SOLID is an acronym for five design principles intended to make software designs more understandable, flexible, and maintainable: Single Responsibility Principle, Open/Closed Principle, Liskov Substitution Principle, Interface Segregation Principle, and Dependency Inversion Principle.

Designing a scalable system involves breaking down the system into manageable components, ensuring loose coupling and high cohesion, using load balancing, and considering both vertical and horizontal scaling. It’s also crucial to use appropriate data partitioning and to plan for future growth.

Design patterns provide reusable solutions to common problems in software design. Examples include the Singleton pattern, which ensures a class has only one instance; the Factory pattern, which creates objects without specifying the exact class; and the Observer pattern, which allows an object to notify other objects of state changes.

Ensuring code quality and maintainability involves following best practices, such as writing clean and readable code, adhering to coding standards, conducting code reviews, implementing automated testing, and documenting the code. Additionally, using design patterns and refactoring code regularly helps maintain high quality.

Microservices are an architectural style where a system is divided into small, independent services that communicate over well-defined APIs. Benefits include improved scalability, easier deployment, better fault isolation, technology diversity, and faster time-to-market.

Handling databases in a distributed system involves techniques such as sharding, replication, and the use of distributed databases like Cassandra or MongoDB. Ensuring data consistency through mechanisms like eventual consistency, and employing strategies like partition tolerance and ensuring high availability are also critical.