# Defining Services and RPC Types — gRPC

Source: https://www.skillbyai.com/en/grpc/s-define

> Four kinds of methods.

## Unary and streaming RPCs

A `service` lists `rpc` methods with request and response messages. There are four kinds: **unary** (one request, one response), **server streaming** (one request, a stream of responses), **client streaming** (a stream of requests, one response) and **bidirectional streaming** (both sides stream independently). Use dedicated request and response messages for every method, even if they wrap a single field, so you can add fields later without breaking the signature.

## From contract to running code

Service definitions describe methods; code generators produce clients and servers in each language.

![Three ideas: service definitions, code generation, a unary call end to end.](assets/figures/grpc/section-3-map.svg) — Figure 3.1 — Services, codegen and unary calls.

## An order service

All four RPC kinds.

```protobuf
service OrderService {
  rpc GetOrder(GetOrderRequest) returns (Order);                        // unary
  rpc WatchOrder(WatchOrderRequest) returns (stream OrderEvent);        // server streaming
  rpc UploadItems(stream OrderItem) returns (UploadItemsResponse);     // client streaming
  rpc Chat(stream SupportMessage) returns (stream SupportMessage);     // bidirectional
}

message GetOrderRequest { string id = 1; }
message WatchOrderRequest { string id = 1; }
message OrderEvent { string order_id = 1; OrderStatus status = 2; }
message UploadItemsResponse { int32 accepted = 1; }
message SupportMessage { string text = 1; }
```

## One request and response message per RPC

Sharing a message between methods couples them; separate messages evolve independently.

**Quiz:** Which RPC kind sends one request and receives a stream of responses?

- [x] Server streaming
- [ ] Unary
- [ ] Client streaming
- [ ] Bidirectional streaming

*Answer:* Server streaming. stream on the response side.