Firebase rate limiting in security rules?

Question

I launched my first open repository project, EphChat, and people promptly started flooding it with requests.

Does Firebase have a way to rate limit requests in the security rules? I assume there's a way to do it using the time of the request and the time of previously written data, but can't find anything in the documentation about how I would do this.

The current security rules are as follows.

{
    "rules": {
      "rooms": {
        "$RoomId": {
          "connections": {
              ".read": true,
              ".write": "auth.username == newData.child('FBUserId').val()"
          },
          "messages": {
            "$any": {
            ".write": "!newData.exists() || root.child('rooms').child(newData.child('RoomId').val()).child('connections').hasChild(newData.child('FBUserId').val())",
            ".validate": "newData.hasChildren(['RoomId','FBUserId','userName','userId','message']) && newData.child('message').val().length >= 1",
            ".read": "root.child('rooms').child(data.child('RoomId').val()).child('connections').hasChild(data.child('FBUserId').val())"
            }
          },
          "poll": {
            ".write": "auth.username == newData.child('FBUserId').val()",
            ".read": true
          }
        }
      }
    }
}

I would want to rate-limit writes (and reads?) to the db for the entire Rooms object, so only 1 request can be made per second (for example).

Answer 1

The trick is to keep an audit of the last time a user posted a message. Then you can enforce the time each message is posted based on the audit value:

{
  "rules": {
          // this stores the last message I sent so I can throttle them by timestamp
      "last_message": {
        "$user": {
          // timestamp can't be deleted or I could just recreate it to bypass our throttle
          ".write": "newData.exists() && auth.uid === $user",
          // the new value must be at least 5000 milliseconds after the last (no more than one message every five seconds)
          // the new value must be before now (it will be since `now` is when it reaches the server unless I try to cheat)
          ".validate": "newData.isNumber() && newData.val() === now && (!data.exists() || newData.val() > data.val()+5000)"
        }
      },

      "messages": {
        "$message_id": {
          // message must have a timestamp attribute and a sender attribute
          ".write": "newData.hasChildren(['timestamp', 'sender', 'message'])",
          "sender": {
            ".validate": "newData.val() === auth.uid"
          },
          "timestamp": {
            // in order to write a message, I must first make an entry in timestamp_index
            // additionally, that message must be within 500ms of now, which means I can't
            // just re-use the same one over and over, thus, we've effectively required messages
            // to be 5 seconds apart
            ".validate": "newData.val() >= now - 500 && newData.val() === data.parent().parent().parent().child('last_message/'+auth.uid).val()"
          },
          "message": {
            ".validate": "newData.isString() && newData.val().length < 500" 
          },
          "$other": {
            ".validate": false 
          }
        }
      } 
  }
}

See it in action in this fiddle. Here's the gist of what's in the fiddle:

var fb = new Firebase(URL);
var userId; // log in and store user.uid here

// run our create routine
createRecord(data, function (recordId, timestamp) {
   console.log('created record ' + recordId + ' at time ' + new Date(timestamp));
});

// updates the last_message/ path and returns the current timestamp
function getTimestamp(next) {
    var ref = fb.child('last_message/' + userId);
    ref.set(Firebase.ServerValue.TIMESTAMP, function (err) {
        if (err) { console.error(err); }
        else {
            ref.once('value', function (snap) {
                next(snap.val());
            });
        }
    });
}

function createRecord(data, next) {
    getTimestamp(function (timestamp) {
        // add the new timestamp to the record data
        var data = {
          sender: userId,
          timestamp: timestamp,
          message: 'hello world'
        };

        var ref = fb.child('messages').push(data, function (err) {
            if (err) { console.error(err); }
            else {
               next(ref.name(), timestamp);
            }
        });
    })
}