Tell about MongoDB: its pros and cons compared to PostgreSQL, how they differ, and why they are needed.

MongoDB is a document-oriented NoSQL database, while PostgreSQL is a relational DBMS with SQL support.

Advantages of MongoDB:

• Flexible schema: data is stored in BSON format, allowing easy changes to document structure.
• Well-suited for projects with rapidly changing requirements and unstructured data.
• High scalability and ease of horizontal sharding.

Disadvantages of MongoDB:

• Lack of complex transactions (although recent versions support document-level transactions).
• Less strict data validation, which can lead to errors without additional validation.

Advantages of PostgreSQL:

• Support for complex SQL queries, transactions, and relationships between tables.
• Strict data schema ensuring integrity and predictability.
• Extensibility and support for various data types.

Disadvantages of PostgreSQL:

• Less flexible schema, making it harder to quickly change data structure.
• More challenging to scale than MongoDB.

When to use what:

• MongoDB is suitable for projects with dynamic data where flexibility and scalability are important.
• PostgreSQL is better for systems with well-structured data and complex transactions, such as financial applications.

Thus, the choice depends on data requirements and application architecture.