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.