Capped collections for fixed-size data in MongoDB - Mini Project: Build & Apply

Create and Use a Capped Collection in MongoDB

📖 Scenario: You are managing a logging system that stores recent log messages. To save space, you want to keep only the latest 5 log entries in your database.

🎯 Goal: Build a capped collection in MongoDB that holds exactly 5 log documents. Insert 5 logs, then add one more log to see the oldest log removed automatically.

📋 What You'll Learn

Create a capped collection named logs with a maximum of 5 documents

Insert 5 log documents with fields message and level

Insert a 6th log document to trigger automatic removal of the oldest log

Query the logs collection to confirm only 5 documents remain

💡 Why This Matters

🌍 Real World

Capped collections are useful for storing logs, sensor data, or any data where only the most recent entries matter and storage space is limited.

💼 Career

Understanding capped collections helps in roles involving database management, backend development, and system monitoring where efficient data retention is important.

1

Create a capped collection named logs

Use the db.createCollection method to create a capped collection called logs with a maximum of 5 documents and a size of 10000 bytes.

MongoDB

# Your code here to create the capped collection 'logs' with max 5 documents and size 10000 bytes

Need a hint?

Use db.createCollection("logs", { capped: true, size: 10000, max: 5 }) to create the capped collection.

2

Insert 5 log documents into logs

Insert 5 documents into the logs collection. Each document should have a message field with values "Log 1" to "Log 5" and a level field with value "info".

MongoDB

db.createCollection("logs", { capped: true, size: 10000, max: 5 })
# Your code here to insert 5 log documents with message 'Log 1' to 'Log 5' and level 'info'

Need a hint?

Use db.logs.insertMany([...]) with 5 documents having message and level fields.

3

Insert a 6th log document to trigger removal of the oldest log

Insert one more document into the logs collection with message set to "Log 6" and level set to "warning". This will cause the oldest document to be removed automatically.

MongoDB

db.createCollection("logs", { capped: true, size: 10000, max: 5 })
db.logs.insertMany([
  { message: "Log 1", level: "info" },
  { message: "Log 2", level: "info" },
  { message: "Log 3", level: "info" },
  { message: "Log 4", level: "info" },
  { message: "Log 5", level: "info" }
])
# Your code here to insert the 6th log document with message 'Log 6' and level 'warning'

Need a hint?

Use db.logs.insertOne({ message: "Log 6", level: "warning" }) to add the 6th log.

4

Query the logs collection to confirm only 5 documents remain

Use db.logs.find() to retrieve all documents from the logs collection. Confirm that only 5 documents exist and the oldest log "Log 1" is removed.

MongoDB

db.createCollection("logs", { capped: true, size: 10000, max: 5 })
db.logs.insertMany([
  { message: "Log 1", level: "info" },
  { message: "Log 2", level: "info" },
  { message: "Log 3", level: "info" },
  { message: "Log 4", level: "info" },
  { message: "Log 5", level: "info" }
])
db.logs.insertOne({ message: "Log 6", level: "warning" })
# Your code here to query all documents in 'logs' collection

Need a hint?

Use db.logs.find() to see all documents in the capped collection.