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The Lost Update

Two people opened the same order. Both pressed save. One of those edits is gone and there is no error, no log, and nothing to find later. Refuse the second write instead.

Intermediate20 min on the clock5 graded checks · 100 pointsThe Write Side

Graded by running your code against real cases.

What you build

updateIfUnchanged(rows, id, patch, expectedVersion) — write only if the version still matches
A stale version is a conflict: change nothing, say so
The version increments on a real write, and only then
An id that does not exist is missing, not a conflict
retryWith(...) — re-read, resolve, and write against the current version

Done means

The second save is refused rather than silently winning, and a retry succeeds.

How it is graded

Published in full, before you start — every point is one of these and there is nothing else. Each one runs your code; it is not a search for keywords.

A matching version writes and bumps+20The lost update
A stale version changes nothing+30The lost update
A refused write does not bump the version+15The lost update
An unknown id is missing, not a conflict+10Failing in a shape callers can read
retryWith resolves and succeeds+25The lost update

What it teaches

The lost updateadvanced

When two people edit the same record and both save, the second write silently erases the first unless it carries the version it was based on — and there is no error, no log and nothing to find afterwards.

Failing in a shape callers can readintermediate

A raw exception string tells the caller nothing they can act on; RFC 7807 ProblemDetails gives every failure the same machine-readable shape.

The rest of The Write Side

The Double ChargeBeginner · 15 min The Wiped FieldBeginner · 15 min Orphans and TombstonesIntermediate · 20 min All Or NothingAdvanced · 25 min