Software Architecture Introduction

I work as a Software Architect/Systems Architect and many times when I do job interviews it seems that people simply have no clue about what I do. I can't blame them as such terms have many meanings and if you look in wikipedia links Software Architect and Systems Architect you will find that some of them seem to be completely different tasks.

In fact, the entire problem lies on the fact that almost any decision made before actually writing some code may be seen as architecture. If I decide to create a game, deciding which kind of game will be created is already a decision of architecture. It is not software architecture by itself, yet such an initial decision will affect the programming that will be done later, as different kinds of game require different kinds of decisions.

But we usually start to talk about software architecture when we start to choose the technologies to be used. That is, will we use XNA? Will we use C#? Will we use Javascript?

If we decide it will be C# but not XNA, will it be Windows Forms? WPF? Silverlight?

And, if it is a multi-player game, will we use pure TCP/IP (or UDP) writing all the communication layers/details or will we use a high-level framework like WCF?

This is a very important decision time, as the entire evolution of the application may go better or worse by those initial decisions. Yet, except in the situation that we decide to write the entire communication on our own we are in a moment to "choose" from existing technologies, not to think about how to create them.

And, the worst truth is: Usually, independently on which chooses we make at this point, the application can still be developed. As I just said, the entire evolution of the application may go better or worse, but it will "be possible".

And apparently that's what most architects do: They choose technologies to write new applications. And something that makes me sad is that they usually don't think about the problem at all, they simply use extremely basic conditions as the parameter to their decisions, like:

·         If it is a game, use XNA as it is optimized for games;

·         If it is a local application, use WPF (if they love new technologies) / use Windows Forms (if they prefer old technologies);

·         If the applications need to communicate with each other, use WCF.

And after those decisions (that is, after the initial "architecture"), they keep working, having to find "work-arounds" over usually bad decisions (or the lack of decisions) they did at the initial stage. After all, if the initial decisions were all right and they aren't going to develop their own framework, why will they continue to work in the project?

Frameworks

I was just saying that the initial stage is to usually choose technologies, like WCF, WPF and the like. For web-sites that will be something like ASP.NET + MVC, Web Forms, caching technologies and the like. All of those technologies can be seen as "frameworks" to do one kind of job.

Well, as an architect I usually have the job of creating frameworks like these. My purpose is rarely to choose the best existing framework, but to make the right decisions to create frameworks like these that work correctly (with good performance, memory consumption, ease of use and most important of all: really expandable).

But I think that you may be scared already: If I want to create a game, will I lose time creating all the technology? That's crazy!

I can agree that for a small project it may seem crazy to write an entire technology when there are others already available. But, first, that's my specialty. Maybe it is not what a company is looking for. Second, in many large projects creating the technology, even if it starts redirecting to another one, opens new possibilities. In fact, I started to create frameworks because most of the time I simply considered the architecture of the already existing ones terrible. It doesn't mean they don't work. It simply means they weren't really helping or making things easier and, in many situations, they were limiting what can be achieved.

But before explaining the problems or the solutions, I will try to explain my view on what is a "framework".

What is a framework?

I frequently see a definition that "you call a library, a framework calls you" and, even if it is OK in the sense that when you use a framework you must "obey" its rules, usually filling events or implementing virtual methods that will be called by the framework, it is very problematic in the sense that some classes may be used directly (like a "library") or inherited (so the virtual methods will be called like a "framework").

Also, any DLL is a library (that's the meaning of the last L), which can contain one or more "frameworks".

So, I prefer to say that there are frameworks in the general sense and in the specific sense. That is, a DLL created to contain a framework "is a framework", but in fact such library can contain isolated classes, usable by any applications, the main framework and even "secondary frameworks".

That is, any solution to a kind of problem, be it build of a single very useful class or by a collection of many classes may be considered a framework. A framework usually has many classes, but in your initial use you may only use the basic methods provided by a single class and only later you may use the extra functionalities.

For example: When you use the BinarySerializer class you are using the basic serialization provided by .NET. But you can create your own serializable classes by using the [Serializable] attribute and even by implementing the ISerializable interface. So, there is an entire framework, but in your initial case you may be using it as a simple "library" class.

