.\"
.\"	aegis - project change supervisor
.\"	Copyright (C) 1996, 1998, 1999 Peter Miller;
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.\" MANIFEST: document describing z_bl_lock.so
.\"
.SH THE BASELINE LOCK
The baseline lock is used to ensure that the baseline remains in a
consistent state for the duration of commands which need to read the
contents of files in the baseline.
.PP
The commands which require the baseline to be consistent
(these include the
.IR aeb (1),
.IR aecp (1)
and
.IR aed (1)
commands)
take a baseline
.I read
lock.
This is a non-exclusive lock,
so the concurrent development of changes is not hindered.
.PP
The command which modifies the baseline,
.IR aeipass (1),
takes a baseline
.I write
lock.
This is an exclusive lock,
forcing 
.IR aeipass (1)
to block until there are no active baseline read locks.
.PP
It is possible that
one of the above development commands
will block until an in-progress
.I "\*(n) -Integrate_PASS"
completes.
This is usually of short duration while the project history is updated.
The delay is essential so that these commands receive a consistent view
of the baseline.
No other integration command will cause the above development commands
to block.
.PP
When \*(n)' branch functionality is in use,
a read (non-exclusive) lock is taken on the branch baseline and also
each of the "parent" baselines.
However,
a baseline write (exclusive) lock is only taken on the branch baseline;
the "parent" baselines are only read (non-exclusive) locked.