`context.WithValue(parent, key, val)` returns a child context that carries a key/value pair.

Signature: `ctx := context.WithValue(parent, key, val)`

• Carry request-scoped metadata like request-id, trace-id, user-id.
• Pass small values across API boundaries when you cannot change function signatures.

type ctxKey string
 
const requestIDKey ctxKey = "request_id"
 
func WithRequestID(ctx context.Context, id string) context.Context {
    return context.WithValue(ctx, requestIDKey, id)
}
 
func RequestID(ctx context.Context) (string, bool) {
    v := ctx.Value(requestIDKey)
    s, ok := v.(string)
    return s, ok
}

• Use `WithValue` sparingly. Prefer explicit parameters for business logic.
• Keys should be custom types (not plain string) to avoid collisions.
• Do NOT store large objects (DB connections, big structs) in context.
• Values must be safe to access concurrently (context may be shared).

• Type assertion
• Type switch
• context.Context

• request-scoped /rɪˈkwest skoʊpt/: gắn với 1 request
• metadata /ˈmetəˌdeɪtə/: dữ liệu mô tả
• sparingly /ˈsperɪŋli/: dùng hạn chế
• collision /kəˈlɪʒən/: xung đột
• explicit /ɪkˈsplɪsɪt/: tường minh
• concurrently /kənˈkɝːəntli/: đồng thời