# Messages and Field Numbers — gRPC

Source: https://www.skillbyai.com/en/grpc/p-messages

> The wire format uses numbers, not names.

## proto3 basics

A `.proto` file declares `syntax = "proto3";` (or uses the newer editions syntax), a `package`, and **messages** with typed fields. Each field has a unique **field number** that identifies it in the binary encoding; names exist only in code. Scalar types include `string`, `bytes`, `bool`, `int32`, `int64`, `uint64`, `double`, `float`, and variants such as `sint64` for negative numbers. In proto3, scalar fields have default zero values, and `optional` lets you detect whether a field was set.

## The schema language

Protocol Buffers define messages and services with numbered fields that keep the wire format stable.

![Three ideas: messages and field numbers, richer types, evolving schemas.](assets/figures/grpc/section-2-map.svg) — Figure 2.1 — Messages, types and evolution.

## An order message

shop/v1/order.proto.

```protobuf
syntax = "proto3";

package shop.v1;

option go_package = "example.com/shop/gen/shop/v1;shopv1";

message Order {
  string id = 1;
  string customer_id = 2;
  repeated OrderItem items = 3;
  int64 total_cents = 4;        // money as integer minor units
  string currency = 5;          // ISO 4217 code, e.g. "INR"
  optional string coupon_code = 6;
}

message OrderItem {
  string product_id = 1;
  int32 quantity = 2;
  int64 unit_price_cents = 3;
}
```

## Use numbers 1 to 15 for frequent fields

Field numbers 1 to 15 encode in one byte together with the wire type, so give them to the most common fields.

**Quiz:** What identifies a field in the protobuf binary encoding?

- [ ] Its name
- [x] Its field number
- [ ] Its position in the file only
- [ ] Its comment

*Answer:* Its field number. Names are for generated code.