With the popularity of service oriented architectures and other buzz phrases related to software as service, good API design has become a significant selling point for any software platform in the past 5-10 years.  People make purchasing decisions based on how easy it is to interoperate with your applications and code and as such the number of client / public facing APIs attached to software has skyrocketed.  I’d like to believe the days of dropping strategic text files in directories to trigger some action or another in an application are behind us.

In this article I’m going to talk about the following things

• Why you should choose your method names carefully, and what to call
• Why pretending to be a data access layer is a terrible thing for an API to do
• Talk about the dangers of leaky abstractions in an API
• Explain the benefits of creating a data contract between you and the calling code
• Explain why it’s vital to support standards
• Make sure that your users can retrieve values they’re going to want to modify
• Suggest supplying compiled libraries alongside your API documentation
• Explain why it’s important to keep your API implementation clean
• Talk about the benefits of dogfooding your API
• Consider supporting atomic operations including rollbacks on failure
• Discuss bulk operations
• Try and convince you that both logging and security should be first class citizens
• Beg you to maintain integration tests and most of all to keep it simple!

Your API Sucks

You’ve probably used an API and there’s a good chance you’ve had to write one.  This probably won’t surprise you; most APIs suck.  They’re horrible to use and built around illogical leaky abstractions that leave you flicking through huge wads of documentation just to make the most rudimentary feature work.

About 18 months ago, after a year of struggling with a broken third party API that almost brought a business to it’s knees by placing significant roadblocks in front of in house development, I was part of a team tasked with designing our own client facing API.  With no desire to expose other developers to the cruel and unusual punishments of software design we’d had to endure, we came to the conclusion that it was really important that we god this piece of the system right.  People say first impressions are everything, and your API design can make or break the faith other developers have in your ability to produce software.  Show somebody a shitty API and they’ll perhaps correctly assume the rest of your code sucks too.

The Best Man For The Job

There’s a bit of a trend that I’ve noticed with some of the worst APIs I’ve worked with: they seem to be designed by the wrong people.  The wrong people to design an API are 1) the guy that wrote the internal code to do the job the API is providing access to and 2) the consumer of the API. 

The guy that wrote the code that the API is calling under the hood will be inherently slanted to implement an API which exposes this functionality and will have a predisposition to creating a leaky abstraction.  This is especially bad for the consumer APIs designed by the internal implementer tend to assume the consumer knows far more than he really does, or has access to internal data that in reality, he doesn’t.

Conversely, an API designed by the consumer of the API will have a tendency towards solving problems that are not the concern of the API itself.  The consumer will, either accidently or by intention, attempt to offload some of the work that should be the responsibility of the calling code into the API.

Ideally, the person that’s writing the API will have knowledge of the system internals, but not be the guy that wrote them. A fellow team member with passive experience to the code would be a good person, or ideally, a pair design exercise between the person that originally wrote the code and an API designer, with the consumer as a consultant.

Speaking The Same Language

Like a lot of software development, you make good progress when you get your terminology right and understand exactly what you’re trying to produce.  I’ve consistently found that the best way to think of a client facing API is as a orchestrating thin wrapper that summarises, in code, a set of business processes that you wish to expose to the public.

In order to get your API design right, you need to clearly define and agree on the boundaries of the system with both your internal team, and your consumers.  It’s important that you have a clear understanding of the following:

• The responsibility of the calling code
• The responsibility of the API layer
• The responsibility of the internal code the API makes calls to

This might sound like a really simple suggestion but I’ve taken part in countless discussions where people on both sides of the API just “presumed” that either the calling code or the API would perform specific functions (data cleansing, logging etc) when in fact, this confusion had lead to none of the implementers bothering to write the required functionality.  Make sure you know for certain what your API is responsible for doing.

Defining Your API – Tips and Tricks

Defining your API methods (or the “contract” of the API) is the most important thing to get right and there are several vital things to remember.

• Choose Your Names Wisely Using the language of the business
  
  It’s vital that your API methods speak in terms that the caller is going to understand.  Your API should be readable.  If your users go hunting for the documentation every time they want to use a method, then you’re probably doing it wrong.

