In my JavaScript game (made with jQuery) I have player position stored in a database. When character is moving, i just send request to specyfic URL, I.E. mysite.com/map/x1/y3 (where a character's position is x=1, y=3).
That url send coordinates to the database and checks to see if any other players are near ours. If yes, it sends also JSON object with name and coords of that players.
And here is my question - how to secure it? Some one could look into my JavaScript code and prepare url looking like mysite.com/map/x100/y234, and it will 'teleport' him into some other side of map.
Any data/computation processed in JavaScript in the browser will be insecure since all the code runs on the local machine. I would recommend to list all the parameters critical to a fair experience of play, such as the player position, score, resources... and compute the management of these parameters on the server-side. You would only gather user inputs from the browser and send the updated state to the browser for display.
Even if you choose to compute some values on the browser side to avoid latency, you should not take them into account for the global state shared by players, and you should resynchronize the local state with the global state - always in the direction global to local - from time to time.
Like in a typical form handling, you should also check that the values sent by the browser for user inputs fall into reasonable bounds, e.g. relative movement in one second is less than a certain distance.
You could obfuscate your javascript source code. That will at least deter casual cheats, however there's probably no way to make it completely secure using javascript.
Related
I have a game with a worldwide highscore feature. It uses the firebase database, and writes the user's score if it is the highscore. The rules state that anyone can read or write, so other people can view the highscore.
My problem is that it's easy to manipulate the highscore without actually getting a score. How can I make it so when you achieve a new highscore, it is written to the database, but if you go into the console and change the data, it won't allow you to change it?
if (score > worldScore) {
database.ref().update({highscore: score});
}
You can see that it is very easy to change the data.
In the Firebase console, there is never any restrictions on what you can read or write in the database. Security rules never apply there.
Strictly speaking, unless you involve some serverside component that can one way or another confirm the score was achieved legitimately, this is not possible. Clientside data is always subject to user manipulation; any confirmation checks on that data which you perform on the client would also be subject to user manipulation.
(As for how to actually perform that serverside confirmation: it'll depend on the details of the game, but one way might be to have the client periodically send significant game data to the server; if the server can determine that any of the data has changed in ways that should be impossible according to the game rules -- like a score jumping too far in too short a period of time -- then ignore any future score submissions from that user. Even this isn't perfect: the user can still cheat by manipulating the data that gets sent in that periodic poll, but they'd have to keep their changes at least within the bounds of plausibility.)
Typically you'll want to store not just the score, but also the way the player achieved that score. For example: if it is a board game, write their moves in addition tot he result. If you have both you can:
Verify that the score they wrote is indeed the score that is gotten by applying the moves.
Perform some analysis to detect if the moves seem likely to be computer generated.
Both of these processes are cases of "trusted code", i.e. code that should be running in a trusted environment. For this you can use either an environment you control (a private server, your laptop, etc), Cloud Functions for Firebase, or (in some cases) Firebase's server side security rules. Which ones are feasible depends on your exact use-case, and your available time.
I am creating a simple game in html5 canvas. i run it using javascript . ANd i want it to be a multiplayer game. but first i need to have a database where i can put the x and y position of an object that will run in every 30 milliseconds(it is the keyframes of my game animation.) . i need to save it in a file or database so other players can see the update of x and y position of other players...i hope you get my point...
now i am asking what database or file should i use to do this data position updating . that can be able to update that fast
For a scenario like this, you will probably get more mileage if you cache locations in memory locally, but then periodically "sync" them with the database. This will require ways to resolve conflicts in position (e.g. if the position you predict / have on the client-side JavaScript deviates from the actual position as reported by the database) but will allow you to be more efficient (e.g. updating at a faster rate when players are nearer to your player and less frequently when they are far away, for example). It will also allow you to animate your player's movements more steadily without jank in the event that a particular database request falls outside of your frame rate requirements.
As for the database, itself, there are a lot of databases to choose from. However, if you don't want to write the server-side code to provide an API for interacting with your database, then you may be interested in Firebase, which provides direct access from client-side JavaScript (without the need to create your own server / API layer on top of the database). Of course you can also use any other database -- Google Cloud Datastore, Google Cloud SQL, MySQL, Cassandra, MongoDB -- and write an appropriate API server layer in the language of your choice (which could also be JavaScript) to provide access to the underlying data, as a valid option as well (and, in fact, that might make more sense if you already have or plan to have a frontend webserver).
