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Extending the Open Grid Services Architecture to Intermittently

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					         Extending the Open Grid Services Architecture to
            Intermittently Available Wireless Networks

                                             Alastair Hampshire
School of Computer Science and IT, University of Nottingham, Jubilee Campus, Wollaton Road,
                                Nottingham, NG8 1BB, UK
                                     axh@cs.nott.ac.uk

                                                    Abstract

       The EQUATOR e-Science program is developing techniques for placing sensing devices on the
       grid. One such project, wearable medical devices, aims to expose mobile sensing devices via
       wireless networks to facilitate remote patient monitoring. The current grid model assumes a
       permanently available network connection between client and service, however there is a clear need
       for mobile clients/ services to whom network availability cannot be guaranteed. This paper
       introduces a framework for supporting both mobile grid clients and service in a wireless network
       environment.

                                                            also examines ways to support the various
    Introduction                                            distinctive aspects of grid applications in a
                                                            mobile environment, such as support for state
    The EQUATOR e-Science program is                        management, handle resolution, etc.
    developing techniques for placing sensing
    devices on the grid. There are three parts to the
    program: Wearable medical devices for remote            Normal Grid Service Invocation
    patient monitoring [1]; Remote sensing of a
                                                            A typical grid service invocation makes use of a
    fresh water lake in the Antarctic to facilitate
                                                            fixed network infrastructure to provide
    environmental modelling [2]; Mobile pollution
                                                            communication between clients and services.
    sensors designed to record pollution levels in
                                                            The Grid idiom uses factory services to create
    urban areas [3].
                                                            service instances. A client (or clients) then
    Exposing sensing devices as grid services
                                                            interacts with the service instance, which is able
    allows them to make use of a range of grid
                                                            to maintain state data for the duration of the
    services, such as a database service to archive
                                                            client(s) interaction(s). OGSI defines the use of
    sensor data or a computational service to
                                                            a Grid Service Handle (GSH) and a Grid
    process/ model the data e.g. modelling carbon
                                                            Service Reference (GSR). A GSH is a persistent
    cycles in fresh water Antarctic lakes. Further, a
                                                            handle to the service, but does not contain
    grid interface could be used to manage and
                                                            protocol or location information. The GSR is a
    control a sensing device remotely. These
                                                            transient network pointer with an associated
    techniques make it considerably easier to share
                                                            lifetime, which can be used to locate and invoke
    both sensing devices and their associated data
                                                            the grid service. The GSH can be resolved to a
    between multiple interested organisations.
                                                            GSR using a Handle Resolver Service. OGSI
    All the above projects share the characteristic of      also provides some additional functionality, for
    involving remote and intermittently available           example, service groups and a notification
    sensor services, as well as mobile client               service. Figure 1 shows a typical grid service
    applications. The present approach to grid              invocation.
    computing (OGSI [4]) is designed to support                                                  Factory
    synchronous, always connected services and                 Client (3) Create service
                                                                                                 Service
    cannot easily be used to facilitate the placement
                                                                        (4) Invoke service        Service
    of asynchronous, intermittently connected
                                                                                                 Instance
    services on the grid. Tools, such as the globus
                                                               (2) Lookup GSR
    toolkit assume the availability of a permanent                                (1) Publish GSR
    connection to a grid service.
                                                                               Handle
    This paper introduces a framework for                                     Resolver
    supporting both mobile grid clients and services
    in a wireless network environment. The paper               Figure 1, Normal grid service invocation
Mobile Grid Service Framework                                   available. Support for roaming services and ad-
                                                                hoc networking can be provided by allowing
This framework aims to allow grid service                       services to dynamically discover intermediary
invocations to persist in the presence of                       routers.
intermittent network failure. Stalled invocations
should     be    completed      when     network
reconnection occurs, where these periods of
                                                                Conclusion
disconnection may be of the order of minutes or                 This paper has shown that current Grid
hours or sometimes longer. The framework also                   implementations cannot be used to support
aims to minimise the necessary modifications                    clients and services that exist in intermittently
required to clients and services, minimise the                  available wireless network environments. In
performance overhead involved in using the                      addition the paper has reported a clear need for
mobility framework and be scalable to large                     mobility support in grid computing from a
numbers of clients and services.                                number of motivating eScience applications. A
This mobility framework makes use of                            framework for supporting mobile clients and
intermediary SOAP Routers to queue messages                     services through the use of SOAP level
during periods of network disconnection as well                 intermediaries has been described. An
as providing finer grained retransmission (i.e                  implementation of the framework has been
retransmission from SOAP Router rather than                     produced and tests have shown the framework
complete end-to-end retransmission).                            to support continued operation in the presence
Figure 2 shows a simplification of how the                      of temporary network failure.
framework can be used to support a mobile grid
service. The complete framework supports both                   References
mobile clients and services by routing requests
from client to service via a number of                          1. Carl Barratt et al. Extending the Grid to
intermediary SOAP Routers. This routing                            Support Remote Medical Monitoring. UK e-
information is embedded in a Mobile-GSR and                        Science All Hands Meeting 2003.
published by the mobile grid service to a handle                2. Steve Benford et al, e-Science from the
resolver on a fixed network. Mobile grid clients                   Antarctic to the Grid. UK e-Science All
contact the handle resolver and retrieve the                       Hands Meeting 2003.
Mobile-GSR. Routing information in the
                                                                3. Steed, A et al. e-Science in the Streets:
Mobile-GSR is mapped to header information in
                                                                   Urban Pollution Monitoring. UK e-Science
a SOAP request message, allowing the message
                                                                   All Hands Meeting 2003.
to route itself to the mobile service. Wireless
network hops can be invoked using a reliable                    4. S. Tuecke, K. Czajkowski, I. Foster, J. Frey,
messaging protocol to ensure that packets are                      S. Graham, C. Kesselman, T. Maguire, T.
not lost when the network is unavailable.                          Sandholm, P. Vanderbilt, D. Snelling. Open
Further optimisations can be made to minimise                      Grid Services Infrastructure (OGSI) Version
packet wastage on low bandwidth or pay per use                     1.0.    Global     Grid      Forum      Draft
networks by only allowing senders to send                          Recommendation, 27 June 2003.
information when they think the receiver is

