1. A computer-implemented method of routing requests to hosts, the method comprising: storing a model of a service framework organized in a hierarchy comprising: a first level including a plurality of leaf nodes, individual leaf nodes corresponding to one of a plurality of hosts, and a second level corresponding to a first plurality of zones, wherein individual zones of the first plurality of zones comprise a subset of the plurality of leaf nodes; and for individual service requests to be processed by a host of the plurality of hosts: receiving a service request; discovering a plurality of candidate hosts from the plurality of the leaf nodes for processing the request; assigning a selection probability to individual candidate hosts of the plurality of candidate hosts based, at least in part, on a probabilistic distribution of measured latency data for the plurality of candidate hosts; selecting a candidate host for servicing the service request based, at least in part, on the assigned selection probability; and routing the service request to the selected candidate host.