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Exposure Time Calculator (ETC) as a Web Service

Donald McLean

2004 Technology Open House

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Exposure Time Calculator (ETC) as a Web Service

What is a Web Service?

Why would the ETC be a good candidate for conversion to a web service?

What are some possible uses of an ETC web service?

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What is a Web Service?

• A server that can communicate via a standardized XML messaging system

• Web services protocols are intended to be platform independent

• Most implementations are either Java or .Net based

• Vast variety of vendors, all muddying the waters with different implementations

• Responsible for generating dizzying numbers of new buzzwords

…But What Does That Mean?

• Web services are a new model of distributed computing

• Web services are mainly for use by other applications– A repair shop ticket system could use a web

service of its parts supplier to automatically order needed parts

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Web Services Architecture

Service Requestor

Service Provider

Discovery Agency

Publish Service Description

Find

Interacts

For more information: http://www.w3.org/TR/ws-arch/

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Standards

• Four standards define the critical elements of web services:– Simple Object Access Protocol (SOAP)

describes handshaking with server– Extensible Markup Language (XML)

describes format of the request and response– Web Service Definition Language (WSDL)

allows servers to describe services being offered– Universal Description, Discovery, and Integration

(UDDI)Protocol for listing services in a directory

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ETC

• Previously a Java application, part of Scientists Expert Assistant (SEA) project

• Tomcat web servlet application

• Mostly Javauses external “SYNPHOT” for some key calculations

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Why A Web Application?

• Cultural– Existing ETCs were web applications

• Performance of desktop application– Communication with remote SYNPHOT server

was a serious weakness– Older computers could be frustratingly slow

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Why Not An Application?

• Original implementation (remote SYNPHOT) slow and problem prone

• Local SYNPHOT with remote files worse– No Windows version of SYNPHOT

• Local SYNPHOT with local files– Still no Windows version of SYNPHOT– Huge (500MB) data library– Data library must be kept up-to-date

• Internet connection is more or less required

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Why A Web Service?

• ETC already uses XML– Log of each request

• Used in testing and debugging

– Log of each result• Used to render result page

• Used to render previous result page

• Can throw hardware at performance

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ETC As Web Application

UserHTTP Get Request

XML Request

Log

ETC Servlet Request

Local SYNPHOT

HTML Response

XML Response

Tomcat Web Server

ETC Library

Response

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Adding Web Service

UserHTTP Get Request

ETC Servlet Request

Local SYNPHOT

HTML Response

Tomcat Web Server + WS Extensions

ETC Library

Response

External Application

XML RequestETC Web

Service Request

XML Response Response

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XML Request

Log

XML Response

Possible Use: APT Integration

• The original SEA had an integrated ETC• One cycle 13/phase 1 comment requested access

to the ETC from within APT• APT and VTT use network interfaces to access

other services• Tools could be designed to run multiple scenarios

(another cycle 13/phase 1 comment)• APT users would need to supply information not

currently needed

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Possible Use: VO Search Tool

• Calculate limiting magnitude of an image

• See Voisin et al, “Simulating Instruments For Mining Uncalibrated Archives”, in Optimizing Scientific Return for Astronomy through Information Technologies, edited by Peter J. Quinn, Alan Bridger, Proceedings of SPIE Vol. 5493 (SPIE, Bellingham, WA, 2004) pp 294-301

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Other Uses

• Available to any application that supports WSDL translation

• Web services are an “enabling technology” that make it possible for applications to be used in ways not imagined by their designers

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