Build an Envoy Controller

From the start, IO has been an Envoy controller. It's fair to say that the first thing that IO was, was an Envoy controller. Pros Envoy is powerful, mature, and secure. Envoy has useful configurability and APIs. Envoy's APIs seem stable. Envoy can be built and updated independently of IO. Cons Envoy's APIs use gRPC and take some effort to learn. A single binary might be easier to manage. Sometimes IO's use of Envoy seems to require some gymnastics.