A common error: Creating your own framework is bad

The normal arguments I see against frameworks are:

·         A framework forces your application to work in a specific direction, avoiding you from doing anything different;

·         The code of a framework to solve a problem is harder than solving the problem directly so, in most cases, creating a framework will only add complexity to the project;

·         Using the previous definitions, some people say: "Create a library, not a framework";

·         Your framework will never be as feature complete as a framework made by a company dedicated to do that;

·         If you quit the company, who will maintain the framework? By buying it from a company we have the guarantee that we will have support.

And I must say that I mostly agree with all the arguments. But the truth is: Any big project ends-up having a framework, be it a well architected one, be it a messed-up one made on top of other frameworks (and that's what some developers that hate frameworks usually do).

That is, developers that avoid creating a framework to buy an external one usually finish with their own framework, based on an external one and it usually has the original limitations + the limitations they may have added to it.

The entire idea is that by using software made by a company we have a better support, better quality etc. But a company dedicated to create a technology don't know our specific needs, so they will give us some "generic" solution. Unskilled programmers may try to do the same and they may end-up doing a very poor job. Very experienced developers may make a better solution for the company, even if it is not as feature complete as the one bought from another company.

So, if you are a really experienced developer (or if you have really experienced developers working for you), it may worth to let them create a framework specific to the company's need.

Architecture - The Bad Ones

When I think about a project, I usually start by thinking what I want to do, then I think the things needed to do the job (the concept of a technology, not the technology itself) and only later I think about the existing technologies that may help me in doing that.

But because I already thought about possible needed technologies without thinking about a specific one, I didn't think about any limitations, any technology specific data or any work-arounds. Then, when I see the existing technologies, what usually makes me decide to create a framework on my own is that those technologies expect the application to be done "to use them" and, even if using one technology in this situation is acceptable, I can't put two external technologies (frameworks) to work together, as one doesn't know about the existence of the other and that's a requirement of the other framework.

What I mean by "they expect the application to use them"?

Well, they expect that your code is written to:

·         Inherit from their base classes;

·         Implement their interfaces;

·         Use their attributes;

·         Or anything like this, which requires the code to be compiled with a reference to them.

And so, if you use objects of Framework A (which doesn't know about Framework B) you can't use those objects with the Framework B if you don't create adapters.

Creating adapters work but, in some cases, it is a waste of time. When we use a framework like Serialization we want to "convert object instances to bytes" without caring how to do it. But if we need to create an adapter that's serializable, why not write the serialization by hand? And worst, you may have a really big graph of objects, and only one of the objects may not be marked as [Serializable], even if it is extremely easy to make it serializable. But you still need to recreate the entire "adapted" graph to solve such a problem as you can't change the source code of an external framework.

And, if you think you can create something to automate the entire adapted graph, you will be creating a "framework" to create adapters. So, why not create the right framework directly?

Note: I already talked about how the attributes violate the Single Responsibility Principle in the article Attributes vs. Single Responsibility Principle. There some people argued that [Attribute]s aren't code. That they are attached to classes/properties and not part of them. Yet, consider the problem of third-party libraries. You can't change their source code to add the attributes and you can't add attributes at run-time (well, at least not until .NET 4... I am not sure if that's possible in .NET 4.5).

Also, there other kinds of problem. Usually they aren't as bad, but I consider them to be very annoying. This happens on frameworks that expect to find some configuration directly in the configuration file, without giving you a chance to set such a configuration from code or on frameworks that do some kind of action automatically but don't allow you to extend such an action, only to replace it (and worst, that usually must be made instance by instance when a global extensibility point would be better).

So, which frameworks I consider problematic? Most of them, even if they are world widely used.

This includes:

·         WPF "convert" bindings;

·         The TypeConverters in general;

·         Default binary serialization of .NET;

·         Default XML serialization of .NET;

·         WCF attribute-driven architecture;

·         MarshalByRefObject and all the classes that already inherit from or depend on it;

·         Most ORM frameworks which usually are attribute based, configuration file(s) based and constrained to database-types requiring adapters to be created if we want the data to be presented with application-specific data-types.

I am not saying that those frameworks don't work. They work. But they could be better.

