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Rawkets: An inside look at a multiplayer HTML5 game


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In this talk Rob will take you on a journey through the development of Rawkets, his HTML5 and JavaScript multiplayer space shooter. He will place particular focus on the issues that plagued the early progress, as well as the subsequent solutions that helped make it the ever-so-slightly addictive game that it is today. Expect theory, examples, and plenty of code in this light-hearted look at game development with the latest Web technologies.

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Rawkets: An inside look at a multiplayer HTML5 game

  1. 1. 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 earlydevelopment 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 theoriesinvolved.This talk is definitely light-hearted but I’ll be assuming some prior knowledge, so feel free toraise your hand throughout the talk if you want to explore an area further, or to simply ask aquestion.
  2. 2. 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 developersabout cool new technologies on the Web.
  3. 3. Created by Phil Banks (@emirpprime)Aside from that I spend most of my time experimenting with HTML5 and other cooltechnologies.It’s fun!
  4. 4. EXPERIMENTATION Rawkets is a graduate from my labRawkets is a project that originally came out of this experimentation, from a desire to learnmore 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 theexperiments. It’s something that I plan to develop and support way beyond the original scopeof learning WebSockets.
  5. 5. WHAT IS RAWKETS? Rawkes, WebSockets, and RocketsRawkets stands for Rawkes (my blog), WebSockets, and Rockets.
  6. 6. Rawkets is a multiplayer space game that allows you to shoot your friends in the face withHTML5 technologies.Still not really at a beta release level yet, hence the bugs you might notice in this video.
  7. 7. By now you’ve probably realised that the graphics at the beginning of this talk and on theposters aren’t the real game graphics.They are actually an awesome “artists impression” illustration that I commissioned a guycalled Reid Southen to create, although maybe when WebGL gets better it will become areality. Who knows.
  8. 8. It looks pretty awesome as a 6ft banner. So awesome in fact that my girlfriend actually askedme 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 itlooks small then let me put it into perspective by telling you that it’s about 8ft away. Readinto that what you will.
  9. 9. TECHNOLOGY Some of the tech involved in Web gamingThere 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.
  10. 10. CANVAS 2D graphics platformCanvas for 2D graphics.
  11. 11. FLASH AUDIO Sound effects and background musicFlash audio for game sound effects and background music.I’ll explain why I use Flash over HTML5 Audio further on in the talk.
  12. 12. WEBSOCKETS Multiplayer communicationWebSockets 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 allowsfor fully bi-directional streaming communication.
  13. 13. NODE.JS Game logic and communicationNode is used as the game server, controlling the logic and handling the WebSocketsconnections to the players.It will eventually be used for player authentication and the storage of data so gameplay canpersist 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.
  14. 14. ISSUES Making a game can be a challengeIt’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 ofRawkets.
  15. 15. NETWORKING Not as easy as I thoughtIssues 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 multiplayergaming in general.In the original prototype of the game the network communication was woefully simple andeverything was transmitted in a verbose format with no further thought.In hindsight it’s obvious why I was experiencing massive performance issues with thenetwork communication. I was basically sending way too much data back and forth.
  16. 16. MESSAGE PROTOCOL Structured and short communicationOne of the ways that I solved these problems was by implementing a structured protocol forthe messages that are being sent and received.This included assigning each message type a number and using enumeration to representthose types in the code.
  17. 17. ENUMERATION Message 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 formatwithin the code, but benefit from only sending the one or two digit number whentransmitting a message.This is only possible if both the client and server follow the same protocol in regards to whichnumber refers to which message type.It’s a simple but effective solution and allowed me to cut a large number of characters fromtransmitted messages.
  18. 18. PACKAGE Message 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 JSONrepresentation 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.
  19. 19. COMPRESSION Reducing data as much as possibleData in WebSockets is transmitted as verbose plain text, so it’s important to cut down andcompress it as much as possible.Some of the ways that I’ve done this include rounding numerical values, reducing the lengthof words if they’re only used for reference, and generally removing any data that isn’tnecessary.There is also BISON, which is a binary representation of JSON that can cut down the amountof data sent back and forth even further.
