Open Data Access Protocol

1
Open Data Access Protocol

• Webb Pinner
• May 2, 2005
• CSC 536

2
Acknowledgements

• Peter Cornillan - Graduate School of
  Oceanography, DODS
• James Gallager - OpenDAP.org
• Tom Sgouros - OpenDAP.org
• John Howland - Deep Submergence Lab, Woods Hole
  Oceanographic Institute

3
Outline of Presentation

• Background Information
• The OpenDAP protocol
• Oceanographic Dive Reply System
• Summary
• Questions?

4
Background

• DODS was created in 1993 by URI, GSO, MIT, NASA
  and NOAA for use in oceanographic datasets.
• It's purpose was to "define the structure of a
  client-server based distribution system for
  access to oceanographic data over the Internet

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Background (cont.)

• DODS was quickly adapted for use by the
  atmospheric data community.
• Because DODS no longer dealt with strictly with
  ocean data the name was changed to the Data
  Access Protocol
• 2002 the project was made open source and thus
  OpenDAP was created

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The OpenDAP Architecture

• Provides uniform interface to data
• Allows SQL-like data querying
• Read-only access to data
• Platform independent
• Client/server model
• For use over Internet (TCP/IP)

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OpenDAP Implementation

• Uses http as transport protocol
• Server implemented as CGI extension in Apache
  webserver
• Requests for data are sent to server in the form
  of a URL
• URL sent to server describes exactly what data is
  requested
• Includes which columns and/or data constraints.

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Example URL

• http//dap_server/cgi-bin/nph-dods/data/dataset1
• http - protocol
• dap_server - servername
• cgi-bin/nph-dods - dap server software location
• data - director where dataset is located
• dataset1 - name of dataset
• Extension determines which service is called
• Constraints may be added to request specific
  datasets

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OpenDAP Implementation(Services)

• Three basic services
• Data Descriptor Service (DDS)
• Data Attribute Service (DAS)
• Data (DODS)
• Additional services
• HTML form (HTML)
• ASCII representation (ASCII)

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OpenDAP Implementation(Constraints)

• Constraints specify which fields to display and
  in what order to display them.
• http///data.dods?datetimetemp
• Constraints specify search criteria.
• http///data.dods?tempgt100templt175

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Current Users

• NASA - Global Hydrology Resource Center
• NOAA - National Data Buoy Center (NDBC)
• NOAA - National Oceanographic Data Center (NODC)
• University of Hawaii/SOEST - Sea Level Center
  (SLC)
• Woods Hole - Upper Ocean Mooring Data Archive
  (UOP)

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OpenDAP DEMO
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OpenDAP Middleware/Dist. System?

• Kinda, sorta, not quite
• Uniform interface to data (Middleware)
• User is unaware of OS, storage format or access
  protocol (Middleware)
• Data distributed across multiple servers (Dist.
  System)

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OpenDAP Middleware/Dist. System?

• Read-only Access
• Each Dataset is independent of other datasets
• No such thing as replication, commit protocols,
  concurrency control, etc.

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Oceanographic Dive Replay System
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Oceanographic Dive Replay System

• Replay a dive complete with textual data and
  images
• Retrieve data from remote servers
• Provide a graphical means to interact with large
  ascii datasets
• Provide means to search dataset

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WHOI - Virtual Vans
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WHOI - Virtual Vans

• Replays a dive complete with textual data and
  images
• Retrieves data from WHOI server
• Provide a HTML based GUI
• Provide means to search event data

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Virtual Vans - Shortcomings

• Unable to search by any other dataset
• Unable to create custom datasets for downloading
• GUI can not dynamically accommodate new sensors.
• All datasets stored at one location

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OpenDAP Replay System

• Use DAS, DDS information to dynamically generate
  interface
• Use URL constraints to search based on any data
  type
• Use URL constraints to customize downloaded
  datasets
• Multiple servers may provide datasets

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OpenDAP Replay System
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Summary

• Introduced the OpenDAP protocol, its history,
  architecture and current uses
• Quick OpenDAP demo
• Case Study Dive Replay System
• Looked at existing system and its faults
• Discussed a new OpenDAP-based systsm and its
  advantages

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Questions?