===== WithValue =====

==== What is it? ====
`context.WithValue(parent, key, val)` returns a child context that carries a key/value pair.

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

==== What is it used for? ====
  * Carry **request-scoped** metadata like request-id, trace-id, user-id.
  * Pass small values across API boundaries when you cannot change function signatures.

==== Example (typed key) ====
<code go>
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
}
</code>

==== Notes / pitfalls (very important) ====
  * 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).

==== Related pages ====
  * [[go:data_structures:type_assertion|Type assertion]]
  * [[go:data_structures:type_switch|Type switch]]
  * [[go:concurrency:context|context.Context]]

==== Hard words (English) ====
  * **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