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Multi-Protocol Support

Connect supports multiple protocols by switching on HTTP Content-Types: each protocol uses a distinct set of types, so servers know which protocol the client expects and respond appropriately. Because gRPC, gRPC-Web, and Connect's own protocol have the same semantics, switching protocols doesn't require code changes.

Assuming that you're using Protobuf schemas, the gRPC protocol uses the application/grpc, application/grpc+proto, and application/grpc+json Content-Types. The gRPC-Web protocol uses application/grpc-web, application/grpc-web+proto, and application/grpc-web+json.

Again assuming Protobuf schemas, the Connect protocol uses the application/proto and application/json Content-Types for unary RPCs. For streaming, it uses application/connect+proto and application/connect+json.

Compatible Semantics​

This approach relies on all three protocols having compatible semantics: they're communicating the same information using different HTTP conventions, so the protocol is just an implementation detail. In particular, all three protocols support:

  • Pluggable serialization and compression.
  • Unary RPCs and all three types of streaming.
  • Timeouts.
  • Headers and trailers (though gRPC-Web and the Connect protocol encode trailers into the last portion of the response body).
  • Errors composed of a code, message, and strongly-typed details.

By organizing their APIs around these concepts and avoiding references to the protocols themselves, Connect implementations let you switch protocols freely.