Memory card and where the analogy breaks

The memory card (tear this page out)

#	Question	Memory code
17.1	Transaction?	Seat and payment, together or not at all
17.2	ACID in depth?	Whole, legal, private, permanent
17.3	Lost update?	Don't read, think and write back; let the database subtract
17.4	Read anomalies?	Unfinished, changed, or newly appeared: three ways to read a lie
17.5	Isolation levels?	The stronger the isolation, the more it's like one clerk at a time, and the more you wait or retry
17.6	Pessimistic locking?	Hand on the seat before you sit
17.7	Optimistic locking?	Check it's still free as you sit
17.8	Deadlocks?	Grab seats in the same order and nobody waits in a circle
17.9	Distributed locks?	A lock that can expire can be held twice, so number every holder
17.10	2PC?	Everyone says "ready", then everyone says "done"; one "no" cancels all
17.11	Sagas?	Do it step by step, and keep an undo for every step
17.12	Seat holds?	Hold it, pay for it, or lose it

The whole chapter in one line: All or nothing inside one database, a hand or a last-moment check on every contested seat, and step-by-step with undo across services.

Where the analogy breaks

Two passengers arguing is visible. Two transactions overwriting each other is silent: a lost update leaves no error, just wrong numbers.
A clerk can't be in two places. A transaction can be retried, aborted or replayed, so every step must be safe to run more than once.
A refund feels like an undo. Some actions can't be undone at all (a sent email, a shipped parcel), which is why sagas put them last.

Quick check

Five questions on this chapter. Pick an answer, then check it.

quiz

1[
2 {
3  "q": "Two clerks read stock = 5, each subtracts 1, each writes 4. What's this, and the fix?",
4  "options": [
5   "Dirty read; use caching",
6   "Lost update; let the database do it atomically: UPDATE ... SET stock = stock - 1 WHERE stock > 0",
7   "Deadlock; retry",
8   "Phantom read; add an index"
9  ],
10  "answer": 1,
11  "explain": "Don't read, think and write back; let the database subtract."
12 },
13 {
14  "q": "Optimistic locking works by...",
15  "options": [
16   "Locking the row while the user decides",
17   "Writing only if a version number hasn't changed since you read it",
18   "Using a distributed lock",
19   "Serializable isolation only"
20  ],
21  "answer": 1,
22  "explain": "Check it's still free as you sit."
23 },
24 {
25  "q": "How do you prevent most deadlocks?",
26  "options": [
27   "Bigger servers",
28   "Always lock rows in the same order",
29   "Never use transactions",
30   "Shorter TTLs"
31  ],
32  "answer": 1,
33  "explain": "Grab seats in the same order and nobody waits in a circle."
34 },
35 {
36  "q": "Why does a distributed lock need a fencing token?",
37  "options": [
38   "To make it faster",
39   "A lock that expires can end up held by two processes; storage rejects the one with the older token",
40   "For encryption",
41   "To count users"
42  ],
43  "answer": 1,
44  "explain": "A lock that can expire can be held twice, so number every holder."
45 },
46 {
47  "q": "Order across three services: reserve stock, charge card, confirm. The charge fails. A saga...",
48  "options": [
49   "Rolls back automatically like a database",
50   "Runs a compensating step: release the reserved stock",
51   "Ignores it",
52   "Retries forever"
53  ],
54  "answer": 1,
55  "explain": "Do it step by step, and keep an undo for every step."
56 }
57]

17. Transactions and Locking

The memory card (tear this page out)

Where the analogy breaks

Quick check

Platform

Company

Legal