Creating a Plug-in Architecturein .NET

@OndrejBalas

Ondrej Balas• Husband & Father• Small Guy, Big Data• Owner of UseTech Design• I build software that drives business

Blog: http://www.ondrejbalas.com Twitter: @OndrejBalas

Plug-in Architecture

• An external piece of functionality that may be added to an existing system by abiding by a contract pre-defined by that system.

Plug-in (my definition)

Interface Segregation

Examples of Plug-ins

Benefits

Benefits• Split work across natural boundaries• Incrementally create and deploy features• Deploy only new and updated modules• Don’t need source code for existing

system

New & Old

Licensing Flexibility

• Changes to contracts are difficult to manage• Increased development time for

small projects

Drawbacks

But How?

// Get input – “HELLO”string input = Console.ReadLine().ToUpper(); // "Encrypt" itstring output = "";foreach (char c in input){ byte b = (byte) c; output += (char)(b < 78 ? b + 13 : b - 13);}

// Write output – “URYYB”Console.WriteLine(output);

Dependency Inversion

Tightly Coupled Code

static void Main(string[] args) { // Get input string input = Console.ReadLine().ToUpper();

// "Encrypt" it Rot13 encryptionAlgorithm = new Rot13(); string output = encryptionAlgorithm.Encrypt(input);

// Write output Console.WriteLine(output);}public class Rot13 { public string Encrypt(string input) { string output = ""; foreach (char c in input) { output += (char)(c < 78 ? c + 13 : c - 13); } return output; }}

static void Main(string[] args) { // Get input string input = Console.ReadLine().ToUpper();

// "Encrypt" it IEncryptionAlgorithm encryptionAlgorithm = new Rot13(); string output = encryptionAlgorithm.Encrypt(input);

// Write output Console.WriteLine(output);}

public class Rot13 : IEncryptionAlgorithm { public string Encrypt(string input) { string output = ""; foreach (char c in input) { output += (char)(c < 78 ? c + 13 : c - 13); } return output; }}

public interface IEncryptionAlgorithm{

string Encrypt(string input);}

DecoupledCode

Code Sample

Dependency Injection

Dependency Injection

Dependency Injection

Dependency Injection

Dependency InjectionCode Sample

Plugins!

Code Sample

• Single - Automatically choose one.• One of Many - Like Single, but give the user

the choice. Think smart phone apps• Many Simultaneously – Using the composite

pattern, many plug-ins are treated as one. Like browser plug-ins; they have the potential of overlapping

Plug-in Strategies

• Most IoC containers– Ninject (using ninject.extensions.conventions)– Castle.Windsor– Unity– StructureMap– MEF – Managed Extensibility Framework– MAF – Managed AddIn Framework– Many other IoC containers

Frameworks

MVC ExampleSite Demo

Add Sales Tax to Calculation

Toppings!More Toppings!

RequirementsPlugin Demo

Big Al’s Plug-inCode Sample

Papa’s Plug-inCode Sample

A New ClientCode Sample

• Why not use config files? Config files are for changing the data/data source. Plug-in architecture is for dropping in functionality

• Deciding where to put plug-in points

Discussion

• Recovering from plug-in crashes• Dynamic loading and unloading at run time• Updating an application while it’s running

Endless Possibilities

@OndrejBalas

Thanks!Questions? E-mail or Tweet me

ondrej@ondrejbalas.com

https://github.com/ondrejbalas/plugin-architecture

@OndrejBalas

Thanks!Questions? E-mail or Tweet me

ondrej@ondrejbalas.com

https://github.com/ondrejbalas/plugin-architecture

@OndrejBalas

Thanks!Questions? E-mail or Tweet me

ondrej@ondrejbalas.com

https://github.com/ondrejbalas/plugin-architecture

@OndrejBalas

Thanks!Questions? E-mail or Tweet me

ondrej@ondrejbalas.com

https://github.com/ondrejbalas/plugin-architecture

@OndrejBalas

Thanks!Questions? E-mail or Tweet me

ondrej@ondrejbalas.com

https://github.com/ondrejbalas/plugin-architecture

Any further and you will see my unused slides

• Single - Automatically choose one.• One of Many - Like Single, but give the user

the choice. Think smart phone apps• Many Simultaneously – Using the composite

pattern, many plug-ins are treated as one. Plug-ins can interfere with each other using this strategy. Like browser plug-ins; they have the potential of overlapping

Plug-in Strategies

• Dependency Injection• Composite Pattern

Plug-in Patterns

• Single (Plug-in replaces functionality)• Multiple (Different implementations of the

same thing – examples doing stuff with files, compression, encryption)• Multiple (Each plug-in has different

functionality like iphone apps)• Single > Multiple would use the Composite

Pattern. Multiple should use Composite

Plug-in Architectures in the wild

Code to Interface = Code to Contract

• It’s just broken out into optional assemblies

• Plugins work due to the concept of abstractions or programming to interface and the idea of composable parts

• The basic idea is that your application loads pieces which by default can be nullable but be replaced by the client

The Client - Big Al’s Pizza Pies

• Like most of our clients, wants “mostly custom” without “custom” cost

• Buys our software which is customizable by way of plugin

• Nephew can code in C# and make changes

More toppings

Sales tax for customers on the other side of the river

Delivery Charges Extra

• Uhoh.. We didn’t give them enough plugin points to be able to do what they want to do.

• This shows one of the pain points – if we don’t think of it, they’ll have to wait for the next version that will include it.

• A new client can get hook points in their own branch that can be merged into the master later

• It isn’t perfect, but it’s a good solution that buys us a lot of flexibility. Piecemeal functionality, deployed to customer’s site, and customer can customize the code as needed – to a point

Where to make plugin points

A La Carte

More Benefits

• Replace or accentuate existing code with dropped in code

• Doesn’t require recompiling the original project OR having the source code

• Plugins are usually not required

Config Files?

Text

Why

Applications become more like Frameworks