  Redundancy mechanisms consist in sending several copies of a same job to a
subset of servers. It constitutes one of the most promising ways to exploit
diversity in multiservers applications. However, its pros and cons are still
not sufficiently understood in the context of realistic models with generic
statistical properties of service-times distributions and correlation
structures of copies. We aim at giving a survey of recent results concerning
the stability-arguably the first benchmark of performance-of systems with
cancel-oncompletion redundancy. We also point out open questions and
conjectures.
