Tag Archives: WCF

Exposing a WCF Data Service via the Azure AppFabric Service Bus


The How


The following excerpt shows how to expose a WCF Data Service via the Azure AppFabric Service Bus, using a self-hosted ServiceHost.


using(var host = new DataServiceHost(
                     typeof(PersonsDataService), new Uri[0])){
    typeof (IRequestHandler),
      new WebHttpRelayBinding(
    Console.WriteLine("host is opened, press any key to exit");


  • PersonsDataService is the data service to be exposed;
class PersonsDataService : DataService<MyDataModel>{
    public static void InitializeService(
                           IDataServiceConfiguration config) {


  • IRequestHandler is the WCF Data Services generic service contract (see below);
  • WebHttpRelayBinding is the Azure AppFabric Service Bus binding for the “web programming model”.
  • and WithServiceBusTransportCredentials is an extension method for configuring the service bus transport credentials
public static ServiceEndpoint WithServiceBusTransportCredentials(this ServiceEndpoint edp)        {
    return edp;
public static void 
ConfigureTransportCredentials(ServiceEndpoint edp) {
    var tcred = new TransportClientEndpointBehavior();
    tcred.CredentialType = 
    tcred.Credentials.SharedSecret.IssuerName = IssuerName;
    tcred.Credentials.SharedSecret.IssuerSecret = SharedSecret;



The Why

WCF Architecture

The Windows Communication Architecture is based on two layers:

  • The Channel Model Layer is a stack of channels that operate on messages, composed by
    • Zero or more protocol channels, implementing protocols such as WS-Security.
    • One transport channel that send or receives the messages from the transport medium (e.g. TCP, pipes). This transport channel uses a message encoder/decoder to translate between the messages handled by the protocol channels and the transport’s byte streams.
  • The Service Model Layer is responsible for the translation between messages and service method calls.



WCF Data Services

WCF Data Services are layered on top of the WCF Service Model:


public interface IRequestHandler {    
     WebInvoke(UriTemplate = "*", Method = "*")]
    Message ProcessRequestForMessage(Stream messageBody);
    RequirementsMode = 
     InstanceContextMode = InstanceContextMode.PerCall)
public class DataService<T> : IRequestHandler, IDataService {
    public Message ProcessRequestForMessage(Stream messageBody){...}


Azure AppFabric Service Bus

The Azure AppFabric SDK contains several bindings to expose and consume services via the service bus, namely the WebHttpRelayBinding that uses the Web HTTP programming model. This binding defines a channel stack with a RelayHttpTransportChannelListener that opens a TCP connection to the services bus and listens to messages relayed by it.



Since the Service Bus bindings extend the channel layer and the WCF Data Services are layered on top of the Service Model Layer, their composition is straightforward.

The case of the missing binding

Today, while preparing a training session on WCF, I decided to use Eclipse’s Web Services Explorer (WSE) to view and interact with a WCF-based service (using plain WSHttpBinding).

However, after processing the service’s WSDL description, WSE showed a service with an empty binding list.

Knowing that some web services stacks don’t support WSDL descriptions divided into multiple files, I used the FlatWSDL endpoint behavior authored by C. Weyer. This behavior extends WCF’s WSDL exporting mechanism, by inlining in the WSDL document all the schemas directly or indirectly referenced by it.

Unfortunately, the behavior of WSE remained the same: it continued to show a service with no bindings. After inspecting again the generated WSDL, I observed that it was still divided into two WSDL documents: the first one with the <service> and <binding> elements, and the second one (imported by the first) with the remaining WSDL elements and the flattened schemas.

The reason for this division is explained in this post by Kirill Gavrylyuk: this division is used when there are WSDL elements (<service>, <binding>, <portType>, …) in different namespaces.

So, a solution is to put all the elements in the same namespace, which also is described in the same post.

After this change, all the WSDL elements were contained in a single document and WSE finally showed the service binding.

These are the joys of interoperability.

WCF asynchronous processing and IIS hosting

Recommending the post “WCF Request Throttling and Server Scalability“.

