Transaction limits in DynamoDB - Step-by-Step Execution

Concept Flow - Transaction limits

Start Transaction

↓

Check Item Count <= 100?

No→Reject Transaction

Yes↓

Check Total Size <= 4 MB?

No→Reject Transaction

Yes↓

Execute All Operations

↓

Commit Transaction

↓

End

A DynamoDB transaction must have 100 or fewer items and total size under 4 MB to proceed; otherwise, it is rejected.

Execution Sample

DynamoDB

TransactWriteItems(
  Items: 3,
  TotalSize: 1.2MB
)

This transaction tries to write 3 items with total size 1.2 MB, which is within limits.

Execution Table

Step	Check	Value	Result	Action
1	Number of items <= 100?	3 <= 100	True	Proceed
2	Total size <= 4 MB?	1.2 MB <= 4 MB	True	Proceed
3	Execute all operations	3 items	Success	Commit transaction
4	Transaction ends	-	-	Transaction committed successfully

💡 Transaction ends after successful commit because all limits are met.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
ItemCount	0	3	3	3
TotalSizeMB	0	1.2	1.2	1.2
TransactionStatus	Not started	Checking limits	Limits OK	Committed

Key Moments - 2 Insights

Why can't a transaction have more than 100 items?

Because DynamoDB enforces a hard limit of 100 items per transaction to ensure performance and atomicity, as shown in execution_table step 1 where exceeding 100 would reject the transaction.

What happens if the total size exceeds 4 MB?

The transaction is rejected immediately, as shown in execution_table step 2 where the size check fails if over 4 MB.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of the item count check at step 1?

A False

B True

C Error

D Skipped

💡 Hint

Refer to execution_table row 1 under 'Result' column.

At which step does the transaction commit happen?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Check execution_table row 3 under 'Action' column.

If the transaction had 105 items, what would happen at step 1?

A Proceed to size check

B Commit transaction

C Reject transaction

D Ignore item count

💡 Hint

Refer to concept_flow where item count > 100 leads to rejection.

Concept Snapshot

DynamoDB transactions have two main limits:
- Max 100 items per transaction
- Max total size 4 MB
If either limit is exceeded, the transaction is rejected.
All operations must succeed or none are applied.
Check limits before commit to ensure success.

Full Transcript

In DynamoDB, transactions have limits to keep them efficient and reliable. You can include up to 100 items in a transaction. Also, the total size of all items combined must be 4 megabytes or less. When you start a transaction, DynamoDB first checks if the number of items is 100 or fewer. If not, it rejects the transaction immediately. If the item count is okay, it then checks if the total size is within 4 MB. If the size is too big, it rejects the transaction. If both checks pass, DynamoDB executes all operations together and commits the transaction. This means either all changes happen or none do, keeping your data consistent. This step-by-step process helps ensure your transaction meets DynamoDB's rules before it runs.