Given some form of index distribution, ranking can also be distributed. A query
broker decides how to allocate queries to processors in a network and is responsible
for assembling the final ranked list for the query. The operation of the broker
depends on the form of index distribution. Caching is another form of distribution
where indexes or even ranked document lists from previous queries are left in
local memory. If the query or index term is popular, there is a significant chance
that this information can be reused with substantial time savings.