A function can have several different profiles with respect to the quality of service.
A contract may govern the acceptable error rate. One of the underlying assumptions that distinguishes distributed systems from more traditional systems is that individual elements in the system may fail without total system failure. Many of these failures are masked from the system, itself, by using replication and distribution.
An element in a system can provide many different levels of service. This is part of the behavior of that element. The levels include: