The purpose of this feature is to allow administrators to selectively control which API calls are throttled, without having to rebuild and deploy a new version every time a throttle is changed.

Current throttle implementation

The current frequency throttle is only able to throttle each user's total call frequency using a semaphore. The UserThrottleFilter uses an InMemoryTimeBlockCountingSemaphore that maps from a key(user id in this case) to a SimpleSemaphore, which keeps a count of calls made and the the after which the count will reset. The UserThrottleFilter then compares the user's call count to the maximum allowed.

New throttle implementation

The new throttle will employ a similar mechanism, except that the keys will be userId+normalizedAPI so that each user can be throttled on each API call. 

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When an request comes in, the request URI will be normalized using normalizeMethodSignature() in AccessRecordUtils of the Synapse-Warehouse-Records project (copy it over?).

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