What is MongoDB? MongoDB is a popular open-source, NoSQL database that is designed to store and manage large volumes of unstructured or semi-structured data. Unlike traditional relational databases (such as MySQL or PostgreSQL), which store data in tables with rows and columns, MongoDB stores data in flexible, JSON-like documents. MongoDB includes an aggregation framework for processing data and transforming it in a way that allows us to perform complex data manipulations (e.g., grouping, filtering, and sorting). MongoDB uses replica sets, which provide high availability by replicating data across multiple servers. What is PostgreSQL? PostgreSQL is an advanced, open-source relational database management system (RDBMS) known for its robustness, extensibility, and standards compliance. PostgreSQL follows the SQL standard but also includes many advanced features that make it one of the most versatile and powerful relational databases available. PostgreSQL is often used to power the backend of web and mobile applications, especially for applications requiring complex queries and data integrity. When to Use MongoDB vs PostgreSQL MongoDB: Ideal for projects with large amounts of unstructured data, such as content management systems, IoT applications or real-time analytics. It good in scenarios where horizontal scaling is a priority. PostgreSQL: Best for applications that require complex queries, robust transactions and data integrity such as financial systems, ERP systems or any use case with relational data and strong consistency requirements. Challenges of Using MongoDB & PostgreSQL MongoDB: Lack of strong ACID compliance in some scenarios, making it less suitable for complex transactions. Query optimization can be tricky with very large datasets. PostgreSQL: Less flexible in handling dynamic, unstructured data, and horizontal scaling is more difficult compared to MongoDB.