A Reconfigurable Component-Based Problem Solving Environment

Technical Report DHPC-090
A Reconfigurable Component-based
Problem Solving Environment
K.A. Hawick
Distributed and High Performance Computing Research Group
School of Informatics, University of Wales Bangor
Dean Street, Bangor, Gwynedd LL57 1UT, Wales, UK
Email: hawick@bangor.ac.uk
H.A. James and P.D. Coddington
Distributed and High Performance Computing Research Group
Department of Computer Science, University of Adelaide
Adelaide, SA 5005, Australia
Email: {heath,paulc}@cs.adelaide.edu.au
1 June 2000
Abstract
Problem solving environments are an attractive approach to the integration of calculation and
management tools for various applications. Scientific and engineering applications are becoming
increasingly demanding in terms of both problem size and problem complexity and therefore
benefit from the integrated problem solving approach. However such applications often require
high performance computing components in order to be computationally feasible. It is therefore
a challenge to construct integration technology, suitable for problem solving environments, that
allows both flexibility as well as the embedding of parallel and high performance computing
systems. Our DISCWorld system is designed to meet these needs and provides a Java-based
middleware to integrate distributed computing component applications across wide-area net-
works. Our system supports both long running simulations as well as parameter-search style
applications where many shorter “runs” are used to explore the parameter space of a scientific
or engineering model. Key features of our design are the ability to access remotely stored data;
compose complex processing requests either graphically or through a scripting language; and
execute components on heterogeneous and remote platforms. Operators in task graphs can be
slow (but portable) “pure Java” implementations or wrappers to fast (platform specific) super-
computer implementations. In addition, some operators may represent compound services that
can potentially be expanded into a sub-graph of simpler operators, allowing the simulation to be
reconfigured to run across multiple compute servers if this will lead to a faster execution time.
Our system also supports efficient access to distributed and hierarchical data stores. User’s
processing requests can also be optimised by reusing data that has been created by previous
requests. We describe our prototype and use it as a vehicle to discuss key issues for problem
solving environments in science and engineering.
Keywords: problem solving environments; DISCWorld; high performance computing; grid
computing; distributed data; Java; technology integration.
1

1 Introduction
Problem Solving Environments (PSEs) provide a way of integrating complex computing technologies
so that users need not be computer specialists. Collaborative PSEs (CPSEs) extend this goal so
that groups of users can work together across distributed computing systems without needing to be
aware of the system details. Users can focus on the application domain expertise needed to solve
their problem while the distributed computing “glue” or middleware that makes the system work
can remain transparent to them. This goal of transparency is however rather difficult to achieve and
this is particularly so when a degree of change in the computer system configuration is required for
problem domains. This change or “reconfiguration” in the system is required in all but the simplest
of real world applications. Application scenarios illustrating the need for reconfiguration in CPSE
are described in section 2.
Reconfiguration will typically involve changes to the collaborating users and program sequences
being run in a CPSE. The running system must be able to handle these changes – which may be
unpredictable – gracefully and should not leave broken pointers or references to objects (programs,
data or users) that have changed.
Our solution to the reconfiguration problem in CPSE involves a software architecture and a
prototype implementation for a distributed computing middleware system known as DISCWorld.
We have designed two critical mechanisms in our DISCWorld system for managing remote data and
services in a data flow model as well as a “futures” mechanism for managing data and services that
exist only in potentia when the request for them is first made. We have found these mechanisms
provide a model capable of robustness and stability in a changing runtime environment and also
allow exploitation of parallelism in complex jobs.
A key feature of our architecture is the recognition that the system can be managed in terms
of data processing operators with data items flowing between them. In particular a human user
interacting with the system can be modelled as a particular sort of operator with extreme latency
and reliability properties. Following the standard task graph approach of dataflow systems, we model
applications in terms of Directed Acyclic Graphs (DAGs) of operators at the nodes and simple data
flow along the connecting edges. We describe the essentials of this reconfigurable DAG model in
section 3.
DISCWorld is outlined in section 4 where we explain the DISCWorld Remote Access Mechanism
(DRAM) and DRAM Futures (DRAMF) mechanisms and how these embody the operator/data
control ideas and their expression as DAGs. We have built a number of prototype CPSEs using
our technology, mostly for decision support applications involving access to distributed archives of
geospatial data such as aerial photography, satellite images and mapping data. However we have
focussed mainly on developing the server-side technology to support CPSEs, since we believe that
determining the fundamental architectural principles and interface specifications will be critical to
genuinely wide-area, open, and scalable CPSEs. An objective of our work is to thoroughly explore the
technical issues necessary to propose draft standards for interfacing between distributed components
of CPSEs.
The idea behind users collaborating through shared access to programs, data and each other
has been prevalent for some years [3]. Users on a networked computer system have found ways
to interact using ad hoc mixes of computer technologies. These include email; shared networked
file systems; remote logins to other users’ computers and more recently the use of desktop video
conferencing facilities. Difficulties have arisen in the growing complexity of operations and data that
users wish to share. Particular obstacles have been the plethora of different data formats even for
simple data exchange; the lack of portability of programs across different computer platforms and
the lack of interoperability between programs. Research into CPSEs and their supporting technology
will hopefully open up many new and significantly more complex interactions between users across
disciplines.
2

