SOA with .NET and Windows Azure : WCF Discovery (part 3) - Discovery Proxies for Managed Discovery & Implicit Service Discovery

Discovery Proxies for Managed Discovery

Ad hoc discovery is a suitable approach for static and smaller local service networks, where all services live on the same subnet and multicasting probes or announcements don’t add a lot of network chatter.

Larger environments, with services distributed across multiple subnets (or highly dynamic networks), need to consider a Discovery Proxy to overcome the limitations of ad hoc probing. A Discovery Proxy can listen to UDP announcements on the standard udpAnnouncementEndpoint for service registration and de-registration, but also expose DiscoveryEndpoint via a WCF SOAP binding.

The implementation requirements for a Discovery Proxy can vary from a simple implementation that keeps an in-memory cache of available services, to implementations that require databases or scale-out solutions, like caching extensions provided by Windows Server AppFabric. WCF provides a DiscoveryProxy base class that can be used for general proxy implementations.

Discovering from a Discovery Proxy

Discovering services registered with a Discovery Proxy follows the steps for ad hoc discovery discussed earlier. Instead of multicasting a UDP message, DiscoveryClient now needs to contact the Discovery Proxy. Details about the discovery contract are encapsulated in the DiscoveryEndpoint class. Its constructor only takes parameters for the communication protocol details, binding, and address.

Here we are configuring a discovery client to query the Discover Proxy:

Example 9.

DiscoveryEndpoint proxyEndpoint =
  new DiscoveryEndpoint(
  new NetTcpBinding(),
  new EndpointAddress(proxyAddressText.Text));
this.discoveryClient = new DiscoveryClient(proxyEndpoint);

Implicit Service Discovery

Our coverage of WCF Discovery so far has focused on the explicit discovery of services. However, it is worth noting that WCF Discovery can also perform the same queries behind-the-scenes, when you configure endpoints as DynamicEndpoint programmatically or in the configuration file. This allows for highly dynamic, adaptive environments where virtually no location-specific details are maintained as part of code or configuration.

A client endpoint, for example, can be configured to locate a service that matches on scope and contract. In the following example, we configure DynamicEndpoint to locate a service with matching contract and metadata:

Example 10.

<client>
  <endpoint kind="dynamicEndpoint"
    binding="basicHttpBinding"
    contract="ICustomerService"
    endpointConfiguration="dynamicEndpointConfiguration"
    name="dynamicCustomerEndpoint" />
</client>
<standardEndpoints>
  <dynamicEndpoint>
    <standardEndpoint name="dynamicEndpointConfiguration">
      <discoveryClientSettings>
        <findCriteria duration="00:00:05" maxResults="1">
          <types>
            <add name="ICustomerService"/>
          </types>
          <extensions>
            <MyCustomMetadata>
              Highly Scalable
            </MyCustomMetadata>
          </extensions>
        </findCriteria>
      </discoveryClientSettings>
    </standardEndpoint>
  </dynamicEndpoint>
</standardEndpoints>

					  

With this configuration, the service consumer can create a proxy object to the server with the following code:

Example 11.

ICustomerService svc =
  new ChannelFactory<ICustomerService>
    ("dynamicCustomerEndpoint").CreateChannel();