Dieter Schuller
Multimedia Communications Lab (KOM), Technische Universität Darmstadt,
Rundeturmstr. 10, 64283 Darmstadt, Germany
Keywords: Service-Selection-Problem, Quality of Service, Optimization, Linear Programming, SOA-Security.
Abstract: In Service-oriented Architectures, services can be composed in a loosely coupled manner to realize business
processes. Thereby, mentioned services are not necessarily located only within the borders of the own en-
terprise. In the Internet of Services, multiple service providers offer various services on several service mar-
ketplaces. In case services with comparable functionalities but varying quality levels are available at differ-
ent costs, service requesters can decide, which services from which service providers to select. My research
focuses on this service-selection-problem for complex workflows by formulating a linear optimization prob-
lem, which can be solved optimally using linear programming techniques. As the actual execution path is
probably not known at planning time (e.g., for conditional braches), a worst-case, and an average-case anal-
ysis is performed. In addition to considering non-functional, quantitative service properties, I am working in
my research towards integrating qualitative service features (as, e.g., security) for different, complex
workflow patterns.
In present times, enterprises in highly competitive
markets require to execute their business processes
and workflows (as IT supported business processes)
efficiently. Due to globalization and deregulation of
markets, the business processes have to be flexible
in addition. This is necessary in order to enable en-
terprises to react quickly to market driven environ-
mental changes by adapting their business processes.
As enterprises often have several legacy systems and
a multitude of applications, running on different
platforms and operating systems, implemented with
various programming languages, the required flex-
ibility is hard to achieve. So, due to this heterogenei-
ty of enterprises’ IT architectures, an approach for
integrating the applications and legacy systems of
the enterprises is necessary to realize the mandatory
flexibility and efficiency regarding the enterprises’
business processes.
Considering Service-oriented Architectures
(SOA), these business processes take center stage.
They are realized by composing autonomous, loose-
ly coupled services. Depending on their granularity,
they can provide a more or less complex functionali-
ty (cf. Krafzig, Banke, Slama, 2004). Thereby, the
enterprises do not need to implement all required
services on their own. It is possible to rely on the
expertise of specialists by invoking services from
dedicated, external service providers, enabling en-
terprises to concentrate on their core business com-
petencies. In fact, cross-organizational business
processes gain more and more importance (Ley-
mann, Roller, 2000). In the Internet of Services, var-
ious services are offered by different service provid-
ers on service marketplaces. In case that functionally
equivalent services with varying Quality of Service
(QoS) at different cost levels are available, enter-
prises (as service requesters) can decide, which ser-
vices from which service providers to select. A cer-
tain degree of flexibility and efficiency can be
achieved by selecting those (external) services,
which meet the enterprise’s requirements best with
respect to functional and (especially) non-functional
service properties. The service selection decision
based on QoS properties is addressed in the service-
The service-selection-problem is well known in
the literature and discussed by several authors, re-
cently (Yu, Lin, 2005; Anselmi, Ardagna, Cremone-
si, 2007; Huang, Lan, Yang, 2009). In this problem,
a couple of services (WS – Web service) has to be
Schuller D. (2010).
In Proceedings of the 5th International Conference on Software and Data Technologies, pages 532-538
DOI: 10.5220/0003047905320538
selected based on their QoS values to realize a giv-
en, requested workflow respectively process (PS –
Process steps). It is assumed that a set of services,
which are able to realize the given workflow with
respect to functional requirements, is already identi-
fied. This situation is depicted exemplarily for a se-
quential workflow in Figure 1.
WS 1,m
WS 1,3
WS 1,2
WS 1,1
WS 2,m
WS 2,3
WS 2,2
WS 2,1
WS 3,m
WS 3,3
WS 3,2
WS 3,1
WS n,m
WS n,3
WS n,2
WS n,1
Figure 1: Service-Selection-Problem.
In my research, the service-selection-problem for
complex, cross-organizational workflows is focused.
For this, a linear optimization problem considering
different workflow patterns is formulated, which can
be solved optimally using mixed integer linear pro-
gramming (MILP) techniques from the field of oper-
ations research (cf. Domschke, Drexl, 2007), if such
a solution exists.
In addition, as the invoked services are probably
offered by external service providers, the exchanged
(SOAP-) messages (for the communication between
the external service provider and the requester) leave
the borders of the own enterprise. These messages
could be intercepted and manipulated by an attacker
(cf. Miede, Nedyalkov, Schuller, Repp, Steinmetz,
2009), potentially leading to failures with respect to
the execution of the business processes
(cf. Figure 2).
Therefore, security requires to be considered dur-
ing service selection in order to avoid or at least to
recognize service manipulations before the probably
modified messages (the results of the service execu-
tion) are used and applied in the enterprise’s process.
Figure 2: Message Interception.
