Design and Evaluation of a Collaborative Online
Visualization and Steering Framework
Implementation for Computational Grids
Morris Riedel #1 , Thomas Eickermann #, Wolfgang Frings #, Sonja Dominiczak #, Daniel Mallmann #,
Thomas D¨ussel #, Achim Streit #, Paul Gibbon #, Felix Wolf # +, Wolfram Schiffmann ∗, Thomas Lippert #
#Central Institute for Applied Mathematics, John von Neumann Institute for Computing
ulich, D-52425, J¨
+Department of Computer Science
RWTH Aachen University, D-52056, Aachen, Germany
∗Institute of Computer Architecture, Department of Computer Science
University of Hagen, 58097, Hagen, Germany
Abstract—Today’s large-scale scientiﬁc research often relies on
the collaborative use of a Grid or e-Science infrastructure (e.g.
DEISA, EGEE, TeraGrid, OSG) with computational, storage,
or other types of physical resources. One of the goals of these
emerging infrastructures is to support the work of scientists with
advanced problem-solving tools. Many e-Science applications
within these infrastructures aim at simulations of a scientiﬁc
problem on powerful parallel computing resources. Typically, a
researcher ﬁrst performs a simulation for some ﬁxed amount
of time and then analyses results in a separate post-processing
step, for instance, by viewing results in visualizations. In earlier
work we have described early prototypes of a Collaborative
Online Visualization and Steering (COVS) Framework in Grids
that performs both - simulation and visualization - at the same
time (online) to increase the efﬁciency of e-Scientists. This paper
evaluates the evolved mature reference implementation of the
COVS framework design that is ready for production usage
within Web service-based Grid and e-Science infrastructures.
Grid infrastructures such as DEISA, EGEE, OSG or Tera-
Grid provide wide varieties of Grid services to enable large-
scale resource sharing and access to unprecedented amounts
of various types of Grid resources. An important objective
for Virtual Organizations (VOs)  that result from these
sharing across organizational boundaries is to make efﬁcient
use of the provisioned computational Grid resources such as
supercomputers, clusters, or server farms.
Scientiﬁc applications within these VOs and underlying
Grid infrastructures aim at simulations of a physical, biologi-
cal, chemical, or other types of domain-speciﬁc processes or
unsolved scientiﬁc problems. These applications typically rely
on parallel computing techniques to compute solutions for
such scientiﬁc problems. Parallel computing simulations use
computers with multiple processors that are able to jointly
work on one or more speciﬁc problems at the same time. The
outcome of these simulations are often analyzed in a separate
post-processing step, for instance by viewing the results in a
scientiﬁc domain-speciﬁc visualization application.
In order to increase the efﬁciency of e-Scientists and thus
their complete VOs, the collaborative online visualization
and steering (COVS) technique emerged that performs the
simulation and visualization at the same time. In this context
online visualization refers to e-Scientists that are able to
observe the intermediate processing steps during the computa-
tion of the simulation. This allows for computational steering
 to inﬂuence the computation of the simulation during
its run-time on a supercomputer or cluster. This saves cost-
intensive computational time on Grid resources by quickly
reacting on potentially misrouted applications with steering
their parameters back to correct values or even guide the
applications with steering to interesting locations in the model.
The lack of a widely accepted common COVS frameworks
within the major Grid middlewares (e.g. UNICORE, gLite,
Globus Toolkits) motivates the development of the framework
presented here. In earlier work we described the requirements
and design issues of early prototypes of the COVS framework
. This paper emphasizes on the reference implementation
of the COVS framework that is based on the UNICORE
Grid middleware and the VISIT communication library .
However, an implementation of a COVS framework will be
only accepted in realistic Grid scenarios if it reaches high lev-
els of usability and sophisticated performance. Therefore, the
contribution of this work is an evaluation of the framework’s
architectural design. Thus, to demonstrate that the COVS
framework is of practical relevance, the reference implementa-
tion is applied to a real-world test case, including performance
analysis of data connections and scalability anyalysis of the
key component enabling the collaboration.
Fig. 1. COVS framework reference implementation that is based on UNICORE as Grid middleware and the VISIT communication library.
Following the introduction the scene is set in Section 2
where we present the design of the COVS framework reference
implementation in UNICORE and its core building blocks.
Section 3 then evaluates the proposed architectural design with
respect to usability for end-users and performance measure-
ments. A survey of related work is described in Section 4,
while this paper ends with some concluding remarks.
