Cost-Based Optimization of Integration Flows - Datenbanken ...

2 Preliminaries and Existing Techniques
From the perspective of structural representation, we distinguish four major types.
First, there are directed graphs, where the integration flow is modeled with G = (V, A) as
sets of vertexes V (operators) and arcs A (dependencies between operators). Typically,
such a graph is restricted to directed acyclic graphs (DAG), and XMI (XML Metadata
Interchange) [OMG07] is used as technical representation. Common examples of this type
are UML (Unified Modeling Language) activity diagrams [OMG03] and BPMN (Business
Process Modeling Notation) process specifications [BMI06]. However, many proprietary
representations exist as well. Van der Aalst et al. identified common workflow patterns
for imperative workflows with directed graph structures in order to classify the expressive
power of concrete workflow languages [vdABtHK00, vdAtHKB03]. Second, hierarchies
of sequences are a common representation. There, the flow is modeled as a sequence of
operators, where each operator can be an atomic or complex (composite of subsequences)
operator. Thus, arbitrary hierarchies can be modeled. As the technical representation
XML is used due to its suitable hierarchical nature. This is the most common type for
representing integration flows because this specification is more restrictive, which allows
the automatic compilation into an executable form and all algorithms working on this
representation can be realized as simple recursive algorithms. Examples are BPEL [OAS06]
processes and XPDL (XML Process Definition Language) [WfM05] process definitions.
Example 2.1. In order to make this distinction between directed graphs and hierarchies
of sequences more understandable, Figure 2.4 shows an example integration flow with both
different flow structures. Figure 2.4(a) illustrates an example plan with a directed graph
structure, where we receive a message, execute two filters depending on an expression
evaluation and finally send the result to an external system. Figure 2.4(b) illustrates the
same example with a hierarchy-of-sequences structure. When comparing both, we see that
the directed graph uses only atomic operators and allows for arbitrary temporal dependencies
between operators, while the hierarchy of sequences require complex operators (e.g., the
Switch operator) that recursively contain other operators and therefore is more restrictive.
Receive
Switch
Selection
Selection
Selection
Selection
Write
Receive
Switch
Selection
Selection
Selection
Selection
Write
(a) Directed Graph
(b) Hierarchy of Sequences
Figure 2.4: Integration Flow Modeling with Directed Graphs and Hierarchies of Sequences
In contrast to directed graphs and hierarchies of sequences, there is a third and a fourth
type, both of which are less common nowadays. Some integration platforms allow to
model (to program) integration flows on source code level using specific APIs. Examples
of this type are JPD (Java Process Definition) [BEA08], BPELJ (BPEL extension for
Java) [BEA04], and script-based integration platforms such as pygrametl [TP09]. As an
advantage, arbitrary custom code can be used, while the flow designer is confronted with
high modeling and optimization efforts. Finally, some platforms use fixed integration flows.
A fairly simple example is the concept of so-called message channels, where messages are
received, transformed and finally sent to a single external system. Concrete integration
flows are modeled by configuring these fixed flows. Although this structure allows for
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