Use DynamoDB conditions to reject duplicate creates and stale edits
Application and assignment
The application is saving bookmark bookmark-1. Two callers might both try to create that ID, or edit it from the same old version. Reading first and writing later leaves a gap in which another caller can act. A condition attached to the write lets storage decide atomically.
Run the supplied CLI sequence against one disposable DynamoDB table. Observe the first create, repeated create, valid versioned edit, and stale repeat. The commands bypass an application API to isolate the storage mechanism. Production authentication and HTTP error mapping are not supplied by this lab.
Starting contract
A “check then insert” sequence has a race: two callers can both observe absence. A conditional write evaluates absence at the storage boundary.
Prerequisites: AWS CLI, credentials, selected region, and a unique disposable table name. Commands create billable resources. Run from a terminal where no table with this name already exists.
LAB_DATA_TABLE=interview-conditional-demo
aws dynamodb create-table --table-name "$LAB_DATA_TABLE" --billing-mode PAY_PER_REQUEST --attribute-definitions AttributeName=id,AttributeType=S --key-schema AttributeName=id,KeyType=HASH
aws dynamodb wait table-exists --table-name "$LAB_DATA_TABLE"
aws dynamodb put-item --table-name "$LAB_DATA_TABLE" --item '{"id":{"S":"bookmark-1"},"owner":{"S":"user-1"},"title":{"S":"First"},"version":{"N":"1"}}' --condition-expression 'attribute_not_exists(id)'
Repeat the put. Expect ConditionalCheckFailedException. Read the row; it must remain unchanged. This proves duplicate create is refused, not that every repeated HTTP request is automatically handled correctly.
Now edit only when owner and version match:
aws dynamodb update-item --table-name "$LAB_DATA_TABLE" --key '{"id":{"S":"bookmark-1"}}' --update-expression 'SET title = :title, version = :next' --condition-expression '#owner = :owner AND version = :expected' --expression-attribute-names '{"#owner":"owner"}' --expression-attribute-values '{":owner":{"S":"user-1"},":title":{"S":"Edited"},":expected":{"N":"1"},":next":{"N":"2"}}' --return-values ALL_NEW
Repeat it. The stale expected version must fail. In an application, derive owner from authenticated identity and use a version increment in the update rather than trusting a caller's next version. Add an index or alternate key design before claiming this id-only table can efficiently list an owner's bookmarks.
Junior: explain the conditional write. Senior: compare conflict versus missing/unauthorized responses and test two concurrent writers. Staff: design tenant key distribution and a compatible change to the access pattern.
Clean up:
aws dynamodb delete-table --table-name "$LAB_DATA_TABLE"
aws dynamodb wait table-not-exists --table-name "$LAB_DATA_TABLE"