Guests with diverse needs may wish to acquire and manage multiple leases in a coordinated way. In particular, a guest may choose to aggregate resources from multiple sites for geographic dispersion or to select preferred suppliers in a competitive market.
Brokers play a key role because they can coordinate resource allocation across sites. SHARP brokers are responsible for provisioning: they determine how much of each resource type each guest will receive, and when, and where. The sites control how much of their inventory is offered for leasing, and by which brokers, and when. The site authorities also control the assignment of specific resource units at the site to satisfy requests approved by the brokers. This decoupling balances global coordination (in the brokers) with local autonomy (in the site authorities).
Figure 2 depicts a broker's role as an intermediary to arbitrate resource requests. The broker approves a request for resources by issuing a ticket that is redeemable for a lease at some authority, subject to certain checks at the authority. The ticket specifies the resource type and the number of units granted, and the interval over which the ticket is valid (the term). Sites issue tickets for their resources to the brokers; the broker arbitration policy may subdivide any valid ticket held by the broker. All SHARP exchanges are digitally signed, and the broker endorses the public keys of the service manager and site authority. Previous work presents the SHARP delegation and security model in more detail, and mechanisms for accountable resource contracts [13].