Granc 🦀

⚠️ Status: Experimental

This project is currently in a highly experimental phase. It is a working prototype intended for testing and development purposes. APIs, command-line arguments, and internal logic are subject to breaking changes. Please use with caution.

Granc (gRPC + Cranc, Crab in Catalan) is a lightweight, dynamic gRPC CLI tool written in Rust.

It allows you to make gRPC calls to any server using simple JSON payloads, without needing to compile the specific Protobuf files into the client. By loading a FileDescriptorSet at runtime, granc acts as a bridge between human-readable JSON and binary Protobuf wire format.

It is heavily inspired by tools like grpcurl but built to leverage the safety and performance of the Rust ecosystem (Tonic + Prost).

🚀 Features

• Dynamic Encoding/Decoding: Transcodes JSON to Protobuf (and vice versa) on the fly using prost-reflect.
• Smart Dispatch: Automatically detects if a call is Unary, Server Streaming, Client Streaming, or Bidirectional based on the descriptor.
• Server Reflection: Can fetch schemas directly from the server, removing the need to pass a local file descriptor set file (.bin or .pb).
• Metadata Support: Easily attach custom headers (authorization, tracing) to your requests.
• Fast Fail Validation: Validates your JSON before hitting the network.
• Zero Compilation Dependencies: Does not require generating Rust code for your protos. Just point to a descriptor file.
• Tonic 0.14: Built on the latest stable Rust gRPC stack.

📦 Installation

From Crates.io

cargo install granc

From Source

Ensure you have Rust and Cargo installed.

git clone https://github.com/JasterV/granc
cd granc
cargo install --path .

🛠️ Prerequisites

Granc needs to know the schema of the service you are calling. It can obtain this in two ways:

1. Automatic Server Reflection: If the server has Server Reflection enabled, Granc can download the schema automatically.
2. Local Descriptor File: You can provide a binary FileDescriptorSet (.bin) generated by protoc.

Generating Descriptors (Optional)

If your server does not support reflection, you must generate a descriptor file:

# Generate descriptor.bin including all imports
protoc \
    --include_imports \
    --descriptor_set_out=descriptor.bin \
    --proto_path=. \
    my_service.proto

Note: The --include_imports flag is crucial. It ensures that types defined in imported files (like google/protobuf/timestamp.proto) are available for reflection.

📖 Usage

Syntax:

granc [OPTIONS] <URL> <ENDPOINT>

Arguments

Argument	Description	Required
<URL>	Server address (e.g., http://[::1]:50051).	Yes
<ENDPOINT>	Fully qualified method name (e.g., my.package.Service/Method).	Yes

Options

Flag	Short	Description	Required
--proto-set		Path to the binary FileDescriptorSet (.bin).	No
--body		The request body in JSON format.	Yes
--header	-H	Custom header key:value. Can be used multiple times.	No

Automatic Server Reflection

If you omit the --proto-set flag, Granc will automatically attempt to connect to the server's reflection service to download the necessary schemas.

# Using Reflection (no descriptor file needed)
granc \
  --body '{"name": "Ferris"}' \
  http://localhost:50051 \
  helloworld.Greeter/SayHello

This requires the server to have the grpc.reflection.v1 service enabled.

JSON Body Format

• Unary / Server Streaming: Provide a single JSON object { ... }.
• Client / Bidirectional Streaming: Provide a JSON array of objects [ { ... }, { ... } ].

Examples

1. Unary Call (using local descriptor)

granc \
  --proto-set ./descriptor.bin \
  --body '{"name": "Ferris"}' \
  http://localhost:50051 \
  helloworld.Greeter/SayHello

2. Bidirectional Streaming (Chat)

granc \
  --body '[{"text": "Hello"}, {"text": "How are you?"}]' \
  -H "authorization: Bearer token123" \
  http://localhost:50051 \
  chat.ChatService/StreamMessages

🔮 Roadmap

• Interactive Mode: A REPL for streaming requests interactively.
• Pretty Printing: Enhanced colored output for JSON responses.
• TLS Support: Configurable root certificates and client identity.

⚠️ Common Errors

**1. Service 'x' not found**

• Cause: The service name in the command does not match the package defined in your proto file.
• Fix: Check your .proto file. If it has package my.app; and service API {}, the full name is my.app.API.

**2. Method 'y' not found in service 'x'**

• Cause: Typo in the method name or the method doesn't exist.
• Fix: Ensure case sensitivity matches (e.g., GetUser vs getUser).

**3. h2 protocol error**

• Cause: This often occurs when the JSON payload fails to encode after the connection has already been established, or the server rejected the stream structure.
• Fix: Double-check your JSON payload against the Protobuf schema.

🤝 Contributing

Contributions are welcome! Please run the Makefile checks before submitting a PR:

cargo make ci # Checks formatting, lints, and runs tests

📄 License

Licensed under either of:

• Apache License, Version 2.0 (LICENSE-APACHE)
• MIT license (LICENSE-MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.