An online shop needs to store information about clients and orders.

An online shop that needs to store information about clients and orders with fixed properties and enforced data types, and ensure that each order is associated with an existing client, requires a relational database.

Relational Database: Relational databases, such as MariaDB, are designed to enforce data types and relationships between different entities (e.g., clients and orders). They can ensure referential integrity, which means an order must be associated with an existing client.

The other options are not suitable:

A. An in-memory database like memcached: In-memory databases do not provide the necessary persistence and relational features.

B. An object store like OpenStack Swift: Object stores are designed for unstructured data and do not enforce data types or relationships.

C. A messaging service like OpenStack Zaqar: Messaging services are for communication between services and not for persistent data storage.

D. A NoSQL database like MongoDB: While NoSQL databases can store data, they typically do not enforce strict data types and relationships as effectively as relational databases.

References:

MariaDB Documentation