Q16-Q20

Q16: Tell me something about CQRS? 

Q17: Do we have two different databases in CQRS?

Q18: Techniques to keep two database in Sync?

Q19: Define Database per Microservice Architecture Design pattern? [Microservice Architecture Design patterns -- Database per Microservice, Event Sourcing, CQRS, Saga, BFF, API Gateway, Strangler, Circuit Breaker, Externalized Configuration, Consumer-Driven Contract Tracing]

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Q16: Tell me something about CQRS? 

Answer:

CQRS (Command Query Responsibility Segregation) is a powerful architectural pattern often used in microservices to separate the responsibilities of reading and writing data

Benefits:

>> This separation allows each side to be optimized independently for performance, scalability, and complexity.

CQRS splits the application into two distinct parts:

Command Side (Write Model): Handles operations that change data (Create, Update, Delete).

Query Side (Read Model): Handles operations that retrieve data (Read).

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Q17: Do we have two different databases in CQRS?

Answer:

In CQRS, it's common but not mandatory to use two separate databases for the read and write sides. 

Benefits of using Two databases:

1. Independent Scaling : We can scale READ DB horizontally to handle high query loads.

2. Technology Flexibility: You might use a relational database (e.g., PostgreSQL) for writes. And a NoSQL database (e.g., MongoDB, Elasticsearch) for reads.

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Q18: Techniques to keep two database in Sync?

Answer:

Techniques

1. Event-Driven Synchronization (Most Common)

• How it works: After a write operation, the system emits a domain event (e.g., OrderCreated, CustomerUpdated).
• These events are published to a message broker (like Kafka, RabbitMQ, or Azure Service Bus).
• A consumer service listens to these events and updates the read database (e.g., MongoDB).
• This enables eventual consistency (With little latency). 

2. Dual Writes (Not Recommended)#

The application writes to both databases in the same transaction or sequentially.

⚠️ Cons:

Risk of inconsistency if one write fails.

Hard to manage rollback and error handling.

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Q19: Define Database per Microservice Architecture Design pattern? 

Answer:

Each Microservice has its own dedicated Database and un related microservice can NOT access db of other domain directly. 

Advantages:

1. Understanding of Project and related DB is easy as it has limited tables and data. 

2. Scaling of DB is easy as required. 

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