  Clarity in naming is exceptionally important.  The names of your API methods should succinctly state what action that method call is going to perform.  Don’t fear using long method names, embrace them for clarity.  As a general rule, your pmethods should probably always be in the form DoSomething(object withThis);

  Ensure that when naming methods you reflect business operations in the method names, not the underlying implementation.

  Bad example:     void InsertToTblCustomer(string[] custDataValues);
  Good example:   void AddCustomer(Customer customer);
  Good example:   void DisableAccount(string accountId); 

• Don’t pretend to be a data access layer

  APIs should summarise business operations in a logical and meaningful fashion.  You are not a public facing data access layer and you should never pretend to be.  If your users want raw database access give them read only permissions on some tables and a copy of SQL Management Studio.  So don’t write methods for CRUD operations in your API (unless you’re writing some kind of online file management utility).

  Bad example:     void InsertToTblCustomer(string[] custDataValues);
  Bad example:     void UpdateTblCustomer(string[] custDataValues);
  There are no good examples!

• Avoid leaky abstractions

  This is a fundamental and simple rule – don’t expose your callers to anything that they’re not interested in or won’t understand.  If it’s not important, don’t show it.  Don’t code for things nobody will ever need and don’t require your callers to have intimate knowledge of data types or internal categories in your system.

• Create
  a data contract between you and the calling code
  
  I’m going to borrow some of the terminology from WCF here because I’ve found it an appropriate label.  Create a Data Contract library for use in your API.  This library should summarise the business process and the outward facing view of your software.  It might contain terminology that doesn’t actually exist in the software itself, but in the business processes that the software models.  Either way, this, and only this, should be the language that the API talks to your callers.

  Where possible, create this data contract in a separate assembly that’s entirely decoupled from your core system and distribute it to people that want to use your API.  This is especially beneficial when using WCF as your clients can generate a service proxy and deal in the same data types that you are in your API code.

  It should be the responsibility of the API layer to marshal the data from your data contract into the domain model of your internal components.

  You data contract should contain every type used to communicate with your API and the object model should be named in a way which is meaningful for the consumers.

  Because your data contract is NOT the object model of your internal components, you’re able to add properties and objects that don’t logically exist in your internal components.  This means that you can perform an operation using some internal component, gather the output in your API layer and then compose the output data in a meaningful way using classes written specifically for the data contract.  This way, by the time the user has access to the output data, it’s in a format and language which they understand.

• Support standards!  Don’t reinvent the wheel!
  
  Here’s a true story; while working with an API, my team was faced with the following API method:
  object Run(string request);

  It was the only method on the API, and “covered up” for around 30 methods all made available through one giant black hole in the side of the system.  Underneath that there was an XML format that the request had to be in in order to call the appropriate method.

  If you’re writing an API, stick to some kind of standards.  Ideally, expose a web service endpoint with an accurate WSDL that people can call or use a simple and obvious REST endpoint.  

  Please, please, please, do not make other developer suffer by rolling your own delivery mechanism.  We have enough of them, don’t confuse people by adding some more.  If you’re going to use a raw sockets connection, supply a calling library and stick to some standard middleware like WCF rather than rolling your own. 

  Thousands of people have spent thousands of man years writing code based on existing techniques, you’re not better than all of them combined.

  Reinventing the wheel is never good for anyone.

• Ensure that the user can retrieve values they’re going to want to modify

  The number of times I’ve used APIs that let me set or update objects without returning the current state of the object is mind blowing.  So always remember: If you’re going to let them set it, let them get it.  Providing an update or “upsert” method without allowing your consumers to query the current state of data in the system is a complete waste of time.

• Supply compiled libraries to work with your API documentation

  This may not always be possible, but it’s a really good idea to supply a sample implementation and compiled binaries with your API that covers the most common scenarios of usage.  Not only does this prevent the consumer from struggling to get to grips with you API, but it allows you to outline and illustrate a set of best practices for usage.  In an ideal world, the user could just use your sample code in their application, so ensure you license the code appropriately.