2 CPSE Application Scenarios
In this section we describe some of the application areas we have considered and how they benefit
from our approach to Collaborative Problem Solving Environments (CPSE).
CPSEs are a much sought after solution to the needs of human decision makers to share data,
resources and ideas. Such decision support systems are very important for a wide variety of organi-
sations, in the commercial, government and defense sectors. A classic example is a defense command
and control system that will involve collaboration between many decision makers in different geo-
graphical locations, and data inputs from many sources, including real-time data from the field of
operations as well as data from a number of distributed data archives. In the government sector,
emergency services command centres have a similar functionality and require similar collaborative
decision support systems. Other examples include areas such as environmental analysis and mod-
eling and land use planning, which may require data and specialised analysis from many different
government departments.
We have developed some prototype CPSEs for both defense command and control [6, 8] and
emergency services response [7,18] systems. These are technology demonstrators that illustrate how
powerful Java-based object-oriented CPSEs can be developed using the DISCWorld grid computing
architecture. Our applications have focussed on the use of geospatial (i.e. spatially referenced) data
such as maps, satellite images, aerial photography, elevation, vegetation cover, etc. A typical decision
support application may require large amounts of data from a number of distributed geospatial data
archives.
We have worked with the Australian Defence Science and Technology Organisation (DSTO) in
building systems for geospatial imagery analysis and exploitation [6, 8]. A fundamental aspect of
defense command and control systems is the requirement for geospatial data such as maps and
images from earth observation systems. This requires a collaborative environment linking image
data archives, image processing applications and compute servers, specialised image analysts, and
the decision makers who use the image data. In many situations the image analyst or the decision
maker may want on-demand access to an image for a specific region, and may also request that some
specialised image processing, such as feature detection or map overlay, be done on the image. We
have developed a prototype software environment that supports on-line distributed data access and
processing across a federated collection of geospatial image archives and image processing services.
Figure 1 shows a simple demonstrator system we have built to allow catalog access, process
control and shared access to annotations and other collaboratory information. The architecture
underpinning this demonstration is described in sections 3 (basic reconfigurable dataflow model)
and 4 (software implementation). We focus on the problems of manipulating shared imagery in
CPSEs for the purposes of illustration in this paper, but the data and operators shared in our
architecture are not limited to this area.
Another demonstrator we have developed is a prototype system for geospatial data collection,
visualisation and processing to support emergency services response [7,18]. The example application
we have targeted is response to a wildfire emergency. This requires coordination of a large team of
people including firefighters, police and paramedics at the scene and an emergency services response
unit at a central command centre.
Our prototype system is based on a Web-based Java applet interface connected to a server
that handles input from multiple data sources and manages state information so that all users
can see the same information on screen via the applet interface (this is similar to the approach of
the Habanero [24] and TANGO [25] systems for Web-based collaboration). For example, airborne
imaging systems can transfer information on the extent of the fire front; police and firefighters in
the field (who may be working on a laptop with a modem connection via a mobile phone) can add
information on current positions of fire trucks and roadblocks; decision makers in the command
center can provide recommendations on where to attack the fire or set up evacuation points. Our
3

