Built-in roles (read, readWrite, dbAdmin) in MongoDB - Time & Space Complexity

Time Complexity: Built-in roles (read, readWrite, dbAdmin)

O(n)

Understanding Time Complexity

When using built-in roles in MongoDB, it's important to understand how the time to check permissions grows as the number of operations increases.

We want to know how the system handles permission checks as more database actions happen.

Scenario Under Consideration

Analyze the time complexity of permission checks using built-in roles.

// Example: Checking user permissions for a read operation
const user = db.getUser("alice");
if (user.roles.some(role => role.role === "read")) {
  db.collection.find({});
}
// Roles: read, readWrite, dbAdmin
// Each role grants different access levels
    

This code checks if a user has the 'read' role before allowing a read operation on a collection.

Identify Repeating Operations

Look at what repeats when checking permissions.

• Primary operation: Checking if the user's roles include the required role.
• How many times: Once per operation that requires permission.

How Execution Grows With Input

The number of roles a user has is usually small and fixed, so checking roles stays quick even if many operations happen.

Input Size (n)	Approx. Operations
10 operations	10 role checks
100 operations	100 role checks
1000 operations	1000 role checks

Pattern observation: The time grows linearly with the number of operations, but each role check is very fast because roles list is short.

Final Time Complexity

Time Complexity: O(n)

This means the total time to check permissions grows directly with the number of operations performed.

Common Mistake

[X] Wrong: "Checking roles takes a long time because the database is big."

[OK] Correct: Role checks depend on the user's roles, which are few, not on the database size.

Interview Connect

Understanding how permission checks scale helps you explain security and performance in real applications confidently.

Self-Check

"What if a user had hundreds of roles assigned? How would that affect the time complexity of permission checks?"

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