  This is an exceptionally good way to deal with any authentication your API may require as you have the ability to provide additional helper classes to perform some common tasks (authenticate –> perform action –> logout, for example).

• Keep your implementation clean

  Delegate the API logic to your middleware components / reusable libraries.  Do your best to ensure the API layer doesn’t actually contain the logic required to perform operations, just the logic required to marshal the data from the API format into your internal data structures.  The API should simply orchestrate calls to one or more internal methods because your API should simply be exposing existing functionality.

  If the API is exposing some new, API specific functionality, consider splitting this behaviour into a separate assembly or binary to aid testability.

• Consider dogfooding

  Dogfooding is the act of using the software you’re creating.  I worked on a project where we were developing an order placement system in ASP.net MVC, and as part of the design process we decided that we wanted to have an API that was a first class citizen.  It then dawned on us that in order to produce the API in a way that accurately mirrored the functionality of the website, we should have the website consume the API like any other client would.  The website had it’s own concept of user authentication, and when a user logged in, the web application logged in to the API as the current user.

  Doing this not only ensured that our security model was watertight, but that any additional web functionality would immediately be available to API users because they were actually the same thing.  On top of that, you gain confidence in your own API because you know that it’s called often by your own code, reducing the likelihood of users discovering bugs in your API because it’s not a product you actually use.

• Support atomic operations including rollbacks on failure

  When implementing your API methods, ensure that if an exception occurs or an operation doesn’t complete, the all the changes made by your API call are reversed.  Consider explicitly supporting transaction scopes in your API to let your consumers compose their own “set” of operations.

• Support bulk operations where appropriate

  Building support for bulk operations into your API can often prevent performance issues occurring later when a user tries to, for example, insert 10,000 customers sequentially.  Consider pluralising your methods, so instead of providing an AddCustomer(Customer customer) method, provide only a AddCustomers(List<Customer> customers); method.  Doing this prevents callers from overloading your system by bulking data through your API in unintended ways, allowing you to properly cache required data and cater for these bulk operations.

  This isn’t always appropriate, however I’d always strongly suggest offering pluralised versions of methods that you suspect may be used in bulk, in order to help optimise your API calls and reduce the amount of data being transferred over the wire.

• Logging as a first class citizen

  Don’t wait until somebody asks about API usage to decide to log it.  Build logging into your API wrapper, from the start, at the point of every method call.  It doesn’t need to be fancy,
  and you can use a number of freely available components to handle these logs and log rotation (consider using log4net or log4j for simple log rotation).

  Log each method call and some summary or identifiable element of the data passed to it.  This’ll help you profile API usage, and identify how data changed in your previously closed system.

• Security as a first class citizen

  Consider the security of your API from the start of the project.  Understand who will have access to your API, which organisations and which individuals.  Do you require roll based security?  Do you need a way to disable API support for specific customers?  Are you transferring data that needs to be encrypted over the wire?

  Beware of over baking your security.  WS-* offers some very robust packet level security features, but if your API doesn’t need them, or is restricted to an internal network, then don’t bog down your implementation in unneeded security.  Beware of making security choices that tie you down to a specific protocol or technology stack – you want to keep your API usable for the consumers.  Do the simplest thing that works.

• Have integration tests!

  Make sure you have integration tests with mocking at the business logic layer. These tests are for your API wrappers, NOT your logic. The logic should be tested independently, you’re just ensuring your API layer, marshalling and method calls operate correctly at this level.

  With any luck, your business logic should already be tested as part of your existing test suite (which you have right?) but if not, ensure the business logic is tested separate from the API code.

  Consider using a TDD or BDD approach to designing your API calls, designing the specification first in the form of some calling code, then write the code required to make your usage examples compile.  This will help you understand exactly what calls the client will have to make for to your API to achieve specific functionality.  These tests can happily double up as regression tests when you make changes to the API.

• Keep it simple!  If all else fails, do what the big boys do.