So, explaining each point:

·         If you don't specify a Converter in a WPF binding, it is able to do some automatic conversion. I think it is able to use the [TypeConverter]s, which are already limited. But you can't register a converter from type A to B to be global to your application if those types are from unrelated assemblies;

·         The [TypeConverter]s in theory can convert any type to any type, but they require the attribute to be used in one of the two types (be it the source or the destination) and a single type-converter must know all the possible conversions. So, considering that we may have types that are easy to convert from one to the other but are coming from unrelated libraries, we are stuck. So those type-converters end-up used only to convert to and from strings or some of the primitive types;

·         The .NET Binary serialization can't serialize a type that's not marked as [Serializable]. It is not important if you know how to serialize it. This is even worse if only the deeper level of a big graph is non-serializable;

·         The .NET xml serialization doesn't share the binary serialization attributes, so if you create a class that can be serialized by both you need to remember to use the attributes for both;

·         What can I say, you can't get a component that's already made to work as a service (for example, stateless and using only basic data-types) if it doesn't use all the "contracts" expected by WCF. I will explain a little more on this later;

·         The entire idea of the MarshalByRefObject is that all the calls become virtual, even if you mark the class as sealed, so they could be "replaced". Well, interfaces are purely virtual, but with an interface you have the option to use the calls as purely virtual or to continue using the rightly typed class, avoiding any virtual call. With the MarshalByRefObject you always end-up doing virtual calls, even when you don't want to. So, tell me how many times did you open a file (FileStream) and really expected it to be replaced by another class? Why not use a Stream (or better, if it existed, an IStream) when you want any stream, and use non-virtual calls to the FileStream when you know its exact type? Unfortunately, by being a MarshalByRefObject you can declare the variable with the real sealed type and the virtual calls will continue to be done.

IoC - Inversion of Control

Maybe I am getting a little off-topic here, but another thing that annoys me is the now popular idea of Inversion of Control. In fact, I already consider it a bad name. If the correct architecture is to "invert control" and people respects it, then it becomes the "normal", not the "inverted" architecture.

To allow such an inversion of control it is recommended that you only depend on "interfaces" not on "implementations", but such a solution is not the best solution all the time. Some components may expect to work only with their "family" components, not with any other component. So, if you use IoC with them, you must use IoC for the entire family, effectively being able to replace one family by another one, not to replace individual components.

That's the case with ADO.NET connections, commands, parameters and the like. You can replace the entire SQL Server family by the entire Oracle family, but you can't replace only the connection without replacing the other components. So, if you are not writing the application that uses components like that, but writing such components (I mean, any component family or framework) you don't really require to make one component to talk to the others only by the interface. Having the interface is good to avoid the need for adapters if you want to replace the entire "family", but the components can talk to each other knowing by their right types.

In fact, the best architecture in such a case is to have the interfaces declared in a common assembly (DLL) and to implement the specific "families" in other DLLs. The users will then be able to depend only on the common assembly and thanks to an IoC container choose which "family" to use at run-time. But each family can be written depending directly on their family components, avoiding the interfaces, the virtual calls, having access to internal fields, properties and methods and also avoiding the IoC completely.

So, if you think that you should make every class only talk to other classes by interfaces, well, think again.

A Note About "Component Families"

I just talked about ADO.NET to explain the component families and a thing I see very frequently is a solution that "loads" drivers using a "rigid" rule.

That is, ADO.NET uses entries in the configuration file (and the Machine.config) to search for database drivers by name if you use the DbProviderFactories.GetFactory() method. That's a extremely rigid rule. The application can't tell how to search the drivers differently.

Even if you can load the drivers without using the DbProviderFactories, remember about such a problem if you create your own "basic solution" capable of loading drivers. You may look for drivers locally or by using some rigid rule like that but, if one isn't found, allow an event to do the search. The AppDomain.AssemblyResolve is an example of how you can create an event to solve that "missing information" problem and it already allows some clever usages, like embedding the libraries into the application while allowing them to be found only when requested.

Architecture - First "Fix"

I know that most of us simply can't solve the architecture problems of already existing frameworks. But, if you work in the creation of some framework, there's an "easy-fix" to most of the problems, and it is very similar to the AssemblyResolve event: Call an event to try to do the job before failing.

If we see what's happening in most cases, it is like this: A framework wants some more information to finish its job and to find such an information it may:

·         Read a configuration file;

·         Read an attribute;

·         Cast your instance to an interface.

And, if it isn't able to do that, it simply fails/throws an exception.

So, why not call an event at that moment, giving all the information you already have (that is, the instance you are working on, the action you want and the parameters you already have, like a conversion from a value X to a specific type) and let the event tell you if it was able to do the job or not?

Only in a situation where the event doesn't do the job you generate the error/exception.

