TTL use cases (sessions, logs, cache) in DynamoDB

Introduction

TTL helps automatically remove old data from your database. This keeps it clean and saves space without you doing extra work.

You want to keep user login sessions only for a limited time.

You need to store logs but only want to keep recent ones.

You cache data to speed up your app but want old cache to expire.

You want to automatically delete temporary data after some time.

You want to reduce storage costs by removing outdated records.

Syntax

DynamoDB

TTL attribute: A timestamp attribute in your DynamoDB table that marks when an item should expire.
Enable TTL on the table and specify the attribute name.
DynamoDB automatically deletes items after the timestamp passes.

The TTL attribute must be a number representing Unix epoch time in seconds.

Expired items are deleted within 48 hours after the timestamp passes, not instantly.

Examples

This example shows a TTL attribute named session_expiry that stores when a user session should expire.

DynamoDB

TTL attribute: session_expiry
Value example: 1686000000 (Unix time in seconds)

Logs can have a log_expire_at attribute to remove old logs automatically.

DynamoDB

TTL attribute: log_expire_at
Value example: 1686003600

Cache items use cache_ttl to expire cached data after some time.

DynamoDB

TTL attribute: cache_ttl
Value example: 1686007200

Sample Program

This example creates a table for user sessions with a TTL attribute session_expiry. The session expires at the specified Unix timestamp.

DynamoDB

CREATE TABLE UserSessions (
  userId STRING HASH KEY,
  sessionId STRING SORT KEY,
  sessionData STRING,
  session_expiry NUMBER
);

-- Enable TTL on attribute 'session_expiry' in AWS Console or via CLI

-- Insert a session that expires at a specific Unix timestamp
INSERT INTO UserSessions (userId, sessionId, sessionData, session_expiry) VALUES (
  'user123', 'sess456', 'data here', 1725152400
);

Important Notes

TTL does not guarantee immediate deletion; it happens asynchronously within about 48 hours.

Expired items are not returned in queries after deletion but may still appear briefly before removal.

Use TTL to save costs and keep your database tidy without manual cleanup.

Summary

TTL automatically deletes old data based on a timestamp attribute.

Common uses: sessions, logs, and cache expiration.

TTL helps manage storage and keeps data fresh without manual work.

Practice

1. What is the main purpose of TTL (Time To Live) in DynamoDB?

easy

A. Automatically delete items after a specified time to save storage

B. Encrypt data for security

C. Backup data to another region

D. Increase read capacity automatically

Solution

1. Step 1: Understand TTL functionality

   TTL marks items with an expiration timestamp to delete them automatically after that time.

2. Step 2: Identify the main benefit

   This automatic deletion helps save storage and keeps data fresh without manual cleanup.

3. Final Answer:

   Automatically delete items after a specified time to save storage -> Option A

4. Quick Check:

   TTL = automatic deletion [OK]

Hint: TTL deletes expired data automatically to save space [OK]

Common Mistakes:

• Confusing TTL with encryption
• Thinking TTL backs up data
• Assuming TTL changes capacity settings

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

easy

A. String representing a date

B. Boolean indicating expiration

C. Binary data

D. Number representing epoch time in seconds

Solution

1. Step 1: Check TTL attribute requirements

   DynamoDB TTL requires a Number attribute that stores the expiration time as epoch time in seconds.

2. Step 2: Eliminate other types

   String, Boolean, or Binary types are not valid for TTL expiration timestamps.

3. Final Answer:

   Number representing epoch time in seconds -> Option D

4. Quick Check:

   TTL attribute = epoch seconds number [OK]

Hint: TTL attribute must be a number with epoch seconds [OK]

Common Mistakes:

• Using string dates instead of numbers
• Setting TTL attribute as Boolean
• Using binary data for TTL

3. Given a DynamoDB table with TTL enabled on attribute expireAt, what happens to an item with expireAt set to a timestamp 10 minutes in the past?

medium

A. The item is automatically deleted by DynamoDB

B. The item remains until manually deleted

C. The item is archived automatically

D. The item is updated with a new timestamp

Solution

1. Step 1: Understand TTL expiration behavior

   Items with TTL timestamps in the past are marked for deletion by DynamoDB automatically.

2. Step 2: Confirm deletion timing

   DynamoDB deletes expired items typically within 48 hours after expiration, often sooner.

3. Final Answer:

   The item is automatically deleted by DynamoDB -> Option A

4. Quick Check:

   Expired TTL item = auto delete [OK]

Hint: Expired TTL items get deleted automatically [OK]

Common Mistakes:

• Thinking expired items stay forever
• Assuming items get archived
• Believing TTL updates timestamps automatically

4. You enabled TTL on attribute sessionExpiry but items are not being deleted after expiration. What is the most likely cause?

medium

A. TTL only works on string attributes

B. The table has no primary key defined

C. The sessionExpiry attribute is missing or not a number

D. DynamoDB does not support TTL

Solution

1. Step 1: Check TTL attribute correctness

   TTL requires the attribute to exist and be a number representing epoch time; missing or wrong type prevents deletion.

2. Step 2: Verify other options

   Tables must have primary keys, but that doesn't affect TTL. TTL does not work on strings. DynamoDB supports TTL.

3. Final Answer:

   The sessionExpiry attribute is missing or not a number -> Option C

4. Quick Check:

   TTL attribute missing or wrong type = no deletion [OK]

Hint: TTL attribute must exist and be a number [OK]

Common Mistakes:

• Assuming TTL works on strings
• Ignoring missing TTL attribute
• Thinking DynamoDB lacks TTL support

5. You want to use TTL to automatically expire cached items after 1 hour. Which approach correctly sets the TTL attribute cacheExpire when inserting an item?

hard

A. Set cacheExpire to current date string plus 1 hour

B. Set cacheExpire to current epoch time plus 3600 seconds

C. Set cacheExpire to Boolean true

D. Do not set cacheExpire attribute

Solution

1. Step 1: Calculate TTL expiration timestamp

   TTL requires a number representing epoch time in seconds, so add 3600 seconds (1 hour) to current epoch time.

2. Step 2: Confirm correct attribute setting

   Setting a string or Boolean won't work; omitting the attribute disables TTL for that item.

3. Final Answer:

   Set cacheExpire to current epoch time plus 3600 seconds -> Option B

4. Quick Check:

   TTL = current time + seconds [OK]

Hint: TTL value = current epoch seconds + expiration seconds [OK]

Common Mistakes:

• Using date strings instead of epoch seconds
• Setting TTL as Boolean
• Not setting TTL attribute at all