  Always strive to keep your API simple.  Pretend your the consumer at all times.  If you’re unsure of how to proceed, I’ve always found inspiration, both for what to do and what not to do, from reading the API documentation of some large companies that have widely used APIs.  It’s safe to say that the likes of Amazon, Google and Microsoft have had to put some thought into their API designs.  Beware of trusting their decisions blindly, but liberally borrow anything you, as a consumer, would find pleasing in your API.

I’m not going to try and convince you that by following my advice that your API design will be flawless.  I’m really hoping for a little discussion on this topic as it seems like something that is rarely covered and often “felt out” by the people left to implement APIs for the public.  These are just some lessons I’ve learnt on the way while implementing several public facing APIs.

Want to talk about APIs?  Send me an email!

A common complaint when faced with localizing ASP.net MVC pages is that littering your code with tonnes of runat=”server” tags breaks the “purity” of the MVC model.  Regardless of how meaningful that debate is, there is a way to achieve globalisation without server controls.

I’m going to walk you through a sample implementation which should hopefully make this clearer.  As a standard disclaimer, none of this code has been tested in a production environment and I wouldn’t advise implementing it blindly.

I’m going to attempt to play this out mostly in screenshots…

The Idea

• The required language is stored in a database / session / extrapolated from the Url route information
• Your view pages derive from the type TranslatableViewPage

• This page adds support for a LanguageCode property that you can make use of inside the view along with adding a class implementing ITranslator, a class that provides hooks into a translation database.

• This controller add a method called ViewInLanguage(string languageCode) that you use instead of View() or View(model) to return your Asp.net MVC view.

Class Layout

The following is built upon the standard Asp.net MVC starter project for the sake of illustration.

Implementation

First, configure the translation settings and register your translator…

You need to configure a fail-safe default language code and a type that implements ITranslator.  This translator will be responsible for doing the heavy lifting.  Ideally we’d add some inversion of control here to allow you to define the translator implementation at configuration time, but that’s outside of the scope of this example.  I’ve implemented a very crude resource .resx translator for the example.

Next, make your controllers inherit from TranslatableController

Once inherited, switch from using the View(); method to ViewInLanguage() passing in your desired language code (gathered from the user session / database / Url route).

Then set your view type (or derive your view from) TranslatableViewPage…

Once your view is inheriting from TranslatableViewPage, you’ll have access to an instance of your specified Translator and the LanguageCode inside the view which you can use in a manner similar to the built in HtmlHelper class to access your translated strings.

How It Works

The TranslatableController provides you with ViewInLanguage and when called generates the standard MVC ActionResult.  This ActionResult will be a ViewResult, which is then wrapped in a LocalizedViewResult wrapper class adding a LanguageCode.

When ExecuteResult is called on the LocalizedViewResult to render the view, the LanguageCode is placed in the TempData array in the TranslatableViewPage.  Then, when the OnInit(EventArgs e) method is called on the TranslatableViewPage, this LanguageCode is extracted and placed in the LanguageCode property on the page.  In addition to this the page provides a constructed instance of ITranslator which you can use in your views to source translated data as the page is rendered.

Source Code

Download Here

A quick code snippet.  These two extension methods (C# 3.0+) enables you to return custom attributes from types, both on a specific type and any data type.  They both return null if the attribute is not present.  

The first extension is useful for extending a type of your choice.  It’s especially useful when considering Enums and the DescriptionAttribute, or other enumeration attributes (especially custom attributes).  The usage examples below should make the use more obvious.

The second extension allows you to return an Attribute from ANY type.  Use with caution, but I find it interesting that it’s actually possible to write an extension method that works that way.

    public static class ElementExtensions
    {    

        public static T GetAttributeValue<T>(this SomeConcreteType val)
        {
            var attributes = val.GetType().GetField(val.ToString()).GetCustomAttributes(typeof(T), false);

            T response = default(T);
            if(attributes.Length > 0)
            {
                response = (T)attributes[0];
            }

            return response;
        }

        public static TReturnType GetAttributeValueFromType<TInstanceType, TReturnType>(this TInstanceType val)
        {
            var attributes = val.GetType().GetField(val.ToString()).GetCustomAttributes(typeof(TReturnType), false);