Figure 1: This figure shows a prototype image analysis and processing tool that enables users to edit
and annotate a geospatial image in a collaborative setting. All users see the results of the changes,
and they can add their own annotations or comments to the collaborative effort. The image, along
with the annotation transcript, can be saved and replayed for later viewing by a decision-maker.
system allows users to overlay many types of geospatial information including roads, terrain elevation,
vegetation types, satellite images and aerial photography, positions of emergency services personnel
and equipment, and the extent of the fire front. It also allows users to access data processing
services, including the ability to run fire simulation models to estimate the spread of the fire, least
path algorithms to suggest optimal routes for emergency services vehicles, and algorithms that use
roadmap and population density information to suggest evacuation strategies.
We are presently considering other application areas such as those in collaborative science and
engineering where we can provide a collection of component operators and can experiment with how
collaborative users can best deploy them. Typically users wish to steer large and complex simulations
or calculations; visualise complex datasets; or carry our parameter searches, coordinating the results
of many runs or jobs. A CPSE which allows both graphical and scripting control is a good solution
to this problem.
Figure 2 shows a fundamental idea behind our approach. Collaborating users can combine
reconfigurable component operators into processing tasks, sharing data, operator codes and results
with one another. Users can compose component operators graphically, but the system uses a
scripting language representation of the graphical composition so that it may manipulate, decompose
and schedule parts of the computation.
A key issue which has emerged from all these application areas is the need to handle complexity
and change in a running system. We believe a number of proprietary systems have provided superfi-
cial solutions to the graphical CPSE problem but none have based their system on a well-considered
architectural model capable of supporting reconfiguration.
4

Figure 2: Simple graphical operator composition. Available operators are listed in the palette on
the left-hand side of the display. Users select the appropriate operators by dragging them onto the
canas at the right-hand side of the screen. When operators are released onto the canvas they are
drawn in boxes with smaller rectangles representing the inputs they require and all the outputs they
produce. Operators are joined by the user; the joins represent data flow between operators. When
executed operators may be placed onto different machines in the distributed system.
3 Reconfigurable DAGs
We model all user processing requests in our system with Directed Acyclic Graphs (DAGs). A DAG
is a graph consisting of nodes (operators) and vertices (data passed between operators). It is directed,
meaning that data flows in one direction only. Furthermore, the graph is acyclic, prohibiting the
presence of cycles within the structure. This allows us to treat the data items flowing between
operators simply; we do not have to use any advanced dataflow mechanisms such as token colouring
to distinguish between data originating from different iterations through the same graph [31].
The Directed Acyclic Graph (DAG) model we employ for expressing combinations of program
components is a simple but powerful model. It is used in a number of other systems such as Java’s
Advanced Imaging (JAI) [32] toolkit and also some standards interfaces for distributed imagery ac-
cess and exploitation [35,36]. It allows us a well-defined mechanism for configuring and reconfiguring
operators at runtime. This is particularly useful when an operator may be an interactive program
component – representing the “human in the loop” in a CPSE.
Figure 3 shows a parameterised operator component as employed in our model architecture.
Operators have none or more inputs and one or more outputs. Parameters can be raw data, in
which case they have no “producing operator” or derived, in which case they do.
An input can be modelled as either an edge or as a parameter. For the purposes of illustration
we use both to explain our system. The embedding context for operators can supply parameters
5

Parameter
(Property)
(lazy or eager)
edge input (incoming data)
queue size = 1
edge output (outgoing data)
queue size = 1
property or argument)
Operator (parameterised by
Figure 3: Parameterised operator showing data flow along edges for input or output.
and in a sense these are the same as special parameter-type input edges. This is shown again in
figure 4 a) and b) where an equivalent DAG can be set up with two special input edges and two
parameters or with four input edges and no parameters. This point is important for considering
humans interacting with components. Humans will typically interact with a GUI component and
will typically input parameters by typing or clicking a mouse or through some other input device.
Our point here is that while the front end environment will have to provide the necessary support
infrastructure for these input parameters, we can still model the dataflow process as though there
were no parameters and just simple data flow edges.
Operators could be relatively simple program components such as: read an image from store;
display an image; crop an image producing a new one; superpose two images producing a third;
split an image into high and low pixel intensity values producing two new images. Alternatively
operators could be entire complex programs involving a large set of inputs and/ or outputs. We
describe how we implement operators either as reusable (Java) components or as (Java) wrappers
to complete programs in section 4.
There are a number of component primitives that can be used to model the various activities in
the CPSE. Figure 5 shows the five principle ones. We can implement base component classes for
these five: source; sink; filter; fork/split; and merge/join. In a simple system where we employ only
one base level data type such as an all encompassing base image class, it is very simple to construct
DAGs from operators based on these five primitives. Any output can feed onto any output since
the data type is the same. It is surprising how many simple but useful systems can be built on
this assumption. Allowing different data types makes the model considerably richer, but it becomes
more difficult to establish rules for interconnecting operators to ensure type safety.
Systems like AVS [2] provide a graphical solution to the type connectivity problem. The front
end GUI colour codes the different data types present in a particular graph and enforces connections
only between matching colours. This solves the problem partially for relatively simple systems but
not for examples where two inputs to an operator are of the same type but have different semantics.
There are considerable arguments one can get into about how rigorously the type system needs to
be defined in such cases. For our experimental system we do not claim to have solved this problem.
This underpins one of the key issues for wide-area CPSEs however. It is necessary to decide how
6

