mirror of https://github.com/hashicorp/consul
93 lines
3.7 KiB
Markdown
93 lines
3.7 KiB
Markdown
# RPC
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Consul uses two RPC systems for communication between components within the
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cluster and with other clients such as Envoy: [gRPC](https://grpc.io/)
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and Go's [`net/rpc`](https://pkg.go.dev/net/rpc) package.
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Communication between client agents and servers uses a mix of both gRPC and
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`net/rpc`. Generally, gRPC is preferred because it supports modern features
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such as context deadlines/cancellation, streaming, and middleware - but Consul
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has been around for a while so the majority of RPC endpoints still use `net/rpc`.
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## Multiplexed "Server" Port
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Most in-cluster communication happens over the multiplexed "server" TCP port
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(default: 8300). Consul servers implement a custom protocol for serving
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different kinds of traffic on the same port, whereby the first byte sent
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indicates the protocol (e.g. gRPC, `net/rpc`, Raft).
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Servers also implement [TLS ALPN](https://en.wikipedia.org/wiki/Application-Layer_Protocol_Negotiation)
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on this port, for situations where wrapping the real protocol with a byte prefix
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isn't practical (e.g. cross-DC traffic over mesh gateways).
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The diagram below shows all the possible routing flows:
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[server port]: https://www.consul.io/docs/agent/config/config-files#server_rpc_port
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<sup>[source](./routing.mmd)</sup>
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The main entrypoint to connection routing is `handleConn` in [agent/consul/rpc.go].
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[agent/consul/rpc.go]: https://github.com/hashicorp/consul/blob/main/agent/consul/rpc.go
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### Development
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Multiplexing several protocols over a single server port helps to reduce our
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network requirements, but also makes interacting with Consul using local
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development tools such as [grpcurl] difficult.
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[grpcurl]: https://github.com/fullstorydev/grpcurl
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You can get a "plain" TCP connection to the gRPC server using this proxy script:
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```
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$ go run tools/internal-grpc-proxy/main.go localhost:8300
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Proxying connections to Consul's internal gRPC server
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Use this address: 127.0.0.1:64077
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```
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Pass the returned proxy address to your tool of choice.
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## Private vs Public vs Internal vs External
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When working on Consul's gRPC endpoints you may notice we use private/public and
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internal/external slightly differently.
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Private and public refer to whether an API is suitable for consumption by
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clients other than Consul's core components.
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Private gRPC APIs are defined in the `proto` directory, and should only be used
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by Consul servers and agents. Public gRPC APIs are defined in the `proto-public`
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directory and may be used by 3rd-party applications.
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Internal and external refer to how the gRPC APIs are exposed.
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Internal gRPC APIs are exposed on the multiplexed "server" port, whereas
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external APIs are exposed on a dedicated gRPC port (default: 8502).
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The reason for this differentiation is that some private APIs are exposed on the
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external port, such as peer streaming/replication; this API isn't (yet) suitable
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for consumption by 3rd-party applications but must be accessible from outside
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the cluster, and present a TLS certificate signed by a public CA, which the
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multiplexed port cannot.
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## RPC Endpoints
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This section is a work in progress, it will eventually cover topics like:
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- net/rpc - (in the stdlib)
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- new grpc endpoints
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- [Streaming](./streaming)
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- [agent/structs](https://github.com/hashicorp/consul/tree/main/agent/structs) - contains definitions of all the internal RPC protocol request and response structures.
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## RPC connections and load balancing
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This section is a work in progress, it will eventually cover topics like:
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Routing RPC request to Consul servers and for connection pooling.
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- [agent/router](https://github.com/hashicorp/consul/tree/main/agent/router)
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- [agent/pool](https://github.com/hashicorp/consul/tree/main/agent/pool)
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