===== context.Context =====

==== What is it? ====
`context.Context` is a standard way to carry:
  * deadlines,
  * cancellation signals,
  * and request-scoped values
across API boundaries and between goroutines.

==== What is it used for? ====
  * Cancel work when a request is no longer needed (client disconnected, timeout, shutdown).
  * Enforce timeouts/deadlines for I/O (HTTP calls, DB queries, RPC).
  * Pass request-scoped metadata (trace IDs, request IDs) carefully.

==== Core functions ====
  * `context.Background()` — root context for main/service.
  * `context.TODO()` — placeholder when you don’t have a context yet.
  * `context.WithCancel(parent)` — manual cancellation.
  * `context.WithTimeout(parent, d)` — cancel after duration.
  * `context.WithDeadline(parent, t)` — cancel at specific time.
  * `context.WithValue(parent, key, val)` — attach request-scoped values (use sparingly).

==== Example: timeout for work ====
<code go>
package main

import (
    "context"
    "time"
)

func main() {
    ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
    defer cancel()

    _ = ctx // pass ctx into HTTP/DB/RPC calls
}
</code>

==== Example: cancellation (stop goroutine) ====
<code go>
package main

import (
    "context"
    "time"
)

func worker(ctx context.Context) {
    for {
        select {
        case <-ctx.Done():
            return
        default:
            time.Sleep(10 * time.Millisecond)
        }
    }
}

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    go worker(ctx)

    time.Sleep(50 * time.Millisecond)
    cancel()
    time.Sleep(20 * time.Millisecond)
}
</code>

==== Notes / pitfalls ====
  * Context is NOT for global configuration.
  * Do NOT store large values in context; keep values small (IDs).
  * Use typed keys to avoid collisions.
  * Always call `cancel()` to release resources (timers) for timeout/deadline contexts.

==== Related pages ====
  * [[go:concurrency:goroutine|Goroutine]]
  * [[go:stdlib:http_client_struct|http.Client]]
  * [[go:stdlib:http_server_struct|http.Server]]

==== Hard words (English) ====
  * **deadline** /ˈdedlaɪn/: hạn chót
  * **cancellation** /ˌkænsəˈleɪʃən/: huỷ
  * **signal** /ˈsɪɡnəl/: tín hiệu
  * **request-scoped** /rɪˈkwest skoʊpt/: gắn với 1 request
  * **metadata** /ˈmetəˌdeɪtə/: dữ liệu mô tả
  * **API boundary** /ˌeɪ piː ˈaɪ ˈbaʊndri/: ranh giới API
  * **collision** /kəˈlɪʒən/: xung đột
  * **sparingly** /ˈsperɪŋli/: dùng hạn chế