Param 3
Param 1 Param 2
Param 4
of DAG
Param 1 Param 2
Param 4
Param 3
Param 4
Param 3
Param 2
Param 1
Encapsulation
a) b) c)
Figure 4: a) Parameterised DAG using single edge type and b) Equivalent Unparameterised DAG
using differently typed edges and c) Rubber Banding to encapsulate a DAG
users and their systems will exchange both data and operator components that they did not write.
There is a clear need for some interface standards for such components if CPSE users are to have
the freedom to access and exchange libraries of component services. This issue is discussed further
in section 6.
Interactive users of a CPSE will construct complex operations or queries to their system con-
sisting of a choice of a palette of operators drawn from libraries belonging to themselves or others.
Interactions and changes in the system environment due to users’ own inputs or those of others in
a CPSE will require the system to be dynamically reconfigured. This reconfiguration is needed to
support the need to change and allow late change of the DAGs which express the computations [20].
Figure 4 shows a simple DAG expressed as either a two input DAG with two parameterised
operators or as a an unparameterised four-input DAG. It also illustrates one of the important
concepts in our system whereby an entire DAG can be collapsed down to a single sub-graph or
indeed down to a single operator, in this case a four-input or four parameter operator.
The collapse of DAGs in this way supports the idea of combining useful operators or DAGs
into reusable components. An underlying issue is how operators and DAGs are named. Naming
and binding to data and operators is a difficult problem. We have made some progress using
a simple name space where every data item and component is assumed to have a unique name.
Implementation of this assumption is generating interesting research as there are various possible
algorithms and models to enable this [10].
Collapsing the sub-graphs into operators is important to allow users to work on large scale
and interesting problems and to reuse the components previously constructed by them or their
collaborators or available in libraries or databases. We have experimented with the concept of a
“CodeServer” [23] which can be remotely queried using metadata and which will deliver a suitable
operator packaged as executable Java bytecode. This allows programs as well as data to be easily
migrated around a wide-area network. This can be very useful for sharing modules in collaborative
work, and also for PSEs that involve processing of large distributed data sets, where it is more
7

Split/Fork
Source
Sink
Filter
Merge/Join
Figure 5: Primitive operator components: source, sink, filter, fork/split and merge/join. Operators
including humans interacting with component modules can be modelled as combinations of these
primitives.
efficient to move the program to the data rather than the data to the program. The main issue
underpinning the success of such a system is how to treat and standardise the metadata describing
coded operators.
Figure 6 shows the converse idea of expanding an operator or part of a graph into a sub-graph in
its own right. These collapsing and expanding ideas are vital for producing a powerful reconfigurable
system. Expanding operators is useful to decompose complex operators into parts that can be easily
farmed out to achieve parallel execution for performance reasons, or indeed to allow decisions to be
made by different computers or users in the collaboration network as to how a job should actually be
scheduled. This latter point is vital for both scalability and reconfiguration reasons in a distributed
system.
One reason for using CPSEs is to be able to exploit and share remote resources that may be
owned by collaborators. These are often specialist or high-performance resources and access to such
scarce resources is often a major driving force behind collaboration. It is therefore evident that
efficiency and scalability of the underlying distributed computing software infrastructure are also
important issues for a CPSE.
Figure 7 shows how a DAG consisting of many operators can be executed in different orders
– lazily or eagerly – resulting in different execution profiles. The flexibility to allow this sort of
reconfiguration is particularly important in a system involving human interacting components where
it may be important that other components execute as independently as possible of a slow and
unpredictable human. The flexible execution profile allows system control components to make
sensible scheduling decisions [21] about when and where to execute operators.
Scheduling is important for accessing data resources as well as computational ones. An applica-
tion area we have studied at length involves access to satellite imagery and bulk data sets that may
be too expensive or simply too large to store in their entirety at more than one or two locations.
These data sets might be accessible only via tape robots or in some cases through human operators
who are involved in some way in loading and unloading tape sets. In a sense the tape or resource
8