            TReturnType response = default(TReturnType);
            if(attributes.Length > 0)
            {
                response = (TReturnType)attributes[0];
            }

            return response;
        }       
    }   

Some usage examples….

    public enum RegexEnum
    {
        [RandomAttribute("^(([a-zA-Z]{1}[0-9]{3,4})|([a-zA-Z]{2}[0-9]{2,3})|[0-9]{4})$")]
        SomeEnumValue,
    }   

    public class RandomAttribute: Attribute
    {
        public string Expression { get; set; }

        public RandomAttribute(string expression)
        {
            Expression = expression;
        }
    }

    private static void Example()
    {
        // Both of these are equivalent
        foreach(var enumMember in Enum.GetValues(typeof(RegexEnum)))
        {
            var value = enumMember.GetAttributeValue();   
            var valueFromAnyType = enumMember.GetAttributeValueFromType<RegexEnum, RandomAttribute>();
        }       
    }

Some nice syntactic sugar for use with enumerations.

I’ve previously provided a code snippet illustrating how to use WiX (the “new” Windows Installer framework) to install certificates onto the target machine.

I’ve had some good feedback about the post, but also had a request for a working example (in order to illustrate how the code fragment is used in an Installer.wxs file).

I’ve cooked up a sample visual studio solution that installs a test certificate (generated using the MSDN makecert certificate example “makecert -sk XYZ -n “CN=XYZ Company” testXYZ.cer “).

You’ll probably need to re-path references to the WixIISExtension and WixUIExtension in relation to your development environment but hopefully this should help you.

The previous post is the third most accessed post in my blog (which probably speaks as much for the number of people that read this as it does it’s utility) so this should be useful to someone.

It’s worth noting that this example installs the junk certificate as a Root CA so you might want to change that or ensure you remove it once you’re done.

Download the Sample Solution

[NOTE: This post relates to an old version of MobileTFL - Please see http://www.davidwhitney.co.uk/software for the latest version]

I’ve just compiled what I hope is the final version of this application barring London growing extra underground lines.

I’ve written a small Windows Mobile application for phones running WinMo 5.0+ with Compact Framework 2.0+.  It syndicates the Transport for London live data in similar way to the pre-existing iPhone and Android applications allowing you to view the current tube network status, planned engineering outages and local station notifications at the push of a finger on your Windows Mobile.

It’s currently living at a holding page at http://www.davidwhitney.co.uk/software (soon to become a larger site) and it’s a free download for anyone that uses Windows Mobile.

The 1.0.0.4 update is a reaction to some feedback I’ve received over at http://www.xda-developers.com and comes complete with coloured glyphs representing each tube line and enhanced resolution support for devices with “unusual” resolutions such as the Sony X1.  I’ve also had a stab at some font and UI element scaling so hopefully the interface will look quite natural on all your devices.

I’ve only managed to test it in the wild on a Touch Diamond, Touch Cruise and the Windows Mobile 5.0 Virtual Machine (along with in Windows).  It seems to run admirably in those conditions.

Obviously it requires a data plan so beware of any provider costs.

You can download the .cab directly at this link: http://www.davidwhitney.co.uk/software/repository/MobileTFL/1.0.0.4/MobileTFL.Setup.CAB

A very quick late night post.

I’m a big fan of HTC’s Touch Diamond once you remove all the cruft that the major phone networks like to cram onto their devices.  It’s a pocket sized, powerful smart phone.  I picked one up only a few months ago as a replacement for my HTC Touch, which was starting to feel very very slow compared to some of the handsets on the market, and the addition of HDSPA on the Diamond was the clincher.

Anyway, as a result the good lady has been quite impressed with the functionality of the Diamond.  She works in the media and the ability to access the internet with an almost desktop like experience (and without an iPhone) was very appealing.  While we were in town this weekend sorting out a new contract for her, I noticed that Google’s G1 ships with a London Tube service status application.  Seeing as Eleanor lives in London it seemed like an instantly cool thing to have on hand, but to my surprise, there isn’t anything comparable available for Windows Mobile (or there is and it’s too difficult to track down).

