Rolling Rate Limiter

Description

This is an implementation of a rate limiter in node.js that allows for rate limiting with a rolling window.

This means that if a user is allowed 5 actions per 60 seconds, any action will be blocked if 5 actions have already occured in the preceeding 60 seconds, without any set points at which this interval resets. This contrasts with many existing implementations, in which a user could make 5 requests at 0:59 and another 5 requests at 1:01.

It can use either in-memory storage or Redis as a backend. If Redis is used, multiple rate limiters can share one instance with different namespaces, and multiple processes can share rate limiter state safely without race conditions. The implementation uses what I believe to be a novel algorithm, with sorted sets.

Examples

In-memory

  
  /*
    Setup:
  */

  var RateLimiter = require("rolling-rate-limiter");

  var limiter = RateLimiter({
    interval: 1000 // in miliseconds
    maxInInterval: 10,
    minDifference: 100 // optional: the minimum time (in miliseconds) between any two actions
  });

  /*
    Action:
  */

  function attemptAction(userId) {

    // Argument should be a unique identifier for a user if one exists.
    // If none is provided, the limiter will not differentiate between users.
    var timeLeft = limiter(userId) 
    
    if (timeLeft > 0) {

      // limit was exceeded, action should not be allowed
      // timeLeft is the number of ms until the next action will be allowed
      // note that this can be treated as a boolean, since 0 is falsy
    
    } else {
    
      // limit was not exceeded, action should be allowed
    
    }

  }

  /*
    Note that the in-memory version can also operate asynchronously.
    The syntax is identical to the redis implementation below.
  */

With a redis backend

This allows multiple processes (e.g. multiple instances of a server application) to use a single redis to share rate limiter state. Make sure that the limiters have identical configurations in each instance.

  
  /*
    Setup:
  */

  var RateLimiter = require("rolling-rate-limiter");
  var Redis = require("redis");
  var client = Redis.createClient(config);

  var limiter = RateLimiter({
    redis: client,
    namespace: "UserLoginLimiter", // optional: allows one redis instance to handle multiple types of rate limiters. defaults to "rate-limiter-{string of 8 random characters}"
    interval: 1000,
    maxInInterval: 10,
    minDifference: 100
  });

  /*
    Action:
  */
  
  function attemptAction(userId, cb) {
    limiter(userId, function(err, timeLeft, actionsLeft) {
      if (err) {
        // redis failed or similar.
      } else if (timeLeft) {
        // limit was exceeded, action should not be allowed
      } else {
        // limit was not exceeded, action should be allowed
      }
    });
  }

As a middleware

You can easily use this module to set up a request rate limiter middleware in Express.

  var limiter = RateLimiter({
    redis: redisClient,
    namespace: "requestRateLimiter",
    interval: 60000,
    maxInInterval: 100,
    minDifference: 100
  });

  app.use(function(req, res, next) {

    // "req.ipAddress" could be replaced with any unique user identifier
    // Note that the limiter returns the number of miliseconds until an action
    // will be allowed.  Since 0 is falsey, this can be treated as a boolean.
    limiter(req.ipAddress, function(err, timeLeft) {
      if (err) {
        return res.status(500).send();
      } else if (timeLeft) {
        return res.status(429).send("You must wait " + timeLeft + " ms before you can make requests.");
      } else {
        return next();
      }
    });

  });

Method of operation

• Each identifier/user corresponds to a sorted set data structure. The keys and values are both equal to the (microsecond) times at which actions were attempted, allowing easy manipulation of this list.
• When a new action comes in for a user, all elements in the set that occurred earlier than (current time - interval) are dropped from the set.
• If the number of elements in the set is still greater than the maximum, the current action is blocked.
• If a minimum difference has been set and the most recent previous element is too close to the current time, the current action is blocked.
• The current action is then added to the set.
• Note: if an action is blocked, it is still added to the set. This means that if a user is continually attempting actions more quickly than the allowed rate, all of their actions will be blocked until they pause or slow their requests.
• If the limiter uses a redis instance, the keys are prefixed with namespace, allowing a single redis instance to support separate rate limiters.
• All redis operations for a single rate-limit check/update are performed as an atomic transaction, allowing rate limiters running on separate processes or machines to share state safely.