What is context.Context in Go, what does it carry, and how do you create one?

context.Context is Go's standard mechanism for propagating three things across API boundaries and goroutine boundaries: cancellation signals, deadlines/timeouts, and request-scoped values. Every blocking or long-running function should accept a Context as its first parameter.

// The full context.Context interface:
type Context interface {
    Deadline() (deadline time.Time, ok bool) // zero time if no deadline
    Done()     <-chan struct{}                // closed on cancel/timeout
    Err()      error                         // nil, Canceled, or DeadlineExceeded
    Value(key any) any                       // request-scoped value lookup
}

// Root contexts (start of a context tree)
ctx := context.Background() // never cancelled, no deadline — use at program start
ctx  = context.TODO()       // placeholder when context not yet known

// Derived contexts — each returns a cancel function
ctx1, cancel1 := context.WithCancel(context.Background())
defer cancel1()  // ALWAYS defer — prevents goroutine leak in the monitor goroutine

ctx2, cancel2 := context.WithTimeout(context.Background(), 5*time.Second)
defer cancel2()

ctx3, cancel3 := context.WithDeadline(context.Background(),
    time.Now().Add(30*time.Second))
defer cancel3()

// Attaching a value (use typed key to avoid collisions)
type ctxKey string
const traceIDKey ctxKey = "traceID"
ctx4 := context.WithValue(ctx, traceIDKey, "abc-123")
traceID := ctx4.Value(traceIDKey).(string)

// Propagate context to downstream functions
func processRequest(ctx context.Context, req Request) error {
    if err := validateInput(ctx, req); err != nil {
        return fmt.Errorf("validation: %w", err)
    }
    return persistToDB(ctx, req) // ctx carries cancellation + deadline
}

Context tree: cancelling a parent cancels all descendants. A child context with a shorter deadline does not extend the parent's deadline — the effective deadline is always min(parent, child).

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