Designing a messaging app using Kafka at a high level involves several components and considerations. Below is a simplified design for a messaging app architecture leveraging Kafka:

1. **User Interface (UI)**:
  - Users interact with the messaging app through a user interface, which can be a web or mobile application. The UI allows users to send messages, view conversations, manage contacts, and receive notifications.

2. **Backend Application**:
  - The backend application serves as the middleware between the UI and the messaging infrastructure. It handles user authentication, authorization, message processing, and interaction with Kafka.

3. **Authentication and Authorization**:
  - Users authenticate themselves to the messaging app using credentials (e.g., username/password, OAuth tokens). Authentication and authorization mechanisms ensure that users have appropriate permissions to access their conversations and perform messaging actions.

4. **User Profile and Contacts Management**:
  - The messaging app maintains user profiles containing information such as usernames, display names, profile pictures, and contact lists. Users can add, remove, and manage contacts within the app.

5. **Messaging Infrastructure with Kafka**:
  - Kafka serves as the messaging backbone of the application, handling message storage, distribution, and processing. It consists of several Kafka topics for different purposes:
    - **Conversation Topics**: Each conversation between users is represented by a Kafka topic. Messages exchanged within a conversation are stored in the corresponding topic partitions.
    - **User Activity Topics**: These topics track user activity, such as user login/logout events, message read receipts, and online status changes.
    - **Notification Topics**: Notification messages (e.g., new message notifications, contact requests) are published to notification topics for delivery to users.

6. **Producer and Consumer Services**:
  - Producer services are responsible for publishing messages to Kafka topics. When a user sends a message or performs an action that generates a notification, the producer service publishes the message to the appropriate Kafka topic.
  - Consumer services subscribe to Kafka topics and process incoming messages. For example, a consumer service listens to conversation topics to retrieve and display messages in real-time to users. Another consumer service processes notification messages and sends them to users via push notifications or other channels.

7. **Message Delivery and Notifications**:
  - When a message is sent, it's published to the Kafka topic corresponding to the conversation between sender and recipient(s). Consumer services fetch new messages from these topics and deliver them to the respective users in real-time.
  - Notification messages are published to notification topics and delivered to users to alert them of new messages, contact requests, or other relevant events. Notifications can be delivered via push notifications, email, or other communication channels.

8. **Message Persistence**:
  - Kafka retains messages for a configurable period, providing message durability and replayability. Messages are stored in Kafka topics, allowing users to retrieve message history and replay conversations.

9. **Scalability and Fault Tolerance**:
  - The messaging app architecture built on Kafka is inherently scalable and fault-tolerant. Kafka clusters can be scaled horizontally by adding more brokers to handle increased message throughput and storage requirements. Kafka's distributed nature ensures high availability and fault tolerance, minimizing the risk of message loss or downtime.

10. **Monitoring and Management**:
   - Monitoring tools and dashboards track the health and performance of Kafka clusters, producer and consumer services, and other components of the messaging app. Metrics such as message throughput, latency, and error rates help detect and troubleshoot issues proactively.
   - Operations teams manage Kafka clusters, configure topics, monitor resource usage, and perform maintenance tasks to ensure optimal performance and reliability.

This high-level design outlines the key components and considerations for building a messaging app using Kafka as the messaging infrastructure. Actual implementations may vary based on specific requirements, scalability needs, and technology choices. Additionally, integrating additional features such as message encryption, multimedia messaging, and message search capabilities can further enhance the functionality and user experience of the messaging app.