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.