So I wrote one.

I’ve put up a really REALLY retro holding page at http://www.davidwhitney.co.uk/software for the purpose of releasing this application, hopefully over the coming months I’ll use it as a gather place for all the applications I’ve written, both free and pay-for, but for the moment, feel free to go and pick up a copy of the ingeniously named MobileTFL (after the transport-for-London website, where the application sources its data).

If you’re too lazy to click through one link:

Download Mobile TFL for Windows Mobile Here (Cab file)

You’ll have to forgive the exceptionally low-fi website and hilarious low res Visual Studio 2008 virtual machine screenshot.  I’ve only really tested the app on the Diamond I have sitting on my desk right now and it works a treat.

Obviously it requires you have a data plan that’s from a company that believes in Mobile internet rather than customer robbery but it only uses a tiny amount of data (it’s just a Http request).

It also requires the Compact Framework.  I’ve built it under CF3.5 but I suspect it’ll run just fine under CF2.0 (they’re binary compatible after all).

Feedback is more than welcome, just send me an email about it.  I’m especially interested on how the rendering looks on your devices, as the rendering code was written quite quickly and as soon as it looked ok in the VM and on my device I pretty much packaged it up.

I doubt it’ll brick your device, but if it does, you know the drill, it’s your problem.

Time for sleep now.

[Update: Microupdate to version 1.0.0.3 to fix a few bugs and aestetics, the above link has changed, revisit it for the new version]

It occurs to me that I wasn’t thinking straight when I wrote the previous entry regarding my reluctance to use .Net 3.5 SP1 in ‘Encounters’ for hybrid networking.

In a clearer state of mind, it occurs to me that because only the Windows version of the game is going to use the WCF networking implementation, that only the Windows version of the game will require the 3.5 SP1 version of the framework as a result.

The Xbox 360 appears to use a version of the Compact Framework 3.5, however it’s worth noting that 3.5 is a binary compatible set of additional assemblies that run in the .Net 2.0 environment.

What this effectively means is that I can use functionality in 3.5 SP1 (specifically the ability to serailize objects that aren’t marked as DataContracts or Seralizable) for the Windows version of the networking stack without contaminating the code of the game and preventing it from running on a 360.

Even better, it means I don’t need to mark-up my data model with any kind of attributes to support network play on Windows and I shouldn’t need to use a dubious set of byte array wrappers for data that can be typesafe, effectively letting me maintain a “purer” game model.

I’ll move my development environment to 3.5 SP1 and test this theory later, but I suspect I can simplify the networking stack on windows without the need for any messy hacks because of SP1.

Sometimes sleeping on a problem really is the answer; I was about to do something pretty stupid.

I’ve jumped right in to the deep end with my game project (which is going under the working title of “Encounters” – I needed to call the solution something!) and one of the core design goals of the project is the multiplayer focus of the game.

I’m a huge fan of Id Software.  John Carmack is probably my favourite “celebrity computer programmer”.  He’s incredible smart and consistent and has made some of both the most influential and my own favourite games.

Standing on the shoulders of giants – The “Quake” model

I’ve always thought the “Quake model” of single player and multiplayer game development to be a good one.  For those that don’t know, one of the simple design tenants of the Quake engine is that everything is a multiplayer game.  When you play the campaign in the original Quake what the game actually does is start a local game server which you then connect to.  Due to the proliferation of the Quake engine and other engines that have been inspired by it’s design this became quite a common way to build first person shooters with multiplayer support from the offset.  It reduced the implementation of multiplayer to simply having a second player connect to the active session.

Standing on the shoulders of giants, I’ve decided that this is the model I wish to follow for Encounters, especially seeing as the primary work-in-progress game design desires 4-player coop as the main campaign (with NPC assistants if you play with fewer humans).  Because of the way supporting multiplayer from the offset effectively means that all the games “thinking” is done in the server component (collision detection, cheat prevention, state management..) I decide that it’d be the best place to start the implementation.

