1. OPeNDAP/Hyrax Interfaces
James Gallagher

2. What are...?
OPeNDAP: A not-for-profit corporation which steers the growth of the DAP. All software it develops is open source.
DAP: Data Access Protocol - a simple request-response protocol which uses HTTP to move structured (scientific) data from a server to a client.
Hyrax: A data server developed by OPeNDAP that supports DAP.

3. Types of Interfaces
Application
Server
Software
libraries
Component

4. Application Level DAP Requests made with a URL
DAP Responses returned as MIME documents
DAP is stateless; implementation using REST
DAP response form is a ‘universal representation’ (canonical) for data - it’s generated by a server (nominally) on the fly without any need to modify the original data.
DAP servers for over a dozen very different formats exist
DAP Servers also support ‘Web’ features
Other protocols that work with the Web (e.g., THREDDS) easily coexist with DAP
See Data Access Protocol (DAP), version 2

5. Server: Hyrax
Front end (OLFS); Java; Processes requests and uses the BES
Back end (BES); C++; Builds responses and responds to OLFS commands
N-Tier architecture
OLFS and BES are extensible
See Hyrax

6. Hyrax: Many Geometries
Simplest... one host or two
OLFS
BES
OLFS
BES
One OLFS talks to many BES daemons
BES
OLFS
WCS
Many OLFS front-ends talk to one BES

7. Components: BES Format handlers: Read data
Response handlers: Build new responses
Reporters: Record usage information
Aggregators: Build aggregate responses
Implement C++ classes
New modules are loaded at runtime
See Hyrax: For Developers, Extending the BES

8. Components: OLFS Extending the OLFS New Dispatch handlers
See Writing Custom DIspatch Handlers

9. Software Libraries From OPeNDAP: C++, C, Java OPeNDAP/Unidata: NetCDF
Other groups: Python

10. OPeNDAP, DAP and Hyrax
OPeNDAP: Runs the open-source project developing and maintaining DAP. OPeNDAP makes a conscious effort to work with other groups.
DAP: A simple protocol, able to take advantage of HTTP’s wide-spread use, that has proven to work well with a variety of data.
Hyrax: A modular, extensible, multi- protocol server for DAP.

11. More Detail about DAP…

12. DAP Request Processing
DAP requests are made using URLs
URL: <pathname>.<request>[?<constraint>]
Pathname: names a unique data source
Request: what the client wants
Constraint: optional modification to the request

13. Request Processing
The OLFS parses the URL into the pathname, request and constraint
It uses its knowledge of the BES commands & capabilities to instruct the BES to build the response that will satisfy the request
It packages the resulting response from the BES (e.g., builds a HTTP ‘text/plain’ response document and inserts the correct content).

14. DAP Constraints
DAP characterizes data sources as a collection of variables (name-type-value tuples).
Constraints are used to ‘project’ and ‘select’ particular variables with a data source.
Using a constraint, part of one variable may be accessed/read/returned from a data source with many variables.
NB: DAP also uses ‘attributes’ to characterize data sources, but attributes don’t play a direct role in the access of/to binary data values.

15. BES Handler Interaction
Handlers can:
Build responses directly from data
Use output from other handlers
Handlers respond to commands from the ‘command handler’
The command handler can be replaced to replace the BES’s default command syntax
Handlers are not limited to DAP

16. DAP Objects
Metadata
Syntax - essentially variable declarations that describe the names and types of the variables in the data source. Referred to as DDS - Dataset Descriptor Structure.
Use - Information about those variables. Often read from the ‘attributes’ in a file (e.g., HDF4) or from some other source. Referred to as DAS - Dataset Attribute Structure.
Data (referred to as ‘DataDDS’)
Combines DDS for the requested variables along with binary data values. Binary data are encoded in XDR.

17. DAP Responses DAP responses are returned by HTTP as ‘documents’
Metadata - both DAS and DDS use text/plain
Data - uses application/octet-stream
Alternatives: Content of both metadata documents may be returned as a single XML response (text/xml) and Data as a multipart document

18. Building Responses
Responses to requests are built from the one or more of the basic objects.
Handlers in the BES ‘pass around’ DAP objects, using them as a generalized representation of the data source.
Handlers in the BES that generate responses can effectively be combined with any ‘format’ handler.’

19. Other Responses
Data may be processed and it’s return form changed (e.g., ‘return as ASCII’)
Metadata may be used to build ‘info pages’ or Web Uis
Other information may be returned (software & protocol version numbers, data source organization, et c.)