This will solve the .NET binary serialization, the XML serialization, may enable WCF to use types that don't have the right attributes and all of that. And, the best of all: As it is not a change to existing methods, but a new event, it will not cause a breaking change as old code will simply ignore the existence of such an event.

Only to finish explaining the fix comparing to the previously presented problems, the MarshalByRefObject is from another kind, which can be solved by using interfaces. And about the ORMs, well, there are many ORMs with different kinds of problems, some of them will benefit from such an event call.

Improvement to the first fix.

Note that the first fix is subjective already. For example, the event could be used to know if a type is [Serializable] or not, even if it doesn't has the attribute. This will solve the problem for types that have a serializable structure but not the attribute (and can be even considered a source of bugs if used incorrectly), but it will not help with types that don't have a valid structure but could be serialized by an user-made algorithm.

So, calling the event asking to serialize a type that's not serializable (instead of trying to consider it [Serializable]) would be much more appropriate. Yet MulticastDelegates aren't optimized to have a single answer. That is, there could be more than 30 (or even 300) event handlers attached, each one dedicated to a single type. Should we execute all the handlers all the time?

That means we may require another solution (well, at least if we want an optimal solution, as by simply having the event it is already possible to build a better solution on top of it). Mine solution for the serialization problem is to try to find a serializer for such a type, and then register such serializer in a dictionary. That is, I don't ask to serialize a given instance, I ask if there's a serializer for such a type and, if there is, I know that I can serialize other instances of the same type without having to call the event again (yes, I wrote my own serialization framework).

Well, for the entire concept I wrote an article called Actionless Frameworks, so check that article if you are interested.

Architecture - Second "Fix"

The first fix by itself may suffer from another problem: Be too local.

That is, for the serialization problem we may create a solution where the serializer has an event to serialize types it is not naturally capable of serializing. But will we add the handler per instance?

Even if you think it is appropriate (and it usually is), it is also very important to avoid repetitions and so, by the same way a type that has the [Serializable] attribute doesn't need to be "added" as valid per serializer instance, it is very important to have global solutions.

In fact, we can say that the first "fix" should exist as a static solution so it can work globally. If you add local and global solutions or only global solutions is not that important, as a global solution, if well written by the user of your code, could work correctly for local situations. The opposite, unfortunately, isn't true.

Architecture - Services

Now I will stop focusing on the fact that I like to create frameworks or on the problems of existing framework as you may be the kind of person that says that you will not create a framework and you will accept the limitations of the existing ones.

So I will talk about SOA (Service Oriented Architecture). It is a common idea now that we should use SOA as such architecture allows every service to be created as a separate application, even using different languages if necessary, and allowing many advantages like distributed processing, real isolation of failure points and many others.

The only thing that SOA really requires is communication. And, even if SOA already means architecture, every service also requires an inner architecture and, at least in .NET, the most accepted technology to allow the SOA to work is WCF.

Well, I just complained about WCF being too attribute based, but you may consider it OK as you will create a new WCF service and implement it as WCF from the start. So, the fact that it uses WCF specific attributes is not a concern at all... right?

And here is where I consider that many applications have a big lack of architecture.

A service is created to do some kind of job/solve some kind of problem. Such kind of solution may work very well as a [web] service. Yet the solution can (and I dare to say that in most cases it should) exist independently of the communication framework that's used.

One of the possible reasons is: Imagine that you decide that for a particular application you will embed the service in it or even that you will use a complete different service technology. Wouldn't it be much better if such "service" is a simple "library", without any WCF specific data?

So, the WCF part could be completely stripped away without problems. That is, the basic architecture may be: "Create any service as a library". Then, if you want to make it accessible as a real web-service, you create another application that's bound to the service and only fills the information needed to expose the library as a service.

And that's my problem with WCF. While it could be possible to transform a normal library that's already stateless into a service by simply "registering" the type as a service (the old and almost obsolete .NET remoting supports that) in WCF we should have a class full of attributes which, in a situation like the one I am describing, means it is necessary to have an "adapter" class per service class, only to add the needed attributes and redirect to the original, attribute-free, library.

But the worst problem I see is that many people will simply write all the code inside the service directly and, if needed, will import the service with a lot of unnecessary attributes to "embed" a service into an application.

Note: I am not discussing the fact that WCF can use different transfer protocols and all of that. I myself created a framework that allows local communications over Memory Mapped Files that's almost 30 times faster than the best WCF configuration I found for local communication. To me, WCF is very optimized for remote communications and do great jobs, but it is far from ideal for local communication, independently of its support for binary communication and pipes.

