There is a continuum of alternatives for adaptive resource allocation with leases. The most flexible model would permit actors to renegotiate lease contracts at any time. At the other extreme, a restrictive model might disallow any changes to a contract once it is made. Shirako leases may be extended (renewed) by mutual agreement. Peers may negotiate limited changes to the lease at renewal time, including flexing the number of resource units. In our prototype, changes to a renewed lease take effect only at the end of its previously agreed term.
The protocol to extend a lease involves the same pattern of exchanges as to initiate a new lease (see Figure 2). The service manager must obtain a new ticket from the broker; the ticket is marked as extending an existing ticket named by a unique ID. Renewals maintain the continuity of resource assignments when both parties agree to extend the original contract. An extend makes explicit that the next holder of a resource is the same as the current holder, bypassing the usual teardown/setup sequence at term boundaries. Extends also free the holder from the risk of a forced migration to a new resource assignment--assuming the renew request is honored.
With support for resource flexing, a guest can obtain these benefits even under changing demand. Without flex extends, a guest with growing resource demands is forced to instantiate a new lease for the residual demand, leading to a fragmentation of resources across a larger number of leases. Shrinking a slice would force a service manager to vacate a lease and replace it with a smaller one, interrupting continuity.
Flex extends turned out to be a significant source of complexity. For example, resource assignment on the authority must be sequenced with care to process shrinking extends first, then growing extends, then new redeems. One drawback of our current system is that a Shirako service manager has no general way to name victim units to relinquish on a shrinking extend; COD overloads configuration properties to cover this need.