Capped collections for fixed-size data in MongoDB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

Output of inserting documents into a capped collection

Given a capped collection with a max size of 1000 bytes and max 3 documents, what will be the count of documents after inserting 4 documents?

MongoDB

db.createCollection('logs', { capped: true, size: 1000, max: 3 });
db.logs.insertMany([
  { event: 'start' },
  { event: 'process' },
  { event: 'end' },
  { event: 'cleanup' }
]);
db.logs.countDocuments();

A 4

B 3

C 2

D 1

💡 Hint

Remember capped collections keep only the most recent documents up to the max limit.

Capped collections automatically remove oldest documents when the max document count or size is exceeded. Here, max is 3, so after inserting 4 documents, only the latest 3 remain.

🧠 Conceptual

intermediate

Behavior of capped collections on document update

What happens when you update a document in a capped collection that increases its size beyond the allocated space?

A The update fails with an error because documents cannot grow in capped collections.

B The document is moved to a new location to accommodate the size increase.

C The document is truncated to fit the original size limit.

D The collection automatically increases its size to fit the updated document.

💡 Hint

Think about the fixed-size nature of capped collections.

In capped collections, documents cannot grow in size after insertion. Updates that increase document size will fail with an error.

📝 Syntax

advanced

Correct syntax to create a capped collection with size and max documents

Which of the following commands correctly creates a capped collection named 'events' with a max size of 5000 bytes and max 100 documents?

A db.createCollection('events', { capped: true, size: 5000, max: 100 });

B db.createCollection('events', { capped: true, maxSize: 5000, maxDocs: 100 });

C db.createCollection('events', { capped: true, size: 5000 }); db.events.setMaxDocuments(100);

D db.createCollection('events', { capped: true, size: '5000', max: '100' });

💡 Hint

Check the exact option names and data types for capped collection options.

The correct options are 'size' for bytes and 'max' for max documents. They must be numbers, not strings. There is no method setMaxDocuments.

❓ optimization

advanced

Optimizing capped collection size for high insert rate

You have a capped collection with size 1MB and max 1000 documents. Your application inserts 2000 documents per minute. What is the best way to avoid losing important recent data?

A Disable capping and use a normal collection with TTL indexes.

B Decrease the capped collection size to force faster removal of old data.

C Increase the capped collection size and max document count to hold more data.

D Insert documents in batches of 1000 to reduce overhead.

💡 Hint

Think about how capped collections manage storage and document retention.

Increasing size and max documents allows the capped collection to hold more data before removing old documents, preserving recent data better.

🔧 Debug

expert

Identifying error when inserting large document into capped collection

You created a capped collection with size 1024 bytes. When inserting a document of 1500 bytes, what error will MongoDB raise?

A No error, document is truncated automatically

B CappedPositionLost: document cannot fit in capped collection

C BSONObjectTooLarge: document exceeds maximum size

D WriteError: document size exceeds capped collection size

💡 Hint

Consider how capped collections handle documents larger than their size limit.

MongoDB raises a WriteError indicating the document size exceeds the capped collection size because capped collections cannot store documents larger than their allocated size.

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