Capped collections for fixed-size data in MongoDB - Step-by-Step Execution

Concept Flow - Capped collections for fixed-size data

Create capped collection with size limit

↓

Insert documents

↓

Check if collection size exceeds limit?

No→Continue inserting

Yes↓

Automatically remove oldest documents

↓

Collection size stays within fixed limit

This flow shows how MongoDB capped collections keep data size fixed by automatically removing oldest documents when size limit is reached.

Execution Sample

MongoDB

db.createCollection("logs", { capped: true, size: 1000 })
db.logs.insert({ event: "start" })
db.logs.insert({ event: "process" })
db.logs.insert({ event: "end" })

Create a capped collection named 'logs' with max size 1000 bytes and insert three event documents.

Execution Table

Step	Action	Collection Size (bytes)	Documents in Collection	Notes
1	Create capped collection 'logs' with size 1000	0	[]	Empty collection created
2	Insert { event: 'start' }	approx 50	[{ event: 'start' }]	First document added
3	Insert { event: 'process' }	approx 100	[{ event: 'start' }, { event: 'process' }]	Second document added
4	Insert { event: 'end' }	approx 150	[{ event: 'start' }, { event: 'process' }, { event: 'end' }]	Third document added
5	Insert large documents repeatedly	1000 (max)	[oldest documents removed automatically]	Oldest docs removed to keep size <= 1000
6	Try to insert document exceeding size	1000	[documents fitting size]	Insert fails if document too large
7	Stop inserting	<=1000	[documents within size limit]	Collection size fixed by capped limit

💡 Execution stops because capped collection size limit is enforced, oldest documents removed automatically to keep size fixed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
Collection Size (bytes)	0	50	100	150	1000	1000
Documents in Collection	[]	[{ event: 'start' }]	[{ event: 'start' }, { event: 'process' }]	[{ event: 'start' }, { event: 'process' }, { event: 'end' }]	[oldest removed to keep size]	[documents within size limit]

Key Moments - 2 Insights

Why does the collection remove oldest documents automatically?

Because the collection is capped with a fixed size (see execution_table step 5), MongoDB removes oldest documents to keep the total size within the limit.

Can a document larger than the capped size be inserted?

No, as shown in execution_table step 6, if a document is larger than the capped size, the insert will fail because it cannot fit.

Visual Quiz - 3 Questions

Look at the execution_table at step 4, how many documents are in the collection?

A 3

B 1

C 2

D 4

💡 Hint

Check the 'Documents in Collection' column at step 4 in the execution_table.

At which step does the collection size reach its maximum capped size?

A Step 6

B Step 3

C Step 5

D Step 7

💡 Hint

Look at the 'Collection Size (bytes)' column to see when it reaches 1000.

If you try to insert a document larger than the capped size, what happens?

A Oldest documents are removed to fit it

B The insert fails

C The collection size increases beyond the cap

D The document is split into smaller parts

💡 Hint

Refer to step 6 in the execution_table where large document insert is attempted.

Concept Snapshot

Capped collections store documents with a fixed max size.
When size limit is reached, oldest documents are removed automatically.
Insert fails if a single document is larger than the capped size.
Use db.createCollection with { capped: true, size: <bytes> } to create.
Ideal for logs or fixed-size queues where order and size matter.

Full Transcript

This visual execution shows how MongoDB capped collections work. First, a capped collection is created with a fixed size limit. Documents are inserted one by one, increasing the collection size. When the size limit is reached, MongoDB automatically removes the oldest documents to keep the collection size within the limit. If a document is too large to fit, the insert fails. This behavior ensures the collection size stays fixed, useful for logs or queues.

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