level-throttle

level-throttle is a dead-simple token-bucket-based throttling mechanism built on levelup.

level-throttle was initially designed to provide throttled access control to API endpoints, but can in fact be used to regulate access to any resource.

level-throttle works by simply counting the number of "tokens" remaining in the "bucket" associated with an arbitrarily-specified "throttle key". Each time a throttled resource is accessed, a token is removed from the associated bucket. If the number of tokens in the bucket reaches zero, the consumer has exhaused access to the throttled resource. The bucket is automatically refilled when appropriate.

Usage

Initializing

require and invoke the module while specifying some configuration options:

// boostrap level
var level = require('level');
var db    = level('./mydb', { valueEncoding : 'json' });

// initialize the `throttle` function
var throttle = require('level-throttle')({
  db        : db,
  namespace : 'the-app-name' ,
  limit     : 100,
  ttl       : 1000 * 60 * 60, // one hour
});

Whereby:

• db is any levelup-compliant handle.
• namespace is a namespace that should prefix all leveldb keys. This makes it possible to use this module to throttle multiple applications without the risk of interference among applications. It is probably sensible for namespace to be the name of the app itself.
• limit is the maximum number of throttled calls that may be made within ttl.
• ttl (time-to-live) is the interval after which the bucket should be re-stocked with tokens. Its value must be specified in milliseconds.

An Error will be thrown if any of the configs are invalid.

Invoking

Next, simply invoke the initialized function:

throttle('some-throttle-key', function(err, tokens) {
  // if `tokens` is `0`, respond with an HTTP `429`
});

The function does one thing: it reports the number of tokens remaining in the bucket for the specified throttle key. If there are 0 tokens remaining, the user has consumed all of his allocated throttle requests for the ttl.

Overriding Config

The ttl and limit values can optionally be overridden when the function is invoked, thus making it possible to set different access controls per throttle key. To do so, simply pass the function a map of override options thusly:

throttle('different-throttle-key', { ttl : 60000, limit: 1337 }, function(err, tokens) {
  // this user will be allotted 1337 tokens per minute
});

Bucket TTLs

ttl values will be passed to the underlying levelup handle with which level-throttle was initialized (db). This means that, if the handle makes use of a module like level-ttl to automatically purge expired buckets from the database, that expiration mechanism will work as expected. The use of such a module is not required, however.