Hi, I’m Rob Hawkes and I’m here today to give an inside look at the development of Rawkets, my HTML5 and JavaScript multiplayer space shooter.

Throughout this talk I plan to take a journey through some of the issues that plagued early development of the game, and cover the subsequent solutions that helped resolve them.

There’ll be some code, but I’m more interested in highlighting the concepts and theories involved.

This talk is definitely light-hearted but I’ll be assuming some prior knowledge, so feel free to raise your hand throughout the talk if you want to explore an area further, or to simply ask a question.

For those who don’t know much about about me…

I’m a Technical Evangelist at Mozilla, which means that it’s my job to engage with developers about cool new technologies on the Web.

Created by Phil Banks (@emirpprime)

Aside from that I spend most of my time experimenting with HTML5 and other cool technologies.

It’s fun!

EXPERIMENTATIONRawkets is a graduate from my lab

Rawkets is a project that originally came out of this experimentation, from a desire to learn more about WebSockets in regards to multiplayer gaming.

Now, the game is much more mature and I’d consider it as a separate entity aside from the experiments. It’s something that I plan to develop and support way beyond the original scope of learning WebSockets.

WHAT IS RAWKETS?Rawkes, WebSockets, and Rockets

Rawkets stands for Rawkes (my blog), WebSockets, and Rockets.

Rawkets is a multiplayer space game that allows you to shoot your friends in the face with HTML5 technologies.

Still not really at a beta release level yet, hence the bugs you might notice in this video.

http://rawkets.com

By now you’ve probably realised that the graphics at the beginning of this talk and on the posters aren’t the real game graphics.

They are actually an awesome “artists impression” illustration that I commissioned a guy called Reid Southen to create, although maybe when WebGL gets better it will become a reality. Who knows.

It looks pretty awesome as a 6ft banner. So awesome in fact that my girlfriend actually asked me if I was going to put it up in our flat our not. She seemed pretty down about me saying no (it smells of ink and solvents).

This is a photo of me in front of the banner at my university end-of-year show. If you think it looks small then let me put it into perspective by telling you that it’s about 8ft away. Read into that what you will.

TECHNOLOGYSome of the tech involved in Web gaming

There are a few key technologies that are involved in the development of the game.

All of them bar one are tightly related to HTML or JavaScript.

CANVAS2D graphics platform

Canvas for 2D graphics.

FLASH AUDIOSound effects and background music

Flash audio for game sound effects and background music.

I’ll explain why I use Flash over HTML5 Audio further on in the talk.

WEBSOCKETSMultiplayer communication

WebSockets is used for the communication between each player and the game server.

For anyone not up to speed with WebSockets, it’s an upgraded HTTP connection that allows for fully bi-directional streaming communication.

NODE.JSGame logic and communication

Node is used as the game server, controlling the logic and handling the WebSockets connections to the players.

It will eventually be used for player authentication and the storage of data so gameplay can persist over multiple game sessions.

This is all made relatively easy with great third-party modules, like Socket.IO for WebSockets, and others that handle Redis and MongoDB for storage, for example.

ISSUESMaking a game can be a challenge

It’s not all plain sailing when making a gaming using HTML5 and JavaScript.

I’m going to cover a few of the main issues that tripped me up during the development of Rawkets.

NETWORKINGNot as easy as I thought

Issues with networking have plagued development of the game right from the beginning.

This probably stems from my lack of prior experience with socket connection and multiplayer gaming in general.

In the original prototype of the game the network communication was woefully simple and everything was transmitted in a verbose format with no further thought.

In hindsight it’s obvious why I was experiencing massive performance issues with the network communication. I was basically sending way too much data back and forth.

MESSAGE PROTOCOLStructured and short communication

One of the ways that I solved these problems was by implementing a structured protocol for the messages that are being sent and received.

This included assigning each message type a number and using enumeration to represent those types in the code.

ENUMERATIONMessage protocol

types = {

PING: 1,

SYNC: 2,

SYNC_COMPLETED: 3,

NEW_PLAYER: 4,

UPDATE_PLAYER: 5,

UPDATE_INPUT: 6,

REMOVE_PLAYER: 7

};

By enumerating the messages types like this I was able to refer to them in a verbose format within the code, but benefit from only sending the one or two digit number when transmitting a message.

This is only possible if both the client and server follow the same protocol in regards to which number refers to which message type.

It’s a simple but effective solution and allowed me to cut a large number of characters from transmitted messages.

PACKAGEMessage protocol

{

z: 1,

id: 1234567890,

s: {

x: 5,

y: 34,

v: 3,

a: 0.46

}

}

Put together with the message types, a full message package is put together as a JSON representation of a JavaScript object.

