Support multiple header names in middleware

[dashkan]

First of all, thank you for the great project. I use it in all my net core projects.

I have a unique situation where I need to integrate with legacy APIs and cannot change the source. Our new APIs use the the standard x-correlation-id header and older APIs use a different header. I need the ability to first try reading x-correlation-id and then fallback to old header name.

Are you open to extending the middleware via the CorrelationIdOptions by adding a callback property

        /// <summary>
        /// A callback to resolve the correlation ID. 
        /// </summary>
        public Func<IHeaderDictionary, StringValues> RequestHeaderCallback { get; set; }

And update the middleware

            StringValues cid;
            if (_options.RequestHeaderCallback != null)
                cid = _options.RequestHeaderCallback(context.Request.Headers);
            else
                context.Request.Headers.TryGetValue(_options.RequestHeader, out cid);
            
            var hasCorrelationIdHeader = !StringValues.IsNullOrEmpty(cid);

This simple change would allow users to use custom logic to read the correlation header. I can even make the callback more broad by passing the HttpContext and maybe even IServiceProvider in case people need to use DI to get their correlation id value.

Happy to submit a PR.

[stevejgordon]

Hi @dashkan. Thanks for raising this suggestion and the offer of a PR!

This feels like a useful addition for situations such as yours. I'm wondering if it would be enough to simply support an array of header names to attempt to read, with the first match returning the correlation ID?

[dashkan]

Hi @stevejgordon. That would work definitely work for me. I was just trying to expand the api to cover more complex workflows.

Totally understand if you want to keep it simple.

Happy to submit PR in either case.

[wdolek]

If I may suggest, this could be also done by providing sequence of headers to be used when reading correlation ID, and then simply iterating over it and trying to find given header in request. Assuming that such iteration won't be expensive since you will configure just few headers, and lookup in HttpRequest.Headers is cheap as it is dictionary.