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.

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

The core of this feature is a new table listing all of the calls that will be throttled.new throttle will use a similar mechanism, except that the keys will be userId+normalizedAPI so that each user can be throttled on each api call. 

The throttle limits for each api call will be defined by a SQL table, allowing administrators to set the throttling of any api calls dynamically.

Table Schema

This table would contain the api calls that administrators would want to throttle, the maximum number of calls per period, and the period of time after which the max calls would reset.

...

CREATE TABLE 'THROTTLED_CALLS'(
	'THROTTLE_ID' int(20) PRIMARY KEY,              --id of the throttle rule
	'NORMALIZED_CALLURI' varchar(256) NOT NULL,        --normalized api URL, numbers such as {id} replaced with #
	'MAX_LOCKS' int(20) NOT NULL,                   --maximum number of locks per time block
	'LOCK_TIMEOUT_SECONDS' int(20) NOT NULL,        --duration of each time block in seconds
	'THROTTLE_EXPIRATION' bigint(20) DEFAULT NULL,  --optional expiration of the rule in unix timestamp. maybe not necessary?
	UNIQUE ('NORMALIZED_CALL')
)

UserThrottleFilter will have a in-memory cached version of this table. It would periodically check the THROTTLED_CALLS table and update its cached version to reflect the information in the database.

The cached in the database.memory version will be a Map from the normalized uri to a pair of values for maxlocks and locktimeoutSeconds. 

Proposed ways to update the cached throtttles:

• By having UserThrottleFilter periodically read the table (via a DAO)
• Make the Map a synchronized singleton. Allowing a worker in the background update it.

To enforce the Throttle, UserThrottleFilter will use a singleton InMemoryTimeBlockCountingSemaphore, which has already been implemented.

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