Vertex (operator)
Expand as...
Directed Acyclic
Graphs (DAGs)
or operator networks
Edge (data)
Figure 6: DAGs can contain compound services, which are expanded at run-time. This is possible due
to the lazy evaluation of services in the DISCWorld environment. Compound services are represented
by their own self-contained sub-graph, which can in turn contain other compound services.
operator is a human operator in the CPSE. We are investigating how storage services can be man-
aged separately in a transparent fashion from other computational aspects of CPSEs. Our approach
is to use file system software technology to make remote data and hierarchical storage systems such
as cached tapes and tape robots appear as remote file systems or databases [28].
In this section we have discussed the model on which our architectural approach is based. We
now discuss how we have implemented this model in a prototype of our DISCWorld system.
4 DISCWorld - An Outline
Our Distributed Information Systems Control World (DISCWorld) project was started in 1996 with
the long term goals of researching the underpinning issues for a metacomputing system. Recently the
popular term “Computational Grid” [12] has superceded that of metacomputer but the ideas remain
the same – providing a high-level middleware system that enables transparent access to high-end
compute servers and data storage distributed over a wide-area network. We aim to further explore
the algorithmic and systems issues involved in building, running and maintaining such systems, which
form the basis of PSEs. CPSEs extend the goal to cope with more interactive multi-user systems,
but as explained above we believe that interacting users can be modelled as operator components
in our system.
The key to a working system is the software design of the DISCWorld servers, the protocols they
use to communicate and the interoperability mechanisms with legacy applications and data systems.
9

H
A
C D
E F G
I J K
L
M N
O
B
H
A
C D
E F G
I J K
L
M N
O
B
a) b)
Figure 7: Two different orderings of execution of the operators in a DAG
We have approached the problem by placing a DISCWorld daemon on every server that participates
in a DISCWorld community. The software for the daemon is intended to be a minimal framework
for loading up customised modules for specialist operations. These may be Java [33] byte code or
wrappers to specialist native platform codes. The design is motivated by a need for scalability to
a large number of cooperating nodes, with as little central decision making as possible. The local
administrators or owners of a resource participating in a DISCWorld can decide what services they
will offer by setting various policy and environment information in its local database.
DISCWorld acts essentially as a software glue, providing interoperability between new and ex-
isting services. Figure 8 illustrates the key ideas behind the DISCWorld daemon “DWd”. A dual
network is present between all servers in principle. A control network is used to communicate small
lightweight messages that specify how servers are interacting and cooperating, and a bulk data trans-
fer mechanism is used to transfer data products and partial products between nodes. In practice
these mechanisms may be implemented on the same hardware network, or on separate networks
when available. The control information is lighter but needs a reliable network, whereas the bulk
data needs high bandwidth but can always be retransmitted if a network is blocked. Daemons are
used to provide all the necessary control information to wrap up legacy codes and systems as well as
being a framework for interfacing to new codes. We have also built a file-system based mechanism
to provide the support services necessary to support legacy applications for which we do not have
source code access [27]. Software adapters are used to bridge firewalls as well as interface to other
metacomputing software systems.
The daemon’s operations are best explained by a discussion of the sequence of events that take
place when a client contacts it.
• A user query is posed to the daemon (through a CPSE front end), in the form of a well defined
request for some computation to be performed on some data (using the DAG/operator model).
• The request must be mapped onto the available resources (operators mapped to particular
compute systems) that the daemon knows about and even though the daemons are peer-based,
10

DWD
Browser
WWW
Legacy Application P1 P2 P3
NFS
HFS
Control Channel
Bus Adaptor
Custom
Application
Launch
Other nodes
TCP
Bus Adaptor
Bus Adaptor
TCP
Firewall
Bulk Data
Transfer Bus
DISCWorld Daemon (DWD)
DWD
DWD
DWD
Figure 8: DISCWorld Software Architecture Overview
the daemon in question acts as a manager for the purpose of brokering this request.
• Management of the request consists of monitoring and supervising the ongoing request once
it has been initiated. The daemon must also provide the user with an ability to revoke the
request should it be necessary.
• The daemon is also responsible for brokering the safe delivery of the results to the user, or
temporary storage of the results and notification of the user. It can also organise the collection
and storage of performance data for the purposes of predicting future performance and costing.
The daemon is initiated as a program by the system administrator or as a system daemon and,
once initiated, it reads some local state information from a persistent store (e.g. database) to
configure itself and subsequently listens to a particular port for user requests. Each daemon has a
“gossip” mechanism to allow it to determine what services and hosts are available to it. Each daemon
stores its own internal database of servers offering component services as well as their associated
characteristics, and maintains a priorities list according to some locally defined policy.
The daemon is highly configurable, and can be customised to reflect the local server/services it
provides to other DISCWorld daemons. It also has a configurable policy for where to seek assistance
from its peers in satisfying incoming user queries.
DISCWorld was primarily motivated to deal with relatively large computations that need to be
carried out on large datasets, and to allow use of distributed high-end computing, communications
and storage resources to execute these problems in a timely fashion. This is particularly useful
for decision support systems and CPSEs, which generally require timely access and processing of
multiple distributed data sources.
The major services/servers that each DISCWorld daemon knows about were originally substan-
tive programs running on high-performance platforms. Nevertheless, the control information and
“glueware” needed to build such a system are complex but not so demanding of computational per-
formance. We have based the development of our CPSEs on the DISCWorld environment, which has
been written in Java. Whereas up until now we have mainly implemented operators as Java wrappers
11

