1. A non-transitory machine readable medium storing a program that when executed by at least one processing unit implements a forwarding element that processes packets, the program comprising: at least one upcall handler to generate and install flows in a cache, wherein each flow is generated using at least one rule, from a flow table, which specifies performing a set of actions on incoming packets; and at least one at least one revalidator, which operates independently of the upcall handler, to validate one or more of the flows that are installed in the cache by removing or modifying each flow that has a different set of actions than a corresponding rule in the flow table.