I am writing a javascript web game that has a 3 x 3 viewport similar to the one Urban Dead uses. The "world map" is stored as a 100 x 100 2D array server-side (nodejs), each pair of coordinate defines a "room". So, the 3 x 3 viewport shows the name and content of 9 of the rooms.
User's location are stored server-side as coordinates. E.g. Bob = { x : 2, y : 3 }. So Bob's x-coordinate is Bob.x. The client (browser) can retrieve this and work out the coordinates of the other 8 rooms, and asks the server for the content of those rooms. These are then displayed in the viewport. This is suppose to look like the viewport in Urban Dead (top left corner).
Question
How should I think about making the viewport "refresh" or update? I am thinking of doing it this way...
1) When the player moves from coordinates (2,3) to (1,3). The client asks the server about the content of the 9 rooms again and re-draw/display everything.
2) When the content of one of the room changes, the server (nodejs) runs a client-side function that tells it to ask the server for the content of the 9 rooms and re-draw/display everything.
Being new to programming, I wonder if this implementation would be too naive or inefficient?
The former option would be the best imo, purely for the reason that as your map expands it will be easier for you to set-up trigger regions that cause your client to load specific areas of the map. You could even only load the rooms that they can see or have access to.
This would also be of benefit, for example, if you were to have multiple characters. Imagine you have your main character in one location, and a secondary character elsewhere. It would be far better for client to know where they are and to understand what each character can see and only request this information from the server. This is a much more expandable solution than having the server just constant broadcast all room information to whoever is listening.
With regards to changes in room content, this could be flagged as an event from the server to all clients - but based purely on room co-ordinates and minimal data. If this event covers one of the rooms the client can currently see, a request by that client for the room information can occur. So in part this involves something of your option two, but shouldn't broadcast much.
As an analogy, this is more akin to having clients/users requesting only the resources they want from a website at the time they want them and possibily signing up to mail alerts about content that's interesting to them. Rather than having the user sign-up to an RSS feed and being notified when anything changes on the site. The former is more optimal and can be controlled in specific ways, the latter is better if your user is interested in overviewing the whole content of the site (which is not the case usually in games -- unless you're designing bots that have to cheat).
Overall, after talking it through implementing part of both approaches would be useful. But if it's a choice the first one will give you more control.
I wouldn't worry about too naive or inefficient solution. The difficult part is to make game playable and fun - optimize later.
Looking at the example, the amount of data to transfer doesn't look very big. You should be able to fetch full 9 room set for update.
Are you pulling the data from server with intervals (polling) or are you somehow pushing changes from server to client?
The question is do you care of AFK being notified, and how important is information about next room.
If you think a strategy can change if a room is updated ? In this case update every time a update is done.
If a player don't care then change are done when pass to next room.
A middle way solution is to ask a "delta" update ( just element who have been modified ) every 10s or 30s depending of the game. In this case us a central clock can be funny ( multiple player will update a same time ), and this create a turn-style gameplay.
As a RPG player ( paper RPG ) I will think the third is a good way. You can even mix solution : short time update for current room, and perception based time ( a bling and deaf will update exterior room only at major event ).
Let's say I'm making an HTML5 game using JavaScript and the <canvas> The varaibles are stored in the DOM such as level, exp, current_map, and the like.
Obviously, they can be edited client-side using Firebug. What would I have to do to maximize security, so it would be really hard to edit (and cheat)?
Don't store the variables in the DOM if you wish a reasonable level of security. JavaScript, even if obfuscated, can easily be reverse engineered. That defeats any local encryption mechanisms.
Store key variables server-side and use https to maximize security. Even so, the client code (JavaScript) is quite vulnerable to hacking.
You can use Object.freeze or a polyfill or a framework which does the hiding for you.
Check out http://netjs.codeplex.com/
You could also optionally implement some type of signing system but nothing is really impenetrable. For instance objects locked with Object.freeze or Object.watch can still be manually modified in memory.
What are you really trying to accomplish in the end?
What you could do is send a representation of the matrix of the game or the game itself or a special hash or a combination of both and tally the score at the server... causing the user to not only have to modify the score but to correctly modify the state of the game.
Server-side game logic
You need to keep the sensitive data on the server and a local copy on the browser for display purposes only. Then for every action that changes these values the server should be the one responsible for verifying them. For example if the player needs to solve a puzzle you should never verify the solution client side, but take for example the hash value of the ordered pieces represented as a string and send it to the server to verify that the hash value is correct. Then increase the xp/level of the player and send the information back to the client.
Anything that is living in the client can be modified. That is because in MMORPG the character's data is living on the server, so players can't hack their characters using any memory tools, hex editor, etc (they actually "can", but because the server keep the correct version of the character's data is useless).
