SQL Azure; under the hood
the basic setup in SQL Azure is this; each database occupies multiple nodes through a single primary database that processes your queries and two secondary copies that provide HA (and the guaranteed %99.9 SLA). If the primary fails for any reason, secondary copies pick up immediately and carry on. When it comes to computational capacity; SQL Azure cluster contain 100s of nodes that provides massive scale. Each node serves many tenants. Through load balancing and throttling mechanisms we keep the clustered balanced.
(from this thread)