The server maintains the entire MOO database in main memory, not on disk. It is therefore necessary for it to dump the database to disk if it is to persist beyond the lifetime of any particular server execution. The server is careful to dump the database just before shutting down, of course, but it is also prudent for it to do so at regular intervals, just in case something untoward happens.
To determine how often to make these checkpoints of the database, the
server consults the value of #0.dump_interval. If it exists and its
value is an integer greater than or equal to 60, then it is taken as the number
of seconds to wait between checkpoints; otherwise, the server makes a new
checkpoint every 3600 seconds (one hour). If the value of
#0.dump_interval implies that the next checkpoint should be scheduled at
a time after 3:14:07 a.m. on Tuesday, January 19, 2038, then the server instead
uses the default value of 3600 seconds in the future.
The decision about how long to wait between checkpoints is made again
immediately after each one begins. Thus, changes to #0.dump_interval
will take effect after the next checkpoint happens.
Whenever the server begins to make a checkpoint, it makes the following verb call:
$checkpoint_started()
When the checkpointing process is complete, the server makes the following verb call:
$checkpoint_finished(success)
where success is true if and only if the checkpoint was successfully written on the disk. Checkpointing can fail for a number of reasons, usually due to exhaustion of various operating system resources such as virtual memory or disk space. It is not an error if either of these verbs does not exist; the corresponding call is simply skipped.