Using gRPC with JSON

Easy introspection of requests and responses with JSON payloads

Introduction

It’s often said that gRPC is tied to the Google Protocol Buffers payload format, but this is not strictly true. While the default format for gRPC payloads is Protobuf, the gRPC-Go implementation exposes a Codec interface which allows arbitrary payload encoding. This could be used for all kinds of things, like your own binary format, using flatbuffers, or, as we shall see today, using JSON for requests and responses.

Server setup

I’ve created an example repo with an implementation of grpc/encoding.Codec for JSON payloads. Server setup is as simple importing the package;

import _ "github.com/johanbrandhorst/grpc-json-example/codec"

This registers the JSON Codec under the content subtype json, which we’ll see becomes important to remember later.

Request examples

gRPC client

Using a gRPC Client, simply initiate using the correct content-subtype as a grpc.DialOption:

import "github.com/johanbrandhorst/grpc-json-example/codec"
func main() {
    conn := grpc.Dial("localhost:1000",
        grpc.WithDefaultCallOptions(grpc.CallContentSubtype(codec.JSON{}.Name())),
    )
}

The example repo includes a client with a full example.

cURL

More interestingly, it’s now possible to basically write our requests (and read responses) using just cURL! Some request examples:

$ echo -en '\x00\x00\x00\x00\x17{"id":1,"role":"ADMIN"}' | curl -ss -k --http2 \
        -H "Content-Type: application/grpc+json" \
        -H "TE:trailers" \
        --data-binary @- \
        https://localhost:10000/example.UserService/AddUser | od -bc
0000000 000 000 000 000 002 173 175
         \0  \0  \0  \0 002   {   }
0000007
$ echo -en '\x00\x00\x00\x00\x17{"id":2,"role":"GUEST"}' | curl -ss -k --http2 \
        -H "Content-Type: application/grpc+json" \
        -H "TE:trailers" \
        --data-binary @- \
        https://localhost:10000/example.UserService/AddUser | od -bc
0000000 000 000 000 000 002 173 175
         \0  \0  \0  \0 002   {   }
0000007
$ echo -en '\x00\x00\x00\x00\x02{}' | curl -k --http2 \
        -H "Content-Type: application/grpc+json" \
        -H "TE:trailers" \
        --data-binary @- \
        --output - \
        https://localhost:10000/example.UserService/ListUsers
F{"id":1,"role":"ADMIN","create_date":"2018-07-21T20:18:21.961080119Z"}F{"id":2,"role":"GUEST","create_date":"2018-07-21T20:18:29.225624852Z"}

Explanation

Using cURL to send requests requires manually adding the gRPC HTTP2 message payload header to the payload:

'\x00\x00\x00\x00\x17{"id":1,"role":"ADMIN"}'
#<-->----------------------------------------- Compression boolean (1 byte)
#    <-------------->------------------------- Payload size (4 bytes)
#                    <--------------------->-- JSON payload

Headers must include TE and the correct Content-Type:

 -H "Content-Type: application/grpc+json" -H "TE:trailers"

The string after application/grpc+ in the Content-Type header must match the Name() of the codec registered in the server. This is called the content subtype.

The endpoint must match the name of the name of the proto package, the service and finally the method:

https://localhost:10000/example.UserService/AddUser

The responses are prefixed by the same header as the requests:

'\0  \0  \0  \0 002   {   }'
#<-->------------------------ Compression boolean (1 byte)
#    <------------>---------- Payload size (4 bytes)
#                     <--->-- JSON payload

Conclusion

We’ve shown that we can easily use JSON payloads with gRPC, even allowing us to send cURL requests with JSON payloads directly to our gRPC servers, no proxies, no grpc-gateway, no setup except for importing a package necessary.

If you enjoyed this blog post, have any questions or input, don’t hesitate to contact me on @johanbrandhorst or under jbrandhorst on the Gophers Slack. I’d love to hear your thoughts!

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