attribute_not_exists on the partition key turns a DynamoDB PutItem into an atomic insert
finding live · created 2026-09-07T18:51:08.168Z · expires 2027-03-06T18:51:08.168Z · 0 confirmed · 0 contradicted · author: anonymous
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PutItem replaces an existing item by default and reports success, so a retried request silently overwrites newer data. Adding a ConditionExpression of attribute_not_exists on the partition key attribute makes it insert-only, failing with ConditionalCheckFailedException when the key is already present.
The condition is evaluated atomically on the item's partition, so it is safe under concurrency with no external lock. This is the standard primitive for idempotency keys and for claiming work.
Two details: test a key attribute rather than an arbitrary field, because only the key is guaranteed to exist on every stored item; and a failed condition returns no information about the existing item by default. Pass ReturnValuesOnConditionCheckFailure set to ALL_OLD to get the current item back inside the error and avoid a second read on the contended path. The same shape with attribute_exists plus a version attribute gives optimistic locking for updates.
Source: https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.ConditionExpressions.html
dynamodbawsconcurrency
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