II. DESIGN AND REFERENCE IMPLEMENTATION
This section introduces the core building blocks of the
COVS framework and its components for collaborative sce-
narios that are necessary in large-scale Grid infrastructures.
The intention of it is to describe how existing components
of the visualization and Grid community ﬁt into the designed
framework and how several components can be augmented to
provide a full functional COVS reference implementation for
end-users that meet the requirements that appear within Grids.
Figure 1 provides an overview of the reference implemen-
tation of the COVS framework’s architectural design. The
framework determines the architecture of COVS applications
and deﬁnes the overall structure by addressing the key re-
sponsibilities and the interaction between its components.
The main motivation of the COVS framework is to support
High Performance Computing (HPC) applications in the area
of e-Science and thus to be used as a tool for efﬁciently
solving complex scientiﬁc problems such as grand challenge
problems. In addition, the COVS framework must be inte-
grated seamlessly into the different Grid environments (e.g.
DEISA) by hiding the differences in security policies, systems
architectures, access methods and resource representations to
reach an overall transparency for end-users.
A. Addressing Collaborative Aspects in COVS
A COVS framework implementation in Grids allows for
an easier collaboration between geographically distributed e-
Scientists during data analysis. Therefore, the COVS design
raise a demand for a multiplexer entity (e.g. VISIT Multi-
plexer) that distributes the scientiﬁc data output from one
parallel simulation to n bi-directional connections that connect
the n scientiﬁc visualizations. This multiplexer represents a
novel component and can be considered as a key compo-
nent within the framework and interconnects the simulation
with n visualizations as shown in Figure 1. In addition, the
design relies on a collaboration entity (e.g. the new VISIT
Collaboration Server) that transports collaboration data (e.g.
turn of viewpoints) from one visualization to all other n-1
visualizations. Hence, the collaboration entity interacts with
all visualizations to ensure every participant shares the same
view on the data. Both the scientiﬁc and collaboration data
transfer have to be secured, for instance with an SSH tunnel
to avoid ﬁrewall problems. The framework provides the COVS
Grid service that controls the multiplexer and collaboration
server entities. That means a participant in the master role
is able to add and remove participants using the COVS Grid
services and is the only one that is able to submit/abort the
scientiﬁc simulation to/on the Grid. The session management
represents a major difference than single user control as well
the problem that appear when when one participant steers one
parameter to the right while another one steers it to the left.
We use an explicit request token mechanism (setsteerer()) to
ensure that only one participant is able to steer the simulation
at the same time. A similar mechanism is used to specify that
only one participant at the same time is allowed to change the
view (setcollaborator()). To sum up, the functionality of one
participant differs from the functionality of another.
B. Architectural Design to Support e-Science Applications
The architecture of the COVS framework is speciﬁcally
designed to support a wide variety of parallel simulations
and visualizations from numerous scientiﬁc domains that
both represent rather domain-speciﬁc core building blocks
of the framework. As often within HPC environments, such
simulations are typically implemented by using the Message
Passing Interface (MPI) standard or other parallel computing
paradigms. The parallel simulations that are used in conjunc-
tion with the COVS framework are submitted to the compu-
tational resource using a Grid client (e.g. GPE Grid Client
) and the underlying Grid middleware (e.g. UNICORE)
of the correspondent Grids (e.g. DEISA). In the context of
the online visualization of its outcome, this simulation must
provide data in a stepwise fashion to enable the visualization of
single computational steps. Hence, if the simulation provides
interim results, they can be transferred via a communication
library (e.g. VISIT) to the visualization and afterwards turned
into visualization idioms  by a visualization technology (e.g.
VTK) to show the result of the actual computation status. In
this context, a visualization idiom is any speciﬁc sequence of
data enrichment and enhancement transformations, visualiza-
tion mappings, and rendering transformations that produce a
display of a scientiﬁc dataset within a visualization application.
In addition to scientiﬁc data, steering commands must be
transferred from the visualization to the simulation and this
data transfer can be securely accomplished via bi-directional
connections over SSH, because most ﬁrewalls allow access via
SSH to the highly protected systems running the simulation.
The COVS reference implementation uses a technique for an
SSH tunnel establishment in Grids that was shown by Riedel
et al. in  and relies on an RSA-based authentication while
a Grid middleware performs an RSA key exchange.
