type Job interface {
    Execute(ctx context.Context) error
}

type JobQueue struct {
    jobs    chan *TrackedJob
    wg      sync.WaitGroup
}

func (q *JobQueue) Submit(job Job) {
    q.wg.Add(1)
    q.jobs <- NewTracked(job)
}

func (t *TrackedJob) ExecuteWithRetry(ctx context.Context) {
    for t.retries < t.MaxRetries {
        ctx, cancel := context.WithTimeout(ctx, t.Timeout)
        err := t.Job.Execute(ctx)
        cancel()
        if err == nil { return }
        t.retries++
        time.Sleep(backoff(t.retries))
    }
}

type Persister interface {
    Save(job *TrackedJob) error
    Update(jobID string, status string) error
    LoadPending() ([]*TrackedJob, error)
}

q := NewJobQueue(10, 3)
q.Submit(&EmailJob{To: "[email protected]"})
q.Submit(&ResizeJob{File: "pic.png"})
q.Wait()

func TestAPI(t *testing.T) {
    t.Run("User", func(t *testing.T) {
        t.Run("Create", func(t *testing.T) { /* ... */ })
        t.Run("Delete", func(t *testing.T) { /* ... */ })
    })

    t.Run("Order", func(t *testing.T) {
        t.Run("Create", func(t *testing.T) { /* ... */ })
        t.Run("Cancel", func(t *testing.T) { /* ... */ })
    })
}