It contains important information regarding WCF’s asynchronous processing when hosting on IIS. For example, I was unaware that

So in the release of .Net 3.0 and 3.5, WCF implemented synchronous versions of HTTP module and handler instead of asynchronous ones

meaning that even with asynchronous service implementation (asyncpattern=true), a thread will be blocked for the duration of the request’s processing. This thread will be the ASP.NET’s thread from the CLR ThreadPool.

Since I do most of my tests and experiences using self-hosting, I didn’t have noticed this.

However, the SP1 of .NET 3.5 adds support for asynchronous HTTP modules and handlers.

Using Zermatt’s claims model on WCF: ClaimsPrincipal.Current

In the last post, I briefly presented Zermatt’s claims model. This model can be used in both WCF based services or ASP.NET based web applications.

Beginning with this post, I will describe how this new claims model is integrated into WCF, using WCF’s extensibility points, and also what are some consequences of this integration.

Once again, keep in mind that these are empirical observations of the first beta release.

One way of using Zermatt’s claims model on a service, without needing to change WCF’s default behavior, is via the ClaimsPrincipal.Current static property. In WCF, the getter method will first check if the incoming message properties (available in the current operation context) contains a property whose name is typeof(IClaimsIdentity).FullName. If not, the getter method will create and return a ClaimsPrincipal instance based on the following procedure:



  1. Retrieves all the IAuthorizationPolicy objects from the current operation context.
  2. Creates a default AuthorizationContext with the claim sets produced by the evaluation of the above IAuthorizationPolicy set.
  3. Creates one ClaimsIdentity instance for each ClaimSet in the above AuthorizationContext. This ClaimsIdentity instance will contain one claim for each claim in the claim set whose with right equals Rights.PossessProperty (this means that claims with right = Rights.Identity will not be reflected in Zermatt’s claims model).
  4. Creates one ClaimsPrincipal referring the above ClaimsIdentity collection.


Before returning, the property’s getter method also adds the created ClaimsPrincipal instance to the incoming message properties, so that subsequent calls will simply return this ClaimsPrincipal cached instance.

In other words, the IClaimsPrincipal returned by ClaimsPrincipal.Current represents a view, using the “new” Zermatt’s claims model, of the “old” (System.IdentityModel) claims model.

Note however that this IClaimsPrincipal must be explicitly requested and is not referenced by Thread.CurrentPrincipal neither by ServiceSecurityContext.PrimaryIdentity.



Authorization policies in WCF: from tokens to claim sets

In a previous post, I presented some classes for handling security tokens in WCF. This post describes the process that begins with the authentication of a token and ends with a collection of claim sets, available at the AuthorizationContext (see this previous post).

One of the classes referred in the last post is the SecurityTokenAuthenticator class, containing the following method

public ReadOnlyCollection<IAuthorizationPolicy> ValidateToken(
    SecurityToken token

This method “Authenticates the specified security token and returns the set of authorization policies for the security token

Surprisingly, this method does not return a set of claims. Instead it returns a collection of IAuthorizationPolicy.

What is an authorization policy?

An authorization policy defines a rule for adding claims to a claim evaluation context. More precisely, the IAuthorizationPolicy interface has an Evaluate method that receives an EvaluationContext.


This class represents the context on which the authorization policies are evaluated. It exposes a property with a read-only collection of claim sets – the claim sets previously added to the context – and a method to add claims sets to the context. So, an authentication policy, via the Evaluate method, conditionally adds claim sets into the context based on the claim sets already there. This conditional addition can be used to implement claim normalization or claim inference policies.

An example of claim normalization is a policy that adds the claim “Teacher of X” to the context if it contains the claims “Professor of X” or “Teaching Assistant of X”. In this case, normalization means to represent a set of syntactically different (but semantically equal claims) by only one claim.

An example of claim inference is a policy that adds the claim “Authorized to post CS101 grades” if the context already contains the claim “Teacher of CS101”.

Unconditional policies

A particular case of policy is the unconditional authorization policy, i. e., a policy that always add the same claims sets to the evaluation context, independently of the claim sets already there. These are the type of authorization policy typically returned by token authenticators.

