TTL use cases (sessions, logs, cache) in DynamoDB - Step-by-Step Execution

Concept Flow - TTL use cases (sessions, logs, cache)

Item Created

↓

TTL Attribute Set

↓

Time Passes

↓

DynamoDB Checks TTL

↓

TTL Expired?

No→Item Remains

Yes↓

Item Deleted Automatically

When an item is created, a TTL timestamp is set. DynamoDB checks this timestamp over time and deletes the item automatically when TTL expires.

Execution Sample

DynamoDB

PutItem {"sessionId": "abc123", "userId": "user1", "ttl": 1686000000}

// TTL is a timestamp in the future

// DynamoDB deletes item after TTL expires

This example shows creating a session item with a TTL timestamp. DynamoDB will delete it after the TTL time passes.

Execution Table

Step	Action	TTL Value	Current Time	TTL Expired?	Result
1	Create item with TTL=1686000000	1686000000	1685990000	No	Item stored
2	Time passes	1686000000	1686000001	Yes	Item deleted by DynamoDB
3	Check item existence	-	-	-	Item no longer exists

💡 At step 2, current time exceeds TTL, so DynamoDB deletes the item automatically.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
Item	None	Exists with TTL=1686000000	Deleted	Deleted
Current Time	1685990000	1685990000	1686000001	1686000001
TTL Expired?	No	No	Yes	Yes

Key Moments - 3 Insights

Why doesn't DynamoDB delete the item immediately after creation?

Because TTL is a timestamp in the future. DynamoDB only deletes items after the current time passes the TTL value, as shown in execution_table step 2.

What happens if the TTL attribute is missing or set to a past time?

If TTL is missing, DynamoDB never deletes the item automatically. If TTL is in the past, DynamoDB deletes the item soon after detection, similar to step 2.

Can TTL be used for caching data?

Yes, TTL automatically removes expired cache entries, keeping the table clean without manual deletion, as shown by the automatic deletion in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the TTL Expired? status at Step 1?

A Yes

B Unknown

C No

D Not applicable

💡 Hint

Refer to the 'TTL Expired?' column in execution_table row for Step 1.

At which step does DynamoDB delete the item automatically?

A Step 2

B Step 1

C Step 3

D Never

💡 Hint

Check the 'Result' column in execution_table to see when the item is deleted.

If the TTL value was set to a time in the past at creation, what would happen?

A Item would be deleted immediately

B Item would be deleted after some delay

C Item would never be deleted

D Item would cause an error

💡 Hint

TTL deletion happens after DynamoDB detects expired TTL, which may take some time after current time passes TTL.

Concept Snapshot

TTL (Time To Live) in DynamoDB lets you set an expiration timestamp on items.
DynamoDB automatically deletes items after TTL expires.
Common uses: session expiration, log cleanup, cache eviction.
Set TTL attribute as a Unix epoch timestamp.
DynamoDB checks TTL periodically; deletion is automatic and asynchronous.
No immediate deletion on item creation; deletion happens after TTL passes.

Full Transcript

TTL in DynamoDB is a feature that lets you set an expiration time on items using a timestamp attribute. When you create an item, you add a TTL attribute with a future time. DynamoDB checks this TTL over time and deletes the item automatically when the current time passes the TTL. This is useful for sessions that expire, logs that should be cleaned up, or cache data that should be removed after some time. The execution table shows that at creation, the TTL is not expired, so the item stays. After time passes and TTL expires, DynamoDB deletes the item automatically. Variables like the item existence and TTL expired status change accordingly. Beginners often wonder why deletion is not immediate; it's because TTL is a future timestamp and deletion happens only after that time. If TTL is missing, the item never expires. TTL is a simple way to keep your database clean without manual deletes.

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