Transcript Four current methods of distributed computing

Distributed Computing Technologies –
Selecting an Appropriate Approach
Philip Maechling
Web Services Workshop
UNAVCO/IRIS Joint
Workshop
8 June 2005
Outline
• Discuss different approaches to distributed
computing with examples from real life.
• Based largely on: Seismic Hazard Analysis Using
Distributed Computing in the SCEC Community
Modeling Environment : March/April 2005 Electronic
Seismologist Column
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A New Computing Model for Scientific
Computing
• Can we utilize a distributed, componentbased computing model to do science?
• What technologies are available?
• What are the advantages/disadvantages
for each?
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A New Computing Model for Scientific
Computing
• Premise is that there is no “one-best” approach
that fits all Projects or needs (unfortunately).
• Encourage Discussion and groups experiences
with these technologies
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A New Computing Model for Scientific
Computing
Program 1 Program 2 Program 3 Program 4 Program 5
USC
P1
P2
IRIS
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P3
SDSC
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SCEC
P5
JPL
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A New Computing Model for Scientific
Computing
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Why is this a hard problem ?
• Multiple programming languages
• Multiple hardware platforms
• Multiple operating systems
• Multiple data types and data formats
• Reliability/Stability of components
• Discovering what is available
• Communications bandwidth and reliability
• Practical issues of support, development, tools,
infrastructure, funding etc.
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Characteristics of Technologies that impact
selection
•
Where does program run (Client Side or Server Side)
•
Input and output and output data types
•
How a components is called (API)
•
Support for Objects
•
Statefulness of components
•
Blocking versus non-blocking calls
•
Communication Protocol
•
Speed
•
Robustness
•
Scalability Number of Clients by a Server
•
Server Security
•
Client and Server Program languages
•
Peer to Peer between computers or Client Server Model
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Desirable Characteristics of a Solution
• Distributed
• Secure
• Multi-organizational
• Multi-language
• Loose coupling
• High cohesion
• Minimal modifications to existing codes
• Minimal costs
• Minimal training time
• Standards-based
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Component Characteristic
• Stateful
– A stateful component maintains information
about previous calls.
• Object Oriented
– An object oriented component allows you to
create an stateful instance of the
component. It provides user defined
interfaces to the component.
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Technologies Not Considered
Existing Distributed Computing technologies not consider
here due to limited inroads into geophysics community:
• Remote Procedure Calls (RPC)
• DCOM
• Enterprise Java Beans
• .Net
• Common Component Architecture (CCA)
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Client Side Techniques
Example Client Side Technologies
• Java Applets
• JavaScript
• Macromedia Flash
• Active X
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Client Side Techniques
• Java Applet Advantages
– Download the program into your browser
– Program runs on your computer
– Rich User Interfaces
– Reasonably Secure
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Client Side Techniques
• Java Applet Disadvantages
– Uses CPU on the client
– Distribution of code only, not shared computing
processing
– Required JVM in browser on client
– Security limitations on what applet can do (e.g. accessing
files on local machine may be restricted)
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Considering Using Java Applets When….
• Want rich interface including graphics
• Want users to use their own computer
• Want secure run-time on users platform
• Want to distribute new versions of code
• Want platform independence
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Common Distributed Programs
• Web Browsers
– Variety of Client side and Server Side execution
– Standards-based (W3C)
– Great Interoperability for non-dynamic content
– Interoperability issues typically on client side
technologies (JavaScript, Flash etc)
– Where’s the click stream
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Common Gateway Interface (CGI Scripts)
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HTTP based Program calls
• Parameters are passed as parameter_name=parameter_value pair
during invocation of cgi-bin or servlet
Examples:
• http://www.SOMEWHERE.org/cgi-bin/ice_cream.plx?flavor=mint
• http://66.102.7.104/search?q=cache:AU7xpnWlL0wJ:pyre.thirdbit.com/2004-fall/tomcat.ppt+History+of+Servlet+Containers&hl=en
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CGI Disadvantages
• May overwhelm server
• Security issues
• Not stateful
• Practical issue relating to web server
technologies such as web server capacities
and communications timeouts.
