What is the context package used for and how do you use it correctly?

context.Context carries deadlines, cancellation signals and request-scoped values across API boundaries and goroutines. It is how cancellation and timeouts propagate through a call tree.

func handler(ctx context.Context) error {
    ctx, cancel := context.WithTimeout(ctx, 2*time.Second)
    defer cancel()

    req, _ := http.NewRequestWithContext(ctx, "GET", url, nil)
    _, err := http.DefaultClient.Do(req)
    return err
}

Key rules:

• Cancellation propagates down the tree: cancelling a parent cancels every derived context.
• Always call the cancel function, usually with defer, to release timers and resources.
• Pass ctx as the first parameter and check ctx.Done() in loops and blocking selects.
• Use WithValue sparingly, only for request-scoped metadata such as trace IDs, never for optional parameters.
• Never store a Context inside a struct; pass it explicitly through calls.

Common bugs are ignoring cancellation so work continues after a client disconnects, and misusing values as a hidden dependency injection channel.