To measure the response time, for example, the observer at
P1 captures the time stamp when the request leaves the client and the time stamp when the
response is received by the client. The QoS monitor calculates, then, the response time by
performing the difference between the two time stamps, and reports the measured value to
the QoS information manager. A violation is detected by the QoS monitor if the measured
value is below a threshold value agreed with the provider.
QoS Adaptation
When the agreed QoS is violated, adaptation mechanisms should be employed in order to
adapt the violated QoS to a level agreed or acceptable by the client. The role of QoS adaptation
is to maintain, as much as possible, the continuity of a service when the initially-contracted
QoS is not preserved. In distributed systems, it is often very hard to sustain QoS over a
long period of time. Adaptive methods could be used as alternatives and enhancements to
resource reservation-based QoS provisioning. Adaptation takes various forms, and could
affect one or more QoS parameters of the Web services.
Scalability.in.QoSMA
The scalability in QoSMA architecture is achieved via replication. Web services are deployed
on various servers, which could be local replica servers and/or deployed on separate
geographical servers.
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