Prototyping

So far so good, I spent an hour or two last night designing a simple state managing game server and retrofitting it to my previous prototype (an arena with a player-controlled unit moving around it) and moving all the logic and validation to the server side.

At this point the “game server” was a singleton class that the game accessed though a fake “proxy” class (left empty for eventual network implementation) and it worked pretty well.  The collision detection worked on the “server side” when the server was running in the same application domain and as a proof of concept everything was quite sound. I’m still left facing a few issues surrounding the frequency of syncing with the server and client side prediction, but they’re all relatively well solved problems in gaming (and there’s certainly some prior art to take inspiration from in XNA tutorials around the web).

Building a network stack

Once the proof of concept was working I started looking into the networking support provided by XNA and hit the mother of all roadblocks.  Because I’m targeting both the 360 and Windows, networking becomes significantly more complicated.  See, Microsoft offer no direct network access on the 360, via XNA, or even to their licensed partners (internet hearsay claims).  They provide Xbox Live APIs as part of the XNA framework however, which seemed like a decent solution until I realised that these networking APIs function only on a 360 as part of Xbox live.

Not so multiplatform really

So I’ve started thinking around the problem.  I do a lot of work with WCF in my day-job writing distributed systems so my obvious inclination was to provide a WCF client-server architecture for the Windows targeted version of the game.  This adds an additional set of issues.  For a start it looks as though the WCF assemblies (System.ServiceModel) are probably not available on the 360.  You’d think that’s not much of a problem if you’re going to use the Live APIs, but then you realise that if you’re using a version of .Net 3.5 prior to the first service pack, you need to annotate any classes you want to transfer via WCF with the DataContract attribute.  Which is in the System.ServiceModel namespace.

I’m not sure (as I currently don’t have a subscription to deploy my test code onto my 360) what version of the framework the 360 is running, but I’d hazard a guess that it’s pre SP1.  I’m going to have to further research this problem, because if you can use SP1 on the 360 (which supports serialization without any attributes on the classes) then we’re home free.

Multiplatform networking code using Inversion of Control

In the interim I’ve devised a cunning solution (or perhaps workaround, I’m not sure).  See, all of these issues lead me to need two entirely separate network stacks for my game.  One that supported the Windows, and the other that supported Xbox live, without contaminating my data model with mark-up that’s useable in only one or the other of the stacks.

Castle Windsor to the rescue.

This is purely speculative as I haven’t attempted to implement the 360 network stack yet, but I intend to use the Castle Windsor IoC (inversion of control) container to load a separate network stack at runtime, depending on the platform.

My idea is that the game knows about two interfaces IGameServer and IGameServerConnection.  I then create two entirely separate assemblies, one with the networking code for the PC, the other with the Live networking code.  The PC version contains a class called WcfGameServer which implements IGameServer (I’ve already written this code), and a class called WcfGameServerConnection which implementes IGameServerConnection (and acts as a hand crafted WCF proxy).  The 360 implementation will feature XBL counterparts to these classes.

As far as the game is concerned, it’ll use the Windsor container to load an instance of IGameServer, call the StartListening(); method, then use Windsor to load an instance of IGameServerConnection() and call JoinGame() on that connection.  This way, the specific networking implementation is entirely removed from the game and hidden behind these two simple interfaces.

I’ve currently got a good way through implementing the WCF version of this model though I’ve hit a few snags on the way.  Because I’m deliberately developing in a pre-SP1 environment for the sake of this exercise, WCF doesn’t like serializing the game model to send over the wire.  As a result I’ve had to produce an inelegant hack to work around my desire to keep the System.ServiceModel assembly reference clear of my game model.

Trying to keep it light

I’ve marked up my model with Serializable attributes (.NET 1.1, that’ll be fine) and inside my WCF implementation of IGameServerConnection I’m marshalling all my data into byte[]’s before sending it over the wire.  This isn’t ideal as it requires the WCF implementation to manually deserialize the byte[]’s into their correct data types in the service implementation and the client library, but it does work.  Unfortunately at the moment these byte arrays are being stored as XML before being sent over the wire (as is the default WCF way) so I’m going to need to force WCF to binary serialize all it’s data rather than bloat my packets (and as a result, the game latency).  I’m using Net.Tcp so it’s pretty lightweight as far as protocols go but I suspect I’ll need to do some additional fine tuning to make the WCF implementation viable.