My research aims at the development of a smart
workflow broker, which accepts (complex)
workflow requests in service-based environments,
computes an optimal execution plan, performs its
execution, and delivers the results back to the initial
requester. Thereby, the service selection decision is
focused. The question is which services – realizing
the requested functionality – from which service
providers to select in order to minimize the total
costs for the invoked services and to satisfy prede-
fined constraints with respect to QoS (specified by
the requester). Further, how can the execution of an
optimal execution plan be recovered in case of ser-
vice failures? Therefore, my research concentrates
on the following topics:
Aggregation functions aggregating the non-
functional service properties for (combina-
tions of) complex workflow patterns.
Linear optimization problem formulation with
respect to non-functional service properties for
complex workflows, which can be solved by
applying MILP, considering a worst-case and
an average-case analysis.
Heuristic algorithms in order to compute almost
optimal solutions with acceptable computa-
tional efforts, if the number of process steps in
the workflow is increasing.
Quantification of qualitative service properties
as, e.g., security in order to be able to consider
such properties as quantitative QoS parame-
ters for the optimization.
Re-planning strategies at design and execution
time to (rapidly) react to positive and negative
deviations of actual QoS values from the ex-
pected ones.
Thereby, a scenario of distributed workflows with a
large amount of service providers and service /
workflow requesters is considered. My research fo-
cuses on the formulation of a linear optimization
problem to compute the optimal solution to the ser-
vice-selection-problem as well as on the develop-
Service Requester
Service Provider A
Service Provider B
Service Marketplace
ment of (efficient) algorithms providing nearly op-
timal solutions. Here, not only sequential but com-
plex workflow patterns are regarded.
In order to develop the mentioned workflow broker
accepting workflow requests and computing –
dependent on the workflow length (number of
process steps) – (nearly) optimal solutions, the fol-
lowing contributions are planed or already realized.
To start with, an aggregation model for the ag-
gregation of different (aggregation) types of QoS
parameters addressing the service-selection-problem
for several complex workflow patterns was devel-
oped (an extension for further complex workflow
patterns is currently ongoing). This model is trans-
formed in a linear optimization problem. Applying
MILP techniques, an optimal solution can be com-
puted, if such a solution exists. The respective im-
plementation is currently ongoing. As the considered
complex workflow patterns include conditional
branches, the ultimately executed path (through the
workflow) is not known at planning time. So, it has
to be decided, which of the paths to consider for the
optimization. If all possible paths require satisfying
the predefined constraints, a worst-case analysis is
performed. An average-case analysis leads to an
execution plan, where the constraints are satisfied in
average. In a best-case analysis, only one of the po-
tential paths would have to satisfy the restriction for
a certain QoS parameter, which is not of practical
interest and will therefore not be considered further.
As the calculation of the optimal solution re-
quires strong computational efforts, heuristic algo-
rithms are currently being developed and imple-
mented as well.
Aside from these contributions regarding the op-
timization techniques, a first approach for the quan-
tification of qualitative service properties (as, e.g.,
for security) has been developed. The aim is to be
able to consider also qualitative service properties
for the service selection by integrating the quantified
qualitative values as additional quantitative QoS
parameters into the optimization problem. Regarding
the projected smart workflow broker, security as
qualitative service property is focused. Referring to
Figure 3, the Security Module makes sure that (po-
tentially) requested security requirements (e.g., inte-
grity and confidentiality) are satisfied.
To increase the robustness of the broker towards
service failures, a re-planning mechanism has been
developed and implemented prototypically. Here, a
service monitor triggers the interruption of the cur-
rently executed service and its replacement – by an
alternative (functionally equivalent) service based on
semantic annotations – if a negative deviation of the
measured form the expected (in Service-Level-
Agreements (SLAs) contracted) execution time is
detected. This is also done, if problems are encoun-
tered with the initial service invocation. The re-
planning mechanism – working on execution time as
QoS parameter – is planed to be extended for further
QoS parameters, for additionally recognizing posi-
tive SLA deviations, and for operating supplementa-
ry at design time. Here, experienced data about the
QoS values of particular services come into play by
(rather) considering these values for the optimization
instead the ones committed in SLAs.
As already mentioned, the re-planning mechan-
ism is based on semantic annotations. This leads to
the presupposition respectively requirement that the
service descriptions considered in the smart
workflow broker scenario are enriched with seman-
tic information about the services’ functionalities.
Here, an extension for the prototypically imple-
mented semantic matchmaker is planned.
An architectural blueprint of the targeted smart
workflow broker is depicted in Figure 3.
Figure 3: Smart Workflow Broker.
For the development of the smart workflow broker,
more than one research methodologies are applied.
