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If there are many calls being throttled, the throttle could potentially use up a lot of memory. With N throttled calls and M users, the throttle's map for call counts could have up to M x N entires. Additionally, the map will not remove entries for users that are are no longer making calls so memory will not be freed until an administrator calls clearAllLocks().
Updates to the throttle rules will not immediately take effect because they are only written into the SQL table will not be immediately reflected in the UserThrottleFilter. . The actual enforcement of the throttle will not happen until UserThrottleFilter updates its cached version of the rules.