System design: messaging platform

The question requires you to design an online messaging platform. Designing such a system involves considering factors such as scalability, high availability, data persistence, security, and the ability to handle real-time communication. The interviewer is primarily interested in your ability to reason about system design concepts and apply them to solve complex problems. When answering this question, ensure to include the following key points:

1. Define the functionality and goals of the system: Consider basic functionality such as sending one-on-one messages, creating group chats, sending multimedia messages, and real-time delivery.
2. Design the high-level system architecture: Discuss the main components of the system, such as the client app, server application, database systems, etc.
3. Discuss the database design: Address how you would structure the data for efficient storage and retrieval of messages.
4. Discuss how the system scales: Address scalability issues and how the platform can handle increasing loads.
5. Address Security: Talk about how you would ensure data privacy and secure communication.

A messaging platform's foremost requirements would be to provide real-time messaging and support a large number of users concurrently. To start with, we should consider using a client-server model where users (clients) connect to a server to send and receive messages.

For the database, we could use NoSQL to handle large quantities of data and high traffic loads. We could use Cassandra, given its excellent write and read scalability, and data could be structured in the form of "MessageID", "SenderID", "ReceiverID", "Message Content", "Timestamp". We could use a distributed file storage system like HDFS to store multimedia messages.

To handle real-time delivery, we could use WebSockets, which provide full-duplex communication. To ensure high-availability, we could have multiple copies of each server and use load balancers to distribute traffic among them.

Regarding security, messages should be encrypted using end-to-end encryption, for instance, with the Signal Protocol.

By monitoring the system regularly, we can identify bottlenecks and scale appropriately, for instance, by sharding databases or adding servers.

For this system, I would use a combination of microservices, a real-time database, effective caching, and prioritization of data. The system would need services for user authentication, message handling, multimedia handling, online presence, and notifications.

For database design, NoSQL seems a suitable fit, possibly DynamoDB, as it provides seamless scalability and consistent performance. Messages could be stored in such a way that they belong to a "conversation" between two users or a group, ensuring efficient message retrieval.

For real-time communication, rather than polling the server for new messages, we could use a publisher-subscriber model, possibly using a service like Apache Kafka.

The system would need to be horizontally scalable, where we can alleviate surges in traffic by merely adding more instances of a service. Coupled with load balancing, this would ensure our system can handle large amounts of traffic.

The security of the system could be ensured by using HTTPS and secure WebSockets. Moreover, we could use JWT (JSON Web Tokens) for user authentication and secure sessions.

System monitoring and logging would be integral to preemptively finding and resolving performance-related issues. We could use ELK Stack for this purpose.