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Using OData Actions to implement server-side behavior

  • Article

The Open Data Protocol (OData) enables you to define a service action on a data service. A service action is a special operation defined on a data service. Service actions provide a way to inject behaviors into an otherwise data-centric model. Service actions enable you to invoke business logic in OData, where the logic must be bound to a give resource. Service actions differ from regular endpoint-based service operations in the following ways:

  • Bound to resources

    Unlike service operations that are defined on a fixed endpoint, service actions are usually bound to resources, entity sets (feeds) or individual entities. These resources are known as binding parameters. In WCF Data Services, binding parameters can only be entity types and collections of entity types. Nonbinding parameters can also be supplied to service actions. However, these parameters can only be of a primitive type, complex type, or a collection of primitive or complex types. When a service action is bound to a resource, the service action is exposed in the entity serialization as an action resource.

  • Has side-effects

    Business logic invoked by a service action affects the system, such as changing data or state or by invoking some other business process. Because service actions always have side-effects, they can only be invoked by using a POST request. Unlike service operations, nonbinding parameters are supplied to service actions in the body message, rather than in the URI. These nonbinding parameters are supplied as JavaScript Notation (JSON)-encoded values in the body of the request.

  • Cannot be further composed

    Service actions can have the same return types as service operations. However, unlike service operations, actions cannot be further composed. This means that the system query options cannot be applied to a service action.

Consider a digital movie as a resource, there are many things you may do with a digital movie: check-out, rate/comment, or check-in. These are all examples of Actions that may be implemented by a WCF Data Service that manages digital movies. Actions are described in an OData response that contains a resource on which the Action can be invoked. When a user requests a resource that represents a digital movie the response returned from the WCF Data Service contains information about the Actions that are available for that resource. The availability of an Action can depend on the state of the data service or resource. For example once a digital movie is checked out it cannot be checked out by another user. Clients can invoke an action simply by specifying a URL. For example http://MyServer/MovieService.svc/Movies(6) would identify a specific digital movie and http://MyServer/MovieService.svc/Movies(6)/Checkout would invoke the action on the specific movie. Actions allow you to expose you service model without exposing your data model. Continuing with the movie service example, you may wish to allow a user to rate a movie, but not directly expose the rating data as a resource. You could implement a Rate Action to allow the user to rate a movie but not directly access the rating data as a resource.

Implementing an action

To implement a service action you must implement the IServiceProvider, IDataServiceActionProvider, and IDataServiceInvokable interfaces. IServiceProvider allows WCF Data Services to get your implementation of IDataServiceActionProvider. IDataServiceActionProvider allows WCF Data Services to create, find, describe and invoke service actions. IDataServiceInvokable allows you to invoke the code that implements the service actions’ behavior and get the results, if any. Keep in mind that WCF Data Services are Per-Call WCF Services, a new instance of the service will be created each time the service is called. Make sure no unnecessary work is done when the service is created.

IServiceProvider

IServiceProvider contains a method called GetService(Type). This method is called by WCF Data Services to retrieve a number of service providers, including metadata service providers and data service action providers. When asked for a data service action provider, return your IDataServiceActionProvider implementation.

IDataServiceActionProvider

IDataServiceActionProvider contains methods that allow you to retrieve information about the available actions. When you implement IDataServiceActionProvider you are augmenting the metadata for your service which is defined by your service’s implementation of IDataServiceMetadataProvider with Actions and handling dispatch to those actions as appropriate.

AdvertiseServiceAction

AdvertiseServiceAction(DataServiceOperationContext, ServiceAction, Object, Boolean, ODataAction%) is called to determine what actions are available for the specified resource. This method is only called for actions that are not always available. It is used to check if the action should be included in the OData response based upon the state of the resource being requested or the state of the service. How this check is accomplished is completely up to you. If it is an expensive to calculate availability and the current resource is in a feed, it is acceptable to skip the check and advertise the action. The inFeed parameter is set to true if the current resource being returned is part of a feed.

CreateInvokable

CreateInvokable(DataServiceOperationContext, ServiceAction, array<Object[]) is called to create a IDataServiceInvokable that contains a delegate that encapsulates the code that implements the action’s behavior. This creates the IDataServiceInvokable instance but does not invoke the action. WCF Data Service Actions have side effects and need to work in conjunction with the Update Provider to save those changes to disk. The Invoke() method is called from the Update Provider’s SaveChanges() method is called.

