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					32                                                                      Economy Informatics, 1-4/2005



                               Grid Computing and Web Services
                                   Lect. Carmen TIMOFTE, PhD
                          Academy of Economic Studies, Bucharest, Romania


Grid computing makes it possible to dynamically share and coordinate dispersed, heterogeneous
computing resources. Flexibility and ubiquity are essential characteristics of Web services tech-
nologies such as WSDL (Web Services Description Language), SOAP (Simple Object Access
Protocol), and UDDI (Universal Description, Discovery, and Integration).
Keywords: OGSA, UDDI, SOAP, WSDL.


I  ntroduction
   Grid is a type of parallel and distributed
system that enables the sharing, selection, and
                                                      Large community of possible GRID users:
                                                      high energy physics, planet earth’s health
                                                      studies (geology, environmental studies,
aggregation of resources distributed across           earthquakes forecast, geologic and climate
"multiple" administrative domains based on            changes, ozone monitoring), biology, ge-
their (resources) availability, capability, per-      netics, earth observation, astrophysics, new
formance, cost, and users' quality-of-service         composite materials research, astronautics.
requirements.
If distributed resources happen to be managed      OGSA
by a single, global centralized scheduling sys-    The Open Grid Services Architecture (OGSA)
tem, then it is a cluster. In cluster, all nodes   combines technologies to unlock and exploit
work cooperatively with common goal and ob-        grid-attached resources. OGSA defines
jective as a centralized, global resource man-     mechanisms to create, manage, and exchange
ager performs the resource allocation. In Grid,    information between Grid Services, a special
each node has its own resource manager and         type of Web service. The architecture uses
allocation policy.                                 WSDL extensively to describe the structure
                                                   and behavior of a service. Service descriptions
Why Grid?                                          are located and discovered using Web Services
The reasons to choose grid are the following:      Inspection Language (WSIL). By combining
   The scale of the problems human beings          elements from grid computing and Web ser-
   have to face to perform frontier research in    vices technologies, OGSA establishes an ex-
   many different fields is constantly increas-    tensible and interoperable design and devel-
   ing;                                            opment framework for Grid Services that in-
   Performing frontier research in these fields    cludes details for service definition, discovery,
   already today requires world-wide collabo-      and life-cycle management.
   rations (multi domain access to distributed
   resources);                                     OGSA defines and standardizes a set of
   GRIDs naturally address this need for col-      (mostly) orthogonal multipurpose communica-
   lecting and sharing resources (CPUs, data       tion primitives that can be combined and cus-
   storage, data ), providing – thanks to al-      tomized by specific clients and services to
   ways growing throughputs and QoS in the         yield powerful behavior. OGSA defines stan-
   underlying networks – unprecedented pos-        dard interfaces (portTypes in WSDL terminol-
   sibilities to access large data processing      ogy) for basic Grid services.
   power and huge data storage and data ac-        Grid applications use grid tools and middle-
   cess possibilities;                             ware components to interact with the fabric.
Economy Informatics, 1-4/2005                                                                   33




Relationship between grids, services and          operable "standards" and difficult-to-reuse
OGSA                                              "implementations." The OGSA uses Web ser-
OGSA introduces several service concepts that     vices technologies like WSDL, SOAP, and
need to be adopted in order to qualify as Grid    WSIL to abstract platform and implementation
services. Necessary components are factories,     differences, giving transparent access to grid
registries, and handle maps. Additional           services.
mechanisms that can be used to build our data     Fundamental to OGSA are WSDL and inter-
grid are notification and lifetime management.    faces for dynamic discovery and life-cycle
From a software-design perspective, a grid can    management for a specific type of Web service
be viewed as a collection of self-contained       - a Grid Service. WSDL conventions and ex-
computing services that can be described, pub-    tensions are used to describe and structure ser-
lished, located, and invoked over any type of     vices, while core service activities are ex-
network. Services are location-independent        pressed using WSDL interfaces and behaviors.
and dynamic, span multiple computing archi-       Every Grid Service instance has a unique and
tectures, and reach across administrative do-     immutable name called the Grid Service Han-
mains. The design principles of OGSA reflect      dle. Lifetime management is provided by
a combination of elements from Web services       mandatory support for the operations Destroy
and grid computing:                               and SetTerminationTime.
    Resource virtualization: each grid compo-
    nent is considered a service.                 GridService
    Standard interface definition mechanisms:     A Grid Service is a WSDL-defined service
    enabling multiple protocol bindings and       that follows a set of conventions relating to its
    transparency between local or remote ser-     interface definitions and behaviors. The speci-
    vices.                                        fication details how clients create, discover,
    Standard foundation services: defining        and interact with a Grid Service. Three charac-
    service semantics, reliability and security   teristics separate a Grid Service from a regular
    models, and core functions such as life-      Web service:
    cycle management, or discovery.                   A Grid Service is an instance of a service
    Implementation independence: support for      implementation of a service type, a collection
    multiple development languages and host-      of specific interfaces.
    ing environments including Java, COBOL,           The instance implements a Grid Services
    C, J2EE, CICS, and .NET.                      Handle. This is a Uniform Resource Indicator
Since a grid consists of both existing and new    (URI) for the service instance and is bound to
hardware and software, multiple variations of     a Grid Service Reference (GSR). The GSR is
communication protocols, security schemes,        similar, for example, to the Interoperable Ob-
and transaction management systems exist and      ject Reference (IOR) in CORBA and contains
cooperate at the same time. Today's grids tend    the necessary information to bind and use the
to be a patchwork of protocols and noninter-      service. The HandleMap interface is used to
34                                                                         Economy Informatics, 1-4/2005


