Transactions

What are Transactions?

DynamoDB transactions let you perform all-or-nothing operations across multiple items and tables.¹ Either all writes succeed together, or they all fail and roll back—no partial updates.

Key capabilities:

• Group up to 100 items across one or more tables in a single transaction
• Read or write items atomically with ACID guarantees
• Prevent race conditions with automatic conflict detection

Example: Transfer $100 between accounts—debit Account A and credit Account B must both succeed or both fail.

The Cost

DynamoDB transactions cost 2x normal read/write operations.¹

• Standard write: 1 WCU per KB
• Transactional write: 2 WCUs per KB
• Standard read: 1 RCU per 4 KB (eventual) or 2 RCUs (strong)
• Transactional read: 4 RCUs per 4 KB (always strong consistency)

Note: Transactions are not recommended for single-item operations. Use standard PutItem, UpdateItem, or DeleteItem with conditional expressions instead—they provide atomicity for a single item at half the cost.

When to Use Transactions

Use transactions ONLY when you need:

1. ACID guarantees across multiple items/tables

   • Transfer funds between two accounts (debit one, credit another)
   • Update inventory count AND create order record atomically
   • Prevent double-booking of resources (hotel rooms, appointments)

2. Coordinated updates

   • Update multiple denormalized copies that must stay in sync
   • Maintain aggregates (update item + increment counter atomically)

3. Check-and-set operations across items

   • “Only update if both Item A and Item B meet conditions”
   • Prevent race conditions in distributed workflows

Important: If you use GSIs, remember they are eventually consistent and should not be included in transactions. For transaction-critical reads, always try to query the base table directly using the primary key—never rely on a GSI that might have stale data.