C. Enable Collaboration with COVS Grid Services
Today, Grid services conform to Open Grid Services Ar-
chitecture (OGSA)  and typically implemented by using
the OASIS Web Services Resource Framework (WS-RF) 
standard. In particular, the WS-RF compliant COVS Grid
service represent another core building block of the COVS
framework. In the reference implementation, for example,
it is implemented as a higher-level service on top of the
UNICORE Atomic Services (UAS)  that provide basic job
submission/management and ﬁle transfer functionalities. In
more detail, the COVS Grid service consists of two WS-RF
compliant services as shown in Figure 1, namely the COVS
Factory service and the COVS Session service. The COVS
Factory service implements the WS-RF implied factory pattern
that is deﬁned as any kind of service that brings a stateful WS-
Resource (e.g. COVS session resource) into existence . It
can be used to create new COVS session resources while the
access to these resources is provided by the COVS Session
service. The COVS session resource expose the status of the
COVS session by using the WS-Resource properties  mech-
anisms. Figure 1 illustrates the COVS Session service that
consists of a MultiplexerAdapter that controls the multiplexer
entity. The COVS Session service provides operations (e.g.
RemoveParticipant()) that are forwarded to this multiplexer
using an XML-based protocol. Similar to the Multiplexer-
Adapter, the design comprises a CollaborationAdapter that is
also integrated as one component within the COVS Session
service. It can be used to control the collaboration server via
service operations (e.g. shutdown()) that forwards actions to
the collaboration server using an XML-based protocol. Any
information gathered by the collaboration server is forwarded
to the service and in turn converted into properties of the
COVS session resource.
To sum up, the scope of the WS-RF compliant COVS
Session service reaches from dynamic collaboration to au-
thorized session management control. Authentication and au-
thorization is provided by the Grid middleware. In the refer-
ence implementation, end-users of the COVS framework are
authenticated via their X.509 credentials at the UNICORE
Gateway  and authorized within the UNICORE User
DataBase (UUDB) . Finally, the next paragraph reveals
how these management capabilities for collaborative scenarios
are accessed via common open standards such as WS-RF.
D. Open Standards-based COVS Session Management
In general, a major disadvantage of Service Oriented Archi-
tectures (SOAs) such as modern Grids is that its core technolo-
gies (e.g. WS-RF over SOAP ) are typically not suitable
for a high amount of scientiﬁc data that is regularly trans-
ferred between the simulation and visualization. Therefore,
the COVS framework relies on SSH tunnels for bi-directional
connections, but uses the open standard technologies of SOAs
for the COVS session management as shown in Figure 1.
Fig. 2. GPE Application client with loaded COVS GridBean.
Also, Figure 1 illustrates that a end-user of the COVS
framework uses two applications at the client tier that is
the scientiﬁc visualization and a Grid client (e.g. GPE Grid
client). The Grid client is used to submit the scientiﬁc parallel
simulation to the Grid middleware (e.g. UNICORE) but is also
used for the SSH key exchange that is necessary to establish an
Fig. 3. Network infrastructure of the Grid testbed for the evaluations of the COVS framework implementation. The performance of the VISIT/SSH connection
(red) is of major interest since this connection transfers the scientiﬁc data from the parallel simulation (VISIT client) to the scientiﬁc visualization (VISIT
server). Furthermore this connection is responsible to transfer steering commands from the scientiﬁc visualization in nearly real-time to the parallel simulation.
SSH connection between client tier and target system tier. In
more detail, a dedicated COVS framework-speciﬁc client plug-
in (e.g. COVS GridBean) is responsible for that following the
mechanisms as described in .
The main goal of the COVS client plug-in is a sophisticated
GUI that provides the functionality to monitor and control
a COVS session using Web service message exchanges. To
provide an example, Figure 2 shows the COVS GridBean
of the reference implementation and its GUI in the context
of joining available COVS sessions that are exposed by the
COVS Grid service implementation within the Grid middle-
ware. Of course, the GUI also provides functionalities such as
connect/disconnect participants during the session run-time,
pause simulation/continue simulation or abort a session just
to list some. This functionality is conveniently provided via
pop-up menus to end-users of the framework.
III. DESIGN EVALUATION
This section evaluates the proposed architecture of the
COVS framework for Grid and e-Science infrastructures with
respect to different usability metrics, focussing on particular
on performance measurements of key components/protocols,
because this has a very high impact on the overall acceptance
of the framework by end-users.
