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RPC

Consul uses two RPC systems for communication between components within the cluster and with other clients such as Envoy: gRPC and Go's net/rpc package.

Communication between client agents and servers uses a mix of both gRPC and net/rpc. Generally, gRPC is preferred because it supports modern features such as context deadlines/cancellation, streaming, and middleware - but Consul has been around for a while so the majority of RPC endpoints still use net/rpc.

Multiplexed "Server" Port

Most in-cluster communication happens over the multiplexed "server" TCP port (default: 8300). Consul servers implement a custom protocol for serving different kinds of traffic on the same port, whereby the first byte sent indicates the protocol (e.g. gRPC, net/rpc, Raft).

Servers also implement TLS ALPN on this port, for situations where wrapping the real protocol with a byte prefix isn't practical (e.g. cross-DC traffic over mesh gateways).

The diagram below shows all the possible routing flows:

^source

The main entrypoint to connection routing is handleConn in agent/consul/rpc.go.

Development

Multiplexing several protocols over a single server port helps to reduce our network requirements, but also makes interacting with Consul using local development tools such as grpcurl difficult.

You can get a "plain" TCP connection to the gRPC server using this proxy script:

$ go run tools/internal-grpc-proxy/main.go localhost:8300
Proxying connections to Consul's internal gRPC server
Use this address: 127.0.0.1:64077

Pass the returned proxy address to your tool of choice.

Private vs Public vs Internal vs External

When working on Consul's gRPC endpoints you may notice we use private/public and internal/external slightly differently.

Private and public refer to whether an API is suitable for consumption by clients other than Consul's core components.

Private gRPC APIs are defined in the proto directory, and should only be used by Consul servers and agents. Public gRPC APIs are defined in the proto-public directory and may be used by 3rd-party applications.

Internal and external refer to how the gRPC APIs are exposed.

Internal gRPC APIs are exposed on the multiplexed "server" port, whereas external APIs are exposed on a dedicated gRPC port (default: 8502).

The reason for this differentiation is that some private APIs are exposed on the external port, such as peer streaming/replication; this API isn't (yet) suitable for consumption by 3rd-party applications but must be accessible from outside the cluster, and present a TLS certificate signed by a public CA, which the multiplexed port cannot.

RPC Endpoints

This section is a work in progress, it will eventually cover topics like:

net/rpc - (in the stdlib)
new grpc endpoints
Streaming
agent/structs - contains definitions of all the internal RPC protocol request and response structures.

RPC connections and load balancing