around high-performance legacy code, the rapid improvement in Java Virtual Machine performance
has prompted us to build more component services using pure Java. A Java implementation allows
for a number of capabilities that can be readily implemented using the Remote Method Invocation
(RMI) Java package.
The multi-threaded mechanisms in Java allow the daemon to be implemented with a prioritised
set of threads to handle the user and server-server interactions. The use of Java threadgroups
coupled with the Java security manager mechanism enables threads to be placed into groups with
controlled access between the groups. This enables the daemon control threads and groups of user
threads to be kept apart and scheduled separately within the daemon. The daemons communicate
via typed messages. These are received at a central point within each daemon, allowing messages to
be routed to the appropriate module. Some of these modules within our current daemon prototype,
include a server/services database manager, a system administration console manager, a computation
organiser and a local execution manager. These only accept control level messages from the central
point, although the DISCWorld model allows for a high bandwidth direct communication channel
between modules. Control messages contain instructions for a single module, and if modules within
the same daemon wish to communicate they must sent messages via the central point.
The use of the Java object serialisation and dynamic loading and binding allows arbitrary data
structures to be passed between both servers and clients in the DISCWorld. The code for these data
structures may be loaded dynamically. This includes operations on the data structure which means
that the same data structure can have different method implementations depending on the data
contained within – thus allowing polymorphic operations. At present, serialisation is limited by the
need for both communicating parties to use Java, which requires Java wrappers around non-Java
code. Wrappers can be done using the Java Native Interface (JNI) API. We believe the additional
effort of implementing Java wrappers is outweighed by the ease with which objects with state can
be stored and communicated.
Using Java coupled with native methods to embed other programs does not mean that the system
itself is no longer portable. The glue that controls the system remains portable although the native
method services will not be portable. This is in keeping with our intention that a particular daemon
should be configurable by the local administrator to reflect the locally available server/services.
In fact, a limited portability can be achieved by careful loading of the native library for some
applications. The library that can be loaded can be dependent of the architecture and operating
system version, which information can be made available to the daemon as part of its configuration.
This is not completely dependable, however, and it is an attractive possibility to find a mechanism
whereby a growing collection of application components can be maintained as portable modules.
The Java Bean mechanism is a useful way to approach this. Beans are essentially codes conform-
ing to a strict application programming interface (API) through which the system can manipulate
them. Native methods as well as Java code and also data can be encapsulated as Java Beans. We
use the Java Bean mechanism to implement our portable code modules which are saved on a code-
server [23] and are transmitted as high level instructions to servers at the request of clients. Use of
well-defined APIs for components means that they may have a basic portable Java implementation,
and an optimised version using native code, or even parallel code for a high-performance parallel
computer, all of which can be invoked by the CPSE using the same interface.
The DISCWorld model for distributed, high-performance computing is readily implemented in
Java. Performance was originally achieved primarily from embedding native application components
in a Java software glue rather than from code running in Java itself. However, Java Virtual Machine
(JVM) implementations have improved sufficiently so that high performance Java application com-
ponents themselves can be utilised in a distributed environment. We have investigated some critical
benchmark operations such as numerical analysis and image processing operations in pure Java and
as native methods on various supercomputer platforms which can act as performance accelerators
or specialist compute servers in a DISCWorld environment [22].
12