  20. 20. RATE LIMITING Cutting down on communicationAside from the message protocol, one of the biggest issues with networking has been dealingwith the sheer number of messages being sent back and forth during the lifetime of a game.
  21. 21. MESSAGES IN MESSAGES OUT 1 1 1 1Having 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 theplayer with details from the server.
  22. 22. MESSAGES IN MESSAGES OUT 2 4 1 2 2 1So say we now have two players, there is still only 1 message in from each player, but noweach player receives 2 messages back from the server; one for them, and one for the otherplayer.This isn’t too bad, but notice how the server is having to send 4 messages – 2 for eachplayer.
  23. 23. MESSAGES IN MESSAGES OUT 4 16 1 4 4 1 1 4 4 14 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 thenumber of players.But 16 messages out is alright, it’s hardly going to tax the server.
  24. 24. MESSAGES IN MESSAGES OUT 30 900 1 30 30 1 1 30 30 1So 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 – NINEHUNDRED – messages going out, every update. That’s a silly amount of data for just 30people.But let’s go further still…
  25. 25. MESSAGES IN MESSAGES OUT 100 10000 1 100 100 1 1 100 100 1Say 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’sbearable.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.
  26. 26. INTELLIGENCE Letting the game prioritise messagesFortunately there is a way around this that cuts down the amount of messages sent; you justneed to send data only for players visible to another player, in essence filtering out gamedata that doesnt affect the current player.Another trick I used is to only send updates when a player is active and moving. If theyhaven’t moved since the last frame and nothing else has changed then why bother sendingan update and wasting bandwidth?These are such simple solutions, but ones that I never even considered at first.
  27. 27. RESPECTING TCP WebSockets uses TCP. Deal with itSomething else that I discovered is important to be aware of when making a game withWebSockets 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 toexpect a certain set of issues.By the way, I should point out that that you could argue that the icon that I’ve used couldrepresent WebSockets, but that’s not why I used it. It’s the US plug symbol and I just thoughtit was funny because it looks like a surprised face. The UK plug symbol is boring incomparison.
  28. 28. OBEY THE ORDER You can’t do much about itOne issue with TCP is that packets will come through in order and get queued up if there areany significant connection issues.This can be a problem with a real-time game as it can cause hang-ups in the transmissionand 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.
  29. 29. FORCED DELAY Giving time for mistakes to be correctedI’ve not attacked the TCP issues head on yet, but one possible way to approach them is tointroduce 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, and100ms 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.
  30. 30. USE SOCKET.IO It intelligently drops messagesI use Socket.IO for all the WebSockets communication for Rawkets simply because it’s easy touse 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 conceptof volatile messages.These volatile messages are completely voluntary, and messages sent with them arentactually sent if the client is having network problems, saving bandwidth for other moreimportant messages.It’s not an ideal solution, but it’ll certainly help.
  31. 31. CHEATERS A blessing and a curseThere’s no denying it, your code is going to be visible to anyone who wants to look at thesource.I experienced this early on in the development of the game with players adding in their ownfeatures, like invincibility, epic speed, rapid-fire, and even creating completely new weaponslike cluster bombs!Now don’t get me wrong, I actually appreciate the cheaters because they highlighted all theerrors of my ways, for free. One of the benefits of the open nature of JavaScript is that it canbe looked at and poked very easily by others, which means that I can fix bugs quicker than ifI was testing on my own.
  32. 32. GLOBALS & CLOSURES Keeping the code wide open is badThere 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 anyclosures, I was practically inviting people to come in and edit the game code. It was too easyto do!It was so easy in fact that after a few hours of releasing the first prototype, players werealready sharing code snippets that others could paste into their browser console to get newfeatures. Annoying, but pretty cool.
  33. 33. (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 deliberatelyutilising closures and private variables I was able to further frustrate efforts by cheaters tooverwrite 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 asdifficult as possible.
  34. 34. CLIENT AUTHORITY Power isn’t always a good thingI’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 theclient, the player, made all the decisions regarding its position and then sent those positionsto the server.The server than trusted those positions and transmitted them to all the other players, whichis fine until the client edits their position and increments it by 100 pixel per frame, ratherthan 5. Bad times.This can be referred to as the “Here I am” approach.