All the other pieces of data are attached to the object with a key that is as short as possible.

The z key that you can see above is a reserved key that is used solely for the message type.

The other keys in this example are the player id and the player state.

COMPRESSIONReducing data as much as possible

Data in WebSockets is transmitted as verbose plain text, so it’s important to cut down and compress it as much as possible.

Some of the ways that I’ve done this include rounding numerical values, reducing the length of words if they’re only used for reference, and generally removing any data that isn’t necessary.

There is also BISON, which is a binary representation of JSON that can cut down the amount of data sent back and forth even further.

http://kaijaeger.com/articles/introducing-bison-binary-interchange-standard.html

RATE LIMITINGCutting down on communication

Aside from the message protocol, one of the biggest issues with networking has been dealing with the sheer number of messages being sent back and forth during the lifetime of a game.

1 1

MESSAGES IN

1MESSAGES OUT

1

Having only one player in the game is easy, you have one message coming in to the server, saying the player has moved, for example, and one message coming back out, updating the player with details from the server.

1 2

2 1

MESSAGES IN

2MESSAGES OUT

4

So say we now have two players, there is still only 1 message in from each player, but now each player receives 2 messages back from the server; one for them, and one for the other player.

This isn’t too bad, but notice how the server is having to send 4 messages – 2 for each player.

1 4

4 1

1

4

4

1

MESSAGES IN

4MESSAGES OUT

16

4 players now, look how the server is having to send 16 messages, yet it only receives 4.

If you haven’t already noticed, the messages out from the server are the square of the number of players.

But 16 messages out is alright, it’s hardly going to tax the server.

MESSAGES IN

30MESSAGES OUT

9001 30

30 1

1

30

30

1

So imagine if we now move into properly multiplayer territory.

30 players in the game would mean 30 messages coming in to the server, and 900 – NINE HUNDRED – messages going out, every update. That’s a silly amount of data for just 30 people.

But let’s go further still…

MESSAGES IN

100MESSAGES OUT

100001 100

100 1

1

100

100

1

Say we go massively multiplayer and have 100 players in the game at one time.

It’s not so bad for each individual player, they send one message in and get 100 back – that’s bearable.

But check out the server, it’s getting 100 messages in and is having to send out 10,000 back, every update. That’s just a mentally stupid number that’s going to cause a lot of grief.

INTELLIGENCELetting the game prioritise messages

Fortunately there is a way around this that cuts down the amount of messages sent; you just need to send data only for players visible to another player, in essence filtering out game data that doesn't affect the current player.

Another trick I used is to only send updates when a player is active and moving. If they haven’t moved since the last frame and nothing else has changed then why bother sending an update and wasting bandwidth?

These are such simple solutions, but ones that I never even considered at first.

RESPECTING TCPWebSockets uses TCP. Deal with it

Something else that I discovered is important to be aware of when making a game with WebSockets is that you’re using TCP.

This is a problem as such, but it means that you need to play by a certain set of rules, and to expect a certain set of issues.

By the way, I should point out that that you could argue that the icon that I’ve used could represent WebSockets, but that’s not why I used it. It’s the US plug symbol and I just thought it was funny because it looks like a surprised face. The UK plug symbol is boring in comparison.

OBEY THE ORDERYou can’t do much about it

One issue with TCP is that packets will come through in order and get queued up if there are any significant connection issues.

This can be a problem with a real-time game as it can cause hang-ups in the transmission and subsequently a hang-up in the graphic display.

In short, the ordering issue can result in jumpy gameplay. Not fun.

With UDP this wouldn’t be so much of a problem, but we don’t have that luxury yet.

FORCED DELAYGiving time for mistakes to be corrected

I’ve not attacked the TCP issues head on yet, but one possible way to approach them is to introduce some sort of delay into the game.

What I mean by this is that you would give every player a 100ms buffer in communication, meaning that they are always 100ms behind the server in regards to what is going on.

If everyone is experiencing this buffer then it shouldn’t be too much of a problem, and 100ms is plenty of time for the game to catch up with resent packets and the like.

As for how to implement this, I’ve not looked into it yet.

USE SOCKET.IOIt intelligently drops messages

I use Socket.IO for all the WebSockets communication for Rawkets simply because it’s easy to use and it has Flash fallback for browsers that don’t support the technology yet.

The cool thing about the latest version of Socket.IO, 0.7, is that is has introduced the concept of volatile messages.

These volatile messages are completely voluntary, and messages sent with them aren't actually sent if the client is having network problems, saving bandwidth for other more important messages.

It’s not an ideal solution, but it’ll certainly help.

