Capped collections for fixed-size data in MongoDB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a capped collection in MongoDB?

A capped collection is a fixed-size collection that maintains insertion order and automatically overwrites the oldest documents when it reaches its size limit.

beginner

How do you create a capped collection with a maximum size of 1MB in MongoDB?

Use the command:
db.createCollection('myCapped', { capped: true, size: 1048576 })
This creates a capped collection named 'myCapped' with a size limit of 1 megabyte.

beginner

What happens when a capped collection reaches its size limit?

When the capped collection reaches its size limit, MongoDB automatically overwrites the oldest documents with new ones, preserving the insertion order.

intermediate

Can you delete documents individually from a capped collection?

No, individual document deletion is not allowed in capped collections. You can only insert new documents, and old ones are overwritten automatically when the size limit is reached.

intermediate

Why are capped collections useful for fixed-size data?

They are useful because they provide high-performance inserts and maintain a fixed storage size, making them ideal for logging, caching, or real-time data streams where only recent data is needed.

What is a key feature of a capped collection in MongoDB?

A It automatically overwrites oldest documents when full

B It allows unlimited growth without size limits

C It supports individual document deletion

D It stores data in random order

Capped collections have a fixed size and overwrite the oldest documents automatically when they reach their size limit.

How do you specify a capped collection size when creating it?

A Using the 'max' option

B Using the 'capacity' option

C Using the 'limit' option

D Using the 'size' option in bytes

The 'size' option specifies the maximum size in bytes for the capped collection.

Which operation is NOT allowed on capped collections?

A Inserting new documents

B Querying documents in insertion order

C Deleting individual documents

D Automatically overwriting old documents

Individual document deletion is not allowed in capped collections.

What is a common use case for capped collections?

A Logging and caching recent data

B Storing large archival data

C Storing user profiles

D Running complex transactions

Capped collections are ideal for logging and caching where only recent data is important.

If you want a capped collection to hold a maximum of 1000 documents, what else must you specify?

A The 'limit' option

B The 'max' option along with 'capped: true'

C Only the 'size' option

D No additional options needed

You can specify the maximum number of documents using the 'max' option when creating a capped collection.

Explain what a capped collection is and why it is useful for fixed-size data.

Think about a collection that never grows beyond a set size and keeps only the newest data.

Describe how to create a capped collection in MongoDB and what options you need to set.

Focus on the command and the key options to limit size and documents.

Practice

1. What is the main purpose of a capped collection in MongoDB?

easy

A. To store data only in memory without writing to disk

B. To allow unlimited data growth without any size restrictions

C. To store data with a fixed size limit and automatically remove old entries

D. To create collections that support complex transactions

Solution

1. Step 1: Understand capped collection purpose

   Capped collections are designed to hold a fixed amount of data, automatically removing the oldest entries when full.

2. Step 2: Compare options with this behavior

   Only To store data with a fixed size limit and automatically remove old entries describes fixed size and automatic removal of old data, matching capped collection behavior.

3. Final Answer:

   To store data with a fixed size limit and automatically remove old entries -> Option C

4. Quick Check:

   Capped collection = fixed size + auto-remove old [OK]

Hint: Remember capped means fixed size with auto-removal [OK]

Common Mistakes:

• Thinking capped collections grow unlimited
• Confusing capped with in-memory storage
• Assuming capped collections support complex transactions

2. Which of the following is the correct command to create a capped collection named logs with a maximum size of 1MB?

easy

A. db.createCappedCollection('logs', 1048576)

B. db.logs.create({ capped: true, size: 1048576 })

C. db.createCollection('logs', { size: 1048576 })

D. db.createCollection('logs', { capped: true, size: 1048576 })

Solution

1. Step 1: Recall syntax for capped collection creation

   The correct syntax uses db.createCollection with options including capped: true and size in bytes.

2. Step 2: Check each option

   db.createCollection('logs', { capped: true, size: 1048576 }) matches the correct syntax. db.logs.create({ capped: true, size: 1048576 }) uses wrong method. db.createCollection('logs', { size: 1048576 }) misses capped: true. db.createCappedCollection('logs', 1048576) is invalid.

3. Final Answer:

   db.createCollection('logs', { capped: true, size: 1048576 }) -> Option D

4. Quick Check:

   Create capped = createCollection + capped:true + size [OK]

Hint: Use createCollection with capped:true and size in bytes [OK]

Common Mistakes:

• Omitting capped:true option
• Using wrong method like createCappedCollection
• Specifying size without capped:true

3. Given a capped collection cache with size 100 bytes, what will be the result of inserting 3 documents each of size 50 bytes?

medium

A. Only the last 2 documents will be stored, the first is removed

B. All 3 documents will be stored without removal

C. Insertion will fail due to size limit

D. Only the first document will be stored

Solution

1. Step 1: Understand capped collection size behavior

   The collection size is 100 bytes. Each document is 50 bytes, so two documents fit exactly.

2. Step 2: Analyze insertion of 3 documents

   When inserting the third document, the oldest document (first) is removed to keep size within 100 bytes, so last two remain.

3. Final Answer:

   Only the last 2 documents will be stored, the first is removed -> Option A

4. Quick Check:

   Size limit causes oldest removal [OK]

Hint: Capped size limits storage; oldest removed when full [OK]

Common Mistakes:

• Assuming all documents fit ignoring size
• Expecting insertion failure instead of removal
• Thinking only first document stays

4. You tried to create a capped collection with db.createCollection('events', { capped: true }) but got an error. What is the likely cause?

medium

A. The collection name 'events' is reserved and cannot be used

B. Missing the required size option specifying the maximum bytes

C. Capped collections cannot be created with createCollection

D. You must specify max documents instead of size

Solution

1. Step 1: Recall required options for capped collections

   When creating a capped collection, the size option (max bytes) is mandatory.

2. Step 2: Analyze the error cause

   Omitting size causes an error. Collection name and method are valid. max is optional.

3. Final Answer:

   Missing the required size option specifying the maximum bytes -> Option B

4. Quick Check:

   Capped creation requires size option [OK]

Hint: Always specify size when creating capped collections [OK]

Common Mistakes:

• Forgetting the size option
• Thinking collection names are restricted
• Confusing max documents with required size

5. You want to store recent user activity logs in a capped collection named userLogs with a max of 1000 documents and size limit 5MB. Which command correctly creates this collection?

hard

A. db.createCollection('userLogs', { capped: true, size: 5242880, max: 1000 })

B. db.createCollection('userLogs', { capped: true, max: 1000 })

C. db.createCollection('userLogs', { size: 5242880, max: 1000 })

D. db.createCollection('userLogs', { capped: true, size: 1000 })

Solution

1. Step 1: Identify required options for capped collection with limits

   Must specify capped: true, size in bytes, and max documents.

2. Step 2: Evaluate each option

   db.createCollection('userLogs', { capped: true, size: 5242880, max: 1000 }) correctly includes all three: capped true, size 5MB (5*1024*1024=5242880 bytes), and max 1000. Others miss required options or have wrong values.

3. Final Answer:

   db.createCollection('userLogs', { capped: true, size: 5242880, max: 1000 }) -> Option A

4. Quick Check:

   Capped + size in bytes + max docs = correct creation [OK]

Hint: Use capped:true with size in bytes and max docs for limits [OK]

Common Mistakes:

• Omitting size or max options
• Using size as number of documents
• Not setting capped:true