  Reconfigurable optical topologies are emerging as a promising technology to
improve the efficiency of datacenter networks. This paper considers the problem
of scheduling opportunistic links in such reconfigurable datacenters. We study
the online setting and aim to minimize flow completion times. The problem is a
two-tier generalization of classic switch scheduling problems. We present a
stable-matching algorithm which is $2\cdot (2/\varepsilon+1)$-competitive
against an optimal offline algorithm, in a resource augmentation model: the
online algorithm runs $2+\varepsilon$ times faster. Our algorithm and result
are fairly general and allow for different link delays and also apply to hybrid
topologies which combine fixed and reconfigurable links. Our analysis is based
on LP relaxation and dual fitting.
