Web Services and Grids

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Web Services and Grids
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• Grids are complex beasts and hard to use by the
  ordinary scientist.
• Does provide a common set of services and
  capabilities across resources.
• However
• Primarily accessed through command line
  interface.
• User must marshal all of the resources.
• No support for making application grid enabled.
• Scientist must monitor application for faults and
  deal with faults.

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• Grid computing is moving toward a
  service-oriented model.
• The Grid is defined by the services it provides.
• Provides for reuse of existing components and
  information resources.
• Greater flexibility in assembling components.
• Desirable to access Grid services over the WWW
• Leads to Grid portals
• Web based application server enhanced with
  necessary software to communicate with Grid
  services and resources.
• Provides application scientists a customized view
  of software and hardware requirements from a web
  browser.

4

• Leads to development of application-specific and
  user-specific portal categories.
• Application-specific portals provide a
  specialized subset of Grid operations within a
  given domain.
• Astrophysics Simulation Collaboratory.
• Diesel Combustion Collaboratory.
• User-portals provide site-specific services for a
  particular community or research center.
• HotPage user portal.
• UNICORE.

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• Astrophysics Simulation Collaboratory.
• The ASC
• assembles the required code,
• estimates resource requirements,
• locates an appropriate set of resources and
• performs the computation.
• User-portals provide site-specific services for a
  particular community or research center.
• HotPage user portal.
• Provides access to high-performance computers for
  scientific research.

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Web Services

• Software components designed to provide specific
  operations (services) accessible using standard
  Internet technology.
• For machine interaction over a network.
• Usually through SOAP (simple Object Access
  Protocol) messages carrying XML documents, and a
  HTTP transport protocol.

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Basic client-server model
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Web Services

• Example Chain of stores with one centralized
  catalog.
• Publish catalog through a Web service.
• Clients would contact Web Service (in the portal
  server) and request catalog.
• Web Service would send catalog in Service
  Response.

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Web Services
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From http//www.globus.org
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Steps in Accessing Web Services

• Find Web Service that meets Requirements.
• -UDDI Registry (Universal Description Discovery
  and Integration).
• The UDDI registry replies with list of servers
  that can provide us the service.
• Now know the location of a Web Service, but not
  how to invoke it. We have to ask the Web Service
  to describe itself (i.e. tell us how exactly we
  should invoke it)
• The Web Service replies in a language called
  WSDL.
• Web Services Description Language.
• Describes a service in XML (Extensible Markup
  Language).

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• Know where the Web Service is located and how to
  invoke it.
• The invocation itself is done in a language
  called SOAP (Simple object access protocol).
• Send a SOAP request asking for (for example) the
  temperature of a certain city.
• The Web Service will reply with a SOAP response
  which includes the temperature we asked for or an
  error message if SOAP request was incorrect.

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Service-Oriented Architecture
Service registry
1. Publish
2. Find
3. Bind
Service requester
Service provider
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Advantages of Web Services

• Similar to CORBA and RMI except
• Platform-independent.
• Language-independent (use standard XML
  languages).
• Client program can be programmed in C and
  running under Windows, while the Web Service is
  programmed in Java and running under Linux.
• Use of HTTP for transmitting messages.
• Most of the Internet's proxies and firewalls
  won't mess with HTTP traffic (unlike CORBA, which
  usually has trouble with firewalls).

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Disadvantages of Web Services

• Overhead Transmitting data in XML.
• Lack of versatility.

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Some Details

• Addressing URIs (Uniform Resource Identifiers).
• Example URI http//webservices.mysite.com/weather
  /us/WeatherService

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• Service Discovery This part of the architecture
  allows us to find Web Services which meet certain
  requirements. This part is usually handled by
  UDDI (Universal Description, Discovery, and
  Integration).
• Service Description Web Services are
  self-describing. This means that, once you've
  located a Web Service, you can ask it to
  'describe itself' and tell you what operations it
  supports and how to invoke it. This is handled by
  the Web Services Description Language (WSDL).
• Service Invocation Invoking a Web Service (and,
  in general, any kind of distributed service such
  as a CORBA object or an Enterprise Java Bean)
  involves passing messages between the client and
  the server. SOAP (Simple Object Access Protocol)
  specifies how we should format requests to the
  server, and how the server should format its
  responses.
• Transport Finally, all these messages must be
  transmitted somehow between the server and the
  client. The protocol of choice for this part of
  the architecture is HTTP (HyperText Transfer
  Protocol).

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Web Services Stack
XML
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Issues in Building Grid Portal

• Develop portal that can interact with users to
  find and execute services.
• Security.
• Determine services to be provided.
• Provide infrastructure to make services grid
  enabled.
• Communication mechanisms.
• Distributed file system.
• Scheduler

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• Read section 3.4, and chapters 9 and 27.
• Think about developing grid portal.