HeaderPropagation 1.0.2

This is a backport to ASP.NET Core 2.1 (and 2.2) of the HeaderPropagation middleware I had recently contributed to the ASP.NET Core project. All code is licensed under the Apache License, Version 2.0 and copyrighted by the .NET Foundation.

Motivation

I believe it is a common use case which deserves to be included in ASP.NET Core. Its main use case is probably to track distributed transaction which requires the ability to pass through a transaction identifier as well as generating a new one when not present.

Given the ASP.NET Core 3.0 release is quite far away, and the current policy doesn't allow to backport new features to already shipped releases, I have created this package as recommended so it can be used today on projects based on ASP.NET Core 2.1 or 2.2.

Usage

In Startup.Configure enable the middleware:

app.UseHeaderPropagation();

In Startup.ConfigureServices add the required services, eventually specifying a configuration action:

services.AddHeaderPropagation(o => o.Headers.Add("User-Agent", new HeaderPropagationEntry
{
    DefaultValue = "Mozilla/5.0 (trust me, I'm really Mozilla!)",
}));

If you are using the HttpClientFactory, add the DelegatingHandler to the client configuration using the AddHeaderPropagation extension method.

services.AddHttpClient<GitHubClient>(c =>
{
    c.BaseAddress = new Uri("https://api.github.com/");
    c.DefaultRequestHeaders.Add("Accept", "application/vnd.github.v3+json");
}).AddHeaderPropagation();

See samples/WebApplication.

Behaviour

HeaderPropagationOptions contains a dictionary where the key represent the name of the header to consume from the incoming request.

Each entry define the behaviour to propagate that header as follows:

• OutboundHeaderName determines the name of the header to be used for the outbound http requests.

• When present, the ValueFactory is the only method used to set the value. The factory should return StringValues.Empty to not add the header.

• When not present, the value will be taken from the header in the incoming request named as the key of this entry in HeaderPropagationOptions.Headers or, if missing or empty, it will be the values specified in DefaultValue or, if the DefaultValue is empty, the header will not be added to the outbound calls.

Please note the factory is called only once per incoming request and the same value will be used by all the outbound calls.