                                                                                           Wireless

                                                Fixed Network               (5) Wireless network
                          Client                                            hop invoked connection
                                       (4) Client routes                    using reliable          Mobile
                                       request via                          messaging               Factory
(3) Client builds routing header       intermediary router     SOAP         protocol                Service
for soap message from gsr                                      Router
<Header>                                                                                                   Mobile
                                         (2) Lookup
 <path>                                                                                                    Service
  <to> Mobile Factory Service </to>      mobile
                                                             (1) Publish mobile service gsr               Instance
  <fwd>                                  service gsr
   <via> SOAP Router <via>                                   <service name="MobileServiceFactory">
  </fwd>                                                      <port binding=“…
  <rev>                                                         <address location=“Mobile Factory Service Url"/>
   <via> Client <via>                         Handle            <address location=“Soap Router Url"/>
  <rev>
 </path>
                                             Resolver         </port>
</Header>                                                    </service>

                                   Figure 2, Mobile grid service architecture

				
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Description: OGSA (Open Grid Services Architecture) is called next-generation grid architecture, which is in the original "five hourglass structure" based on the combination of the latest Web Service technology initiative. OGSA, including two key technologies that grid technology and Web Service technology. With grid computing research, it is increasingly found important grid architecture. Grid architecture is about how to build grid technology, including the grid and the basic function of each part of the definition and description of the relationship between the various parts of the grid and integration method provides an effective operational mechanism characterization of the grid. Clearly, the grid architecture is the skeleton and soul of the grid, the grid is the core technology, only a reasonable grid architecture to be able to design and build a good grid to be able to function effectively mesh.