An overview of the contribution and the research
methodology used is given in the following list:
Conceptual work to analyze the research prob-
lem and to design the architecture
Measurements of Web services in a test-bed to
record actual QoS behavior for re-planning
ICSOFT 2010 - 5th International Conference on Software and Data Technologies
Analytical evaluation and optimization of
worst-case and average-case service selection
as well as analytical models for QoS aggrega-
Simulation to evaluate worst-case and average-
case optimization and the re-planning mechan-
Prototypical implementation of the architecture
and the re-planning mechanism
Combining these methodologies allows treating the
addressed research questions in an integrated man-
In this section, the contributions of my hitherto re-
search are presented in summary. Here, the service-
selection-problem is addressed performing an aver-
age-case and a worst-case analysis. In addition, ini-
tial concepts have been developed to be able to inte-
grate security and other qualitative QoS parameters
into the optimization, and a basic re-planning me-
chanism has been implemented for sequential
workflow patterns.
5.1 QoS Aggregation
In order to solve the service-selection-problem, it is
necessary to be able to aggregate the QoS values of
the eligible services, i.e. the services capable of rea-
lizing the process steps of the considered business
process respectively workflow. Otherwise, it is not
possible to compare different service selections for
the workflow, which is necessary to compute the
optimal solution.
How the QoS values have to be aggregated de-
pends on the considered workflow pattern and on the
aggregation type of the regarded QoS parameter.
E.g., the costs for the whole workflow are calculated
by summing up the costs of the selected services,
whereas the workflow reliability is determined by
multiplying the reliabilities of these services. In my
research, the aggregation types summation, multipli-
cation and the Min/Max-operator are considered
covering quite all kinds of QoS parameters. In
(Schuller, Papageorgiou, Schulte, Eckert, Repp,
Steinmetz, 2009), generic aggregation functions for
these aggregation types have been developed consi-
dering the workflow pattern sequence. In (Schuller,
Eckert, Miede, Schulte, Steinmetz, 2010), aggrega-
tion functions for the workflow patterns parallel split
(AND-split), synchronization (AND-join), exclusive
choice (XOR-split), simple merge (XOR-join), and
arbitrary cycles (loops) are shown for a worst-case
and an average-case analysis.
A specification of further – in my research (re-
garding the QoS aggregation) yet unconsidered
workflow patterns is available in (Aalst, Hofstede,
Kiepuszewski, Barros, 2003).
5.2 Optimization
Based on the abovementioned aggregation functions,
a linear optimization problem is formulated in
(Schuller et al., 2009) for sequential workflow pat-
terns. In (Schuller et al., 2010), combinations of the
workflow patterns parallel split, synchronization,
exclusive choice, simple merge, and arbitrary cycles
are considered for this purpose. In Figure 4, a (com-
plex) example workflow for a potential combination
of these patterns is shown (in Business Process
Modeling Notation).
Figure 4: Example Workflow.
The challenge for the formulation of the linear opti-
mization problem lies in the combination of the ag-
gregation functions for the regarded complex
workflows and in the linearization of the partly non-
linear aggregation functions with respect to the deci-
sion variables. As, for instance, the reliability of the
eligible services has to be multiplied, the respective
decision variables – stating whether the currently
considered service is selected or not – are multiplied,
too. Having such conditions in the problem formula-
tion, MILP techniques to compute an optimal solu-
tion to the service-selection-problem are no longer
applicable. Therefore, a linearization step for the
mentioned non-linear aggregation functions is man-
datory in order to generate linear constraints. For
this purpose, an approximation, proposed by
(Heckmann, 2008), is applied to the multiplicative
non-linear functions. Conditions with the Min/Max-
operator are transformed into a set of equivalent
linear constraints. As here, the amount of added
conditions grows exponentially with the number of
process steps within a conditional branch, increasing
the restriction strength is proposed in (Schuller et al.,
Having computed the optimal execution plan,
aggregating the QoS values of the selected services
to calculate the workflow QoS does not lead to ex-
actly one value per QoS parameter, but to a set of
values, depending on the executed path. So, due to
multiple execution paths, (only) a range of values
representing the workflow QoS can be calculated.
5.3 Quantification of Qualitative
Service Properties
As the exchanged messages (between service pro-
vider and requester) could be intercepted and mani-
pulated by an attacker (cf. Figure 2) if services from
external services are invoked, security mechanisms
as, e.g., encryption, digital signatures and checksums
require to be considered. To integrate these qualita-
tive service features into the optimization, a quanti-
fication of these properties is necessary. Therefore,
the security features have to be ascertained and con-
sistently assigned to numeric values, to consider
them as quantitative QoS parameters for the optimi-
zation. By the term “consistently” it is meant that the
same value is always assigned to a certain ascer-
tained property. In this context, Jaquith defines a
good metric in (Jaquith, 2007) as a “consistent stan-
dard for the measurement”. It should be possible to
express the metric as a numeric value rather than
having qualitative labels as “high”, “medium” or
“low” (Jaquith, 2007).
