Envoy Proxy Fundamentals

Envoy is a self-contained network proxy. It receives traffic from a downstream client, applies a configured processing pipeline, and forwards traffic to an upstream endpoint. That sounds ordinary. The useful difference is that Envoy makes routing, security, load balancing, and telemetry available outside the application process.

You can place Envoy at an application edge, beside an application as a sidecar, or between internal services. The placement changes who is downstream and upstream. The core model does not change.

Start with the request path

A listener binds an address and port and accepts downstream connections. Envoy chooses a filter chain for that connection. Network filters then process the connection. For HTTP traffic, the HTTP connection manager creates an HTTP filter chain for each request. The router filter selects a route and cluster, then forwards the request to an endpoint in that cluster.