What are capped collections and when would you use them in MongoDB?

Q: What are capped collections and when would you use them in MongoDB?

Learn the answer to "What are capped collections and when would you use them in MongoDB?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a backend engineer at a social media company. You are building a new service that needs to store a log of the most recent user activity.

You need to find a way to store the logs in a way that is efficient and that automatically removes old logs.

The Challenge

Explain what capped collections are in MongoDB and how you would use them to solve this problem. What are the key benefits of using capped collections?

Wrong Approach

A junior engineer might try to solve this problem by using a regular collection and then writing a separate script to periodically remove old logs. This would be a complex and inefficient solution.

Addresses symptoms, not root cause

Right Approach

A senior engineer would know that a capped collection is the perfect tool for this job. They would be able to explain what a capped collection is and how to use it to automatically remove old logs.

Step 1: Understand What Capped Collections Are

A capped collection is a fixed-size collection that automatically overwrites its oldest entries when it reaches its maximum size.

Step 2: The Key Features of Capped Collections

Feature	Description
Size	Capped collections have a fixed size, which you specify when you create the collection.
Ordering	Capped collections maintain the insertion order of the documents.
TTL	Capped collections do not have a TTL (Time To Live) index.
Updates	You cannot update documents in a capped collection if the update would cause the document to grow in size.
Deletes	You cannot delete documents from a capped collection.

Step 3: Use a Capped Collection

Here’s how we can create a capped collection to store our logs:

db.createCollection("my_logs", { capped: true, size: 100000 })

In this example, we create a new capped collection called my_logs with a maximum size of 100,000 bytes.

The Benefits of Using Capped Collections

Benefit	Description
Performance	Capped collections are very performant, because they are designed for high-throughput logging.
Simplicity	Capped collections are easy to use, and they automatically handle the removal of old logs.

Systematic, production-ready debugging

Practice Question

You are building a service that needs to store a log of the most recent user activity. Which of the following would be the most appropriate?

Questions