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docs: update "How to write gRPC SDK for GreptimeDB" #262

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30 changes: 29 additions & 1 deletion docs/developer-guide/how-to/how-to-write-sdk.md
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# How to write a gRPC SDK for GreptimeDB

GreptimeDB builds its gRPC interface on top of [Apache Arrow Flight](https://arrow.apache.org/docs/format/Flight.html). If you want to write a gRPC SDK for GreptimeDB in a programming language you are familiar with, read on!
There are two gRPC services exposed by GreptimeDB. One is defined by GreptimeDB, the other is built on top
of [Apache Arrow Flight](https://arrow.apache.org/docs/format/Flight.html). If you want to write a gRPC SDK for
GreptimeDB in a programming language you are familiar with, read on!

> Currently, we only have gRPC SDK written in Java, and you can find more details [here](../user-guide/java-sdk.md).

## `GreptimeDatabase` Service

GreptimeDB defines a custom gRPC service called `GreptimeDatabase`. You can find its protobuf
definitions [here](https://github.com/GreptimeTeam/greptime-proto). It contains two RPC methods:

```protobuf
service GreptimeDatabase {
rpc Handle(GreptimeRequest) returns (GreptimeResponse);

rpc HandleRequests(stream GreptimeRequest) returns (GreptimeResponse);
}
```

The `Handle` method is for unary call: When a `GreptimeRequest` is received and processed by the GreptimeDB
server, it responds with a `GreptimeResponse` immediately.

The `HandleRequests` acts in
a "[Client streaming RPC](https://grpc.io/docs/what-is-grpc/core-concepts/#client-streaming-rpc)" style. It ingests a
stream of `GreptimeRequest`, and handles them on the fly. After all the requests have been handled, it returns a
summarized `GreptimeResponse`. Through `HandleRequests`, we can achieve a very high throughput of requests handling.

However, currently the `GreptimeDatabase` service can **only** handle insert requests. For query requests, we need to
implement the [Apache Arrow Flight](https://arrow.apache.org/docs/format/Flight.html) gRPC client as well.

## [Apache Arrow Flight](https://arrow.apache.org/docs/format/Flight.html) Service

First, you can find our protobuf definitions for GreptimeDB requests and responses in this [repo](https://github.com/GreptimeTeam/greptime-proto#for-sdk-developers). The section named "For SDK developers" in the README of that repo is worth reading.

Then check your programming language to see whether Arrow Flight RPC officially supports it. Currently, it supports C++, Java, Go, C# and Rust and might add more supported languages, so stay tuned with its [Implementation Status](https://arrow.apache.org/docs/status.html#flight-rpc). If you can't find the language you are using, you have to write a client from sketch (starting from Arrow Flight's raw gRPC service protobuf [definition](https://arrow.apache.org/docs/format/Flight.html#protocol-buffer-definitions)).
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