Either way, I’ve got a good feeling about the model, subtle hacks aside, and I think this is quite a good way to target both platforms with minimal impact to your game code.

So this is day 3 of development (I’m sure I’m going to loose count pretty quickly).

[Footnote]

It occurs to me that I got a few fundamental things wrong when this post was originally written regarding the requirements for .Net 3.5 SP1.  These mistakes almost complicated the design of the networking stack.  Read more in my follow up here.

It was always a matter of time before my addiction to games and my addiction to programming eventually collided in a horrible mistake.

I’ve decided to start writing a game, using a home grown game engine hopefully with “a little help from my friends”.

I won’t spoil anything too much (as I don’t really want to play my hand too early) but the gist is a 2d, top down, multiplayer action game, with heavy RPG tendencies.  Think of it as a cross between 2d Zelda games, Cannon Fodder and high end raiding in World of Warcraft.  The game design is up in the air at the moment, but hopefully it’ll be derived from a system my good friend Matt has started fleshing out (with the potential for a more RPG leaned game mode, and a more arcade oriented game mode).

The only problem is, I’ve never attempted to write a game before.

So hopefully we can learn together.  I have a rough hit-list of the ground I’m going to have to cover to pull this off, and a rough idea of the technology involved (partially from my other programming pursuits, partially from my enthusiastic following of the games industry) but actually implementing it from the ground up is going to prove an entertaining learning experience.

I’ll tag these posts so people can avoid or watch them at their leisure- there’s a good chance I’ll be dropping into lots of technical detail with complete disregard for the experience levels of my audience. That said, I’m going to be learning a lot of this stuff from scratch (collision detection, graphics rendering, effective ways to script games) while dragging in my bread and butter (distributed systems, C#, etc) to hopefully fill in around the edges.

I’m going to be working in the XNA framework, partially to lower my barrier to entry (seeing as .Net is the day-job and the hobby) and partially because I enjoy the ability to target both the PC and the Xbox360 platform using roughly the same code.

It’s going to be a rocky ride I’m sure, I’ve been working on proof-of-concept code for about a week on and off and it looks like it’s something you can stumble in to (especially with some of the fantastic guides available online).  I’m more than willing to take pointers or tips, so if anyone knows of any especially trustworthy resources on authoring 2d games I’d love to hear some suggestions.

Recently I’ve been trying to pay a debt of sorts.  I use the software provided on the fantastic XDA-Developers frequently on my Windows Mobile devices and have been doing for almost three years now.

So currently, if I have a little bit of free time, I’ll nip past their development section and try and fulfil a random request.

Today’s request might come in useful to a little bit of a wider audience so I’ll post it here.  The initial problem is outlined in this thread and the upshot was that someone needed a very simple Http server that could run on Windows Mobile, written in C#’, and was programmable.

What the guy really seemed to be looking for was something that used the Http protocol to return random computed data, so after a tiny bit of googling and a simple MSDN example, I’ve built a really simple web server for Windows Mobile.

Amusingly, my sample doesn’t behave much like a web server at all.  It just tells you what you requested, but it should be enough to get you going in the right direction.  It’s derived from the MSDN example with some additional sugar, and I certainly wouldn’t suggest that it’s got a threading model that’d stand up in a production environment (at a glance it looks like it’d process requests in sequence…) but hopefully it’s useful to somebody looking to produce a simple server, or who is just interested in how Http works.

I’m not really sure if I can accurately call it a webserver, seeing as it doesn’t even support a full set of Http Verbs, but you get the idea.

I’ve not actually bothered compiling this on a mobile device yet (lack of inclination) however seeing as it was explicitly based on a socket programming for Windows Mobile MSDN example, I suspect it’ll work just fine.  No warranty, do what you will with it.

Download Simple C# Http Server for Windows Mobile (Source Code Only) (7kb)