  We study the problem of evaluating persistent queries over streaming graphs
in a principled fashion. These queries need to be evaluated over unbounded and
very high speed graph streams. We define a streaming graph data model and query
model incorporating navigational queries, subgraph queries and paths as
first-class citizens. To support this full-fledged query model we develop a
streaming graph algebra that describes the precise semantics of persistent
graph queries with their complex constructs. We present transformation rules
and describe query formulation and plan generation for persistent graph queries
over streaming graphs. Our implementation of a streaming graph query processor
shows the feasibility of our approach and allows us to gauge the high
performance gains obtained for query processing over streaming graphs.