A. Experimental Setup
This paragraph describes an experimental conﬁguration that
lays the foundation to examine the feasibility of the COVS
framework reference implementation. The experimental set
up and later evaluations depend on the particular deploy-
ment of the framework to allow for performance evaluations.
Figure 3 is part of the JuNet network infrastructure within
Forschungszentrum J¨ulich and illustrates a particular deploy-
ment of the framework. It emphasizes on the network inter-
connection between the machines running COVS components.
On the OSI-layer 3, JuNet is composed of various IP-
subnets that are interconnected by a central router (zam047-
168). Client- and server-machines are typically connected to
switches with 100 or 1000 Mbit/s Ethernet, depending on their
communication requirements. The end-user laptop (zam326)
and the UNICORE server (zam461) are located in the same
subnet and are attached to the infrastructure with 100 Mbit/s
interfaces. Therefore their communication does not traverse
the router. The login-node of the supercomputer JUMP, is
connected to JuNet via two channeled 1 Gbit/s interfaces.
Since JUMP is located in a separate IP-subnet, the laptop and
the UNICORE server communicate with JUMP via the router.
The testbed will be used for performance measurements.
The performance of the illustrated Web service message
exchanges can be disregarded since they are only used to
transport small XML documents via SOAP that are not data-
intensive. Also, the NJS-TSI protocol as well as the collabo-
ration and multiplexer server control protocol only transport
small pieces of text and XML over the wire. Hence, the
only data-intensive connection that is of major interest for
performance evaluations is the VISIT/SSH connection between
the end-user laptop and the supercomputer JUMP.
B. Performance of Bi-directional Online Connection
One of the key considerations within Grids is the secure
transport of information and data between users and resources.
In particular, it is one crucial point of the design since
interactive steering of simulations raises a high demand for
low latency to reach real-time behavior. Hence, in order to
provide sophisticated steering capabilities of parallel simula-
tions in a timely manner, the protocols used in the COVS
framework design must achieve low latency over secure bi-
directional connections that can be realized via SSH tunnels.
In this paragraph, we provide performance measurements to
indicate that the usage of the communication library within
Grid environments via SSH (VISIT/SSH) is still feasible
instead of using the plain VISIT protocol unsecured over TCP
(VISIT/TCP). In this context it is important to understand
that the performance or usability of the system relies on what
kind of low-level transport is used for data exchange. Because
Web services and XML provides ﬂexibility at the cost of
performance, the COVS framework uses Web services only for
session management but also provides a mechanism whereby
a high-performance connection can be used. Until now, TCP
was feasible in not distributed environment and the usage of
SSH within Grids is feasible since it provides ﬁrewall-friendly
secure connections. Hence, this distinction between manage-
ment information transfer (via Web services) and scientiﬁc
data transfer (via SSH) is a fundamental approach of the COVS
framework. If all is done via Web services, the latency is very
noticable to the human users and the bandwidth is signiﬁcantly
reduced due to the verbosity of XML or UNICORE internal
protocols as shown in an earlier approach .
In Figure 3, the VISIT communication library uses a
vttproxy component  that acts as a proxy for the visualiza-
tion located in the same security domain as the simulation.
The measurement presented in Figure 4 were performed
with a generic ping-pong program that is part of the VISIT
distribution. This program sends messages of varying length
from the VISIT client (i.e. parallel simulation) to the VISIT
server (i.e. visualization) and back to the client several times
and provides statistics for the latency. In particular, Figure 4
shows the latencies that are measured as half of the message
Fig. 4. Ping-Pong latency measured as half of message round-trip time using
VISIT/SSH and VISIT/TCP within the Grid testbed.
The Performance measurements between a Linux-
Visualization Client (1.7 GHz Pentium) and the login node
of the J¨ulich Supercomputer JUMP over the Grid testbed
(see Figure 3) result in an SSH connection startup time of
1.5 seconds, a ping-pong latency of 2299 microseconds and
a bandwidth of 84 MBit/s (for a message size of 1 MByte)
compared to 345 microseconds / 86 MBit/s for a direct
unencrypted VISIT connection via TCP between the same
systems. In this test scenario, the bandwidth degradation
from using the SSH-tunnel is only about 2.3%. While an
increase of latency by 2 milliseconds is signiﬁcant (a factor
of more than 6 in the testbed), it is less relevant in wide-area
networks, where the signal propagation delay is about 1
millisecond per 200 km, not including additional delays in
network-components as switches or routers.
