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.

Practice

1. What is the maximum number of operations allowed in a single TransactWriteItems request in DynamoDB?

easy

A. 50 operations

B. 25 operations

C. 5 operations

D. 10 operations

Solution

1. Step 1: Understand DynamoDB transaction limits

   DynamoDB limits the number of operations in a transaction to ensure performance and consistency.

2. Step 2: Recall the specific limit for TransactWriteItems

   The limit for TransactWriteItems is 10 operations per transaction.

3. Final Answer:

   10 operations -> Option D

4. Quick Check:

   TransactWriteItems max ops = 10 [OK]

Hint: Remember: Write transactions allow up to 10 ops only [OK]

Common Mistakes:

• Confusing write and read transaction limits
• Assuming more than 10 operations are allowed in write transactions
• Mixing up TransactWriteItems with TransactGetItems limits

2. Which of the following is the correct syntax to start a transaction with multiple write operations in DynamoDB using TransactWriteItems?

easy

A. dynamodb.transactWriteItems({ Items: [...] })

B. dynamodb.transactWriteItems({ TransactItems: [...] })

C. dynamodb.transactWrite({ Operations: [...] })

D. dynamodb.transactWrite({ Items: [...] })

Solution

1. Step 1: Identify the correct parameter name for transaction items

   The DynamoDB API for TransactWriteItems requires the parameter TransactItems to specify operations.

2. Step 2: Match the syntax with the correct parameter

   Only dynamodb.transactWriteItems({ TransactItems: [...] }) uses TransactItems correctly inside the method call.

3. Final Answer:

   dynamodb.transactWriteItems({ TransactItems: [...] }) -> Option B

4. Quick Check:

   Use TransactItems for write transactions [OK]

Hint: Look for 'TransactItems' key in write transaction syntax [OK]

Common Mistakes:

• Using 'Items' instead of 'TransactItems'
• Calling a non-existent method like 'transactWrite'
• Confusing parameter names for transaction operations

3. Consider a DynamoDB transaction using TransactGetItems with 30 get operations. What will happen when this transaction is executed?

medium

A. The transaction fails because it exceeds the 25 operation limit

B. The transaction succeeds and returns all 30 items

C. Only the first 25 items are returned, others ignored

D. The transaction partially succeeds with a warning

Solution

1. Step 1: Recall the operation limit for TransactGetItems

   DynamoDB limits TransactGetItems to a maximum of 25 operations per transaction.

2. Step 2: Analyze the effect of exceeding the limit

   Submitting 30 operations exceeds this limit, causing the entire transaction to fail.

3. Final Answer:

   The transaction fails because it exceeds the 25 operation limit -> Option A

4. Quick Check:

   TransactGetItems max ops = 25 [OK]

Hint: TransactGetItems max 25 ops; more causes failure [OK]

Common Mistakes:

• Assuming partial success with extra operations
• Thinking the limit is 30 or more
• Believing the transaction truncates results automatically

4. You wrote a DynamoDB transaction with 12 write operations inside TransactWriteItems. The transaction fails. What is the most likely cause?

medium

A. Exceeded the maximum of 25 operations allowed in TransactWriteItems

B. Exceeded the maximum size of 4 MB per item

C. Exceeded the maximum of 10 operations allowed in TransactWriteItems

D. Used incorrect parameter name 'Items' instead of 'TransactItems'

Solution

1. Step 1: Check the operation count limit for TransactWriteItems

   The maximum allowed operations in TransactWriteItems is 10.

2. Step 2: Identify the cause of failure with 12 operations

   Since 12 exceeds 10, the transaction fails due to exceeding the operation limit.

3. Final Answer:

   Exceeded the maximum of 10 operations allowed in TransactWriteItems -> Option C

4. Quick Check:

   More than 10 ops in write transaction causes failure [OK]

Hint: More than 10 writes in transaction causes failure [OK]

Common Mistakes:

• Confusing operation count with item size limits
• Assuming 25 ops limit applies to write transactions
• Ignoring parameter name errors as cause

5. You want to perform a transaction that reads 20 items and writes 8 items atomically in DynamoDB. How should you structure your requests to stay within transaction limits?

hard

A. Use one TransactWriteItems with 8 writes and one TransactGetItems with 20 reads separately

B. Use a single TransactWriteItems with 28 operations combining reads and writes

C. Use one TransactGetItems with 20 reads and add 8 writes to it

D. Use a single TransactGetItems with 28 operations combining reads and writes

Solution

1. Step 1: Understand separate transaction types and their limits

   DynamoDB separates read and write transactions: TransactWriteItems for writes (max 10 ops), TransactGetItems for reads (max 25 ops).

2. Step 2: Plan transactions to respect limits

   Perform writes (8 ops) in one TransactWriteItems and reads (20 ops) in one TransactGetItems to stay within limits.

3. Final Answer:

   Use one TransactWriteItems with 8 writes and one TransactGetItems with 20 reads separately -> Option A

4. Quick Check:

   Separate read/write transactions to respect limits [OK]

Hint: Split reads and writes into separate transactions [OK]

Common Mistakes:

• Trying to combine reads and writes in one transaction
• Exceeding operation limits by mixing types
• Assuming one transaction can handle both reads and writes