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January 12, 2023
5 min read

Introducing buf curl - Call your gRPC endpoints with the simplicity of buf

Authored by Josh Humphries

Today, we’re introducing a new tool for Protobuf APIs: buf curl. This addition to the Buf CLI makes it easy to invoke gRPC, gRPC-Web, and Connect endpoints when debugging, when testing, or in cases where using a generated RPC client is too heavy. It is designed to match the same workflow as curl, even providing many of its familiar flags. Upgrade to v1.12.0 of buf to use buf curl today!

Why buf curl?

buf curl makes it effortless to call RPC endpoints using the same setup and workflow as for building, linting, breaking change detection, and code generation - if your code builds with buf, you can use buf curl right away.

There have been previous tools that allow you to call gRPC endpoints. In fact, in a previous life, I built a popular one named grpcurl. buf curl is the next generation of grpcurl - with grpcurl, you either need your servers to support server reflection (which comes with its own security considerations), or you have to practically re-create your Protobuf build system in the form of -proto and -import-path arguments. With buf curl, you don't have to think about any of that. While buf curl does support server reflection, it also seamlessly integrates with the Buf Schema Registry, eliminating the need for server reflection or for a local Protobuf schema.

buf curl is to grpcurl as buf is to protoc - all the power that you need without any complexity. For teams that already use Buf, buf curl is sure to make workflows simpler.

Examples

To get a sense for how buf curl works, let’s take a look at a few examples.

First, let’s use buf curl to invoke a gRPC endpoint with local Protobuf sources and no server reflection. This uses the module defined in the proto directory of github.com/bufbuild/connect-demo to invoke our demo ElizaService:

$ git clone https://github.com/bufbuild/connect-demo && \
  cd ./connect-demo && \
  buf curl --protocol grpc --schema ./proto \
    https://demo.connect.build/buf.connect.demo.eliza.v1.ElizaService/Say \
    -d '{"sentence":"Hello."}'
Cloning into 'connect-demo'...
remote: Enumerating objects: 446, done.
remote: Counting objects: 100% (314/314), done.
remote: Compressing objects: 100% (195/195), done.
remote: Total 446 (delta 187), reused 185 (delta 89), pack-reused 132
Receiving objects: 100% (446/446), 120.50 KiB | 1.45 MiB/s, done.
Resolving deltas: 100% (197/197), done.
{"sentence":"Hello, how are you feeling today?"}

We also push this module to the Buf Schema Registry, located at buf.build/bufbuild/eliza. Using modules hosted on the Buf Schema Registry is a snap:

$ buf curl --protocol grpc --schema buf.build/bufbuild/eliza \
    https://demo.connect.build/buf.connect.demo.eliza.v1.ElizaService/Say \
    -d '{"sentence":"Hello."}'
{"sentence": "Hello, how are you feeling today?"}

Finally, let’s try a request that uses the Connect protocol (the default) and this time retrieve the RPC schema using server reflection (which is enabled on our demo ElizaService). In this scenario, you don’t need any protocol or schema flags:

$ buf curl \
    https://demo.connect.build/buf.connect.demo.eliza.v1.ElizaService/Say \
    -d '{"sentence":"Hello."}'
{"sentence": "Hello, how are you feeling today?"}

How it works

Protobuf-based RPCs binary-encode messages and wrap streamed messages in a binary envelope, making it difficult to craft ad-hoc requests using tools like curl and wget.

buf curl differs from normal curl by handling the relevant RPC protocol details for the user so they don’t need to remember the right way to craft a request (such as which HTTP method and headers need to be specified). It also takes care of message formatting and framing by accepting a simple JSON document (or multiple JSON values concatenated together for request streams) and translating the messages to the Protobuf binary format. For responses, it does the reverse, translating the Protobuf binary format into JSON to make the response data easier for humans to read and, combined with tools like jq, easier to include in scripts.

In order to do this format conversion, the Protobuf schema for the RPC service is needed since the Protobuf binary format cannot be interpreted without a schema. The buf curl command can automatically download the schema from the server if it supports server reflection. If the server does not support server reflection, the tool can use a compiled descriptor set or even compile Protobuf sources on the fly, pulling the sources from a module in the Buf Schema Registry, a Git repo, or files on disk.

The buf curl command supports three protocols, selected via command-line option: Connect, gRPC, and gRPC-Web. If your environment doesn't support HTTP/2 end-to-end, including through all load balancers and reverse proxies, then the gRPC protocol won't work. For that reason, buf curl also supports Connect and gRPC-Web, which don’t require HTTP/2 and can work over HTTP/1.1.

The Connect protocol was developed to be friendlier both for humans and commonplace HTTP software libraries. It includes out-of-the-box support for JSON encoding and does not use framing or message enveloping for unary RPCs (i.e., non-streaming RPCs that have one request message and one response message). However, Connect still needs framing for streaming RPCs, which means it can be challenging to use with vanilla curl. buf curl is a one-stop shop for interacting with RPCs, even when using a simpler protocol like Connect.

The command-line interface and options for buf curl were designed to mirror the standard curl command as closely as possible, so users who are familiar with curl will be able to easily find their way around buf curl.

Try it out

Take buf curl for a spin! More details about the command can be found on our documentation site or by running buf help curl after installing v1.12.0. If you have any requests or find a bug, please let us know by filing a GitHub issue.

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