CHEATERSA blessing and a curse

There’s no denying it, your code is going to be visible to anyone who wants to look at the source.

I experienced this early on in the development of the game with players adding in their own features, like invincibility, epic speed, rapid-fire, and even creating completely new weapons like cluster bombs!

Now don’t get me wrong, I actually appreciate the cheaters because they highlighted all the errors of my ways, for free. One of the benefits of the open nature of JavaScript is that it can be looked at and poked very easily by others, which means that I can fix bugs quicker than if I was testing on my own.

GLOBALS & CLOSURESKeeping the code wide open is bad

There are two reasons why cheating was so prevalent and so easy to do.

The first is that by keeping all the game code in the global namespace and not using any closures, I was practically inviting people to come in and edit the game code. It was too easy to do!

It was so easy in fact that after a few hours of releasing the first prototype, players were already sharing code snippets that others could paste into their browser console to get new features. Annoying, but pretty cool.

(function() {

var rawkets = rawkets || {},

r = rawkets;

rawkets.namespace = function(namespace_str) {

var parts = namespace_str.split("."),

parent = rawkets,

i;

if (parts[0] === "rawkets") {

parts = parts.slice(1);

};

for (i = 0; i < parts.length; i++) {

if (typeof parent[parts[i]] === "undefined") {

parent[parts[i]] = {};

};

parent = parent[parts[i]];

};

return parent;

};

window.rawkets = window.r = rawkets;

})(window);

By adding my own “rawkets” namespace I was able to hide code away, and by deliberately utilising closures and private variables I was able to further frustrate efforts by cheaters to overwrite game functionality.

Plus the new namespace makes code so much neater.

Code manipulation isn’t something that I can prevent entirely, but I can at least make it as difficult as possible.

CLIENT AUTHORITYPower isn’t always a good thing

I’m not going to lie, the first version of Rawkets was way too trusting.

I used what is referred to as the authoritative client model, which basically means that the client, the player, made all the decisions regarding its position and then sent those positions to the server.

The server than trusted those positions and transmitted them to all the other players, which is fine until the client edits their position and increments it by 100 pixel per frame, rather than 5. Bad times.

This can be referred to as the “Here I am” approach.

SERVER AUTHORITYRelinquish that power

The solution is to make the server authoritative, which means that you prevent manipulation of the client’s code from doing any damage.

All the movement logic is now performed on the server, meaning that when a client moves it simply lets the server know which direction it wants to move. From there the server calculates the new position and sends it back to the client.

This can be referred to as the “Where am I?” approach, and if done right it can completely remove the ability to cheat.

INHERENT LATENCYServer authority

Input Move Update+0 +40 +80

40ms 40ms

80ms total round-trip

However, the problem with the authoritative server model is that there is some inherent latency within the system.

What I mean by this is that it obviously takes some time for a movement to be sent from the client to the server, then for the server to move the client, and then for the server to send the new position back again.

In the example here imagine that there is a 40ms latency between the client and server, which means that a message sent to the server will take a total of 80ms to make the round-trip.

The problem here is what happens during that 80ms period that you’re waiting for the updated position? If you do nothing then there’s going to be an 80ms delay between you pressing the up arrow and your rawket moving forward. Not good.

CLIENT PREDICTIONServer authority isn’t enough

To solve the latency issues with the authoritative server you need to implement some element of prediction on the client.

What I mean by prediction is an ability for the client to guess, quite accurately, where it should move the player before the message comes back from the server detailing the new position.

INSTANT MOVEMENTClient prediction

Input Move Update+0 +40 +80

40ms 40ms

Prediction happens here

The prediction happens as soon as the client performs some sort of movement (a key-press, etc), before the server has received the input.

All the prediction does is run the same physics as the server, based on the new input.

This is exactly as if were using the authoritative client model, apart from one important difference.

CORRECTIONWhen prediction goes wrong

Whereas the authoritative client model would be in control, with the authoritative server model and client prediction, the server is in control.

The whole point of using the authoritative server is because the client can’t be trusted. So it makes sense that prediction can’t be trusted either.

To get around this you use periodically check the client position against the server and perform a correction if necessary.

This may sound simple in concept, but it’s one of the hardest aspect of multiplayer gaming to get right. Simply because it’s obvious when you get it wrong.

var correction = function(time, state, input, entity, rk4) {

...

if (Math.abs(state.x - lastMove.state.x) > 2) {

...

var currentTime = time,

currentInput = input;

entity.setState(state); // Rewind entity state

var move, // Current move

frameTime; // Time between correction and stored move

for (m = 0; m < moveCount; m++) {

move = moves[m];

frameTime = (move.time - currentTime) / 1000;

// Update physics based on corrected time, input and state

...

currentTime = move.time;

currentInput = move.input;

move.state = entity.getState();

};

};

};

This is the Gaffer on Games approach by Glen Fiedler.

