TransactWriteItems in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the result of this TransactWriteItems operation?

Given a transaction that attempts to put an item and delete another in DynamoDB, what is the outcome if the condition on the delete fails?

DynamoDB

TransactWriteItems({
  TransactItems: [
    {
      Put: {
        TableName: "Users",
        Item: { "UserId": { S: "123" }, "Name": { S: "Alice" } }
      }
    },
    {
      Delete: {
        TableName: "Users",
        Key: { "UserId": { S: "999" } },
        ConditionExpression: "attribute_exists(UserId)"
      }
    }
  ]
})

A The entire transaction fails and no changes are made because the delete condition is not met.

B The put operation succeeds but the delete operation fails, so the transaction partially applies.

C The delete operation is ignored and the put operation succeeds.

D The transaction retries automatically until the delete condition passes.

💡 Hint

Remember that TransactWriteItems is atomic; all operations succeed or none do.

In DynamoDB, TransactWriteItems ensures atomicity. If any condition fails, the entire transaction is rolled back and no changes are applied.

📝 Syntax

intermediate

Identify the syntax error in this TransactWriteItems request

Which option contains a syntax error that will cause the TransactWriteItems request to fail?

DynamoDB

TransactWriteItems({
  TransactItems: [
    {
      Update: {
        TableName: "Orders",
        Key: { "OrderId": { S: "abc123" } },
        UpdateExpression: "SET #st = :s",
        ExpressionAttributeNames: { "#st": "Status" },
        ExpressionAttributeValues: { ":s": { S: "Shipped" } }
      }
    }
  ]
})

A ExpressionAttributeValues key ':s' should be a string without colon.

B Key attribute value should be Number type, not String.

C UpdateExpression uses invalid syntax; should be 'SET Status = :s'.

D Missing comma after ExpressionAttributeNames object.

💡 Hint

Check punctuation carefully in JSON objects.

In JSON, commas separate key-value pairs. Missing a comma after ExpressionAttributeNames causes a syntax error.

❓ optimization

advanced

Optimizing TransactWriteItems for multiple conditional writes

You want to update 5 items conditionally in a single transaction. Which approach is most efficient and correct?

A Send 5 separate UpdateItem calls with ConditionExpression for each item.

B Use one TransactWriteItems call with 5 Update operations each having its own ConditionExpression.

C Use BatchWriteItem with 5 Put requests and check conditions client-side.

D Use one TransactWriteItems call with 5 Put operations without conditions.

💡 Hint

TransactWriteItems supports multiple operations atomically with conditions.

Using one TransactWriteItems call with multiple conditional updates ensures atomicity and efficiency compared to multiple calls or batch writes without conditions.

🔧 Debug

advanced

Why does this TransactWriteItems call fail with a TransactionCanceledException?

Given this transaction, why does DynamoDB return TransactionCanceledException with cancellation reasons?

DynamoDB

TransactWriteItems({
  TransactItems: [
    {
      Put: {
        TableName: "Products",
        Item: { "ProductId": { S: "p1" }, "Stock": { N: "10" } },
        ConditionExpression: "attribute_not_exists(ProductId)"
      }
    },
    {
      Update: {
        TableName: "Inventory",
        Key: { "InventoryId": { S: "inv1" } },
        UpdateExpression: "SET Quantity = Quantity - :dec",
        ExpressionAttributeValues: { ":dec": { N: "5" } },
        ConditionExpression: "Quantity >= :dec"
      }
    }
  ]
})

A The Put operation fails because ProductId 'p1' already exists, causing the transaction to cancel.

B The Update operation fails because Quantity is less than 5, causing the transaction to cancel.

C The transaction fails because TransactWriteItems does not support Update operations.

D Both Put and Update operations fail due to syntax errors in ConditionExpression.

💡 Hint

Check the condition expressions and existing data state.

If the Put operation's condition attribute_not_exists(ProductId) fails because the item exists, the entire transaction is canceled with reasons.

🧠 Conceptual

expert

What is a key benefit of using TransactWriteItems in DynamoDB?

Choose the best description of a key benefit of TransactWriteItems compared to individual write operations.

A It bypasses all condition checks to speed up writes.

B It automatically scales throughput for each write operation individually.

C It guarantees atomicity, so all writes succeed or none do, preventing partial updates.

D It allows unlimited number of operations in a single transaction.

💡 Hint

Think about what atomicity means in transactions.

TransactWriteItems ensures all operations in the transaction succeed or none do, preventing partial updates and maintaining data integrity.