From last time
• Grid computing concept started in mid-90’s• Fosters 3 point checklist of a grid system:
– coordinate shared resources – standard, open, general-purpose protocols and interfaces– deliver nontrivial qualities of service
• Grid definitions focus on:– distributed computing– common set of interfaces, tools and APIs– inter-institutional Virtual Organizations– virtualization of resources, services, people, orgs, etc.
Standards Bodies
The primary standards-setting body is1:
• Global Grid Forum (GGF)– Started in 1998– Meets three times a year, GGF1, GGF2, GGF3 …– More than 40 organizations involved and growing …
Others:
• W3C consortium (Worlds Wide Web Consortium)– Working on standardization of web-related technologies such as XML– See http://www.w3.org
• OASIS (Organization for the Advancement of Structured Information Standards)
• IETF, DMTF
Grid computing
• With the use of the Internet interconnection technology, implementation now based upon Internet technologies.
• Now uses a form of web services.
• Enables using existing protocols, security mechanisms, etc.
Standards in the Web Services World
• XML introduced (ratified) in 1998• SOAP ratified in 2000• Web services developed• Subsequently, standards have been are
continuing to be developed:– WSDL– WS-* where * refers to names of one of many
standards
Standards in the grid computing world
• Open Grid Services Architecture (OGSA)
• First announced at GGF4 in Feb 2002
• OGSA does not give details of implementation.
Grid computing software
• Started before standards became accepted.
Gone through several development cycles:
• Originally own protocols were developed (e.g. GT2)
Then
• OGSA (Open Grid Services architecture) standard and a specification called OGSI (Open Grid Service Infrastructure) was developed. Extended web service was invented called a grid service to embody state and transience. (GGF) Implemented in GT3.
And
• Now relies more directly upon developing web service standards (GT 4)
1996-2002
2002-2004
2005 -
Grid computing standards
Open Grid Services Architecture(OGSA)
• Defines standard mechanisms for creating, naming, and discovering service instances.
• Addresses architectural issues relating to interoperable services for grid computing
Web Services
• Introduced in the 2000 • Web services build upon earlier distributed computing
concepts • One of the underlying concepts is the client-server model• Software components designed to provide specific operations
(“services”) accessible using standard Internet technology.• For machine interaction over a network.• Concept similar to Remote Procedure Call (RPC), Remote
Method Invocation (RMI), only applied over HTTP• Usually through SOAP (simple Object Access Protocol)
messages carrying XML documents, and a HTTP transport protocol.
Basic client-server model
Client/Server Model
•Starts first
•Waits for contact from a client
•Responds to requests
•Starts second
•Contacts a server with a request
•Waits for response from server
Server Client
Types of Servers
A server can be:
Stateful
Stateless
Iterative
Concurrent
iterative stateful
concurrent stateful
iterative stateless
concurrent
stateless
Stateful Server
• Maintains some information between requests
• Requires smaller messages, since some information is kept between contacts
• May become confused if a connection terminates abnormally (if the design is not fault tolerant)
• Example: FTP
Stateless Server
• Requires larger messages. That is, the message must contain all information about the request since no state information is kept.
• Example: HTTP
Iterative Server
while (1) {
accept a connection (or request) from a client service the client
close the connection (if necessary)
}
Concurrent Server
while (1) {
accept a connection/request from client
start a new thread to handle this client
/* the thread must close the connection! */
}
Internet Addressing
http://web.info.uvt.ro/~petcu/
• Same as IP address 194.102.62.5• A Domain Name Server (DNS) may be called to find the IP
address of comp.uark.edu• Each IP machine is usually configured with the name of a
DNS server.• Some IP names and addresses can also be stored in
/etc/hostfile
Find the home page of user aaponFind the home page of user aapon
Contact the HTTP server on the computer named Contact the HTTP server on the computer named web.info.uvt.roweb.info.uvt.ro
Internet Addressing
“http” says: send the message to port 80• An IP address includes both a host address and a port
number!• The HTTP server listens to port 80• The HTTP server responds when a client contacts it You can write a server that listens to any port not
already in use!• A port number is a 16-bit integer. Ports below 1024 are
reserved for system use.• Well-known ports include FTP, Telnet, SMTP, etc.
RPC and RMI
• Remote Procedure Call (RPC)
• Extends the client/server concept by making the definition of an interface more formal, clarifying rules for parameter passing, adding capabilities for locating services, etc.
• RMI (Remote Method Invocation) is RPC applied to object. We will study Java RMI
Remote Procedure Call
Early distributed computing system introduced in the 1980’s.Allows a local program to execute a procedure on a remote computer and
get back results from the procedure.Basis of certain remote operations such as mounting remote files in a
shared file system.We need to know how and where to make the call.The basic RPC requires the calling program to know details about how to
make the call (meaning and types augments and return value)The calling program also needs to know where to send the request.RPC introduced the concept a service registry a third party used to
identify location of “service” (procedure).Using a service registry is now part of what is now called a Service-
Oriented Architecture.