Architecture - Program to interfaces - Real situations

A common expression that I usually hear and see is that we should "write to interfaces, not to implementations". This is usually justified for things like IoC, testing and a lot of "amazing" things. Yet, as I explained in the IoC topic, simply making every component talk to others by interfaces is bad. Family of components expect to work with their relatives.

Yet a very common situation that I see frequently is people trying to do globalization by using resources directly. That is, the code is dependent on the resources API and is not capable of working with non-resource solutions.

I can go one step further and say that globalization is a kind of feature we should consider the use of a "framework" or a "service" (or even both, depending on some kind of configuration).

So, how can we achieve such a support for both? I just answered that. Interfaces.

Having all "services" seen as interfaces locally allows those services to be implemented differently without breaking your code. That is, a basic application may implement the service to respond that it doesn't find any translation (and I am already considering the program uses some language, like English, by default), a little better implementation may use a text file to find translations, some other implementations may use specific resource files and some others may redirect to an external service or even find those translations using a database.

So, one of the good things that programming to SOA do is that references to other services are usually already implemented by the use of interfaces. So, if you have an interface, you can change the actual implementation without problems.

Architecture - Application

When talking about SOA I said that one of the advantages is that services usually are presented as interfaces, so the code is already prepared to be "replaced" by another implementation.

But that's a half truth. Surely by using interfaces we can replace one instance by another one. But how are we getting our instances?

A common architecture problem of SOA consumers is that they call the service library directly to create the service instances and so, even with interfaces that allow the implementation to be replaced, they are completely bound to the technology that implement those interfaces. The code simply can't replace one implementation by another one, as the "start point" is already the service library (be it WCF or another one).

So, following the same principle that we should make our service as a library and only later, if needed, create the service (as a separate program that uses such a library) we should program the application in a manner that it doesn't directly see the communication layer/technology. That is, when you program your application, it can't ask an instance of the service IMyService to WCF (so, your application should not see the ChannelFactory, the ClientBase or the System.ServiceModel.dll directly).

That is, you can use an IoC container or you can create your own class that will work as your "factory", which can use an event to create the implementations to the interfaces (services) you will ask. Then your code should only use such IoC container or factory as the starting point. With this extra "layer" you will be able to replace the creation of a service from a specific library to a "generic" one, and so you will be able to replace the implementation at any moment (including a local service instead of a remote one) without breaking all the places that instantiate the service.

Façades

Now that I presented the case that to call a service you should not ask directly to the service library to create the service instances, I will talk about something that's a little counter-intuitive.

I was just saying that we should use interfaces so the code can be easily replaced. But, for many situations, it is better to give some sealed solutions. Especially when talking about web services, as it is a common practice to pass all the needed parameters per call.

Compare this with normal objects that are created, their properties are filled and only later one or more calls are done, without any parameters or with a very reduced list of parameters.

So, to achieve this, we should use façades. We should create local objects that have a "local approach" to use the services, even if they internally redirect to one of those interfaces that have many parameters (and to which you may want to use some default values).

As I said, this may seem counter-intuitive as I was just saying to program to interfaces, to avoid adapters and all, and that will be an "adapter" that uses sealed or even static classes. But that doesn't mean that you will be bound to an implementation, as such façade will still use your IoC container or configurable factory. This will only mean that the users will not see the interfaces and the factory all the time. The developers creating the service and configuring the factory will see that, but the developers that will only use the service will simply see local classes that do the work correctly, without having to bother about condensed method calls and interfaces.

Conclusion

I hope that after reading this article you can see that home-made (or company made) frameworks aren't that bad. That worldwide known frameworks aren't necessarily more prepared to help your application evolve than a framework that you can write and that you can see that home-made frameworks can benefit from using actually existing frameworks while keeping the capacity to completely replace an old external framework by a new one without implying changes to the application itself, only requiring to fill some adapters if such frameworks aren't already prepared to adapt to your code.

And the most important conclusion of all is that if you write a framework, allow such a framework to be used in applications that already reference other libraries that aren't going to change, so allow any information that's required by your code to be found using different methods by creating an event to fill such an information if it wasn't already given to your framework by other means.

Reference From 

http://www.codeproject.com/Articles/680661/Software-Architecture

By Paulo Zemek