# Metadata — gRPC

Source: https://www.skillbyai.com/en/grpc/x-metadata

> Headers for RPCs.

## Key-value pairs alongside messages

**Metadata** is a set of key-value pairs sent with a call, like HTTP headers: authentication tokens, request IDs, tenant IDs, tracing context. Keys are lowercase ASCII; binary values use keys ending in `-bin`. Servers can send **initial metadata** (headers) and **trailing metadata** (trailers, alongside the status). Keep business data in messages and use metadata for cross-cutting information.

## Cross-cutting concerns

Metadata carries context, TLS and tokens secure calls, and interceptors apply shared logic to every RPC.

![Three ideas: metadata, authentication, interceptors.](assets/figures/grpc/section-6-map.svg) — Figure 6.1 — Metadata, TLS/auth and interceptors.

## Sending and reading metadata in Python

A sketch.

```python
# client
order = stub.GetOrder(
    order_pb2.GetOrderRequest(id="o-1001"),
    metadata=[("x-request-id", "req-7f3a"), ("authorization", f"Bearer {token}")],
    timeout=2.0,
)

# server
def GetOrder(self, request, context):
    md = dict(context.invocation_metadata())
    request_id = md.get("x-request-id", "")
    context.set_trailing_metadata([("x-served-by", "orders-7d9c")])
    ...
```

## Keep metadata small

Metadata is sent as HTTP/2 headers with size limits; large payloads belong in messages.

**Quiz:** What suffix marks a binary metadata key?

- [ ] _b64
- [ ] -binary
- [x] -bin
- [ ] -raw

*Answer:* -bin. Binary values are base64-encoded on the wire.