MelbJS - Inside Rawkets

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MelbJS - Inside Rawkets

  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.
  2. 2. If you don’t already know, I work at Mozilla.My official job title is Technical Evangelist, but I prefer Rawket Scientist, which is what it sayson my business card.Part of my job is to engage with developers like you and me about cool new technologies onthe Web.
  3. 3. Throughout this talk I plan to take a quick journey through some of the issues that plaguedearly development of the game, and cover the subsequent solutions that helped resolvethem.
  4. 4. ion t ta lab e n erim from m y Exp ate adu a gr is kets RawRawkets 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. ts ? ke aw ckets is R Ro at ts, d an Wh ocke bS s, We ke RawRawkets stands for Rawkes (a merging of my names), as well as WebSockets, and Rockets.
  6. 6. Rawkets is a multiplayer space game that allows you to shoot your friends in the face withHTML5 technologies.It’s still not really at a beta release level yet, hence the bugs you might notice in this video.http://rawkets.com
  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.Perhaps when WebGL gets better it will become a reality. 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 pretty horrible).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.
  9. 9. es u ss ge I n lle ha ac be an esc gam aking MIt’s not all plain sailing when making a game using HTML5 and JavaScript.I’m going to cover a few of the main issues that tripped me up during the development ofRawkets.
  10. 10. io n a t n im ontrol g a r in c akin ow se we he br T uttin gt POne of the simplest fixes is to stop using setTimeout or setInterval and to userequestAnimationFrame instead.If you use setTimeout or setInterval and don’t manage it then you put a huge amount ofstress on the CPU and continue that stress even if you switch tabs or minimise the browser.By using requestAnimationFrame you give the browser control over when a new animationloop should occur, reducing load on the CPU and saving battery life on mobile devices.requestAnimationFrame also automatically limits the number of updates if you switch toanother tab or minimise the browser, again saving resources and keeping your players happy.Right now you can’t easily set a specific framerate when using requestAnimationFrame but solong as you use time-based updates (not frame-based) in your game then you’ll be fine.
  11. 11. in g o rk etw ug ht N Itho y as as se ta NoIssues 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.
  12. 12. col to n ro atio e p sag mun ic es tcom M hor ds an ured ct StruOne 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.
  13. 13. Enumeration 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.
  14. 14. Message package MSG_ID PLAYER_ID TIMESTAMP X Y 1 | 1234567890 | 1316763202872 | 5 | 34Put together with the message types, a full message package is put together as a simplestring 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 MSG_ID that you can see above is a reserved key that is used solely for the message type.The other items in this example are the player id, timestamp, and the player position.
  15. 15. io n s es ible pr ss om po C uch as asm ata gd cin du ReData in WebSockets is normally transmitted as verbose plain text, so it’s important to cutdown and compress 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.
  16. 16. ge s essa ion m icat ary mun Bin ste rcom , fa er rt sho en EvI never got around to implementing this but there is now binary message support inWebSockets.By switching to binary you can reduce the size of your messages by a noticeable amountwhile also increasing the amount of data that you can transmit at a single point in time.http://hobbycoding.posterous.com/websockt-binary-data-transfer-benchmark-rsulthttp://hobbycoding.posterous.com/the-fastest-websocket-module-for-nodejs
  17. 17. in g it lim ation ate un ic R om m c n on ow gd uttin CAside 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.
  18. 18. 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.
  19. 19. 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.
  20. 20. 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.
  21. 21. 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…
  22. 22. 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.
  23. 23. nce lige es tel essag In se m iti prior e am theg tting LeFortunately 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.
  24. 24. TC P ing ect lw ith it esp .D ea R sesT CP etsu ck So WebSomething 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.
  25. 25. e r ord th e ou t it ey ab Ob om uch ’t d can YouOne 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. Althoughsimilar protocols are in the pipeline and may make their way into our lives relatively soon,things like Media Streaming APIs and WebRTC.
  26. 26. ters ea Ch urse ac and ssing ble AThere’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.
  27. 27. a d b a re pen als ide o lob ew G pc od ee ’t k DonThere 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 usinganything like closures I was practically inviting people to come in and edit the game code. Itwas too easy to 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 actually pretty cool.
  28. 28. rity o th hing a u nt oo dt Clie ys ag a alw ’t isn er PowI’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.
  29. 29. ri ty o th wer a u r ve ish th at po er S elinqu RThe 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.
  30. 30. Inherent latency 40ms 40ms Client Server Client +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.
  31. 31. io n dict h pre no ug nt ’t e Clie hority isn ut vera SerTo 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.
  32. 32. Instant movement 40ms 40ms Client Server Client +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.
  33. 33. ion ct re rong or C oesw ng tio redic p hen WWhereas 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.
  34. 34. ility b ta ning S un er am theg ping KeeKeeping 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.I also needed a way to scale the game and keep it running as fast as possible.
  35. 35. ForeverI use a little Node module called Forever. It’s amazing!https://github.com/nodejitsu/forever
  36. 36. Forever forever start game.jsAll I have to do now is make sure the game process quits on a catastrophic error and Foreverwill automatically restart it for me.Using Forever is as simple as installing the module with NPM and then starting your Nodescript using the Forever demon. The rest is taken care of for you.
  37. 37. Hook.ioSome of you may also be interested in hook.io, which can help create more stable Nodeapplications.The concept is to decouple your application logic by breaking it into individual processes sothat if one process goes down the rest can continue to run and your entire game doesn’tcrash.You use hook.io through its event system that lets you communicate between these separateprocesses, regardless of whether they’re on the same server or not. It’s a pretty cool concept.https://github.com/hookio/hook.io
  38. 38. Rob Hawkes @robhawkes Rawkes.com Personal website and blog RECENT PROJECTS MORE COOL STUFF Twitter sentiment analysis Rawket Scientist Delving into your soul Technical Evangelist at Mozilla Rawkets.com Slides HTML5 & WebSockets game slideshare.net/robhawkesGet in touch with me on Twitter: @robhawkesFollow my blog (Rawkes) to keep up to date with stuff that I’m working on: http://rawkes.comI’ve recently worked on a project that analyses sentiment on Twitter: http://rawkes.com/blog/2011/05/05/people-love-a-good-smooch-on-a-balconyRawkets is my multiplayer HTML5 and JavaScript game. Play it, it’s fun: http://rawkets.comThese slides are online at http://slideshare.net/robhawkes

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