Playing Traffic Cop With Fly-Replay

Fly.io, a platform for compute, offers an interesting feature called "Fly Replay" which allows applications to respond with a header that triggers the replay of an HTTP request somewhere else. This can be useful in various scenarios such as routing requests to specific apps or VMs within the same organization. The Fly Proxy plays a crucial role in this process by looking for the fly-replay header in responses and replaying the initial HTTP request accordingly. One use case is directing write queries to a leader database near the server, which can significantly improve performance. Additionally, developers can create a "proxy" app that responds with a fly-replay header based on the hostname of the incoming request, effectively routing requests to specific apps or microservices.