Synchronous and Asynchronous Group Communication (long Version)

In distributed systems, high service availability can be achieved by letting a group of servers replicate the service state; if some servers fail, the surviving ones know the service state and can continue to provide the service. Group communication services, such as membership and atomic broadcast, have been proposed to solve the problem of maintaining server state replica consistency. Group membership achieves agreement on the history of server groups that provide the service over time, while atomic broadcast achieves agreement on the history of state updates performed in each group. Since many highly available systems must support both hard real-time and soft real-time services, it is of interest to understand how synchronous (hard real-time) and asynchronous (soft real-time) group communication services can be integrated. We contribute towards this goal by proposing a common framework for describing properties of synchronous and asyn-chronous group communication services and by comparing the properties that such services can provide to simplify the task of replicated programming.