The main architectural styles include:

1. Monolithic
2. Microservices
3. Service-Oriented Architecture (SOA)
4. Event-Driven Architecture
5. Layered Architecture
6. Pipe and Filter
7. Client-Server
8. Peer-to-Peer

SOLID stands for:

• Single Responsibility Principle: A class should have only one reason to change
• Open-Closed Principle: Software entities should be open for extension, but closed for modification
• Liskov Substitution Principle: Objects of a superclass should be replaceable with objects of its subclasses without affecting the correctness of the program
• Interface Segregation Principle: Many client-specific interfaces are better than one general-purpose interface
• Dependency Inversion Principle: Depend upon abstractions, not concretions

1. Horizontal Scaling (Sharding): Distributing data or load across multiple servers
2. Caching: Storing frequently accessed data in memory for faster retrieval
3. Load Balancing: Distributing incoming network traffic across multiple servers

Key characteristics include:

1. Service independence and autonomy
2. Decentralized data management
3. Failure isolation
4. Scalability and flexibility
5. Technology diversity
6. Automated deployment (CI/CD)
7. Business-aligned services

• Creational Patterns: Deal with object creation mechanisms (e.g., Singleton, Factory, Builder)
• Structural Patterns: Deal with object composition to form larger structures (e.g., Adapter, Bridge, Composite)
• Behavioral Patterns: Characterize ways objects interact and distribute responsibility (e.g., Observer, Strategy, Command)

The CAP theorem states that in a distributed system, it's impossible to simultaneously guarantee all three of:

• Consistency: All nodes see the same data at the same time
• Availability: Every request receives a response
• Partition tolerance: The system continues to operate despite network failures

In practice, partition tolerance is necessary, so systems must choose between consistency and availability.

Key considerations include:

1. Defense in depth (multiple layers of security)
2. Principle of least privilege
3. Secure communication (encryption in transit and at rest)
4. Input validation and sanitization
5. Authentication and authorization mechanisms
6. Regular security audits and updates
7. Secure session management
8. Error handling and logging (without exposing sensitive information)

CI/CD contributes to software architecture by:

1. Enabling frequent and reliable software releases
2. Automating build, test, and deployment processes
3. Facilitating early bug detection and faster feedback
4. Promoting modular and testable code design
5. Supporting infrastructure as code practices
6. Enabling easy rollbacks and version control
7. Encouraging consistent development practices across teams