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

2 Preliminaries and Existing Techniques
control-flow modeling [MMLW05]. In detail, SQL activities, Retrieve Set activities, and
Atomic SQL Sequence activities (as transactional boundary for other SQL activities) have
been introduced. For example, this concept was implemented as the BPEL extension
ii4BPEL (information integration for BPEL) [IBM05b] within the IBM Business Integration
Suite that is based on the WebSphere application server. See the survey by Vrhovnik
et al. [VSRM08] for a detailed comparison of SQL support by workflow products. To
summarize, this BPEL/SQL approach provides hybrid modeling semantics in terms of
control flow and data flow. In addition, it enables the efficient execution of data-intensive
processes due to potentially reduced transferred data. We will revisit this approach from
an optimization perspective in Subsection 2.1.5.
In contrast to the BPEL/SQL activities, the BPEL-DT (Data Transitions) [HRP + 07,
Hab09] approach uses the traditional control-flow semantics enriched with so-called data
transitions. It is based on the concept of data-grey-box Web services [HPL + 07] that
represent a specific data framework in order to propagate data between Web services
by reference. Data transitions are used in terms of data-intensive data dependencies
between external Web service interactions within a BPEL process. Such a data transition
is modeled and executed as an integration flow using arbitrary integration platforms such
as ETL tools. In conclusion, arbitrary combinations of the control-flow-oriented BPEL
specification with data-flow-oriented integration flows can be realized.
However, both approaches are still control-flow-oriented because the control-flow-oriented
BPEL is enriched with data-centric constructs, but still exhibits temporal dependencies,
which is the major classification criterion between data-flow and control-flow semantics.
B. Hybrid Flow Structure
In addition to the mentioned hybrid modeling semantics, there are also arguments for a
hybrid modeling structure. For example, when using directed graphs or the hierarchy of
sequences, aspects like complex control flow branches, specific variable initialization or
the preparation of complex queries to external systems are difficult and complex to model.
In contrast, when using source code structures, the overall process specification is hidden
and too fine-grained. For these reasons, a combination of both aspects is advantageous
for certain integration flows.
BPELJ (BPEL for Java) [BEA04] enables such a hybrid modeling structure by combining
the hierarchy of sequences and source code. Therefore, arbitrary Java code snippets
(expressions, or small blocks of Java code) can be included in BPEL process specifications.
This is done via so-called bpelj:snippet activities, where the complete code block is included
in this activity and the activity can be used within the overall process specification.
C. Model-Driven Development of Integration Flows
In the context of complex integration flows, we observe a trend towards applying modeldriven
development techniques from the area of software technology.
First steps towards model-driven development of integration flows were made by Simitsis
et al. in the sense of separating ETL flow modeling into conceptual [VSS02a], logical
[VSS02b] and physical [TVS07] models as well as using transformation rules [Sim05] to
describe the transition from one model to another.
The Orchid project [DHW + 08] enables the generation of ETL jobs from declarative
schema mapping specifications. A so-called Operator Hub Model (OHM) is used in order
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