A generic limiter library; includes basic client identity handling
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We often want to restrict access to the ressources a web application publishes; for example, you want subscribers to have unlimited access, guest users to enjoy a realistic peek into your offerings without handing over the full power of your application and the data therein, and at the same time deter any automated downloads. One part of the equation that can make this happen is User Authentication; the other is identifying anonymous visitors and selectively throttle or block their access, based on automatically collected behavior details and / or manual black- and whitelisting.
A limiter should be completely agnostic as to the task it is limiting—it should help to generally put limits to ressource usage, not, say, be built to specifically limit HTTP requests (a drop-in middleware could do that–one that uses a generic limiter).
Because of its genericity, a limiter should work with arbitrary ID tokens that represent clients—these could be any piece of data (an internal user ID, an IP address, a session ID) that is fit to identify a client in the sense of the application.
Since a limiter should be able to base its behavior on the past behavior of a given client and on the client's ranking (say, customer vs. guest vs. rogue), it must either entertain a suitable datastructure to keep such group affiliation and usage data (or else be fed (some of) these details when called).
Whether or not a limiter should take care of data persistence is an open question; ideally, server restart or redirection to another server process should not impact limiter behavior (this consideration would appear to favor Redis-like data persistence plans).
A limiter should distinguish between and be configurable for at least three behavioral patterns: full access, deferred (throttled) access, and denial of service (the last two with timeouts and permanent).