Service-Oriented Architecture (SOA)
• Client needs to:– Identify location of the required service– Know how to communicate with the service to
get it to provide the actions required.
• Uses service registry - a third party.
Service-Oriented Architecture
(Service requester)Client Server
(Service provider)
Service registry
PublishFind
Bind
Service-Oriented Architecture
Steps:
• Services “published” in a Service registry.
• Service requestor asks Service Registry to locate service.
• Service requestor “binds” with service provider to invoke service.
Later systems
• Later forms of remote procedure calls in 1990’s introduced distributed objects:
– CORBA (Common Request Broker Architecture)
– Java RMI (Remote Method Invocation)
• Universally agreed standardized interfaces
• Inter-operability
• The need to provide flexibility
• Using the Internet as the network communication (Internet standards)
Key aspects of Web services
Has similarities with RMI and other distributed object technologies (CORBA etc.) but::
• Web Services are platform independent
– They use XML within a SOAP message).
– Most use HTTP to transmit message.
XML documentSOAP messageHTTP envelop
Key aspects of Web services
• Should be self-describing: – publish a public interface to the services
• Should be discoverable:– Mechanism for publishing WS’s you have
created– Can be found via a simple ‘find’ mechansim
Web Services “Stack”
• HTTP transport
• SOAP message carrying XML documents
• WSDL (Web Services Description Language used to describe message syntax for invoking a service and its response.
• UDDI (Universal Description, Discovery and Integration) used as web service discovery mechanism.
HTTP
WSDLService description
UDDIService discovery
SOAP + XMLService invocation
Service message transport
Activity Protocol/language
Simple Object Access Protocol (SOAP)
Provides mechanisms for:– Defining communication unit - a SOAP message
– Error handling
– Extensions
– Data representation
– Remote Procedure Calls (RPC’s)
– Document-centric approach for business transactions
– Binding to HTTP
XML Tags
• Must define your own tags using names as names in a programming languages
• As in programming languages, restrictions. Case sensitive. Start with a letter.
• “Elements” have start and end tags.• Start tags can have attributes as in HTML• XML on-line materials
W3C consortium home page: http://www.w3.org/XMLW3Schools XML Tutorial http://www.w3schools.com/xml/
Address of a Web Service
• URIs
Example (URL)
http://www.info.uvt.ro/webservices/math1
This does not exist, and if did, would only be meaningful to software.
Note
• The term “Web Service” is a little misleading as the web service need not be on the web.
• It could be on the same computer as the client, or another computer in a local cluster.
Web Service Application
Client Stub
• Between client code and the network is a client stub, sometimes called client proxy.
• The client stub is responsible for taking a request from the client and converting the request into a SOAP request on the network - marshalling.
• Also responsible for receiving SOAP responses on
network and converting to a suitable form for client.
Server Stub
• Between the service and the network is a server stub, sometimes called a skeleton.
• Responsible for receiving a SOAP request from the client stub and converting it into a suitable form for the service -unmarshalling.
• Also converts the response from the service into a SOAP message for the client stub.
Steps
• Client calls client stub.• SOAP request sent across network• Server stub receives request and sends
request to service• Service send result to serve stub• Server stub sends result across network to
client stub.• Client stub sends result to client.
Web Service Application
Call client stubSOAP
requestRequest service
Result returnedSOAP
responseClient receives result
SOA• SOA is more than a group of web services or any other specific set of
technologies: it's an architecture.– all functions are aggregated as reusable services, each defined by a
service interface. • SOA is the contract to identify the services,
– it contains rules to access them. – All request and response data, exception conditions, and functionality
must be listed as part of this interface. – Service contracts are designed to be coarse-grained– interactions package several function calls and responses into fewer, but
larger, messages.• Service-oriented architectures are not a new thing:
– first SOA for many people was DCOM or Object Request Brokers (ORBs) based on the CORBA specification
SOA
• All services are abstracted from the internal design that achieves the results for the services. – interface should have sufficient information for a
service to be identified and used without needing to know about its internal design, language, or platform implementation.
– A loosely-coupled design also means that services are designed for no particular service consumer.
– The information carried by the service should be agnostic to the purpose and technical objectives of the service consumer.
SOA
• Location of the service(s) should be transparent to the consumer:– registry could serve as the discovery mechanism for
consumers to locate services being offered in a transparent way.
– Irrelevant whether the services are local or remote– responsibility of the system, not the calling application,
to effect and manage the invocation of the service. – Allows for services to be truly independent and
managed.
