QoS-Aware Service Compositions in Cloud Computing - Cloud Data Management - page 118

Fig. 8.3 Data flow graphs

the start point of the composite service. S e represents the end point. The data items
transferred between these abstract application services form a set D Df data i ;1
i d g .
A set of k n concrete application services f s n1 ;s n2 ;:::;s nk n g is available to
execute the abstract service S n . A concrete application service can be executed
on several virtual machines, databases and network services. After mapping each
abstract service to a concrete application service, VM UCSs and Database UCSs
need to be selected for each application service. Network UCSs need to be selected
for each data transfer in the data-flow graph. Assume each VM can only execute
one application service at a time. A late application service can only execute on
the VM after the former application services finish their executions. To sum up, any
solution to a composition problem in cloud computing includes: (1) Map the abstract
application services to concrete application services and corresponding UCSs (VM,
database and network services). (2) Schedule the execution order of the application
services. This execution order is a topological sort [ 114 ] of the data-flow graph, i.e.
a total ordering of the nodes in the DAG that obeys the precedence constraints.

QoS Model

QoS attributes contains (1) ascending QoS attributes, i.e. a higher value is better;
(2) descending QoS attributes, i.e. a smaller value is better; (3) equal QoS attributes,
i.e. no ordering but only equality, e.g. security protocol should be X.509. Four
QoS attributes are considered in this work: response time, price, availability
and reputation. Among them, time and price belong to the descending attributes
while availability and reputation belong to the ascending attributes. Vector Q D

Q 1 ;Q 2 ;Q 3 ;Q 4 denotes all the available QoS attributes. Q i ;1 i 4 represents

time, price, availability and reputation.
QoS values of an application service consist of three parts: execution, network
and storage QoSs. Existing QoS models in SOC [ 238 ] only consider the execution
QoSs. Execution QoS refers to the QoS value for executing an application service in