TTL behavior and timing in DynamoDB - Mini Project: Build & Apply

Understanding TTL Behavior and Timing in DynamoDB

📖 Scenario: You are managing a DynamoDB table that stores user session data. To keep the table clean and efficient, you want to automatically remove expired sessions using DynamoDB's Time To Live (TTL) feature.

🎯 Goal: Build a DynamoDB table setup with TTL enabled on a specific attribute, configure the TTL attribute values, and understand how TTL expiration timing works.

📋 What You'll Learn

Create a DynamoDB table named UserSessions with a primary key SessionId (string).

Add a TTL attribute named ExpirationTime to the table items.

Set the TTL attribute ExpirationTime with Unix epoch timestamps for session expiration.

Enable TTL on the ExpirationTime attribute in the table configuration.

Understand that TTL deletion is approximate and can take up to 48 hours after expiration.

💡 Why This Matters

🌍 Real World

TTL helps automatically remove expired data like sessions, logs, or temporary records to save storage and improve performance.

💼 Career

Understanding TTL is important for database administrators and backend developers managing data lifecycle and cost optimization in DynamoDB.

1

Create the DynamoDB table with primary key

Create a DynamoDB table named UserSessions with a primary key called SessionId of type string.

DynamoDB

# Create the DynamoDB table UserSessions with primary key SessionId (string)
# Your code here

Hint

Use the AWS CLI create-table command with --attribute-definitions and --key-schema to define the primary key.

2

Add TTL attribute to items

Add an item to the UserSessions table with SessionId set to session123 and a TTL attribute called ExpirationTime set to the Unix epoch timestamp 1700000000.

DynamoDB

aws dynamodb create-table \
    --table-name UserSessions \
    --attribute-definitions AttributeName=SessionId,AttributeType=S \
    --key-schema AttributeName=SessionId,KeyType=HASH \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

# Add an item with SessionId 'session123' and ExpirationTime 1700000000
# Your code here

Hint

Use the AWS CLI put-item command to add an item with the TTL attribute as a number.

3

Enable TTL on the ExpirationTime attribute

Enable TTL on the UserSessions table by setting the TTL attribute name to ExpirationTime using the AWS CLI.

DynamoDB

aws dynamodb create-table \
    --table-name UserSessions \
    --attribute-definitions AttributeName=SessionId,AttributeType=S \
    --key-schema AttributeName=SessionId,KeyType=HASH \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

aws dynamodb put-item \
    --table-name UserSessions \
    --item '{"SessionId": {"S": "session123"}, "ExpirationTime": {"N": "1700000000"}}'

# Enable TTL on the ExpirationTime attribute
# Your code here

Hint

Use the AWS CLI update-time-to-live command to enable TTL on the table.

4

Understand TTL expiration timing

Add a comment explaining that DynamoDB TTL deletion is approximate and can take up to 48 hours after the ExpirationTime timestamp to delete expired items.

DynamoDB

aws dynamodb create-table \
    --table-name UserSessions \
    --attribute-definitions AttributeName=SessionId,AttributeType=S \
    --key-schema AttributeName=SessionId,KeyType=HASH \
    --provisioned-throughput ReadCapacityUnits=5,WriteCapacityUnits=5

aws dynamodb put-item \
    --table-name UserSessions \
    --item '{"SessionId": {"S": "session123"}, "ExpirationTime": {"N": "1700000000"}}'

aws dynamodb update-time-to-live \
    --table-name UserSessions \
    --time-to-live-specification Enabled=true,AttributeName=ExpirationTime

# Add a comment about TTL deletion timing
# Your code here

Hint

Write a comment explaining the approximate nature of TTL deletion timing.

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