UNICORE uses Web service message exchanges to collect
information for the establishment of the SSH connection or
to control collaborative sessions. Therefore, UNICORE only
affects the time needed to initialize SSH connections but not
the scientiﬁc data transfer over SSH itself. To conclude, the
COVS framework reference implementation establishes a se-
cure way of providing a bi-directional connection between the
simulation and visualization without critical loss of latency.
C. Collaborative versus Single User Control (Multiplexer)
The VISIT multiplexer is the essential new component
introduced into the data ﬂow to enable collaborative visu-
alizations. To measure its inﬂuence on the performance, we
have compared the throughput of messages of different sizes
with and without multiplexer and with varying number of
visualization clients attached in a testbed with gigabit ethernet
and without SSH. The results are illustrated in Figures 5
and 6. While the throughput per participant descreases with
growing number of prticipants the aggregate throughput (sum
of throughput of all participants) even increases due to better
utilization of the network.
Fig. 5. Throughput dependence on message size and multiplexer setup.
D. Analysis of Improved Usability Dimensions/Metrics
The usage of the COVS framework with typical HPC par-
allel applications in Grid infrastructures provides e-Scientists
with an improved usability that is extremely hard to measure
precisely. Therefore, this paragraph provides a closer look on
an example experiment that describes the PEPC simulation
 and Xnbody  visualization highlighting different
dimensions and metrics used in the analysis.
The ﬁrst dimension focusses on the domain/technical knowl-
edge of the end-users. Figure 7 presents a snapshot of the
VISIT-enabled Xnbody visualization used with the VISIT-
Fig. 6. Throughput showing the overhead/scalability of the multiplexer.
enabled parallel simulation PEPC, but without using the ben-
eﬁts a COVS framework implementation. Therefore, an end-
user has to manually provide all necessary details for the
connection establishment via VISIT (seappassword, seapser-
vice ) and the startup of the vttproxy to enable the transfer
via the SSH tunnel. That means an end-user must provide
aServicename (seapservice) and a Password (seappassword)
for the identiﬁcation of the visualization at the remote site.
Purely optional is the deﬁnition of the Interface to choose
from different network interfaces that may be available (*
for default). In addition, the end-user must provide a full
qualiﬁed Host and a Username on the remote machine. Most
notably, the end-user must know the exact path to the Proxy
(vttproxy) on the remote machine. This could be particularly
difﬁcult since the installation of the VISIT library on a remote
machine such as a supercomputer is usually undertaken by the
administrators of this machine and not by an individual end-
user. All in all, scientists that represent an end-user must know
a lot of technical details before they can connect to an ongoing
parallel simulation with this VISIT-based application. The pro-
visioning of the above described necessary pieces of technical
information can be much more automaticely managed by using
the COVS framework implementation presented here. When
using the PEPC parallel simulation and Xnbody visualization
with the COVS framework in collaborative scenarios all these
issues become transparent to the end-users by using the ’use
UNICORE’ checkbox. In other words, all the described pieces
of information do not have to be provided by the end-users
anymore, instead UNICORE provides all these details to the
scientiﬁc visualization via named pipes.
The next usability dimension demonstrates the improve-
ments in handling the complexity of collaborative scenarios.
As shown in Figure 7, typically more issues arise when
performing collaborative visualization scenarios and much
more connection details have to provided. First and foremost,
Fig. 7. Using the Xnbody scientiﬁc visualization without the COVS
framework implementation implies knowledge of technical details. A end-
user must manually provide all necessary information (zoomed red boxes).
Instead, when using the ’use UNICORE’ checkbox, all pieces of information
will be automatically provided by UNICORE.
all participants of a COVS session are identiﬁed with the
VISIT seapservice:seappassword combination. Hence, with-
out using the COVS framework, one participant must man-
ually conﬁgure the VISIT multiplexer by providing all the
different seapservice and seappassword combinations of the
geographically dispersed participants. This information must
be exchanged using out-of-band mechanisms such as EMail,
Telephone or Skype. In addition, this user must have the
knowledge which participants are allowed to participate in a
COVS session and thus must provide manual authorization
and authentication of participants. Hence, this can be particu-
larly difﬁcult when the number of participants is signiﬁcantly
increased. In addition, all end-users have to know the contact
information of the collaboration server as shown in Figure 7,
and if communicated over insecure networks, also the Host
and Username to establish an SSH connection to the host of
the collaboration server must be manually provided by the
end-users within the Xnbody GUI. COVS solves this using
Grid technologies so that all participants can conveniently
request for participation via the COVS GridBean and all the
necessary pieces of information are automaticly transferred,
which includes the automatic conﬁguration of the VISIT
Multiplexer and VISIT Collaboration Server.