  35. 35. SERVER AUTHORITY Relinquish that powerThe solution is to make the server authoritative, which means that you prevent manipulationof the client’s code from doing any damage.All the movement logic is now performed on the server, meaning that when a client moves itsimply lets the server know which direction it wants to move. From there the server calculatesthe 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 completelyremove the ability to cheat.
  36. 36. INHERENT LATENCY Server authority 40ms 40ms Input Move Update +0 +40 +80 80ms total round-tripHowever, the problem with the authoritative server model is that there is some inherentlatency within the system.What I mean by this is that it obviously takes some time for a movement to be sent from theclient to the server, then for the server to move the client, and then for the server to send thenew 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 theupdated position? If you do nothing then there’s going to be an 80ms delay between youpressing the up arrow and your rawket moving forward. Not good.
  37. 37. CLIENT PREDICTION Server authority isn’t enoughTo solve the latency issues with the authoritative server you need to implement some elementof prediction on the client.What I mean by prediction is an ability for the client to guess, quite accurately, where itshould move the player before the message comes back from the server detailing the newposition.
  38. 38. INSTANT MOVEMENT Client prediction 40ms 40ms Input Move Update +0 +40 +80 Prediction happens hereThe 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 importantdifference.
  39. 39. CORRECTION When prediction goes wrongWhereas the authoritative client model would be in control, with the authoritative servermodel 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 itmakes sense that prediction can’t be trusted either.To get around this you use periodically check the client position against the server andperform a correction if necessary.This may sound simple in concept, but it’s one of the hardest aspect of multiplayer gaming toget right. Simply because it’s obvious when you get it wrong.
  40. 40. 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 QuakeWorldtheory.
  41. 41. STABILITY Keeping the game runningKeeping the game running is massively important, especially while it’s in rapid developmentand 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 wenthorribly wrong.
  42. 42. 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 restartedin Node.Ideally the game shouldnt crash, but this has proven to be a great solution that allows me tocontinue rapidly developing the game without worrying about it going down permanently ifIm not around.
  43. 43. MONIT SCRIPT Keeping the game running check host with address 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 restartJust 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 for an active tcp connection every cycle (60seconds), 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.
  44. 44. INIT.D SCRIPT Keeping the game running #!/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 1This 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 Iuse 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 beoptimised, but it works perfectly for my needs and is pretty damn cool.
  45. 45. WHY FLASH AUDIO? HTML5 Audio isn’t quite ready yetSo 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 gamedevelopment.One particular example is support for looping, which is severely lacking and inconsistentacross most browsers.Opera has the best implementation so far, but the rest all have some kind of pause betweenloops, 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 isfinished, but none are ideal so I went with Flash audio until it gets fixed.
  46. 46. LOADS MORE Multiplayer games teach you a lotStuff 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.
  47. 47. THE FUTURE What I’d like to seeThere 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.
  48. 48. ROB HAWKES @robhawkes Personal website and blog RECENT PROJECTS MORE COOL STUFF Twitter sentiment analysis Mozilla Technical Evangelist Delving into your soul. My job HTML5 & WebSockets game. Web development podcast.Twitter - @robhawkesRawkes -
  49. 49. FOUNDATION HTML5 CANVAS My amazing book on canvas, animation, and making games. Out now Paperback and digital formats Become a canvas master Learn how to animate Make two cool space games RAWKES.COM/FOUNDATIONCANVASBook - available on Amazon right now
  50. 50. DEV DERBY Experimenting with the latest Web technologies Every month This month is HTML5 video Manipulate video with canvas Win prizes (like an Android) Next month is all about touch DEVELOPER.MOZILLA.ORG/EN-US/DEMOS/DEVDERBYAlso, you should definitely take part in the Dev Derby, which is a monthly competition run bythe 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 canmanipulate it using the canvas.The winners get cool prizes, like an Android phone. It’s a great excuse to play around withthese technologies.
  51. 51. Thank you.If you have any questions feel free to grab me on Twitter (@robhawkes), or