Workload Balancing for Highly Available Services: The Case of the N+M Redundancy Model

In today's information based world the demand on highly available services is ever increasing. Fault tolerant systems are capable of providing the expected services even in the presence of a failure. This is achieved through the redundancy of the service providers, where service assignments i.e. workloads are shifted to redundant healthy service providers when a failure occurs. The assignment and the shift are performed according to a redundancy model. A well-known redundancy model is the N+M where we have N active service providers and M standbys. In case of a failure of an active provider, the services are reassigned to its standbys. Maintaining a balanced workload before and after a failure in the N+M redundancy is a challenging task. Especially when the solution is decided at configuration time, and no runtime information is available. This is exactly the issue we tackle in this paper. We present three different approaches aiming at solving this problem with different priorities of the relevant constraints. Our solutions do not require any runtime information and can maintain a balanced workload even after a failure by anticipating the workload redistribution.