Context Package: Complete Interview Guide

Problem Statement

Master context.Context for cancellation, timeouts, and request-scoped values in Go applications.

Context Interview Questions

Q1: Why should context.Context be the first argument?

Answer: Convention established by Go team. Makes it immediately clear that the function supports cancellation. Example: func DoWork(ctx context.Context, args ...)

Q2: Two ways a Context can be cancelled?

// 1. Manual cancel
ctx, cancel := context.WithCancel(parent)
cancel() // Explicit cancellation

// 2. Timeout/Deadline
ctx, cancel := context.WithTimeout(parent, 5*time.Second)
ctx, cancel := context.WithDeadline(parent, time.Now().Add(5*time.Second))

Q3: Does cancelling parent cancel children?

parent, cancelParent := context.WithCancel(context.Background())
child, _ := context.WithCancel(parent)

cancelParent() // Both parent AND child are cancelled

Answer: Yes. Cancellation propagates down the tree.

Q4: Does cancelling child cancel parent?

Answer: No. Cancellation only propagates downward.

Q5: What is context.WithValue used for?

type ctxKey string
const traceIDKey ctxKey = "traceID"

ctx := context.WithValue(parent, traceIDKey, "abc-123")
traceID := ctx.Value(traceIDKey).(string)

Use for: Request-scoped data (trace IDs, auth tokens).
NOT for: Optional function parameters, database connections.

Q6: Is context.Context thread-safe?

Answer: Yes, fully immutable and safe for concurrent use.

Q7: How to handle timeout in select?

select {
case result := <-workChan:
    return result, nil
case <-ctx.Done():
    return nil, ctx.Err() // context.DeadlineExceeded or context.Canceled
}

Q8: What happens with nil context?

var ctx context.Context = nil
ctx.Done() // PANIC: nil pointer dereference

Q9: context.Background() vs context.TODO()?

Answer: Semantically equivalent, both return empty contexts.
- Background(): Use at top-level (main, init, tests)
- TODO(): Placeholder when unsure which context to use

Q10: Should you store Context in a struct?

// ❌ Anti-pattern
type Server struct {
    ctx context.Context
}

// ✅ Pass through function arguments
func (s *Server) Handle(ctx context.Context, req Request)

Answer: No. Context should flow through call stack, not be stored.