Why Capped collections for fixed-size data in MongoDB? - Purpose & Use Cases

The Big Idea

What if your database could clean itself up and never run out of space?

The Scenario

Imagine you have a small notebook where you write down your daily expenses. When the notebook is full, you have to erase old pages manually to make space for new notes.

The Problem

Manually erasing old pages is slow and you might accidentally erase important notes. Also, keeping track of what to erase wastes time and can cause mistakes.

The Solution

Capped collections automatically keep only the newest data up to a fixed size. Old data is removed without any extra work, so your storage never overflows and you always have recent information.

Before vs After

✗ Before

db.expenses.insert({date: '2024-06-01', amount: 20}); // Manually delete old entries when full

✓ After

db.createCollection('expenses', {capped: true, size: 10000}); // Auto-remove old data when size limit reached

What It Enables

You can store continuous streams of data without worrying about running out of space or manual cleanup.

Real Life Example

A chat app uses capped collections to keep only the latest 1000 messages, ensuring fast access and fixed storage size.

Key Takeaways

Capped collections limit data size automatically.

They remove oldest data when full, no manual cleanup needed.

Perfect for logs, caches, or real-time data streams.

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