The Strategy pattern
The Strategy pattern is one of the OOP design patterns which I like the most..
According to wikipedia, “the strategy pattern (also known as the policy pattern) is a behavioral software design pattern that enables selecting an algorithm at runtime.” – source
This UML diagram showcases it Pretty well:
Why I like it?
I believe there are several reasons that make this design pattern one of the most useful ones around..
- Improves the KISS (Keep It Simple and Standard) principle on the code.
- LSP – The “Strategies” are interchangeable, can be substituted by each others. This is a clear aplication of the L in SOLID, the Liskov Substitution Principle or LSP: “Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.” – Source
- Open-Closed – The Strategy implementation through an interface is a clear application of the “Open-Closed Principle: “software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification”. In this case we can extend it by writing a class that extends the behavior but we cannot modify the interface. Even more, if we implement the Strategy using a Plugin implementation we do not even need to modify its source code. It’s a very clean implementation. It also helps into Decoupling the code and responsibilities. – Source
- SRP : We can strongly affirm also say that the Strategy promotes the Single Responsibility Principle as each Strategy implementation should be implemented in a single class.
- DI: And also, the Dependency Inversion Principle: “One should “depend upon abstractions, [not] concretions.” – Source. This is so as the Strategies depend on abstractions, an interface which defines the strategy.
We can easily implement the Strategy pattern with Dependency Injection but this makes that the code of the Strategy to be on the same assembly or executable and thus, coupled. Due to this, I consider this as a sub-optimal implementation which does not fulfill the Open-Closed principle at 100% if we consider the main Executable as a “Software Entity”.
Even more, if we are in a highly regulated Environment, this means that we can add functionality without altering “the main software” which might be subject to a regulated process like FDA-approval, in case of a medical system… that means several months of documentation, testing and waiting for FDA to sign everyhting.
Do you like it already? Wait – there are more Benefits!
In my previous work, at RUF Telematik, I proposed the application of this pattern with a plugin system as part of the Technical Product Roadmap. Basically to decouple the code which interfaces a concrete hardware (type of HW, manufacturer, version…) So the main software would not need to know how to talk with a camera, monitor or communication device in the train System. The responsibility is delegated to a .dll plugin that knows how to do that work and we can dynamically add These Features without altering the main software.
In addition to the software architecture benefits and the code quality, we have some more benefits:
- We can parallelize the development of the Hardware Manager dlls to different developers who can test them separately.
- We can separate the release and test workflows and accelerate the development time.
- We do not need to test the main software every time we add support for a new device or version of a device firmware..
- We do not need to revalidate through industry standards the full software over and over again (usually with an substantial cost of time and money)
In a train we could categorize the different hardware on it on hte following four categories:
- TFT screens
- LCD Screens
- RCOM Communication devices
- Camera devices
Each one has different Vendors, models and version numbers so a bit more complex implementation should be needed but this is an example so we do not Need to build “the real Thing”.
So we could implement an interface like ITrainInstrumentManager that supported methods like:
- and so on…
And then implement a Strategy that fulfills this interface for every Type of Equipment, for every brand/vendor and for every model and version…
With the added Benefit that I can parallelize this work and get several persons to work in different Strategies, one for each new device. This would enable to add support for new hardware devices in no time at all.
And they could be tested separately, with the warranty that if the tests do work, the main tool would work as well.
Without altering or releasing the tool, just adding the plugins in the corresponding folder or loading them dynamically from an online service or location. (if we implement the strategy using the plugin technique, of course)
This presentation showcases some of the points mentioned above, if you are still curious.
Implementation of the Strategy Pattern
One of the best implementations I have ever been part of is when I worked along 2011 and 2012 at Parlam Software where a plug-in architecture was designed and implemented by my now friend Xavier Casals. Then he was my customer and CTO of Parlam (and still is).