There are also other solutions, like Valve’s approach which is based on the old QuakeWorld theory.

http://gafferongames.com/game-physics/networked-physics/http://developer.valvesoftware.com/wiki/Latency_Compensating_Methods_in_Client/Server_In-game_Protocol_Design_and_Optimization

STABILITYKeeping the game running

Keeping the game running is massively important, especially while it’s in rapid development and is prone to crashing (through errors of my own I must add).

I needed a way to automatically restart the game server if it crashed or something went horribly wrong.

To do that I went with Monit scripts that monitor the WebSockets port for activity.

If the port is inactive for 3 separate checks in a row then the game is automatically restarted in Node.

Ideally the game shouldn't crash, but this has proven to be a great solution that allows me to continue rapidly developing the game without worrying about it going down permanently if I'm not around.

http://mmonit.com/monit/

check host rawkets.com with address 127.0.0.1

start program = "/etc/init.d/rawkets start"

stop program = "/etc/init.d/rawkets stop"

if failed port 8000 type tcp for 2 times within 3 cycles then restart

MONIT SCRIPTKeeping the game running

Just as a quick example, here is the entire monit script for keeping the game up and running.

I’d say four lines of code was pretty impressive!

This effectively checks port 8000 on rawkets.com for an active tcp connection every cycle (60 seconds), and will only restart the game if there is a problem for at least 2 cycles.

The time between cycles can be changed in the monit configuration files.

#!/bin/sh

case "$1" in

start)

cd /rawkets

/usr/local/bin/node rawkets.js 2>&1 >> /var/log/node.log &

exit 1

;;

stop)

/usr/bin/pkill -f 'node rawkets.js'

exit 1

;;

esac

exit 1

INIT.D SCRIPTKeeping the game running

This is the init.d script that I use to start and stop the game server from the command line.

All it does is binds a particular set of shell commands to “start” and “stop” keywords, which I use to start and stop the Node process for the game.

I’m not massively experienced with init.d scripts so I’m sure that it can probably be optimised, but it works perfectly for my needs and is pretty damn cool.

WHY FLASH AUDIO?HTML5 Audio isn’t quite ready yet

So why in a proud HTML5 and JavaScript game have I resorted to Flash?

To put it bluntly, HTML5 Audio support is severely lacking in areas for the needs of game development.

One particular example is support for looping, which is severely lacking and inconsistent across most browsers.

Opera has the best implementation so far, but the rest all have some kind of pause between loops, which makes short and constantly looping audio very obvious and annoying.

There are some solutions, like using JavaScript to try and loop the audio just before it is finished, but none are ideal so I went with Flash audio until it gets fixed.

LOADS MOREMultiplayer games teach you a lot

Stuff that I couldn’t fit in…

- Canvas optimisation.- Using events to decouple game logic.- Using the same code for client and server.- setTimeout vs. requestAnimationFrame.- Storage.- Control system, like Seb’s.

THE FUTUREWhat I’d like to see

There are a few things that I’d like to see in the near future to help with game development.

Need a way to benchmark browsers, connections and operating systems.- Like Google Analytics, but for game performance and feature detection.- Measuring FPS.- Measuring network performance.

Better HTML5 audio.

Hardware accelerated canvas on Mac and mobile devices.

Better documentation.

Rawkets.comHTML5 & WebSockets game.

Twitter sentiment analysisDelving into your soul.

RECENT PROJECTS

Rawkes.comPersonal website and blog

MORE COOL STUFF

ExplicitWeb.co.ukWeb development podcast.

Mozilla Technical EvangelistMy job

ROB HAWKES@robhawkes

Twitter - @robhawkesRawkes - http://rawkes.com

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http://rawkes.com/foundationcanvas

DEV DERBYExperimenting with the latest Web technologies

Manipulate video with canvas

DEVELOPER.MOZILLA.ORG/EN-US/DEMOS/DEVDERBY

Every month

This month is HTML5 video

Win prizes (like an Android)

Next month is all about touch

Also, you should definitely take part in the Dev Derby, which is a monthly competition run by the Mozilla Developer Network to see what can be done with the latest Web technologies.

This month the focus is on HTML5 video, which is pretty interesting considering that you can manipulate it using the canvas.

The winners get cool prizes, like an Android phone. It’s a great excuse to play around with these technologies.

https://developer.mozilla.org/en-US/demos/devderby

Thank you.

If you have any questions feel free to grab me on Twitter (@robhawkes), or email rob@rawkes.com