For HTTP/2 after .NET 5, How does HttpClient handle the pending request when connection is disconnected?

[GF-Huang]

We know that HTTP/2 has multiplexing properties. So usually only one TCP connection is established with the same server.

By reading the source code, I roughly understand that HttpClient internally uses Http2Connection to send requests. Http2Connection uses Channel<T> internally to implement the producer-consumer mode to send each request, which ensuring thread safety.

I got lost while reading due to the code complexity of Http2Connection. My question is:

• When there are still multiple requests unsent in Channel<T>, the TCP connection (equivalents such as NetworkStream or Socket) inside Http2Connection is disconnected for various reasons, how are the remaining requests handled?
• For the callers of these requests, will they get an HttpRequestException at the same time?
• Or will Http2Connection[Pool] internally re-establish the connection and send the pending/remaining requests in the Channel<T>?

[huoyaoyuan]

@dotnet/ncl

[ManickaP]

TLDR: the connection gets reestablished automatically

There are multiple things to take into consideration:

• H/2 has a mechanism for graceful connection close from the server side: https://www.rfc-editor.org/rfc/rfc9113.html#GOAWAY. Based on that we finish certain requests and retry the remaining ones on a new connection.
• Requests do not get associated with a particular connection unless we know that the connection has a capacity for it, you can look into GetHttp2ConnectionAsync. So the requests associated with a connection are in flight.
• We do not retry requests in flight unless we get https://www.rfc-editor.org/rfc/rfc9113.html#name-rst_stream telling us we can, so if the connection fails, requests in flight will fail.

Hope this answers your question.