
locked as bytestrings consisting of the conference room name,
the date when it is needed, and the beginning of the
appropriate half-hour slot.
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Lockers are 32-bit numbers, so we might choose to use the
User ID of the individual running the scheduling program. To
schedule half-hour slots, all the application needs to do is
issue a <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> call for the appropriate locker/object pair.
To schedule a longer slot, the application needs to issue a
<a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call, with one <a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> operation per half-hour
— up to the total length. If the <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call fails,
the application would have to release the parts of the time
slot that were obtained.
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To cancel a reservation, the application would make the
appropriate <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> calls. To reschedule a reservation, the
<a href="../api_reference/C/lockget.html" class="olink">DB_ENV->lock_get()</a> and <a href="../api_reference/C/lockput.html" class="olink">DB_ENV->lock_put()</a> calls could all be made inside of a
single <a href="../api_reference/C/lockvec.html" class="olink">DB_ENV->lock_vec()</a> call. The output of <a href="../api_reference/C/lockstat.html" class="olink">DB_ENV->lock_stat()</a> could be
post-processed into a human-readable schedule of conference
room use.
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