DISCWorld therefore provides us with a base platform architecture upon which CPSEs can be
constructed. Two architectural aspects of DISCWorld are worth explaining in more detail and
which underpin the DAG model described in section 3. The DISCWorld Remote Access Mechanism
(DRAM) provides a rich pointer to allow references between operators and data in the running
system. These are used to represent the edges in the DAG and need to be carefully controlled if
the system is to be reconfigured. This is particularly important for long running computations or
in the case of partial failure of a CPSE due to network vagaries or indeed some users going offline
deliberately, such as reaching the end of their work shift. The DRAM facilities and its associated
mechanism for data futures (DRAMFs) [19] provide a general architectural model for specifying how
the system behaves under reconfiguration.
4.1 Remote Access Glue
The DRAM is a rich pointer because besides containing an object reference, it contains additional
metadata that can be used, for example, to decide whether the data can be moved between machines,
or whether it is more feasible to move the data to a remote machine for processing, or to move the
service to the data. DRAMs are unlike other, more traditional approaches to object references; they
do not refer to any memory locations, therefore they are not fragile in the presence of remote server
restarts [17]. This is particularly useful in a collaborative environment as there are no guarantees
that the machine from which the data is being sourced will stay up for the entire collaborative
session. In addition multiple references (DRAMs) can be created to the same DISCWorld object,
thus all the members of a collaborative team can receive updates to the object when changes are
made.
DRAMs can be used to point to data and services. Part of the metadata contained within a
DRAM describes its bulk characteristics, such as the size (in bytes) of the data or program byte-
code. In addition DRAMs for services (DRAMSs) contain rudimentary information on the run-time
characteristics of the services to which they point. These characteristics can be used to create
references to data which has not yet been created. We name these DRAM Futures (DRAMFs) [19].
A DRAM Future contains an estimate of the time that will take to produce the data to which
it refers. This estimate contains tolerances for any other data that may need to be produced from
previous DRAMFs. A DRAM Future represents a guarantee by the server that creates it, that the
server will be able to generate the data in the estimated time frame.
We believe the combination of DRAMs, pointing to data, services and not-yet-created futures,
together with an appropriate scripting language [16] provide the necessary framework to enable the
efficient construction and utilisation of PSEs and CPSEs in a wide-area distributed system.
4.2 Builder Environment
We have largely focussed on developing the server-side software infrastructure of the DISCWorld
metacomputing environment. However we have experimented with some simple client front ends
that can act as DISCWorld nodes in their own right. It is a challenging problem to design an
architecture for the visual environment that allows users to build collaborative applications.
Figures 1 and 2 show examples of our builder interface experiments. Figure 1 is a simplified
example showing how a user might combine component operators together to produce a complete
sequence. The example is restricted to very simple integer calculations to illustrate the builder idea
only. Figure 2 shows how a more complex operator environment might be parameterised in some
way to allow users to interact with particular components.
13

5 Interfacing to Other CPSEs
Technology and tools for PSEs in specific application areas have been around for some years now.
Tools for problem solving in computational algebra and numerical simulations such as Matlab [34],
Mathematica [39], Maple [38] and similar systems are quite mature. More recently efforts have led
to PSEs which enable resource sharing such as NetSolve [4] and Ninf [30]. These were predated by
a little cited research system developed by Mike Rezny which used the mex scripting capability of
Matlab to interact with powerful back end systems to support operations like matrix solving [29].
Other systems such as Habanero [24] and TANGO [25] have exploited the powerful visual and
graphical capabilities of Java to produce collaborative systems using the now well known metaphors
of shared whiteboards, shared text processing and video conferencing. Such systems are able to
tap into the now widespread desktop video camera and microphone technologies and provide an
integrated approach to sharing access to running programs. Fox et al. have produced a demonstrator
system WebWindows [13] showing how many of our everyday applications could be run at server
side rather than on desktop client systems.
Application Service Providers (ASPs) are starting to become more prevalent on the Web and are
providing access to specialist programs or services through the now well established Web client/server
technologies. These include the Common Gateway Interface (CGI) approach; the servlet and smart
applet approach; and web browser plug ins. These are all appropriate for the architectural model of
single client and single server at any one transaction. These technologies are deficient however for a
robust and multi-user CPSE environment.
Metacomputing or grid computing systems offer an ideal platform for supporting collaborative
PSEs. The DISCWorld model shares some similarities with other grid computing systems currently
in development. All of these systems currently use their own APIs, which makes interoperability a
major issue. This is currently being addressed by the Grid Forum [14], who are working on standard
interface specifications.
Globus/Nexus [11] is a system under active research and has a powerful ability to manage inter-
process communication and servers by moving communications endpoints around. We are currently
evaluating this feature of Nexus and trying to determine how Java networking mechanisms can be
used to provide a similar capability. Unlike the DISCWorld model, Globus/Nexus addresses fine-
grained tasks. The external interface made use of an entity called the I-POP which performed the
tasks of user authentication and security assurance. The I-POP has some degree of commonality
with a DISCWorld daemon which must also handle these operations.
DISCWorld is designed to help schedule and run a range of distributed computations. This is an
objective which it shares with the Nimrod [1] system. Nimrod allows a user to create an interface for
managing parameterised simulations on a series of machines using user-supplied code. DISCWorld
is aimed at the type of user who will be less likely to want to add their own code directly, but rather
will want to utilise existing services, albeit in user specified combinations.
Legion [15] also provides an environment for integrating applications across wide-area networks.
We believe Legion tackles the problem at a substantially finer grain than DISCWorld, allowing the
user to link communications mechanisms to their own programs. The SNIPE [9] system also appears
to operate at a finer grain than DISCWorld.
CORBA [26] is an alternative technology to Java in many of the technical aspects required
for a CPSE. We believe current CORBA implementations are more appropriate for “local area
DISCWorlds”, where the resources involved are all owned by one organisation and accordingly
administered. We are currently investigating how some additions to a CORBA based system might
allow ORBs to find each other and exchange server/services information over a wider area and cross
boundaries of ownership and administration, as we use in DISCWorld.
We are aware of other efforts to use Java for wide-area distributed computing, such as InfoS-
pheres [5]. We believe this too differs from DISCWorld in the granularity of application modules
14

