This project implements a distributed systems middleware providing reliable message passing in peer-to-peer networks using Virtual Synchrony (VSync) protocol. Developed as part of the Distributed Systems course at Politecnico di Milano.

• Virtual Synchrony Protocol: Keeps message ordering consistent across nodes
• Fault Tolerance: Checkpoint-based recovery system for handling node failures
• Dynamic Topology Management: View manager for controlling network membership and topology changes
• Reliable Communication: Multi-layer architecture with reliability guarantees
• Automatic Discovery: Protocol for new nodes to join the network automatically
• JSON-based Message Serialization: Efficient message encoding/decoding

The middleware follows a 4-layer architecture:

1. Application Layer: API for sending/receiving messages
2. VSync Layer: Implements Virtual Synchrony with view management and fault tolerance.
3. Reliability Layer: Handles message acknowledgment, retransmission, and ordering.
4. Communication Layer: Manages network I/O, discovery, and serialization.

• Scalar Clock: Logical timestamps for message ordering and causality.
• Acknowledge Map: Tracks delivery confirmations and retransmissions.
• Stable Priority Blocking Queue: Thread-safe queue ensuring deterministic message processing.
• Checkpoint/Recovery System: Periodic snapshots for fault recovery.
• Knowledge Vector: Tracks delivery state across all nodes for consistency.
• View Change Protocol: Coordinates atomic topology updates.
• Client Handler: Asynchronous TCP connection management.

1. Node Startup: Each node starts the middleware stack, initializing all layers
2. Network Discovery: New nodes broadcast discovery messages to find and join existing networks
3. View Management: A designated view manager maintains the network topology and coordinates changes
4. Message Delivery: Application messages flow through VSync → Reliability → Communication layers
5. Fault Tolerance: The system periodically creates checkpoints and can recover from failures
6. Virtual Synchrony: All nodes see the same sequence of messages in the same order, maintaining consistency

Comprehensive unit tests covering:

• Message Serialization: JSON encoding/decoding and polymorphic message types
• Reliability Layer: Acknowledgment protocols, retransmission, and duplicate detection
• Data Structures: Thread-safe priority queues and concurrent collections
• Manual Integration Testing: Multi-node scenarios using Docker Compose

The project includes Docker configuration for easy deployment and testing with multiple nodes.