A good example was Diablo 2: you have actually two different characters: one for single player (and Network playing with other players where one was the server), and one for Battle.net. In the first case, people could "hack" the character's level and points just editing the memory on the fly or the character file with an hex editor. But that wasn't possible with the character you was using on Battle.net.
Another simple example could be a quiz where you have a limited time to answer. If you handle everything on client side, players could hack it and modify the elapsed time and always get the best score: so you need to store the timestamp on the server as well, and use that value as comparison when you get the answer.
To sum up, it doesn't matter if it's JavaScript, C++ or Assembly: the rule is always "Don't rely on client". If you need security for you game data, you have to use something where the clients have no access: the server.
I have a web page that shows a large amount of data from the server. The communication is done via ajax.
Every time the user interacts and changes this data (Say user A renames something) it tells the server to do the action and the server returns the new changed data.
If user B accesses the page at the same time and creates a new data object it will again tell the server via ajax and the server will return with the new object for the user.
On A's page we have the data with a renamed object. And on B's page we have the data with a new object. On the server the data has both a renamed object and a new object.
What are my options for keeping the page in sync with the server when multiple users are using it concurrently?
Such options as locking the entire page or dumping the entire state to the user on every change are rather avoided.
If it helps, in this specific example the webpage calls a static webmethod that runs a stored procedure on the database. The stored procedure will return any data it has changed and no more. The static webmethod then forwards the return of the stored procedure to the client.
Bounty Edit:
How do you design a multi-user web application which uses Ajax to communicate with the server but avoids problems with concurrency?
I.e. concurrent access to functionality and to data on a database without any risk of data or state corruption
Overview:
Intro
Server architecture
Client architecture
Update case
Commit case
Conflict case
Performance & scalability
Hi Raynos,
I will not discuss any particular product here. What others mentioned is a good toolset to have a look at already (maybe add node.js to that list).
From an architectural viewpoint, you seem to have the same problem that can be seen in version control software. One user checks in a change to an object, another user wants to alter the same object in another way => conflict. You have to integrate users changes to objects while at the same time being able to deliver updates timely and efficiently, detecting and resolving conflicts like the one above.
If I was in your shoes I would develop something like this:
1. Server-Side:
Determine a reasonable level at which you would define what I'd call "atomic artifacts" (the page? Objects on the page? Values inside objects?). This will depend on your webservers, database & caching hardware, # of user, # of objects, etc. Not an easy decision to make.
For each atomic artifact have:
an application-wide unique-id
an incrementing version-id
a locking mechanism for write-access (mutex maybe)
a small history or "changelog" inside a ringbuffer (shared memory works well for those). A single key-value pair might be OK too though less extendable. see http://en.wikipedia.org/wiki/Circular_buffer
A server or pseudo-server component that is able to deliver relevant changelogs to a connected user efficiently. Observer-Pattern is your friend for this.
2. Client-Side:
A javascript client that is able to have a long-running HTTP-Connection to said server above, or uses lightweight polling.
A javascript artifact-updater component that refreshes the sites content when the connected javascript client notifies of changes in the watched artifacts-history. (again an observer pattern might be a good choice)
A javascript artifact-committer component that may request to change an atomic artifact, trying to acquire mutex lock. It will detect if the state of the artifact had been changed by another user just seconds before (latancy of javascript client and commit process factors in) by comparing known clientside artifact-version-id and current serverside artifact-version-id.
A javascript conflict-solver allowing for a human which-change-is-the-right decision. You may not want to just tell the user "Someone was faster than you. I deleted your change. Go cry.". Many options from rather technical diffs or more user-friendly solutions seem possible.
So how would it roll ...
Case 1: kind-of-sequence-diagram for updating:
Browser renders page
javascript "sees" artifacts which each having at least one value field, unique- and a version-id
javascript client gets started, requesting to "watch" the found artifacts history starting from their found versions (older changes are not interesting)
Server process notes the request and continuously checks and/or sends the history
History entries may contain simple notifications "artifact x has changed, client pls request data" allowing the client to poll independently or full datasets "artifact x has changed to value foo"
javascript artifact-updater does what it can to fetch new values as soon as they become known to have updated. It executes new ajax requests or gets feeded by the javascript client.
The pages DOM-content is updated, the user is optionally notified. History-watching continues.
Case 2: Now for committing:
artifact-committer knows the desired new value from user input and sends a change-request to the server
serverside mutex is acquired
Server receives "Hey, I know artifact x's state from version 123, let me set it to value foo pls."