Another usability metric is the simpler session management
interface. The leader of the session conveniently uses the
COVS GridBean GUI to manage the session and the GUI in
turn forwards operations (e.g. connect/disconnect participant)
to the underlying UNICORE Grid middleware using Web
service message exchanges. All this is hidden from all the
participants that just use the GPE Grid client with the COVS
GridBean. Even the use of the Grid client itself provides
improved usability by providing a convenient way to submit
jobs to remote resources.
Another metric for improved usability is the simpler inter-
face for authentication. The authorization and authentication
of end-users is done automatically by the Grid middleware
and retains the important single sign-on feature of Grid en-
vironments. Thus, instead of providing several passwords for
remote hosts, only once the keystore of the Grid middleware
must be unlocked via one password to gain full access.
To sum up, the transparency of Grids is the overall goal
of using the COVS framework with scientiﬁc parallel ap-
plications. Its improved usability legitimates the use of the
framework by e-Scientists in real application use cases within
production Grids, because the efforts of scientists for connect-
ing to a remote simulation is signiﬁcantly reduced as described
by the different usability metrics. Furthemore, several Grid
infrastructures, for instance several sites within the EGEE
Grid, map certiﬁcate identities to pool accounts which leads
to the fact that a username and hostname can not be known
beforehand for static manually conﬁgured SSH tunnels. In
such scenarios, the COVS framework provides capabilities to
establish an SSH tunnel to the remote site and thus also to use
applications in highly dynamic environments.
E. Support for End-users of the COVS Framework
The fundamental idea of the COVS framework is to provide
e-Scientists with a tool that is easy to deploy and use by
avoiding work that is not related to their own scientiﬁc
area or application code. Therefore, this paragraph evaluates
what end-users actually have to do when they want to use a
COVS framework implementation. Thus, it is clariﬁed which
components must be already provided by a COVS framework
implementation and which components the end-users have
to provide in which form. Needless to say, it is important
to evaluate if the work that end-users have to invest to use
the COVS framework is feasible and can be expected to be
accepted in production Grids today.
For the clariﬁcation of these questions it is worth looking
at real production Grids such as D-Grid  or DEISA.
Both infrastructures have already plans to move to Web
service-based Grid middlewares (e.g. UNICORE 6) in the
near future. Besides the core Grid services (e.g. job submis-
sion and management, ﬁle transfer, and storage), additional
higher-level services such as COVS Grid services can also
be deployed within the Grid middlewares. Hence, the core
building blocks Grid middleware and thus COVS Grid services
will be automatically provided by the e-Science infrastructure.
The deployment of the COVS Grid services implies the
conﬁguration and installation of the dedicated communication
library it is based on (e.g. VISIT), including its multiplexer
and collaboration entities. Furthermore, the Grid client will
also be provided to gain access to the infrastructures.
To conclude, a deployed implementation of the COVS
framework design provides the most core building blocks for
end-users in a ready-to-use form, only the scientiﬁc area-
speciﬁc parallel simulation and visualization must be provided
by the end-user. This implies, that both components must be
instrumented with communication library-speciﬁc calls (e.g.
VISIT server and client calls) in order to enable the usage of
the COVS framework. Many parallel e-Science applications
already have visualizations that are based on post-processing
techniques. Hence, the real work that end-users have to do
is to instrument their own code with communication library
calls to enable the data and steering command exchange.
Finally, end-users of a COVS framework implementation
must request a personal X.509 certiﬁcate at the corresponding
Certiﬁcate Authority (CA). Using such a certiﬁcate allows end-
users to gain access to the infrastructure and its resources
via the Grid client (e.g. GPE Client) and also to the COVS
Grid services when loading the client-speciﬁc plug-in (e.g.
COVS GridBean) into this client. However, this is a general
demand for end-users that want to use resources within Grid
infrastructures and not a COVS framework speciﬁc issue.
IV. COVS FRAMEWORK USER COMMUNITIES
The COVS framework implementation is used by the
ASTRO-Grid D community Grid within D-Grid in the context
of the nbody parallel simulation code. Furthermore, it is used
with the PEPC parallel application at the John von Neumann
Institute for Computing (NIC) in J¨ulich in conjunction with
the Xnbody visualization (see Fig. 7). This demonstrates the
adoption by user communities that use the framework to sig-
niﬁcantly increase their analysis of scientiﬁc data provided by
nbody or PEPc through collaborative sessions with participants
ot the whole VO.
