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MonoRail Services Architecture

MonoRail uses a set of services to handle specific tasks. The framework is responsible for defining the default implementations, instantiate, start and configure them. The services are made available through a combination of lifecycle interfaces and an implementation of IServiceProvider

Stating the problem

It's impossible to come up with a sophisticated software where the default behavior pleases everyone and integrates with everything. The usual solution is making the software rely on contracts and having the core code as just a coordination of invocations on the contracts implementation. An user is thus capable of replacing one or more contract implementation.

The challenging is implementing an architecture where the parts are easily replaced, configurable and can rely (depend) on other parts.

How MonoRail tackles this problem

The most usual solution to this problem is to use an Inversion of Control container. However, things have to be balanced. For MonoRail, an IoC container would introduce dependencies on assemblies and an longer initialization. In the end, we wouldn't benefit from all IoC container features, so it could be considered too much for our problem.

Instead, we combined what is already on the .Net library and some creative solution.

Basically we create two levels of services registries, per application and per request, and a simple interfaces that defines lifecycles that services optionally implement. This allow the service to start its work when it is supposed to and to gather reference to other services.

How it works

When the web application is started, the ASP.Net modules are initialized. MonoRail has a EngineContextModule which is in charge of

• Read the configuration
• Initialize the services
• Subscribe to ASP.Net application and request level events
• Create a request context (which we'll not cover here)

Services implementation can be defined in the configuration section. After reading the configuration section MonoRail checks for missing definition and register the missing services using the default implementation.

After that it instantiates every service and runs the lifecycle. If everything went well, the framework is properly initialized. All services are registered in the application level container, which happens to be implemented by the EngineContextModule class. We call this the parent container.

When a request starts, MonoRail creates a DefaultRailsEngineContext instance, which also is a container for services. MonoRail sets the parent container on it. This allow the user to override services per request, and resolution of services in the parent.

The initialization flow

1. The configuration is read into the MonoRailConfiguration. It's also registered as a service
2. The services collected are instantiated and registered
3. The lifecycle is executed

Lifecycle interfaces

A service might implement a few interfaces to expose to MonoRail that it behaves in a specific way or that it needs something from the framework.

ISupportInitialize (from System.ComponentModel)

This interface can be implemented if a service wants performs some initialization.

IInitializable (from Castle.Core)

This interface can be implemented if a service wants performs some initialization.

IServiceEnabledComponent (from Castle.Core)

This interface can be implemented if a service uses other services.

Invocation order

IServiceEnabledComponent is the first one to be invoked. This gives a chance to services gather all services references it wants. Then the initialization interface's methods are invoked.

The expected (good) behavior from a service implementation

For service that uses IServiceEnabledComponent lifecycle, the implementor should keep in mind that the initialization lifecycle has not run for all services, so it might not be safe to use other services as they might not be properly initialized at the moment.

The order of service registration and instantiation is not guaranted. So the implementor should not make any assumption regarding it.

Built-in services

The following is a succint list of services and their roles. You can refer to this list to learn more about MonoRail inner workings or to use them when developing extensions and new services.

MonoRailConfiguration

Exposes the MonoRail configuration

ExtensionManager

Manages registered extensions dispatching events from Asp.Net infrastructure and from MonoRail services

IViewSourceLoader

Sits in front of the file system and from assembly resources. It is used by view engines to obtain view streams

IViewEngine

Process view templates

IScaffoldingSupport

Adds scaffold support to a controller

IControllerFactory

Creates the controller instances

IViewComponentFactory

Manages registered ViewComponents and creates their instances

IFilterFactory

Manages registered filters and creates their instances

IResourceFactory

Create resources

IEmailSender

Sends e-mail

IEmailTemplateService

Process e-mail templates using the MonoRail infrastructure

IControllerDescriptorProvider

Inspects Controller types building a descriptor of what has been defined using attributes

IResourceDescriptorProvider

Creates descriptors for resources declared on controllers

IRescueDescriptorProvider

Creates descriptors for rescues declared on controllers

ILayoutDescriptorProvider

Creates descriptors for layouts declared on controllers

IHelperDescriptorProvider

Creates descriptors for helpers declared on controllers

IFilterDescriptorProvider

Creates descriptors for filters declared on controllers

IControllerTree

Manages a binary tree of controllers registered

ICacheProvider

Manages the cache

Registering services

You have to use the monorail configuration node to override or add services to MonoRail.

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