Grokking Modern API Design Interview
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Concurrency and Conditional Requests

Two support agents read the same order.

Agent A changes the shipping instruction. Agent B changes the contact phone number. Each client sends a representation based on the copy it read.

If B writes last, B's stale copy may restore the old shipping instruction. Both requests return success. One person's work disappears.

This is the lost update problem. The API needs a way for a writer to say:

"Apply this change only if the resource is still the state I read."

HTTP provides that condition through an ETag and If-Match.

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