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Considering Using CGI When….
• Want minimal installation and operation
• Small number of users
• Want to use common
programming/scripting languages (perl,
php)
• Dynamic web pages
• Display data from database
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Java Servlets
• Java-based server side technology
• Adds memory and process management
(improves scalability) through the user of a
“servlet engine” or “servlet container”
• Security and performance improvements
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Java Servlet Interface
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Servlet Advantages
• Executable stays in memory once it is called
• Container provides memory and process
management
• Added safety and security
• Easy to add statefulness
• Server side code is Java, so it’s platform
independent.
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Servlet Advantages
• Executable stays in memory once it is called
• Container provides memory and process
management
• Added safety and security
• Easy to add statefulness
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Servlet Disadvantages
• Not Object Oriented Interface
• Not inherently Stateful
• Limitations on input and output types
• Practical issues with size of data in and out
(e.g. web server time-outs)
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Considering Using Servlets When….
• Want higher performance from server
• Want to protect server from being
overwhelmed by calls
• Want to build Dynamic web pages
• Display data from database
• Want platform independence on server side
code
• Want to work in Java including access to all the
Java Libraries
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Web Services
• Standards-based approach to distributed
computing.
• Follows Model of Web Browser/Web Server
Technology
• Establishes Exchange Format (XML),
Communication Protocol (SOAP), and Interface
Definition (WSDL)
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Web Services Interface
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Web Services XML Request/Response
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Web Services Advantages
• Could transform distributed computing like WWW
has transformed information sharing
• Standards-based
• Uses standard ports
• Interoperability between languages
• Hardware independent
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Web Services Disadvantages
• Not Object Oriented Interface
• Not Stateful
• Limitations on input and output types
• Trades performance for interoperability
• Many standards
• Large data transfers typically done out of band losing
interoperability advantages
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Considering Using Web Services When….
• Interoperability is more important
performance
• Want to interoperate between languages
• Want standards based system
• Want access to commercial tools and
techniques
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Common Object Request Broker
Architecture (CORBA)
• CORBA is a specification for distributed object
technology
– Language independent (Supports C, C++, Fortran,
Java, others)
– Full Object Support
– Robust communication Protocol
• Stable, mature CORBA implementations are
available including open source and real-time
versions.
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CORBA Operation
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CORBA Operation
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CORBA Advantages
• Object oriented, robust, well supported
• Many features such as object discovery
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CORBA Disadvantages
• Fairly complex
• May have more features than you need
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CORBA Pro/Cons
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Considering Using CORBA When….
• Multi-language support
• Very high reliability
• Object oriented systems
• Dynamic discovery of object and services
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Java Remote Method Invocation (Java
RMI)
• Java RMI is distributed object technology
developed to work with the Java Language
• Comprehensive, Object-oriented, platform
independent, robust, mature
• Distributed without cost by Sun
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Java RMI Architecture
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Java Remote Method Invocation (RMI
Interface)
import java.rmi.Remote; import java.rmi.RemoteException;
public interface EUStats extends Remote
{
String getMainLanguages(String CountryName) throws
RemoteException;
int getPopulation(String CountryName) throws
RemoteException;
String getCapitalName(String CountryName) throws
RemoteException;
}
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Java RMI Disadvantages
• Java RMI only supports Java
• Proprietary protocol by single vendor
• Requires RMI-lookup
• Requires non-standard port
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Java RMI Advantages
• Full object support
• Cross platform capabilities
• Robust communications
• Large objects
• Security for client and servers
• Distribution/updates of codes
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RMI Pros/Cons
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Conclusions
• Distributed computing allows sharing and
re-use
• Introduces uncertainties
• Explore existing solutions before inventing
your own.
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