provided for.
The developers of Java and Jini have developed a number of new technologies that provide many
of the low level mechanisms and infrastructure that make construction of a DISCWorld significantly
easier. They do not however solve any of the higher level problems we outline.
We are working on a series of software protocol adapters and brokers that may provide a level
of interoperability between DISCWorld servers and some of the systems mentioned above. Our own
efforts with DISCWorld and other research systems such as the Visual Component Composition En-
vironment (VCCE) of Walker, Rana and coworkers [37], and other Computational Grid technologies
such as Globus and Legion are gradually exploring the fundamental issues for such a robust CPSE.
We believe these issues can be divided into two important categories. Firstly a group of low level
issues that mostly involve standardisation and software engineering and for which possible solutions
exist.
• technologies for integration;
• security and authentication;
• networking and ensuring connectivity between users;
• compression and transport of data to make best use of networks;
• remote execution of code on shared resources;
• migration of code and data for performance optimisation;
• exchanging data and format translations;
• exchanging libraries of components or services.
We believe these are potentially solvable issues with present and emerging technologies and with
a degree of standardisation for exchange of component programs and data.
Secondly there are a number of more fundamental problem issues for which it is not obvious (at
least to us) how to formulate a workable solution. These include:
• naming and metadata to describe the components, resources and data;
• resource discovery;
• reliability and robustness against failure;
• achieving full transparent reconfigurability in a system.
We anticipate some years of interesting research and debate in these latter issues.
6 Conclusions and Ongoing Strategy
Metacomputing and grid computing systems provide high-level middleware to support efficient and
transparent access to distributed data and compute resources, and therefore provide an ideal platform
for developing collaborative problem solving environments. We have introduced our DISCWorld
metacomputing environment and shown how this Java-based, object-oriented system has been used
to develop prototype decision support systems and CPSEs that can be used over a wide-area network.
Reconfiguration is an important aspect of Problem Solving Environments in general and for
Collaborative PSEs in particular. Our data flow model is a preliminary step to understanding the
15

technical issues required to build working systems that truly span administrative boundaries and
allow collaboration across wide areas.
Our key observation is that users can be treated in much the same way as programs – they are all
“operators in control network” albeit with different latency and reliability properties. Collaborative
systems involving interacting users rather than a single user making demands of the system present
their own special problems. Reconfiguration lies at the heart of these.
We believe that using modern GUI technology such as Java it is possible to construct sophisticated
and useful CPSE front ends that can exploit our architecture and middleware components. Our
prototype has proved an effective way for image analysts and decision makers to collaborate across
departments in defense and government organisations.
Outstanding issues remain those of constructing a metadata or name space and accompanying
standards to allow user communities to exchange data and programs more easily. We believe that a
decentralised approach to this problem is the only feasible one, but that some loose standards need
to be first established in much the same way that an HTML specification was required to stimulate
open development of information to exchange on the WWW.
We believe the area of CPSEs is at a critical point socially and that the technology is almost
mature enough to allow rapid growth. It will require more successful end-user application examples to
be constructed to unearth the next level of technical issues for interdisciplinary collaborative systems.
We believe the critical issues for future progress in CPSEs are those of component interoperability
and interface standardisation.
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