Virtual synchrony is a concept that allows processes in a distributed system to behave as if they are synchronized, even though they may be running on different machines. This approach enables the design of distributed applications by allowing processes to be grouped and events like broadcasts, group membership changes, and activity migration to appear instantaneous. The ISIS2 system provides a set of tools that facilitate this virtual synchrony, enabling applications to be decomposed into orthogonal components that can be treated independently. This approach is more straightforward, flexible, and likely to yield correct solutions compared to alternative methods. The ISIS2 system includes tools for initiating asynchronous actions, updating replicated data without blocking, and obtaining mutual exclusion using fault-tolerant replicated semaphores. These tools are implemented using a more primitive communication mechanism that provides virtually synchronous process groups. Virtual synchrony allows for the design of distributed algorithms under the assumption that all processes observe events in the same order, which simplifies algorithmic design and enables the use of simple assumptions about process behavior. The system addresses several subproblems, including process groups and group communication, deciding how to respond to requests, concurrency, synchronization, replicated data, detecting and reacting to failures, dynamic reconfiguration, stable storage, recovery, and transactions. These tools are designed to be orthogonal, allowing for the gradual extension of applications into a complete system. The system also provides a variety of tools for managing configuration data, handling group RPCs, executing requests, ensuring concurrency, managing semaphores, replicating data, detecting failures, recovering from failures, reconfiguring the system, and providing a news service. The ISIS2 system is designed to be highly concurrent and asynchronous, with tools that can be used as if they were synchronous. This allows for efficient handling of distributed applications, including a "twenty questions" program that demonstrates the system's capabilities. The system is capable of supporting a large number of queries and updates per second, and is fault-tolerant, with mechanisms for recovery from partial and total failures. The system's approach to virtual synchrony is a key factor in its effectiveness, enabling the design of distributed applications that are both correct and efficient.Virtual synchrony is a concept that allows processes in a distributed system to behave as if they are synchronized, even though they may be running on different machines. This approach enables the design of distributed applications by allowing processes to be grouped and events like broadcasts, group membership changes, and activity migration to appear instantaneous. The ISIS2 system provides a set of tools that facilitate this virtual synchrony, enabling applications to be decomposed into orthogonal components that can be treated independently. This approach is more straightforward, flexible, and likely to yield correct solutions compared to alternative methods. The ISIS2 system includes tools for initiating asynchronous actions, updating replicated data without blocking, and obtaining mutual exclusion using fault-tolerant replicated semaphores. These tools are implemented using a more primitive communication mechanism that provides virtually synchronous process groups. Virtual synchrony allows for the design of distributed algorithms under the assumption that all processes observe events in the same order, which simplifies algorithmic design and enables the use of simple assumptions about process behavior. The system addresses several subproblems, including process groups and group communication, deciding how to respond to requests, concurrency, synchronization, replicated data, detecting and reacting to failures, dynamic reconfiguration, stable storage, recovery, and transactions. These tools are designed to be orthogonal, allowing for the gradual extension of applications into a complete system. The system also provides a variety of tools for managing configuration data, handling group RPCs, executing requests, ensuring concurrency, managing semaphores, replicating data, detecting failures, recovering from failures, reconfiguring the system, and providing a news service. The ISIS2 system is designed to be highly concurrent and asynchronous, with tools that can be used as if they were synchronous. This allows for efficient handling of distributed applications, including a "twenty questions" program that demonstrates the system's capabilities. The system is capable of supporting a large number of queries and updates per second, and is fault-tolerant, with mechanisms for recovery from partial and total failures. The system's approach to virtual synchrony is a key factor in its effectiveness, enabling the design of distributed applications that are both correct and efficient.