Policy evaluation process

The policy evaluation process uses policies from two origins:

  • Policies resulting from token authentication. These policies represent the claims issued to the message originator.
  • External policies, representing additional policy rules (e.g. normalization and inference rules).


The claim sets resulting from the policy evaluation process are inserted into the AuthorizationContext.

Since there might be interdependencies between the policies, the policy evaluation process can call the Evaluate method of the same policy more than once. An example of interdependent policies is:

  • P1 (external policy): Add “Authorized to post CS101 grades” if “Teacher of CS101”.
  • P2 (external policy): Add “Teacher of CS101” if “Professor of CS101” or “Teaching Assistant of CS101”.
  • P3 (token policy): Add “Professor of CS101” unconditionally.

In this example, P2 must be evaluated after P3 adds the “Professor of CS101” claim, and P1 must be evaluated after P2 adds the “Teacher of CS101” claim. This results in more that one call to the Evaluate method, since there is no predefined order between the policies.

When a policy wants to be called again if additional claim sets are inserted by other policies, its Evaluate method must return false. If this method returns true, it means that no more claim sets will be added by the policy.

The policy evaluation process keeps repeatedly evaluating all the policies while

  • At least one policy returns false (meaning that it wants to be called again if new claim sets are added)
  • And the last policy evaluation cycle resulted in at least one new claim set added to the context.

If we model a authorization policy as a function mapping claim sets into claim sets, then the result of this evaluation process is the least fixed point of the policy set.

Security tokens in WCF

In WCF, security tokens are represented by classes derived from the SecurityToken abstract class. However, this class exposes little functionality:

  • A SecurityKeys property, to access the keys associated with this token.
  • Two properties, ValidFrom and ValidTo, with the token’s validity period.
  • A couple of methods for creating and matching key identifiers.

Most of the functionality related to security tokens is associated with three classes:

  • SecurityTokenProvider abstract class – Defines the interface for the creation of security tokens, i. e., for token factories. Typically, there is one concrete derived class for each token type (e. g. X509SecurityTokenProvider).
    Instances of classes derived from this one are used by the security protocol channel, at the message originator side, to create the tokens that are attached to the sent messages.


  • SecurityTokenSerializer abstract class – Defines the interface for serialization/deserialization into/from XML (XmlWriter/XmlReader) of token instances. The majority of token types are handled by the concrete WSSecurityTokenSerializer class.
    Instances with this type are used both at the message originator side and at the message recipient side.


  • SecurityTokenAuthenticator abstract class – Defines the interface for token verification and also for extracting the token’s claims. Typically, there is one concrete derived class for each token type (e. g. X509SecurityTokenAuthenticator).
    Instances with this type are used by the security protocol channel, at the message recipient side, to validate and extract the claims from the tokens attached to the received messages.

The figure below shows these classes, with some members hidden for legibility.



How are the provider/serializer/authenticator instances created?

The WCF runtime uses an object with type SecurityTokenManager (abstract class) to create a provider/serializer/authenticator instance, via the methods:

  • public abstract SecurityTokenProvider CreateSecurityTokenProvider(
        SecurityTokenRequirement tokenRequirement
  • public abstract SecurityTokenSerializer CreateSecurityTokenSerializer(
        SecurityTokenVersion version
  • public abstract SecurityTokenAuthenticator CreateSecurityTokenAuthenticator(
        SecurityTokenRequirement tokenRequirement,
        out SecurityTokenResolver outOfBandTokenResolver

How is the SecurityTokenManager instance created? 

Both the ClientCredentials and ServiceCredentials classes derives from the abstract class SecurityCredentialsManager, that contains a CreateSecurityTokenManager method.

Note that the ClientCredentials and ServiceCredentials classes hold the security configuration settings. These settings are reflected on the used token provider/serializer/authenticator via the chain of factories

  • ClientCredentials and ServiceCredentials creates SecurityTokenManager
  • SecurityTokenManager creates SecurityToken[Provider|Serializer|Authenticator]