Before revision:  In this paper, we present a comprehensive theoretical analysis on delay-optimal scheduling in queueing systemswith replications. In particular, low-complexity replication policies are designed , and are rigorously proven to be delay-optimal or near delay-optimal among all non-preemptive and causal policies.
After revision:  In this paper, we present a comprehensive theoretical analysis on delay-optimal scheduling of replications in both centralized and distributed multi-server systems. Low-complexity scheduling policies are designed , and are rigorously proven to be delay-optimal or near delay-optimal among all non-preemptive and causal policies.