SOA Characteristics• Provides logical, abstracted view:
– programs, dbs, business processes, etc
• Message orientation defines service: – exchanged between provider agents and requester agents– one does not need to know a service is constructed– Legacy code can be "wrapped" in message handling code that allows
it to adhere to the formal service definition.
• Description orientation: – A service is described by machine-processable meta data. – details exposed important for use of service are included – semantics of a service should be documented by its description
• Granularity: – Services use a small number of ops; large, complex messages.
• Network orientation• Platform neutral exchange of messages (XML)
Web Services Architecture
The Web Services Architecture is specified and standardized by the World Wide Web Consortium, the same organization responsible for XML, HTML, CSS, etc.
Web Service Definition Language (WSDL)
A W3C standard XML document that describes three fundamental properties of a service:
• What it is - operations (methods) it provides.• How it is accessed - data format, protocols.• Where it is located - protocol specific network
address.
Parts of a WSDL Document
Parts of an WSDL document::• Root definitions - namespaces
• portType definitions - abstract definition of service
• Message definitions - parameters in method signature
• Type definitions - data types
• Binding definitions - to protocols I.e. SOAP over HTTP
• Service definitions - where service is, ports
Service
Port
Port Type
Operation AMessages (input, output)
Operation BMessages (input, output)
Bindings
port and service
Describe “where” service is.
• port - describes how a binding is deployed at the endpoint of a network
• service - a named collection of ports
portType
Describes “What” - an abstract definition of service operation. Compare to a Java interface.
Uses the elements:
• message definitions - a set of parameters referred to by method signature, decomposed into parts
• type definitions - defines all data types used
Binding
Describes “how” the elements in abstract interface (portType) are converted in actual data representations and protocols e.g. SOAP over HTTP.
Could be more than one binding associated with a portType.
Grid service
The Global Grid Forum (GGF) developed standard interfaces, behaviors, core semantics, etc. for grid applications based upon web services.
GGF introduced the term Grid Service as an extended web service that conforms to the GGF OGSI standard.
• Key aspect is the separation of the (web) service and a resource – conceptually if not actually.
• Provides the ability to have “state” without altering the statelessness of a web service.
Grid Services
• Common interface specification supports the interoperability of discrete, independently developed services
• Based on extensions of Web Services• OGSA – Open Grid Services Architecture• OGSI – Open Grid Services Infrastructure
OGSI
• Service Data – a common mechanism to expose a service instance’s state data for query, update, and change notification
• Grid Services uses a Factory to manage instances – to allow transient and private instances
Changes to Grid Standards
• Introduction of Web Services Resource Framework (WSRF), January, 2004– Web services vendors recognized the
importance of OGSI concept but would not adopt OGSI as it was defined (summer 2003)
– Globus Alliance teamed up with Web services architects and came up with WSRF
– Add the ability to create, address, inspect, discover, and manage stateful resources
Stateless Web Service
• It is generally thought that web services should be stateless – they do not remember or store information themselves from one invocation to the next.
• Any state information needed by the web service to complete a client request or maintained should be held separately.
Web Service
Resource
Resource properties
Client
Web Service Resource Framework(WS-RF)
Holds information retained between accesses.
First attempt as a stateful service
A now defunct attempt at providing a stateful service was embodied in the OGSI standard which was part of the OGSA grid computing standard
• Introduced in 2002
• Abandoned in 2004
Stateless web service + stateful resource
Web services can be front-ends to stateful resources.Concept formalized in a standard called
WS-RF framework.
WS-RF also at the center of recent grid computing software (Globus 4.0).
Provides a way of identifying resource through the web service, and other things such as resource lifetime
WS-Resource Framework
• A specification developed by OASIS, essentially to replace OGSI and make the implementation of a stateful web service acceptable.
• Specifies how to make web services stateful and other feature, without drifting from the original web services concept.
WS-* Standards
Principal web service standards adopted for grid computing:
• WSRF Framework collection of 5 specifications:– WS-ResourceProperties
• Specifies how resource properties are defined and accessed
– WS-ResourceLifetime• Specifies mechanisms to manage resource lifetimes
– WS-ServiceGroup• Specifies how to group services or WS-Resources together
– WS-BaseFaults• Specifies how to report faults
WS-* Standards
• WS-Notification– Collection of specifications that specifies how
configure services as notification producers or consumers
• WS-Addressing– Specifies how to address web services.
– Provides a way to address a web service/resource pair
WS-Addressing
• WS-Resource Framework introduced a standard for addressing WS-Resources, called WS-Addressing
• Introduces an End-Point Reference (EPR) to point to the service, which will provide the URL and possibly additional information about the service.