If the Serverside version of artifact x is equal (can not be less) than 123 the new value is accepted, a new version id of 124 generated.
The new state-information "updated to version 124" and optionally new value foo are put at the beginning of the artifact x's ringbuffer (changelog/history)
serverside mutex is released
requesting artifact committer is happy to receive a commit-confirmation together with the new id.
meanwhile serverside server component keeps polling/pushing the ringbuffers to connected clients. All clients watching the buffer of artifact x will get the new state information and value within their usual latency (See case 1.)
Case 3: for conflicts:
artifact committer knows desired new value from user input and sends a change-request to the server
in the meanwhile another user updated the same artifact successfully (see case 2.) but due to various latencies this is yet unknown to our other user.
So a serverside mutex is acquired (or waited on until the "faster" user committed his change)
Server receives "Hey, I know artifact x's state from version 123, let me set it to value foo."
On the Serverside the version of artifact x now is 124 already. The requesting client can not know the value he would be overwriting.
Obviously the Server has to reject the change request (not counting in god-intervening overwrite priorities), releases the mutex and is kind enough to send back the new version-id and new value directly to the client.
confronted with a rejected commit request and a value the change-requesting user did not yet know, the javascript artifact committer refers to the conflict resolver which displays and explains the issue to the user.
The user, being presented with some options by the smart conflict-resolver JS, is allowed another attempt to change the value.
Once the user selected a value he deems right, the process starts over from case 2 (or case 3 if someone else was faster, again)
Some words on Performance & Scalability
HTTP Polling vs. HTTP "pushing"
Polling creates requests, one per second, 5 per second, whatever you regard as an acceptable latency. This can be rather cruel to your infrastructure if you do not configure your (Apache?) and (php?) well enough to be "lightweight" starters. It is desirable to optimize the polling request on the serverside so that it runs for far less time than the length of the polling interval. Splitting that runtime in half might well mean lowering your whole system load by up to 50%,
Pushing via HTTP (assuming webworkers are too far off to support them) will require you to have one apache/lighthttpd process available for each user all the time. The resident memory reserved for each of these processes and your systems total memory will be one very certain scaling limit that you will encounter. Reducing the memory footprint of the connection will be necessary, as well as limiting the amount continuous CPU and I/O work done in each of these (you want lots of sleep/idle time)
backend scaling
Forget database and filesystem, you will need some sort of shared memory based backend for the frequent polling (if the client does not poll directly then each running server process will)
if you go for memcache you can scale better, but its still expensive
The mutex for commits has to work globaly even if you want to have multiple frontend servers to loadbalance.
frontend scaling
regardless if you are polling or receiving "pushes", try to get information for all watched artifacts in one step.
"creative" tweaks
If clients are polling and many users tend to watch the same artifacts, you could try to publish the history of those artifacts as a static file, allowing apache to cache it, nevertheless refreshing it on the serverside when artifacts change. This takes PHP/memcache out of the game some for requests. Lighthttpd is verry efficent at serving static files.
use a content delivery network like cotendo.com to push artifact history there. The push-latency will be bigger but scalability's a dream
write a real server (not using HTTP) that users connect to using java or flash(?). You have to deal with serving many users in one server-thread. Cycling through open sockets, doing (or delegating) the work required. Can scale via forking processes or starting more servers. Mutexes have to remain globaly unique though.
Depending on load scenarios group your frontend- and backend-servers by artifact-id ranges. This will allow for better usage of persistent memory (no database has all the data) and makes it possible to scale the mutexing. Your javascript has to maintain connections to multiple servers at the same time though.
Well I hope this can be a start for your own ideas. I am sure there are plenty more possibilities.
I am more than welcoming any criticism or enhancements to this post, wiki is enabled.
Christoph Strasen
I know this is an old question, but I thought I'd just chime in.
OT (operational transforms) seem like a good fit for your requirement for concurrent and consistent multi-user editing. It's a technique used in Google Docs (and was also used in Google Wave):
There's a JS-based library for using Operational Transforms - ShareJS (http://sharejs.org/), written by a member from the Google Wave team.
And if you want, there's a full MVC web-framework - DerbyJS (http://derbyjs.com/) built on ShareJS that does it all for you.
It uses BrowserChannel for communication between the server and clients (and I believe WebSockets support should be in the works - it was in there previously via Socket.IO, but was taken out due to the developer's issues with Socket.io) Beginner docs are a bit sparse at the moment, however.