A first approach to a consistent concept for such
a quantification of qualitative QoS parameters –
which is at a very early stage – is presented in
(Schuller, 2010).
5.4 Re-planning
When invoking one of the services in the execution
plan, a service monitor is started, too. As imple-
mented so far, the mentioned monitor measures the
time the invoked service takes to provide its functio-
nality. If the measured time exceeds the – in SLAs –
contracted execution time (negative SLA deviation),
the execution of this service is stopped and a search
for alternative services (with respect to functionali-
ty) is started. Referring to Figure 3, the service di-
rectory is queried to provide alternative services
based on semantic annotations. For this purpose, the
semantic matchmaker is used. With this set of alter-
native services, a new execution plan is compiled by
solving the service-selection-problem for the re-
maining (yet unexecuted) process steps. In other
words, the initial execution plan is re-planned – a
dynamic re-selection of services is performed – tak-
ing the already executed services respectively their
QoS values into consideration for the adaptation of
the restrictions of the (new) optimization problem.
The execution of the initial workflow then continues
by invoking the (re-planned) services. This re-
planning mechanism is presented for sequential
workflow patterns in (Schuller et al., 2009).
In case, the described mechanism is repeated
several times during workflow execution, applying
heuristics is probably advisable due to the mentioned
scalability issues when computing an optimal solu-
The main topic of my research, the service-
selection-problem, is widely studied in the literature.
Heuristic solutions to this problem are proposed and
implemented by several authors (Anselmi et al.,
2007; Jaeger, Muehl, Golze, 2005; Mabrouk, Geor-
gantas, Issarny, 2009). Tree-based algorithms, where
the complex workflow is transformed into a tree-
based execution structure (BPTree) based on the
alignment of the considered workflow patterns are
proposed in (Menascé, Casalicchio, Dubey, 2008).
Unfolding arbitrary cycles and computing optimal
execution plans for each possible path in the com-
plex workflow is proposed in (Ardagna, Pernici,
2007; Zeng, Benatallah, Ngu, Dumas, Kalagnamam,
Chang, 2004). The authors solve the service-
selection-problem for complex workflows by formu-
lating and solving the linear optimization problem
using MILP techniques solely for (the unfolded)
sequential workflows. So, all possible execution
paths have to be known and considered consecutive-
ly for the computation of an optimal solution.
These shortcomings are overcome in my re-
search, where arbitrary and recursive combinations
of (the in section 5.1 mentioned) workflow patterns
can be considered for the optimization without a
preliminary step for identifying all possible execu-
tion paths. The multiple execution paths are directly
taken into account when formulating the optimiza-
tion problem. In other words, an optimal solution to
the service-selection-problem is computed consider-
ing all execution paths conjunctly. So the execution
paths are not considered independently from each
other. This is an improvement regarding the men-
tioned approaches (in the literature) as a consecutive
computation for all paths considers the paths inde-
pendently from each other which probably leads to a
case where different services are assigned to the
ICSOFT 2010 - 5th International Conference on Software and Data Technologies
same process step when computing solutions for
different execution paths.
In highly competitive markets, flexible and efficient
business process execution is indispensable. Follow-
ing the paradigm of SOA, using services from (ex-
ternal) service providers (probably having large ex-
pertise in a certain business field), enables a flexible
and efficient process execution. In case, multiple
service providers offer (quite) equivalent services at
different quality and cost levels on various service
marketplaces (as is probably the case in the Internet
of Services), enterprises have the opportunity to de-
cide which services from which service providers to
select accomplishing the required functionalities
based on their QoS preferences, increasing the
achieved efficiency regarding the business process
Therefore, the service-selection-problem for
complex workflows is addressed in my research. In
this context, I contributed or contribute the follow-
An approach for aggregating QoS values for
(combinations of) complex workflow patterns.
An approach for solving the service-selection-
problem optimally (if an optimal solution ex-
ists) without preliminary identifying all execu-
tion paths.
A heuristic algorithm for a nearly optimal solu-
tion of the service-selection-problem due to
scalability issues considering the optimal solu-
A concept for a quantification of qualitative
service properties in order to integrate security
into the service selection decision.
A re-planning strategy to react upon positive
and negative SLA deviations for complex
In particular, the integration of further complex
workflow patterns as multi-choice (OR-split) and
multi-merge (OR-join) into the optimization prob-
lem – yet insufficiently considered (even in the men-
tioned heuristic approaches) in the literature – and
the development of a reasonable concept for quanti-
fication of security is currently investigated. The
extension of the re-planning mechanism for complex
workflows in combination with the progression of
the semantic matchmaker (and the therefore required
semantic annotations) is addressed as well in my
future research.
This work is supported in part by the E-Finance Lab
e. V., Frankfurt am Main, Germany
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ICSOFT 2010 - 5th International Conference on Software and Data Technologies