Challenge 3: Transaction or Not? — Possible Solution
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(a) Updating a single user's last_login timestamp — NO transaction
needed. This is a single updateOne() against one document in one
collection. A single-document write is already atomic in MongoDB by
default; wrapping it in a transaction would add overhead for zero
extra safety.

(b) Moving a task between two different users' embedded tasks arrays
— YES, this needs a transaction, and it's the trickiest case precisely
because each individual write IS atomic on its own. Removing the task
from User A's document and adding it to User B's document are each,
separately, perfectly safe single-document updates. The problem is the
PAIR of them together: without a transaction, a crash between the two
writes can leave the task removed from User A but never added to User
B (the task vanishes entirely), or added to User B while still present
on User A (the task duplicates). Each write being individually atomic
does not make the overall two-step operation atomic — that's exactly
what a transaction adds.

(c) Transferring loyalty points between two separate customer_accounts
documents — YES, this needs a transaction, for the same reason as the
classic bank-transfer example in this chapter: decrementing one
account's points and incrementing another's are two independent
single-document writes. Without a transaction, a failure between them
can destroy points (deducted from one account, never credited to the
other) or create them out of nothing (credited without ever being
deducted) — both are Atomicity/Consistency failures a transaction
prevents by making both writes succeed or fail as one unit.