OGSA Open Grid Services Architecture Ramya Rajagopalan rr

OGSA : Open Grid Services Architecture Ramya Rajagopalan rr 5 xf@umsl. edu

Introduction Developed by the Global Grid Forum Aims 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 Services Technique 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 OGSA Web 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.

OGSA 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 Details The OGSA Service Model: Basic Premise: Everything is represented by a grid service. Uniform Service Oriented Model Specifies 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 Interface OGSA 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 Handle Every Grid Service instance is assigned a globally unique 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: Factories Services implement an interface to create new Grid service instances. The service that implements this interface is called a factory. Create. Service() operation creates a requested service and returns the GSH and an initial GSR. Again OGSA doesn't specify how the instance is created.

Lifetime Management Soft 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 References GSH 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 Maps Every Grid Service instance is always registered with at least one home handlemap. GSH includes the handle. Map's identity. All handle. Map services are also identified by a URL. Two steps: 1. Identifying a handle. Map service that contains the mapping for the specified GSH. 2. contacting that handle. Map to obtain the desired GSR.

Service Data and Discovery Each Grid service instance has an associated service data(usually a collection of XML elements) Find. Service. Data Operation: querying and retrieving service data. GSH, GSR, primary key, handle. Map etc are service data members.

Service Discovery Process 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

Notification OGSA 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 Management To support the feature Grid Service interfaces must be globally and uniquely named. In WSDL we use the port. Type's qname to identify a Grid Service globally. Changes made in the Grid Service are reflected through new interface names(i. e. , new Port. Types and/or Service. Types) Allows clients to thus find a compatible service.

Other Interfaces Manageability 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 Bindings Four 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 Services Distributed Data Management Services. Workflow Services. Auditing Services. Instrumentation and Monitoring Services. Problem determination services for distributed computing. Security protocol mapping services.

OGSI Open Grid Services Infrastructure Gives a formal and technical specification of what a grid service is. Its a excruciatingly detailed specification of how Grid Services work. GT 3 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 the building looks like. OGSI: (The Specification) is the structural design that the engineer creates to support the architect's vision of the building. GT 3: (The implementation) is the bricks, cement and beams used to build the building with the engineer's specifications.

References http: //www. globus. org/alliance/publications/papers /ogsa. pdf http: //docs. huihoo. com/globus/gt 3 tutorial/ch 01 s 01. html The Grid 2: By Ian Foster and Carl Kesselman