TTL indexes for auto-expiry in MongoDB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a TTL index on the 'createdAt' field that expires documents after 3600 seconds.

MongoDB

db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: [1] })

Drag options to blanks, or click blank then click option'

A3600

B60

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or negative numbers which disable TTL expiration.

Confusing minutes with seconds.

The expireAfterSeconds option sets the time in seconds after which documents expire. Here, 3600 seconds means documents expire after one hour.

2fill in blank

medium

Complete the code to create a TTL index on the 'lastModified' field that expires documents after 86400 seconds (1 day).

MongoDB

db.collection.createIndex({ [1]: 1 }, { expireAfterSeconds: 86400 })

Drag options to blanks, or click blank then click option'

AcreatedAt

BlastModified

Ctimestamp

DexpireAt

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong field name for the TTL index.

Confusing the field with other date fields.

The TTL index must be created on the field that stores the date to expire documents. Here, it's lastModified.

3fill in blank

hard

Fix the error in the TTL index creation code by filling the blank with the correct option.

MongoDB

db.collection.createIndex({ expireAt: 1 }, { [1]: 3600 })

Drag options to blanks, or click blank then click option'

AexpireAfter

BexpireAfterMinutes

CexpireAfterSeconds

DexpireSeconds

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect option names like 'expireAfterMinutes' or 'expireSeconds'.

Misspelling the option name.

The correct option name for TTL expiration time is expireAfterSeconds. Other options are invalid and cause errors.

4fill in blank

hard

Fill both blanks to create a TTL index on 'timestamp' field that expires documents after 7200 seconds (2 hours).

MongoDB

db.collection.createIndex({ [1]: [2] }, { expireAfterSeconds: 7200 })

Drag options to blanks, or click blank then click option'

Atimestamp

B1

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1 or 0 as index direction which is invalid here.

Using wrong field names.

The TTL index must be created on the date field with ascending order (1). Negative or zero values are invalid for index direction.

5fill in blank

hard

Fill all three blanks to create a TTL index on 'expireAt' field with ascending order and expiration after 1800 seconds (30 minutes).

MongoDB

db.collection.createIndex({ [1]: [2] }, { [3]: 1800 })

Drag options to blanks, or click blank then click option'

AexpireAt

B1

CexpireAfterSeconds

Dtimestamp

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong field names or index directions.

Using incorrect option names for expiration.

The TTL index is created on the 'expireAt' field with ascending order (1) and the option 'expireAfterSeconds' sets the expiration time.

Practice

1. What is the main purpose of a TTL index in MongoDB?

easy

A. Automatically delete documents after a specified time

B. Encrypt documents for security

C. Create backups of collections

D. Improve query speed by sorting data

Solution

1. Step 1: Understand TTL index function

   A TTL index is designed to remove documents automatically after a certain time period.

2. Step 2: Compare options with TTL purpose

   Only Automatically delete documents after a specified time describes automatic deletion after time, which matches TTL index behavior.

3. Final Answer:

   Automatically delete documents after a specified time -> Option A

4. Quick Check:

   TTL index = auto-delete documents [OK]

Hint: TTL means auto-delete documents after time [OK]

Common Mistakes:

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

2. Which of the following is the correct way to create a TTL index on a field named createdAt that expires documents after 3600 seconds?

easy

A. db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 })

B. db.collection.createIndex({ createdAt: 1 }, { ttl: 3600 })

C. db.collection.createTTLIndex({ createdAt: 1 }, 3600)

D. db.collection.createIndex({ createdAt: 1 }, { expireAfter: 3600 })

Solution

1. Step 1: Recall correct TTL index syntax

   The correct syntax uses createIndex with the option expireAfterSeconds set to the expiry time in seconds.

2. Step 2: Match options with correct syntax

   Only db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 }) uses createIndex and expireAfterSeconds correctly.

3. Final Answer:

   db.collection.createIndex({ createdAt: 1 }, { expireAfterSeconds: 3600 }) -> Option A

4. Quick Check:

   expireAfterSeconds option = correct TTL syntax [OK]

Hint: Use expireAfterSeconds option in createIndex [OK]

Common Mistakes:

• Using wrong option names like ttl or expireAfter
• Using non-existent createTTLIndex method
• Forgetting to specify expireAfterSeconds

3. Given a collection with documents having a timestamp field and a TTL index created as db.logs.createIndex({ timestamp: 1 }, { expireAfterSeconds: 120 }), what happens to a document inserted with { timestamp: new Date() } after 3 minutes?

medium

A. The document remains indefinitely

B. The document is automatically deleted

C. The document is archived but not deleted

D. The document causes an error on insertion

Solution

1. Step 1: Understand TTL expiry timing

   The TTL index deletes documents after expireAfterSeconds from the timestamp field value.

2. Step 2: Calculate expiry for 3 minutes

   3 minutes = 180 seconds, which is more than 120 seconds, so the document will be deleted.

3. Final Answer:

   The document is automatically deleted -> Option B

4. Quick Check:

   TTL expiry after 120s means deletion after 3 minutes [OK]

Hint: TTL deletes after expireAfterSeconds from date field [OK]

Common Mistakes:

• Thinking document stays forever
• Assuming TTL archives instead of deletes
• Believing TTL causes insertion errors

4. You created a TTL index with expireAfterSeconds: 0 on a field expireAt, but documents are not being deleted. What is the likely cause?

medium

A. TTL indexes only work on _id field

B. expireAfterSeconds cannot be zero

C. You must restart MongoDB for TTL to work

D. The expireAt field is not a date type

Solution

1. Step 1: Check TTL index requirements

   TTL indexes require the indexed field to be a date type to calculate expiry.

2. Step 2: Analyze why documents not deleted

   If expireAt is not a date, TTL cannot determine expiry time, so documents stay.

3. Final Answer:

   The expireAt field is not a date type -> Option D

4. Quick Check:

   TTL needs date field type to delete documents [OK]

Hint: Ensure TTL field is a date type [OK]

Common Mistakes:

• Thinking expireAfterSeconds cannot be zero
• Believing TTL only works on _id
• Assuming MongoDB restart is needed

5. You want documents in a collection to expire exactly at a specific date and time stored in the expireAt field. Which TTL index configuration achieves this?

hard

A. Create TTL index on createdAt with expireAfterSeconds set to document age

B. Create TTL index on expireAt with expireAfterSeconds: 3600

C. Create TTL index on expireAt with expireAfterSeconds: 0

D. Use a background job to delete documents at expireAt

Solution

1. Step 1: Understand TTL with exact expiry date

   Setting expireAfterSeconds to 0 means documents expire exactly at the date/time in the field.

2. Step 2: Compare options for exact expiry

   Only Create TTL index on expireAt with expireAfterSeconds: 0 uses expireAfterSeconds: 0 on expireAt field to expire exactly at that time.

3. Final Answer:

   Create TTL index on expireAt with expireAfterSeconds: 0 -> Option C

4. Quick Check:

   expireAfterSeconds: 0 = expire exactly at date field [OK]

Hint: Use expireAfterSeconds: 0 for exact expiry date [OK]

Common Mistakes:

• Using non-zero expireAfterSeconds for exact expiry
• Confusing createdAt with expireAt field
• Relying on manual background jobs unnecessarily