Workflow Concept of WS-PGRADE/gUSE - Science Gateways for Distributed Computing Infrastructures - page 37

opportunities support this level of parallel execution in gUSE, such as allowing
generator ports and de field generation methods. These

possibilities are discussed in Sect. 3.6.1 . Since any node can be an embedded
workflow in gUSE, the PS node can also be an embedded workflow. We reach the
highest level of parallelism (shown in the right hand-side of Fig. 3.2 ) where the
execution of the same workflow is done in parallel. In fact, such a parallel execution
of workflows can also be initiated by the user submitting the same workflow with
different con

ning various parameter

gurations (in gUSE terminology, in different instances).

3.5 Work fl ow Views

According to the design phases, workflows have different views separated by their
focus. During design time the users are allowed to modify the structure, to add
parameters to the jobs or to set the execution resources. Besides, execution man-
agement requires different views to be able to check states of the certain jobs, to get
their outputs or standard output/error messages and based on them, make decisions
to cancel or continue the execution. These views are introduced in this subsection.

3.5.1 Design Time — Abstract Work fl ow View

As a ne its abstract graph,

using a web-start client-based application called Graph Editor. Figure 3.3 illustrates
a graph created in Graph Editor. As discussed briefly in Chap. 2 , it offers a clear
graphical interface for de

first step of designing a workflow in gUSE, the users de

ning the nodes representing the component applications
within the workflow, and in addition, to associate port entities for each job, name
them, add a short optional description about them, and select their main type (input
or output). These ports can be connected to each other reflecting that the data
generated by the source node and associated to the given port must be transferred to
the sink node. Evidently, an output port can be connected to several input ports, but
an input port must be in connection at most to one output port. If an input port is not
connected to any output port, it represents an input file that should be provided by

the run-time environment where the workflow is executed. For example, it could be

file stored on an FTP server.
However, this obvious semantics of graph creation would allow the creation of
circles in the structure. As this class of structures is not supported by gUSE, loops
are recognized and blocked by the Editor. Therefore, only directed acyclic graphs
(DAG) can be de

ned in gUSE. Finally, the graph can be saved on the portal server.
The editor is an intuitive graphical tool to de ne the abstract part (the graph tag) of

the workflow description.