  Datacenters need networks that support both low-latency and high-bandwidth
packet delivery to meet the stringent requirements of modern applications. We
present Opera, a dynamic network that delivers latency-sensitive traffic
quickly by relying on multi-hop forwarding in the same way as
expander-graph-based approaches, but provides near-optimal bandwidth for bulk
flows through direct forwarding over time-varying source-to-destination
circuits. The key to Opera's design is the rapid and deterministic
reconfiguration of the network, piece-by-piece, such that at any moment in time
the network implements an expander graph, yet, integrated across time, the
network provides bandwidth-efficient single-hop paths between all racks. We
show that Opera supports low-latency traffic with flow completion times
comparable to cost-equivalent static topologies, while delivering up to 4x the
bandwidth for all-to-all traffic and supporting 60% higher load for published
datacenter workloads.