map between the GSH and GSR. Introducing             describing service-specific creation parame-
the GSR in the specification allows the separa-      ters. The interface returns a GSH that uniquely
tion of the service name from the service im-        identifies the instance over time. The soft-state
plementation, facilitating service evolution         lifetime management support avoids clients
such as "non-stop" service upgrades.                 having to tear down services and prevents re-
     The instance also implements a port called      source "leaks" in hosting environments.
GridService supporting three operations:
     o FindServiceData: Allows a client to dis-      Implementations
cover information about the service's state,         From a technical perspective, OGSA is critical
execution environment, and additional seman-         to streamlining and accelerating the creation
tic details not available in the GSR. It is a form   and deployment of grid applications. To ex-
of reflection used by the service consumer to        pand the reach of grid applications beyond the
learn more about the service.                        level of a single enterprise the OGSA needs to
     o Destroy: An operation to explicitly de-       more thoroughly address issues concerning:
stroy an instance                                         Use of WSDL extensions - The extensibil-
     o SetTerminationTime: A method to ex-           ity of WSDL has implications for interopera-
tend the lifetime of a service                       bility with existing WSDL clients and servers.
In addition to the required GridService port,        Full interoperability and accommodation of
OGSA also defines additional, optional service       non-OGSA WSDL clients by grid servers will
ports. These interfaces define properties com-       accelerate the adoption of the extensibility
monly required by distributed systems, such as       elements. This is important since the handling
messaging, discovery and registration, instance      of extensions is spotty at best in most current
creation, and full life-cycle management.            toolkits.
The NotificationSource and NotificationSink               Definition and use of service ports - Dis-
ports handle messaging. This is a simple pub-        covery and life-cycle management deserve a
lish-subscribe system similar to Java's JMS,         separate interface. In the current OGSA speci-
using XML messages. Existing messaging sys-          fication, the services are combined in GridSer-
tems such as Tibco or MQ Series can also be          vice. These services perform distinct roles and
used to implement the service. Error notifica-       are used in different ways. For example, dis-
tion, application monitoring, and dynamic dis-       covery of grid services can be open to anyone,
covery of services are areas where the Notifi-       while instantiation and destroy operations re-
cation ports are useful.                             quire a secure, authenticated connection.
The Registry port is used to bind the metadata            Heterogeneous, end-to-end, security - Ad-
of Grid Service instances, identified by their       dressing concerns around authentication, au-
GSH, to a registry. The registry is a particular     thorization, and trust relations is essential for
type of Grid Service that maintains a set of         grid implementations across organizational
Grid Service Handles and policies associated         boundaries. The OGSA should include provi-
with the collection. Extracting information          sions for "pluggable" security mechanisms that
from the registry service is done using the          can be discovered by the client using a service
FindServiceData method on the GridService            description. Service providers, in turn, can
port.                                                plug in a security architecture that fits their in-
Instances of a Grid Service can be created via       frastructure for Grid Services.
a factory or manually. Similarly, they can be             Grid Service manageability - Currently,
destroyed via soft state termination or explic-      the OGSA does not indicate how resources are
itly. The factory interface operation Create-        exposed or, in an operational setting, how
Service can create transient application service     large sets of Grid Service instances can be
and supports reliable service instantiation          monitored and managed. Grid Services are
(once-and-only-once). Interaction with the fac-      commonly subject to service level agreements
tory requires creation-lifetime information and      that define specific availability, performance,
can be extended to include an XML document           and reliability characteristics.
Economy Informatics, 1-4/2005                                                                    35


A virtual organization (VO) example                  The next figure shows a simple VO layout
The minimal set of services that a grid node         with two sites and a possible set of services:
should have depends on what kind of re-              one site capable of only storage and another
sources the node site offers. If the site has only   capable of only computing. Ovals represent
storage capabilities, it will need most of the       factories and rectangles represent exposed ser-
data management services discussed above. If         vices. The replica manager (RM) factory may
it only has computing capabilities but no stor-      spawn other high-level data management ser-
age capabilities, then it needs only a subset of     vices. There may be many other services such
those, but it will need job management ser-          as monitoring and management services that
vices instead. A site offering both storage and      are not shown in the picture.
computing capabilities will require both data
and job management services.




Conclusion                                             www.sys-con.com/webservices/ Web Ser-
OGSA brings utility-based computing a step           vices Journal, 2004;
closer, and allows the development and de-             www.infoworld.com, Sun combines grid
ployment of standards-based Grid Services to-        computing, Web services, 2003.
day. The combination of Web services and               www-1.ibm.com/grid
grid computing virtualizes networked re-               wwws.sun.com/software/gridware
sources using common computing standards,              www.microsoft.com/
making them accessible to a larger audience.
To extend the reach of the architecture and
maximize its potential benefits, additional ser-
vices, such as security and management, need
to become an integral part of the specification.

References
  www.cio.com, True Grid - Grid Technology,
2004;
  www.cio.com, In the Grid Game - CIO
Magazine May 15,2004.htm;
  www.nwfusion.com/sungrid.html, Sun com-
bines grid computing, Web services.htm,
2003;
  www.worldcommunitygrid.org/ Community
Grid - About Us - Grid Computing Basics.htm,
2004
  www.globus.org/Globus Project Open Grid
Services Architecture.htm, 2004;
  www.nwfusion.com, Grid and the Future of
the Network Machine.htm, 2005;

				
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Description: Web Services Description Language acronym is used to describe a Web service and how to communicate with Web Services XML language. Interface to provide users with detailed instructions.