The early tree contains all activities of G . Fig.1 (a)
depicts a workflow example represented by DAG, in
which the number above the node denotes the minimum
response time of the activity. Table 1 gives the service
pools for workflow activities. The early tree constructed
by ETGA is shown in Fig.1 (b), in which entry node 1
is the root of ET. All nodes with no successor are called
leaves, such as 4, 6, 8, 9, 11, and 14. Time interval for
activity i (1≤ i ≤ n) can be determined by its earliest
start time i s and its earliest finish time i f .