Preface

Every order Northwind takes comes with a promise printed at checkout: delivered by Thursday. This reference explains how an order keeps that promise, station by station, and what the teams do when a station fails.

Read Chapter 1 first; the other chapters stand on their own. Terms in the text such as idempotency key are collected in the index.

The order journey

1.1Five stations#

An order passes five stations in a fixed order: checkout, payment, warehouse, courier and tracking. Figure 1.1 shows them, and Table 1.1 names their owners.

Table 1.1 · Who owns each station

1.2Hand-offs#

A station hands the order on by writing an event; the next station reads it. No station calls another directly, so a slow station never slows the one before it. The events are kept for thirty days.

1.3What the customer sees#

The tracking page reads the same events. It shows one tile per station and the promise under them; see Section 2.1.

Promises and clocks

2.1The promise tile#

The promise is computed when the order is placed and recomputed after every station: computed at checkout. It only moves later when a station is late; it never moves earlier.

2.2Cut-off times#

Table 2.1 · Cut-off times for a Thursday promise

The second row is the one warehouse staff watch: an order packed by 07:10 on Tuesday still makes Wednesday's first wave (Table 2.1).

Payments that time out

3.1The declines were timeouts#

Nine in ten night-time declines were the bank's fraud check timing out, not a decision about the card. A timeout is not a decline: it is a bank that has not answered yet.

3.2One idempotent retry#

var key = order.id();
var result = bank.authorize(card, amount, key);
if (result.isTimeout()) {
    sleep(Duration.ofSeconds(30));
    result = bank.authorize(card, amount, key);
}
return result;

1. The order id is the idempotency key: a retry is the same request.
2. One retry only, thirty seconds later.

The retry changes nothing for the stations after payment; see Section 1.2. The retry is logged with both attempts.

Glossary

The glossary is also in the index.

Index

10 terms. Each number is a section: 2.3 is chapter 2, section 3.