OGSA : Open Grid Services Architecture

Ramya Rajagopalan

rr5xf@umsl.edu

IntroductionDeveloped by the Global Grid ForumAims to define a common, standard, open

architecture for Grid Applications.Defines a set of rules that make up a grid

service.Sharing and Coordinated use of diverse

resources in Dynamic VO's

Background….Two Technologies to build OGSA:Globus Tool Kit Community based Open Architecture, open

set of services and software libraries that support Grids and Grid applications.

Web Services Popular Standards based framework for

accessing network applications.

Web ServicesTechnique for describing software

components to be accessed.Define methods for accessing and

discovering the components.Its another distributed computing

technology which allows us to create client/server apps.

How does this work??

Web Service Standards....Web Services Description

Language(WSDL)Standardized XML Format for describing

network services.Describe the name, location and ways to

communicate.Define Web services as a set of endpoints

operating on messages.

Standards…Simple Object Access Protocol(SOAP)Messaging between service provider and

requester.Follows the HTTP Request-Response

model.XML Envelope over the message to

describe what's in and how to process it.Defines an XML based convention for the

Request-Response.SOAP payloads can also be carried in FTP,

JMS.

Why Web Services??Web services are Platform independent

and Language independent.Major advantage to build Internet scale

applications as they use HTTP for transmitting messages.

WSDL supports Dynamic discovery and composition of services in heterogeneous environments.

Grid Services and OGSAWeb services with improved characteristics

and services.Provides a set of well defined interfaces and

conventions.Interface Addresses: a) Discovery, dynamic Service Creation. b) Lifetime Management, notification.Conventions include: a) Naming Services b) Upgrade ability.

OGSAOGSA defines the semantics of a Grid

Service instance.Defines the basic behavior and does not

specify what a service does and how it does it.

Doesn't address issues of implementation programming model, language, tools and execution environment.

By Defining a set of service semantics OGSA specifies interactions between services in a manner independent of any hosting environment.

OGSA Technical DetailsThe OGSA Service Model:Basic Premise: Everything is represented

by a grid service.Uniform Service Oriented ModelSpecifies a core set of consistent

interfaces from which all Grids are implemented.

Grid Services maintain internal state for the lifetime of the service.

OGSA Service Model....Grid Service instance refers to a

particular instantiation of a service.State oriented service facilitates failure

recovery mechanism.OGSA services can be created and

destroyed dynamically.The existence of state distinguishes one

instance from another.

Grid Service InterfaceOGSA does not define a specific set of

services but defines a set of interfaces for manipulating service models.

A mandatory interface which must be supported by all Grid services.

Interface for manipulating the grid service, handles, reference abstractions that define the OGSA model.

Grid Service HandleEvery Grid Service instance is assigned a

globally unique GSH.GSH carries no protocol specific or

instance specific information.All the instance specific information are

encapsulated in a single abstraction called a Grid Service reference(GSR).

Grid Service Reference(GSR)Unlike GSH, GSRs change during the

service's lifetime.GSR has an explicit expiration time.OGSA mappings define mechanisms for

obtaining an updated GSR.A GSR does not always guarantee access

to a Grid Service, local policy or access control constraints may prohibit servicing a request.

Transient Services: FactoriesServices implement an interface to create

new Grid service instances.The service that implements this interface

is called a factory.CreateService() operation creates a

requested service and returns the GSH and an initial GSR.

Again OGSA doesn't specify how the instance is created.

Lifetime ManagementSoft state approach: Every instance is

created with a specific lifetime.Initial lifetime can be extended by explicit

client request.Client can always know when the Grid

Service will terminate.Resource consumption at hosting

environment is always bounded.

Managing Handles and ReferencesGSH lives forever, but GSR expires.A handle-to-reference mapping interface

takes a GSH and returns a valid GSR.Mapping operations may be controlled

and requests denied.Again a Valid GSR doesn't promise access

to the Grid Service.

Handle MapsEvery Grid Service instance is always

registered with at least one home handlemap.

GSH includes the handleMap's identity.All handleMap services are also identified

by a URL.Two steps: 1. Identifying a handleMap service that

contains the mapping for the specified GSH.

2. contacting that handleMap to obtain the desired GSR.

Service Data and DiscoveryEach Grid service instance has an

associated service data(usually a collection of XML elements)

FindServiceData Operation: querying and retrieving service data.

GSH, GSR, primary key, handleMap etc are service data members.

Service DiscoveryProcess of identifying a subset of GSH's

from a specified set.Registry: Grid service that supports

service discovery.Registry Interface->To register a GSH and

Associated service data element-> Information about GSH's already registered.

Registration is a soft state operation and must be periodically refreshed

NotificationOGSA notification Framework allows

clients to be notified by messages.Interface also includes framework for

asynchronous one way delivery of notifications.

The Source must support the OGSA notificationsource interface.

Clients wishing to receive notifications must implement notificationsink interface.

How does notification work?To start a notification from a particular

service:Invoke the subscribe operation on the

notification source interface.Give it the service GSH of the notification

sink.Stream of messages flows from source to

sink.Sink sends back a series of keepalive

messages to notify the source its interested in getting more messages.

If reliability is desired then we use an appropriate protocol for this service.

Change ManagementTo support the feature Grid Service

interfaces must be globally and uniquely named.

In WSDL we use the portType's qname to identify a Grid Service globally.

Changes made in the Grid Service are reflected through new interface names(i.e., new PortTypes and/or ServiceTypes)

Allows clients to thus find a compatible service.

Other InterfacesManageability interface that supports a

set of manageability operations. Allows potentially large sets of Grid

Service instances to be monitored and manged from management consoles, automation tools and the like.

Concurrency interface to provide concurrency control operations.

Network Protocol BindingsFour Primary requirements for selecting a

network protocol within an OGSA context:

Reliable Transport : Reliable service invocation e.g.,HTTP -R

protocol.Authentication and Delegation: Grid service abstractions require support

for communication of proxy credentials to remote site. One way to do this is to incorporate appropriate support within the network protocol

contd.... Ubiquity Any arbitrary pair of services must be able to

interact.

GSR format Grid service reference can take a specific

binding format, one possible GSR format is WSDL.

Higher Level ServicesDistributed Data Management Services.Workflow Services.Auditing Services.Instrumentation and Monitoring Services.Problem determination services for

distributed computing.Security protocol mapping services.

OGSIOpen Grid Services InfrastructureGives a formal and technical specification

of what a grid service is.Its a excruciatingly detailed specification

of how Grid Services work.GT3 includes a complete implementation

of OGSI.

Interesting Idea!!!Consider you want build a New House.

OGSA: (The Definition) is the blueprints the architect creates to show how the building looks like.

OGSI: (The Specification) is the structural design that the engineer creates to support the architect's vision of the building.

GT3: (The implementation) is the bricks, cement and beams used to build the building with the engineer's specifications.

Referenceshttp://www.globus.org/alliance/publications/papers/ogsa.pdf

http://docs.huihoo.com/globus/gt3-tutorial/ch01s01.html

The Grid2: By Ian Foster and Carl Kesselman