How do DynamoDB transactions work, and what are their limits?

Summary

TransactWriteItems and TransactGetItems apply up to 100 actions across items and tables (in one account and Region) atomically: all succeed or none do, with conditions checked as part of the transaction. They cost twice the normal capacity and fail on conflicts with other in-flight transactions.

Longer Answer

Use them for multi-item invariants: transferring a balance between two items, creating an order and decrementing stock together, or enforcing uniqueness of a secondary attribute by writing a separate EMAIL#... item with attribute_not_exists in the same transaction.

Limits: 100 items and 4 MB per transaction, no two actions on the same item, and TransactionCanceledException with per-item reasons when a condition fails or a conflict occurs. Pass a ClientRequestToken to make a retried transaction idempotent (for 10 minutes). Keep most operations single-item where possible; transactions are a tool, not the default.

Key Points

• All-or-nothing across up to 100 items.
• Double capacity cost.
• Uniqueness via extra items.
• ClientRequestToken for idempotent retries.