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Naive implementation

func placeOrder(o Order) {
    warehouse.ReserveItems(o.Items)

    payment.ProcessPayment(o)

    notifyBuyer(o, OrderPlaced)

    saveOrder(o)
}

Failure modes

func placeOrder(o Order) {
    reserved, err := warehouse.ReserveItems(o.Items)
    if err != nil { // Warehouse service unavailable
        // Retry?
    }

    if !reserved { // Business error requiring user intervention
        notifySeller(o, OrderFailed)
        notifyBuyer(o, OrderFailed)

        return
    }

    // ...
}

We want this

func placeOrder(o Order) {
    warehouse.ReserveItems(o.Items)

    payment.ProcessPayment(o)

    notifyBuyer(o, OrderPlaced)

    saveOrder(o)
}

Reminder

func placeOrder(o Order) {
    warehouse.ReserveItems(o.Items)

    payment.ProcessPayment(o)

    notifyBuyer(o, OrderPlaced)

    saveOrder(o)
}

Determinism

Output value is based entirely on the input.

func add(a, b int) int {
    return a + b
}

func add(a, b int) int {
    // This is not deterministic
    resp := http.Get(fmt.Sprintf("https://add.com/%d/%d", a, b))

    return decodeBody(resp.Body)
}

Workflow replay

func Workflow(ctx workflow.Context, input Input) (Output, error) {
    encodedNumber := workflow.SideEffect(ctx, func(ctx workflow.Context) interface{} {
        return rand.Intn(max)
    })

    sum := input.Number + number

    workflow.Sleep(ctx, 1*time.Second)

    return Output{sum}, nil
}

Intro workshop