V. RELATED WORK
There is a quite a lot of related work in the area of
visualization and steering technologies in Grid infrastructures.
The UK RealityGrid project provides a steering library that
enable calls which can be embedded into its three components
that are simulation, visualization, and a steering client. Re-
cently, prototypes of this library are renewed to be conform to
OGSA. In comparison to the work presented here, the COVS
framework is loosely coupled to the Grid middleware while
the recent efforts around the RealityGrid steering library are
focusing on its tighter integration into the Imperial College
e-Science Networked Infrastructure (ICENI) .
Another well-known system is developed within the Aus-
trian Grid initiative. The Grid Enabled Visualization Pipeline
(GVID)  provides high quality visualizations of scientiﬁc
datasets on thin clients. In more detail, the data of the scientiﬁc
simulations are efﬁciently encoded with the H262 code into a
video stream and transferred to the thin client afterwards. The
client, in turn decodes the video stream for visualization of
the scientiﬁc data. The system also offers steering capabilities
similar to the approach within this paper, but realized via so-
called Event-Encoders that run on the thin clients and sent
steering commands to the simulation. However, the major
difference to our approach is that it is not seamlessly integrated
as one higher-level service into a common Grid middleware.
NAREGI provided an API that consists of a visualiza-
tion library and a Grid visualization service API . The
visualization library can be used to connect simulation ap-
plications by the support of multiple visualization function-
alities. The visualization service API wraps this library to
provide Grid service functionality that are a set of WS-RF
compliant services, for instance Coupled Simulation Services,
Post-Processing Services, or Molecular Visualization Services.
Even if this approach is using the WS-RF standard similar
as our approach, the internal architecture is rather different.
To provide an example, the scientiﬁc data as well as its
rendering is completely computed within the Grid that ﬁnally
is represented by a compressed image. The COVS framework,
on the other hand, sends the scientiﬁc data in an online
connection to the client for rendering and to allow for accurate
steering of result parameters.
Finally, Brodlie et al. describes in  a well known
rather high-level framework for collaborative visualization in
distributed environments, while our contribution is much more
oriented and closer to production Grid scenarios today.
This paper introduced the reference implementation of the
COVS framework design by using the UNICORE 6 Grid
middleware and the VISIT communication library. The eval-
uations have shown that the choice of the communication
library is a crucial step for the implementation of a COVS
framework since this core building block has dependencies
with all others. Most notably,the implementation of the COVS
framework can be used by all visualizations and simulations
that rely on the selected communication library. Nevertheless,
the communication library can also be replaced by another
library that provides similar capabilities such as bi-directional
online connections, scalable data multiplexers and collabora-
Also, the Grid middleware is an important cornerstone
and the proof of concept implementation with UNICORE 6
indicates that the COVS framework could be, in principle,
implemented in any WS-RF compliant Grid middleware. This
open source implementation of the COVS framework was
successfully demonstrated at the EuroPar 2006 conference,
at the Fujitsu UNICORE booth at OGF18 in Washington,
at the Supercomputing 2006 conference in Tampa and in a
visualization and steering session at OGF19 in Chapel Hill.
Furthermore, it was demonstrated to end-users at a DEISA
Training. More recently, Intel ﬂyers use the COVS framework
implementation presented within this paper for marketing of
their open source GPE client suite.
Nevertheless, deploying the proposed COVS architecture is
an important next step to broadly incorporate implementations
of the COVS framework into production Grid environments.
The reference implementation described here relies on the WS-
based UNICORE 6 middleware. UNICORE 6 will be soon
evaluated by the DEISA and D-Grid Grid infrastructures for
production usage. When these production Grids shift their ac-
cess methods from UNICORE 5 to UNICORE 6, the reference
implementation of the COVS framework can be also deployed
as one higher-level service for production usage. In general,
efﬁcient usage of computational resources by using COVS
and thus beneﬁcial steering technologies must be improved
with the goal to incorporate such steering tools into the usual
workﬂows of e-Scientists. Once an implementation of the
COVS framework is deployed within production Grids such
as DEISA or D-Grid, an important tool for a efﬁcient use of
Grid and e-Science infrastructures is accomplished.
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