TTL behavior and timing in DynamoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

When does DynamoDB delete expired items?

In DynamoDB, after an item's TTL timestamp passes, when will DynamoDB delete that item?

A At the end of each day during a scheduled cleanup window.

B Immediately at the exact second the TTL timestamp is reached.

C Only when a read or write operation accesses the expired item.

D Within 48 hours after the TTL timestamp has passed, but timing is not guaranteed.

💡 Hint

Think about how background processes work asynchronously in cloud services.

DynamoDB's TTL feature deletes expired items asynchronously and typically within 48 hours after expiration, but exact timing is not guaranteed.

❓ query_result

intermediate

Effect of TTL on Query Results

You have a DynamoDB table with TTL enabled on attribute expireAt. You run a Query operation immediately after an item's TTL timestamp has passed but before DynamoDB deletes it. What will the Query return?

A The expired item will be returned because it still exists in the table.

B The expired item will not be returned because TTL hides expired items instantly.

C The Query will return an error due to expired items.

D The Query will return only the expired item and no others.

💡 Hint

Consider when DynamoDB physically removes expired items versus when they expire logically.

TTL expiration marks items for deletion, but they remain in the table until DynamoDB deletes them asynchronously. Queries still return these items until deletion.

📝 Syntax

advanced

Correct TTL Attribute Format

Which of the following DynamoDB item attributes is correctly formatted for TTL to work?

DynamoDB

Item example: { "id": "123", "expireAt": ??? }

A "expireAt": "June 1, 2024 12:00:00"

B "expireAt": "2024-06-01T12:00:00Z"

C "expireAt": 1711896000

D "expireAt": true

💡 Hint

TTL expects a specific type of timestamp value.

DynamoDB TTL requires the attribute to be a Unix epoch time in seconds (a number), not a string or boolean.

❓ optimization

advanced

Reducing Delay in TTL Deletion

You want expired items to be deleted as soon as possible after TTL expiration. Which approach can help reduce the delay?

A Increase the TTL timestamp by a few hours to batch deletions.

B Enable DynamoDB Streams and process expired items manually to delete them faster.

C Disable TTL and delete expired items with a scheduled Lambda function only.

D Set the TTL attribute to a string date format for faster processing.

💡 Hint

Think about how you can react to changes in the table immediately.

Enabling DynamoDB Streams lets you detect expired items quickly and delete them manually, reducing delay beyond the default asynchronous TTL process.

🔧 Debug

expert

Why are expired items not deleted after 48 hours?

You enabled TTL on your DynamoDB table with attribute expireAt. Some items have expired more than 48 hours ago but still appear in the table. What is the most likely reason?

A The TTL attribute is missing or has incorrect data type on those items.

B DynamoDB deletes expired items only during monthly maintenance windows.

C The table's read capacity is too low to allow TTL deletions.

D Expired items are never deleted automatically; TTL only marks them.

💡 Hint

Check the attribute format and presence carefully.

If the TTL attribute is missing or not a valid Unix epoch number, DynamoDB cannot identify the item as expired and will not delete it.

Practice

1. What does TTL (Time To Live) do in DynamoDB?

easy

A. Encrypts data stored in the table.

B. Backs up data periodically.

C. Automatically deletes items after their expiration time.

D. Indexes data for faster queries.

Solution

1. Step 1: Understand TTL purpose

   TTL is designed to remove expired data automatically to save storage and keep data fresh.

2. Step 2: Identify TTL behavior

   It deletes items after their expiration timestamp passes, not encrypting or backing up data.

3. Final Answer:

   Automatically deletes items after their expiration time. -> Option C

4. Quick Check:

   TTL = automatic deletion [OK]

Hint: TTL means automatic deletion after expiry [OK]

Common Mistakes:

• Thinking TTL encrypts data
• Confusing TTL with backup
• Assuming TTL speeds up queries

2. Which attribute type must the TTL attribute have in DynamoDB?

easy

A. String representing a date in ISO format

B. Number representing Unix epoch time in seconds

C. Boolean indicating if item is expired

D. List of timestamps

Solution

1. Step 1: Recall TTL attribute format

   The TTL attribute must be a number storing the expiration time as Unix epoch seconds.

2. Step 2: Eliminate other types

   Strings, booleans, or lists are not valid for TTL expiration timestamps.

3. Final Answer:

   Number representing Unix epoch time in seconds -> Option B

4. Quick Check:

   TTL attribute = Unix epoch seconds [OK]

Hint: TTL attribute stores expiration as Unix epoch seconds [OK]

Common Mistakes:

• Using string dates instead of numbers
• Using boolean instead of timestamp
• Trying to store multiple timestamps

3. If an item has a TTL attribute set to 1650000000 (Unix epoch seconds), when will DynamoDB delete it?

medium

A. Within 48 hours after 1650000000 seconds

B. Exactly at 1650000000 seconds

C. Immediately after setting the TTL attribute

D. Only when manually deleted

Solution

1. Step 1: Understand TTL deletion timing

   DynamoDB deletes expired items approximately after the TTL timestamp, not exactly at that second.

2. Step 2: Recall deletion delay

   Deletion can take up to 48 hours after the expiration time to occur.

3. Final Answer:

   Within 48 hours after 1650000000 seconds -> Option A

4. Quick Check:

   TTL deletion delay = up to 48 hours [OK]

Hint: TTL deletion can be delayed up to 48 hours [OK]

Common Mistakes:

• Expecting deletion exactly at expiration time
• Thinking deletion is immediate
• Assuming manual deletion is required

4. You set TTL on an attribute but notice expired items are not deleted after 3 days. What is a likely cause?

medium

A. TTL attribute is not a number with Unix epoch seconds

B. TTL is enabled but the table is in a different AWS region

C. You forgot to enable encryption on the table

D. The table has no primary key

Solution

1. Step 1: Check TTL attribute format

   If the TTL attribute is not a number with Unix epoch seconds, DynamoDB ignores it for expiration.

2. Step 2: Verify other options

   Region difference, encryption, or primary key absence do not prevent TTL deletion.

3. Final Answer:

   TTL attribute is not a number with Unix epoch seconds -> Option A

4. Quick Check:

   TTL attribute format error blocks deletion [OK]

Hint: TTL attribute must be Unix epoch number [OK]

Common Mistakes:

• Assuming region affects TTL
• Thinking encryption affects TTL
• Ignoring TTL attribute data type

5. You want to ensure items expire exactly after 24 hours using TTL. Which approach is best considering DynamoDB's TTL behavior?

hard

A. Disable TTL and run a scheduled job to delete expired items

B. Set TTL attribute to current time + 86400 seconds and manually delete items after 24 hours

C. Use a string date format for TTL attribute for precise timing

D. Set TTL attribute to current time + 86400 seconds and accept up to 48-hour deletion delay

Solution

1. Step 1: Understand TTL timing limits

   DynamoDB TTL deletes items approximately after expiration, with up to 48 hours delay.

2. Step 2: Choose best practical approach

   Setting TTL attribute to current time + 86400 seconds (24 hours) is correct; manual deletion adds complexity and string format is invalid.

3. Step 3: Consider alternatives

   Disabling TTL and scheduling deletes is possible but less efficient than TTL's automatic deletion.

4. Final Answer:

   Set TTL attribute to current time + 86400 seconds and accept up to 48-hour deletion delay -> Option D

5. Quick Check:

   TTL delay means accept approximate expiry [OK]

Hint: TTL deletion timing is approximate, plan accordingly [OK]

Common Mistakes:

• Expecting exact deletion timing
• Using string instead of number for TTL
• Disabling TTL unnecessarily