I would consider adding time-based modified stamp for each dataset. So, if you're updating db tables, you would change the modified timestamp accordingly. Using AJAX, you can compare the client's modified timestamp with the data source's timestamp - if the user is ever behind, update the display. Similar to how this site checks a question periodically to see if anyone else has answered while you're typing an answer.
You need to use push techniques (also known as Comet or reverse Ajax) to propagate changes to the user as soon as they are made to the db. The best technique currently available for this seems to be Ajax long polling, but it isn't supported by every browser, so you need fallbacks. Fortunately there are already solutions that handle this for you. Among them are: orbited.org and the already mentioned socket.io.
In the future there will be an easier way to do this which is called WebSockets, but it isn't sure yet when that standard will be ready for prime time as there are security concerns about the current state of the standard.
There shouldn't be concurrency problems in the database with new objects. But when a user edits an object the server needs to have some logic that checks whether the object has been edited or deleted in the meantime. If the object has been deleted the solution is, again, simple: Just discard the edit.
But the most difficult problem appears, when multiple users are editing the same object at the same time. If User 1 and 2 start editing an object at the same time, they will both make their edits on the same data. Let's say the changes User 1 made are sent to the server first while User 2 is still editing the data. You then have two options: You could try to merge User 1's changes into the data of User 2 or you could tell User 2 that his data is out of date and display him an error message as soon as his data gets send to the server. The latter isn't very user friendly option here, but the former is very hard to implement.
One of the few implementations that really got this right for the first time was EtherPad, which was acquired by Google. I believe they then used some of EtherPad's technologies in Google Docs and Google Wave, but I can't tell that for sure. Google also opensourced EtherPad, so maybe that's worth a look, depending on what you're trying to do.
It's really not easy to do this simultaneously editing stuff, because it's not possible to do atomic operations on the web because of the latency. Maybe this article will help you to learn more about the topic.
Trying to write all this yourself is a big job, and it's very difficult to get it right. One option is to use a framework that's built to keep clients in sync with the database, and with each other, in realtime.
I've found that the Meteor framework does this well (http://docs.meteor.com/#reactivity).
"Meteor embraces the concept of reactive programming. This means that you can write your code in a simple imperative style, and the result will be automatically recalculated whenever data changes that your code depends on."
"This simple pattern (reactive computation + reactive data source) has wide applicability. The programmer is saved from writing unsubscribe/resubscribe calls and making sure they are called at the right time, eliminating whole classes of data propagation code which would otherwise clog up your application with error-prone logic."
I can't believe that nobody has mentioned Meteor. It's a new and immature framework for sure (and only officially supports one DB), but it takes all the grunt work and thinking out of a multi-user app like the poster is describing. In fact, you can't NOT build a mult-user live-updating app. Here's a quick summary:
Everything is in node.js (JavaScript or CoffeeScript), so you can share stuff like validations between the client and server.
It uses websockets, but can fall back for older browsers
It focuses on immediate updates to local object (i.e. the UI feels snappy), with changes sent to the server in the background. Only atomic updates are allowed to make mixing updates simpler. Updates rejected on the server are rolled back.
As a bonus, it handles live code reloads for you, and will preserves user state even when the app changes radically.
Meteor is simple enough that I would suggest you at least take a look at it for ideas to steal.
These Wikipedia pages may help add perspective to learning about concurrency and concurrent computing for designing an ajax web application that either pulls or is pushed state event (EDA) messages in a messaging pattern. Basically, messages are replicated out to channel subscribers which respond to change events and synchronization requests.
https://en.wikipedia.org/wiki/Category:Concurrency_control
https://en.wikipedia.org/wiki/Distributed_concurrency_control
https://en.wikipedia.org/wiki/CAP_theorem
https://en.wikipedia.org/wiki/Operational_transformation
https://en.wikipedia.org/wiki/Fallacies_of_Distributed_Computing
There are many forms of concurrent web-based collaborative software.
There are a number of HTTP API client libraries for etherpad-lite, a collaborative real-time editor.
django-realtime-playground implements a realtime chat app in Django with various real-time technologies like Socket.io.
Both AppEngine and AppScale implement the AppEngine Channel API; which is distinct from the Google Realtime API, which is demonstrated by googledrive/realtime-playground.
Server-side push techniques are the way to go here. Comet is (or was?) a buzz word.
The particular direction you take depends heavily on your server stack, and how flexible you/it is. If you can, I would take a look at socket.io, which provides a cross-browser implementation of websockets, which provide a very streamline way to have bidirectional communication with the server, allowing the server to push updates to the clients.
In particular, see this demonstration by the library's author, which demonstrates almost exactly the situation you describe.