Maarten Balliauw {blog}

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Rewriting WCF OData Services base URL with load balancing & reverse proxy

When scaling out an application to multiple servers, often a form of load balancing or reverse proxying is used to provide external users access to a web server. For example, one can be in the situation where two servers are hosting a WCF OData Service and are exposed to the Internet through either a load balancer or a reverse proxy. Below is a figure of such setup using a reverse proxy.

WCF OData Services hosted in reverse proxy

As you can see, the external server listens on the URL www.example.com, while both internal servers are listening on their respective host names. Guess what: whenever someone accesses a WCF OData Service through the reverse proxy, the XML generated by one of the two backend servers is slightly invalid:

OData base URL invalid incorrect

While valid XML, the hostname provided to all our clients is wrong. The host name of the backend machine is in there and not the hostname of the reverse proxy URL…

How can this be solved? There are a couple of answers to that, one that popped into our minds was to rewrite the XML on the reverse proxy and simply “string.Replace” the invalid URLs. This will probably work, but it feels… dirty. We chose to create WCF inspector, which simply changes this at the WCF level on each backend node.

Our inspector looks like this: (note I did some hardcoding of the base hostname in here, which obviously should not be done in your code)

1 public class RewriteBaseUrlMessageInspector 2 : IDispatchMessageInspector 3 { 4 public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext) 5 { 6 if (WebOperationContext.Current != null && WebOperationContext.Current.IncomingRequest.UriTemplateMatch != null) 7 { 8 UriBuilder baseUriBuilder = new UriBuilder(WebOperationContext.Current.IncomingRequest.UriTemplateMatch.BaseUri); 9 UriBuilder requestUriBuilder = new UriBuilder(WebOperationContext.Current.IncomingRequest.UriTemplateMatch.RequestUri); 10 11 baseUriBuilder.Host = "www.example.com"; 12 requestUriBuilder.Host = baseUriBuilder.Host; 13 14 OperationContext.Current.IncomingMessageProperties["MicrosoftDataServicesRootUri"] = baseUriBuilder.Uri; 15 OperationContext.Current.IncomingMessageProperties["MicrosoftDataServicesRequestUri"] = requestUriBuilder.Uri; 16 } 17 18 return null; 19 } 20 21 public void BeforeSendReply(ref Message reply, object correlationState) 22 { 23 // Noop 24 } 25 }

There’s not much rocket science in there, although some noteworthy actions are being performed:

  • The current WebOperationContext is queried for the full incoming request URI as well as the base URI. These values are based on the local server, in our example “srvweb01” and “srvweb02”.
  • The Host part of that URI is being replaced with the external hostname, www.example.com
  • These two values are stored in the current OperationContext’s IncomingMessageProperties. Apparently the keys MicrosoftDataServicesRootUri and MicrosoftDataServicesRequestUri affect the URL being generated in the XML feed

To apply this inspector to our WCF OData Service, we’ve created a behavior and applied the inspector to our service channel. Here’s the code for that:

1 [AttributeUsage(AttributeTargets.Class)] 2 public class RewriteBaseUrlBehavior 3 : Attribute, IServiceBehavior 4 { 5 public void Validate(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase) 6 { 7 // Noop 8 } 9 10 public void AddBindingParameters(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase, Collection<ServiceEndpoint> endpoints, BindingParameterCollection bindingParameters) 11 { 12 // Noop 13 } 14 15 public void ApplyDispatchBehavior(ServiceDescription serviceDescription, ServiceHostBase serviceHostBase) 16 { 17 foreach (ChannelDispatcher channelDispatcher in serviceHostBase.ChannelDispatchers) 18 { 19 foreach (EndpointDispatcher endpointDispatcher in channelDispatcher.Endpoints) 20 { 21 endpointDispatcher.DispatchRuntime.MessageInspectors.Add( 22 new RewriteBaseUrlMessageInspector()); 23 } 24 } 25 } 26 }

This behavior simply loops all channel dispatchers and their endpoints and applies our inspector to them.

Finally, there’s nothing left to do to fix our reverse proxy issue than to just annotate our WCF OData Service with this behavior attribute:

1 [RewriteBaseUrlBehavior] 2 public class PackageFeedHandler 3 : DataService<PackageEntities> 4 { 5 // ... 6 }

Working with URL routing

A while ago, I posted about Using dynamic WCF service routes. The technique described below is also appropriate for services created using that technique. When working with that implementation, the source code for the inspector would be slightly different.

1 public class RewriteBaseUrlMessageInspector 2 : IDispatchMessageInspector 3 { 4 public object AfterReceiveRequest(ref Message request, IClientChannel channel, InstanceContext instanceContext) 5 { 6 if (WebOperationContext.Current != null && WebOperationContext.Current.IncomingRequest.UriTemplateMatch != null) 7 { 8 UriBuilder baseUriBuilder = new UriBuilder(WebOperationContext.Current.IncomingRequest.UriTemplateMatch.BaseUri); 9 UriBuilder requestUriBuilder = new UriBuilder(WebOperationContext.Current.IncomingRequest.UriTemplateMatch.RequestUri); 10 11 var routeData = MyGet.Server.Routing.DynamicServiceRoute.GetCurrentRouteData(); 12 var route = routeData.Route as Route; 13 if (route != null) 14 { 15 string servicePath = route.Url; 16 servicePath = Regex.Replace(servicePath, @"({\*.*})", ""); // strip out catch-all 17 foreach (var routeValue in routeData.Values) 18 { 19 if (routeValue.Value != null) 20 { 21 servicePath = servicePath.Replace("{" + routeValue.Key + "}", routeValue.Value.ToString()); 22 } 23 } 24 25 if (!servicePath.StartsWith("/")) 26 { 27 servicePath = "/" + servicePath; 28 } 29 30 if (!servicePath.EndsWith("/")) 31 { 32 servicePath = servicePath + "/"; 33 } 34 35 requestUriBuilder.Path = requestUriBuilder.Path.Replace(baseUriBuilder.Path, servicePath); 36 requestUriBuilder.Host = baseUriBuilder.Host; 37 baseUriBuilder.Path = servicePath; 38 } 39 40 OperationContext.Current.IncomingMessageProperties["MicrosoftDataServicesRootUri"] = baseUriBuilder.Uri; 41 OperationContext.Current.IncomingMessageProperties["MicrosoftDataServicesRequestUri"] = requestUriBuilder.Uri; 42 } 43 44 return null; 45 } 46 47 public void BeforeSendReply(ref Message reply, object correlationState) 48 { 49 // Noop 50 } 51 }

The idea is identical, except that we’re updating the incoming URL path for reasons described in the aforementioned blog post.

Enjoy!

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