Rolling Rate Limiter

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

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 some other rate limiter implementations, in which a user could make 5 requests at 0:59 and another 5 requests at 1:01.

Important Note: As a consequence of the way the Redis algorithm works, if an action is blocked, it is still "counted". 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.

This behavior is somewhat counterintuitive, but it's the only way that I have found that uses an atomic MULTI set of commands for Redis. Without this, race conditions would be possible. See more below..

Upgrading from 0.1

Version 0.2 was released August 31 2020. The method of operation remains the same, but the API has changed. A short summary of the changes:

• Library was rewritten in Typescript.
• Rate limiters are now instances of a RateLimiter class.
• Methods now use promises instead of callbacks.
• A wouldLimit method is now available to see if an action would be blocked, without actually "counting" it as an action.
• limitWithInfo and wouldLimitWithInfo methods are available to return more information about how and why an action was blocked or not blocked.
• Tests were rewritten in Jest, and run on both redis and ioredis clients.

Quick start

Basic use in an Express application.

const { RedisRateLimiter } = require('rolling-rate-limiter');

const limiter = new RedisRateLimiter({
  client: redisClient, // client instance from `redis` or `ioredis`
  namespace: 'rate-limiter', // prefix for redis keys
  interval: 60000, // milliseconds
  maxInInterval: 10,
});

app.use(function(req, res, next) {
  limiter.limit(req.ipAddress).then((wasBlocked) => {
    if (wasBlocked) {
      return res.status(429).send("Too many requests");
    } else {
      return next();
    }
  })
});

Available limiters

• RedisRateLimiter - Stores state in Redis. Can use redis or ioredis clients.
• InMemoryRateLimiter - Stores state in memory. Useful in testing or outside of web servers.

Configuration options

• interval: number - The length of the rate limiter's interval, in milliseconds. For example, if you want a user to be able to perform 5 actions per minute, this should be 60000.
• maxInInterval: number - The number of actions allowed in each interval. For example, in the scenario above, this would be 5
• minDifference?: number - Optional. The minimum time allowed between consecutive actions, in milliseconds.
• client: Client (Redis only) - The Redis client to use.
• namespace: string (Redis only) - A string to prepend to all keys to prevent conflicts with other code using Redis.

Instance Methods

All methods take an Id, which should be of type number | string. Commonly, this will be a user's id.

• limit(id: Id): Promise<boolean> - Attempt to perform an action. Returns false if the action should be allowed, and true if the action should be blocked.
• wouldLimit(id: Id): Promise<boolean> - Return what would happen if an action were attempted. Returns false if an action would not have been blocked, and true if an action would have been blocked. Does not "count" as an action.
• limitWithInfo(id: Id): Promise<RateLimitInfo> - Attempt to perform an action. Returns whether the action should be blocked, as well as additional information about why it was blocked and how long the user must wait.
• wouldLimitWithInfo(id: Id): Promise<RateLimitInfo> - Returns info about what would happened if an action were attempted and why. Does not "count" as an action.

RateLimitInfo contains the following properties:

• blocked: boolean - Whether the action was blocked (or would have been blocked).
• blockedDueToCount: boolean - Whether the action was blocked (or would have been blocked) because of the interval and maxInInterval properties.
• blockedDueToMinDifference: boolean - Whether the action was blocked (or would have been blocked) because of the minDistance property.
• millisecondsUntilAllowed: number - The number of milliseconds the user must wait until they can make another action. If another action would immediately be permitted, this is 0.
• actionsRemaining: number - The number of actions a user has left within the interval. Does not account for minDifference.

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.

Local development

Installation

Install dependencies with yarn.

To run tests, you will need to have a Redis server running. You can do this by installing Redis, and running redis-server. Alternatively, you can run the CI build, which includes tests, by installing act. This requires Docker to be running.

Testing

• yarn ci: Runs the CI build, including linting, type checking, and tests. Requires act to run GitHub actions locally.
• yarn lint: Runs ESLint.
• yarn test: Runs Jest.
• yarn typecheck: Runs TypeScript, without emitting output.
• yarn build: Runs TypeScript and outputs to ./lib.