Delegating Handlers

Ocelot allows the user to add delegating handlers to the HttpClient transport. This feature was requested by issue 208 and the team decided that it was going to be useful in various ways. Since then we extended it in issue 264.

How to Use

In order to add delegating handlers to the HttpClient transport you need to do two main things.

First, in order to create a class that can be used a delegating handler it must look as follows. We are going to register these handlers in the ASP.NET Core DI container, so you can inject any other services you have registered into the constructor of your handler.

public class FakeHandler : DelegatingHandler
{
    protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken token)
    {
        // Do stuff and optionally call the base handler...
        return await base.SendAsync(request, token);
    }
}

Second, you must add the handlers to DI container like below:

ConfigureServices(s => s
    .AddOcelot()
    .AddDelegatingHandler<FakeHandler>()
    .AddDelegatingHandler<FakeHandlerTwo>()
)

Both of these AddDelegatingHandler methods have a default parameter called global which is set to false. If it is false then the intent of the Delegating Handler is to be applied to specific Routes via ocelot.json (more on that later). If it is set to true then it becomes a global handler and will be applied to all Routes, as below:

services.AddOcelot()
    .AddDelegatingHandler<FakeHandler>(true)

Finally, if you want Route specific Delegating Handlers or to order your specific and (or) global (more on this later) Delegating Handlers then you must add the following to the specific Route in ocelot.json. The names in the array must match the class names of your Delegating Handlers for Ocelot to match them together:

"DelegatingHandlers": [
  "FakeHandlerTwo",
  "FakeHandler"
]

Order of Execution

You can have as many Delegating Handlers as you want and they are run in the following order:

1. Any globals that are left in the order they were added to services and are not in the DelegatingHandlers array from ocelot.json.

2. Any non global Delegating Handlers plus any globals that were in the DelegatingHandlers array from ocelot.json ordered as they are in the DelegatingHandlers array.

3. Tracing Delegating Handler, if enabled (see Tracing docs).

4. Quality of Service Delegating Handler, if enabled (see Quality of Service docs).

5. The HttpClient sends the HttpRequestMessage.

Hopefully other people will find this feature useful!