Built-in roles (read, readWrite, dbAdmin) in MongoDB

Introduction

Built-in roles help control what users can do in a MongoDB database. They make it easy to give the right access without confusion.

When you want someone to only look at data without changing it.

When a user needs to add, update, or delete data in the database.

When a user should manage database settings but not change data.

When you want to quickly assign common permissions without creating custom roles.

Syntax

MongoDB

db.createUser({
  user: "username",
  pwd: "password",
  roles: ["roleName"]
})

Replace "username" and "password" with the user's login details.

Replace "roleName" with one of the built-in roles like "read", "readWrite", or "dbAdmin".

Examples

This user can only read data from the database.

MongoDB

db.createUser({
  user: "reader",
  pwd: "pass123",
  roles: ["read"]
})

This user can read and write data (add, update, delete).

MongoDB

db.createUser({
  user: "writer",
  pwd: "pass123",
  roles: ["readWrite"]
})

This user can manage database settings but cannot change data.

MongoDB

db.createUser({
  user: "adminUser",
  pwd: "pass123",
  roles: ["dbAdmin"]
})

Sample Program

This example creates a user with readWrite access on the exampleDB database. Then, the user can add a product to the products collection and read it back.

MongoDB

use exampleDB

db.createUser({
  user: "testUser",
  pwd: "testPass",
  roles: ["readWrite"]
})

// Then login as testUser and insert a document
// mongo -u testUser -p testPass --authenticationDatabase exampleDB
// use exampleDB
// db.products.insertOne({name: "apple", price: 1.2})

// To check the inserted document
// db.products.find()

Important Notes

Built-in roles cover most common needs, so you usually don't need to create custom roles.

Always assign the least privilege needed to keep your data safe.

Roles apply per database, so a user can have different roles on different databases.

Summary

Built-in roles control user permissions easily.

Use read to allow only viewing data.

Use readWrite to allow viewing and changing data.

Use dbAdmin to allow managing database settings.

Practice

1. Which built-in MongoDB role allows a user to only view data without making any changes?

easy

A. root

B. readWrite

C. dbAdmin

D. read

Solution

1. Step 1: Understand the read role

   The read role allows users to view data but not modify it.

2. Step 2: Compare with other roles

   readWrite allows changes, dbAdmin manages settings, root has full access.

3. Final Answer:

   read -> Option D

4. Quick Check:

   View-only permission = read [OK]

Hint: View-only means read role [OK]

Common Mistakes:

• Confusing readWrite with read
• Thinking dbAdmin allows data viewing only
• Assuming root is for read-only

2. Which of the following is the correct way to assign the readWrite role to a user on the database mydb in MongoDB shell?

easy

A. db.grantRolesToUser('username', [{role: 'readWrite', db: 'mydb'}])

B. db.createUser({user: 'username', roles: ['readWrite']})

C. db.grantRolesToUser('username', ['readWrite'])

D. db.grantRolesToUser('username', [{role: 'readWrite', db: 'admin'}])

Solution

1. Step 1: Check syntax for granting roles

   The correct syntax uses db.grantRolesToUser with an array of role objects specifying role and database.

2. Step 2: Validate database name and role format

   db.grantRolesToUser('username', [{role: 'readWrite', db: 'mydb'}]) correctly specifies role as 'readWrite' and database as 'mydb'. db.grantRolesToUser('username', [{role: 'readWrite', db: 'admin'}]) wrongly uses 'admin' database.

3. Final Answer:

   db.grantRolesToUser('username', [{role: 'readWrite', db: 'mydb'}]) -> Option A

4. Quick Check:

   Grant role with role+db object array [OK]

Hint: Use role objects with db name in array for granting roles [OK]

Common Mistakes:

• Passing roles as simple strings without db
• Using wrong database name for the role
• Using createUser instead of grantRolesToUser for existing users

3. Given a user with the dbAdmin role on database sales, which of the following actions can the user perform?

medium

A. Create and drop indexes on collections

B. Insert new documents into collections

C. Read documents from collections

D. Modify documents in collections

Solution

1. Step 1: Understand dbAdmin role capabilities

   The dbAdmin role allows managing database settings like indexes, stats, and users, but not data changes.

2. Step 2: Match actions to role permissions

   Creating/dropping indexes is a dbAdmin task. Inserting, reading, or modifying documents require readWrite or read roles.

3. Final Answer:

   Create and drop indexes on collections -> Option A

4. Quick Check:

   dbAdmin manages indexes, not data changes [OK]

Hint: dbAdmin manages indexes, not data content [OK]

Common Mistakes:

• Assuming dbAdmin can insert or modify data
• Confusing readWrite with dbAdmin
• Thinking dbAdmin can read data

4. You tried to assign the readWrite role to a user with this command:
db.grantRolesToUser('alice', ['readWrite'])
But it failed. What is the likely cause?

medium

A. The user 'alice' does not exist

B. The role array must contain objects with role and db fields

C. The readWrite role does not exist

D. You must use createUser to assign roles

Solution

1. Step 1: Check syntax for grantRolesToUser

   The method requires an array of role objects with role and db fields, not just strings.

2. Step 2: Analyze the given command

   The command uses an array of strings, which is invalid syntax and causes failure.

3. Final Answer:

   The role array must contain objects with role and db fields -> Option B

4. Quick Check:

   Roles must be objects with role and db [OK]

Hint: Roles need role+db objects, not strings [OK]

Common Mistakes:

• Passing roles as strings instead of objects
• Assuming createUser is needed to add roles to existing users
• Ignoring user existence errors

5. You want a user to manage database indexes and view data but not modify documents. Which combination of built-in roles should you assign?

hard

A. readWrite only

B. dbAdmin only

C. read and dbAdmin

D. readWrite and dbAdmin

Solution

1. Step 1: Identify roles for viewing data and managing indexes

   read allows viewing data, dbAdmin allows managing indexes and database settings.

2. Step 2: Exclude roles that allow modifying documents

   readWrite allows modifying documents, which is not desired here.

3. Final Answer:

   read and dbAdmin -> Option C

4. Quick Check:

   View + manage indexes = read + dbAdmin [OK]

Hint: Combine read and dbAdmin to view and manage without edits [OK]

Common Mistakes:

• Using readWrite which allows document changes
• Assigning only dbAdmin which lacks read permission
• Assuming dbAdmin includes readWrite