GetServiceActions

This method returns a collection of ServiceAction instances that represent all of the actions a WCF Data Service exposes. ServiceAction is the metadata representation of an Action, that includes information like the Action name, its parameters, and its return type.

GetServiceActionsByBindingParameterType

This method returns a collection of all ServiceAction instances that can be bound to the specified binding parameter type. In other words, all ServiceActions that can act on the specified resource type (also called binding parameter type).This is used when the service returns a resource in order to include information about Actions that can be invoked against that resource. This method should only return actions that can bind to the exact binding parameter type (no derived types). This method is called once per request per type encountered and the result is cached by WCF Data Services.

TryResolveServiceAction

This method searches for a specified ServiceAction and returns true if the ServiceAction is found. If found, the ServiceAction is returned in the serviceAction out parameter.

IDataServiceInvokable

This interface provides a way to execute a WCF Data Service Action. When implementing IDataServiceInvokable you are responsible for 3 things:

  1. Capturing and potentially marshaling the parameters

  2. Dispatching the parameters to the code that actually implements the Action when Invoke() is called

  3. Storing any results from Invoke() so they can be retrieved using GetResult()

The parameters may be passed as tokens. This is because it is possible to write a Data Service Provider that works with tokens that represent resources, if this is the case you may need to convert (marshal) these tokens into actual resources before dispatching to the actual action. After the parameter has been marshalled, it must be in an editable state so that any changes to the resource that occur when the action is invoked will be saved and written to disk.

This interface requires two methods: Invoke and GetResult. Invoke invokes the delegate that implements the action’s behavior and GetResult returns the result of the action.

Service Actions Access Control

In WCF Data Services, service-wide visibility of service actions is controlled by the SetServiceActionAccessRule(String, ServiceActionRights) method on the IDataServiceConfiguration class in the same way that service operation visibility is controlled by using the SetServiceOperationAccessRule(String, ServiceOperationRights) method.

Metadata Definition of Service Actions

Like service operations, service actions are defined as FunctionImport elements in the service metadata. A service action FunctionImport contains zero or more Parameter elements, which represent both binding and nonbinding parameters. In addition, the following attributes on FunctionImport define the behavior of the service action:

  • IsSideEffecting

    Because service actions can always have side-effects, this attribute is always true.

  • IsBindable

    Service actions are usually, but not always bound to a resource. When this attribute is true, the first parameter must be an entity type or entity collection.

  • IsComposable

    Because service actions do not support additional query composition, this attribute is always false.

Invoking a WCF Data Service Action

Actions are invoked using an HTTP POST request. The URL specifies the resource followed by the action name. Parameters are passed in the body of the request. For example if there was a service called MovieService which exposed an action called Rate. You could use the following URL to invoke the Rate action on a specific movie:

http://MovieServer/MovieService.svc/Movies(1)/Rate

Movies(1) specifies the movie you wish to rate and Rate specifies the Rate action. The actual value of the rating will be in the body of the HTTP request as shown in the following example:

POST http://MovieServer/MoviesService.svc/Movies(1)/Rate HTTP/1.1 
Content-Type: application/json 
Content-Length: 20 
Host: localhost:15238
{ 
   "rating": 4 
}

Warning

The above sample code will work only with WCF Data Services 5.2 and later which has support for JSON light. If using an earlier version of WCF Data Services, you must specify the json verbose content-type as follows: application/json;odata=verbose.

Alternatively you can invoke an action using the WCF Data Services Client as shown in the following code snippet.

MoviesModel context = new MoviesModel (new Uri("http://MyServer/MoviesService.svc/"));
            //...
            context.Execute(new Uri("http://MyServer/MoviesService.svc/Movies(1)/Rate"), "POST", new BodyOperationParameter("rating",4) );

In the code snippet above, the MoviesModel class was generated by using Visual Studio to Add Service Reference to a WCF Data Service.

See Also

Concepts

Developing and Deploying WCF Data Services

Custom Data Service Providers (WCF Data Services)

Other Resources

WCF Data Services

Data Service (WCF Data Services)