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This Plugin system enabled adding dynamically Data convertors to third party systems, like different CMS systems as “SDL Tridion”, to where his service connects and works with, so basically he can deliver an interface to anybody that wants to interface with its system and enables an easy implementation as well as testing and deployment. Once the DLL is tested and verified, can be signed for security reasons and added to a folder where it is magically loaded and we get the perfect implementation of Open Closed Principle…
“software entities… should be open for extension, but closed for modification”
I know it is a lot to say but let’s get it done and you tell me after 😉
We will create a .NET standard solution which will have implement 3 Projects:
- StrategyInterface –> A .NET Core Class Library that will hold the Strategy interface, two custom attributes and a custom exception for managing the plugin. This is the basic contract that we will share between the main application that will use the plugin(s) and the plugins themselves.
- Plugins –> This is a project with a simple class that implements the Interface on the StrategyInterface Project/Assembly. I will use the two custom Attributes to add a name and a description so I can programatically go through them with Reflection before creating an instance, which is convenient if I want to avoid creating excessive objects. Note that this Project will have different implementations, in our case I created 4: CAM, LED, RCOM and TFT. Each one will create a DLL in a concrete directory, “D:\Jola\Plugins”.
- StrategyPatternDoneRight –> feel free to discuss with me on the name, comments are open to all ;). This is the main customer/user of the Plugins that implement the Strategy and will load the plugins that match the interface from a concrete location of the filesystem. At the moment I did not put too much logic but just to load all the matching assemblies and execute a simple method which all the plugins provide.
The solution looks like:
The most important here is the interface that defines the Strategy:
There we will create the custom Attributes, one for Name and another for Description:
I created a Plugins Folder to contain them all, then created .NET Standard assemblies and added a reference to the StrategyInterface Project.
Lets say that we create the CAM.Generic Project to implement support for the Train Network Cameras… there we add a class which implements the Strategy Interface and we add the two custom Attributes to it:
Obviously this is a simplification but here we would put all the hardware dependant code for handling complex network operations with the camera…
All the Plugin Projects are customized to have the same Build Output Path, to avoid doing Manual work:
Just be Aware that the output path that you use must exist and be the same for all Plugins.
So, all we have left is to implement the mechanism to recover the assemblies at a concrete filesystem path and load them dynamically into our current execution process. We will do this using Reflection.
I am creating a wrapper class for exposing the strategies implemented by our plugin assemblies.
This class is named StrategyPluginContainer and will expose the two custom Attributes and an instance of the Plugin (really it is an instance of the class that implements the Strategy Interface).
The two key reflection techniques used here are:
- Activator.CreateInstance(Type) – This creates an instance of the specified Type using the default constructor. Note this is not reflection but comes directly from System Namespace.
- Type.GetCustomAttributes(Attribute type, inherit) – this obtains from a type the value of a custom attribute.
Note: green is due to style suggestions from my VS installation to what I do not agree if I want clarity. Expression bodied properties or using ?? are good, reduce space but if somebody is not used to this Syntax readability and understandability are reduced..
Now we can implement the StrategyPluginLoader. This class responsability is to Keep a list of the Plugins that implement the Strategy and it does so by loading them from the Filesystem (could get them from a web service or other mean).
Basically it has a List of StrategyPluginContainer which we just created, exposed through a property.
And populates it by getting all the DLLs from a specific hard disk Folder and loading them with Reflection’s Assembly.LoadFrom(filename).
then we get the types contained on this Assembly and iterate through them to match them against the Strategy Interface. I also check that the two custom Attributes are supported and if everything matches, I create a StrategyPluginContainer instance of this concrete type.
As a final check, I verify if the Plugin is already on the plugin list for not repeating and if is existing I update it in a proper way.
Last but not least, I use all this through a nice console application, I create the StrategyPluginLoader, I execute the command to load all the plugins and iterate through them invoking the only command in the interface which is implemented in separate, decupled assemblies and loaded dynamically at runtime, without no Knowledge or coupling of any Kind in the main application.
The full code can be found in GitHub here.