add X-RateLimit-* headers according to industry standards

Author: artch

lib/middleware.js

@@ -1,4 +1,6 @@
-var rateLimiter  = require('./rate-limiter');
+var rateLimiter = require('./rate-limiter');
+
+var defaultFailMessage = 'Rate limit exceeded, retry after {after}ms';
 
 module.exports = function(opts) {
   var limiter = rateLimiter(opts);
@@ -7,8 +9,24 @@ module.exports = function(opts) {
       if (err) {
         next();
       } else {
+        res.set({
+          'X-RateLimit-Limit': rate.limit,
+          'X-RateLimit-Remaining': rate.current > rate.limit ? 0 : rate.limit - rate.current,
+          'X-RateLimit-Reset': (rate.reset / 1000) | 0
+        });
         if (rate.current > rate.limit) {
-          res.writeHead(429);
+          var after = rate.reset - Date.now();
+          var failMessage = defaultFailMessage;
+          res.set('Retry-After', (after / 1000) | 0);
+          if(opts.failMessage) {
+            if(typeof opts.failMessage === 'function') {
+              failMessage = opts.failMessage(req);
+            }
+            else {
+              failMessage = opts.failMessage;
+            }
+          }
+          res.status(429).send(failMessage.replace(/{after}/g, after));
           res.end();
         } else {
           next();

lib/rate-limiter.js

@@ -10,21 +10,27 @@ module.exports = function(opts) {
          .setex(tempKey, opts.window(), 0)
          .renamenx(tempKey, realKey)
          .incr(realKey)
-         .ttl(realKey)
+         .pttl(realKey)
          .exec(function(err, results) {
            if(err) {
              callback(err);
            } else {
-             if (results[3] == -1) {  // automatically recover from possible race condition
-               opts.redis.expire(realKey,opts.window());
+             var reset, ttl = results[3];
+             if(ttl == -1) {  // automatically recover from possible race condition
+               opts.redis.expire(realKey, opts.window());
+               reset = Date.now() + opts.window() * 1000;
+             }
+             else {
+               reset = Date.now() + ttl;
              }
              var current = results[2];
              callback(null, {
                key: key,
                current: current,
                limit: opts.limit(),
                window: opts.window(),
-               over: (current > opts.limit())
+               over: (current > opts.limit()),
+               reset: reset
              });
            }
          });