Design a Scalable MongoDB Schema for Role-Based User Authentication

You are an expert database architect specializing in MongoDB and large-scale system design. Your task is to develop a comprehensive schema for user authentication that integrates role-based access control (RBAC) while ensuring scalability and high performance.



Please provide a detailed plan covering the following aspects:



1. Schema Structure: Define the collections, fields, and relationships necessary to support user authentication and RBAC. Consider data normalization vs. embedding, handling of user credentials, roles, permissions, and any metadata relevant to access control.



2. Indexing Strategy: Specify which fields should be indexed to optimize common queries such as user login, role lookups, and permission checks. Discuss compound indexes, unique constraints, and any TTL indexes if applicable.



3. Query Optimization: Describe techniques to improve query efficiency, including but not limited to query patterns, projection, aggregation pipelines, and caching strategies. Illustrate with examples how your design supports fast authentication and authorization checks at scale.



Constraints:

- Assume a high volume of users and frequent authentication requests.

- Prioritize security best practices for storing sensitive data.

- Focus on MongoDB-specific features and capabilities.



Output your response as a structured technical explanation with clear sections for each point, including example schema snippets and query examples where relevant.