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Understanding DAP4: Enhancements in Data Access Protocol for Modern Web Services

This presentation explores the key features and improvements of DAP4, the latest version of the Data Access Protocol. It highlights the shift from DAP2, which focused solely on the data model, to DAP4, which integrates specifications for web services, making it a more comprehensive tool for managing and accessing data. Key topics include the Dataset Services Response (DSR), content negotiation, the new definitions of coverages and shared dimensions, and how DAP4 adheres to REST principles while providing enhanced capabilities for data subsetting and filtering.

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Understanding DAP4: Enhancements in Data Access Protocol for Modern Web Services

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Presentation Transcript

  1. DAP4 Introduction James Gallagher OPeNDAP 1/10/14

  2. Agenda • Input on a Developer’s meeting • All about DAP4 • The CDM, too • Comparison and discussion of DAP4 and CDM

  3. DAP3 DAP4 • DAP2: DDS, DAS, Data • DAP4: DSR plus DMR, Data • This looks like the ‘name game’ but it’s a real difference! • While DAP2 was a data model only, DAP4 includes specification of the web services

  4. DAP4 Data Model • Coverages: Replace Grids with a more general model: Arrays, shared dimensions and maps • ‘Coverage’ is slang for ‘discrete coverage’ as defined by OGC’s abstract coverages specification • Maps define the Domain of a discrete function • The Array defines the Range • Shared dimensions provide the connection between Domain and Range • Tabular data: Sequences (a nest-able table) • Projection: subsetting based on data types • Filters: subsetting based on values

  5. The Services Response (DSR) • DSR: Dataset Services Response • DSR: Dataset Services Response is the ‘service endpoint’ • This contains links to all of the other responses available from the server for a specific dataset • A key point for/of REST • In DAP2 the ‘base URL’ was undefined; in DAP4 the base URL for a dataset is the DSR

  6. DSR and Content Negotiation • DSR provides links to all of the other responses • However, a server must implement Content Negotiation. • i.e., a browser asking for the DMR winds up displaying a (HTML) web interface while a smart client parses XML

  7. REST • We have adopted a strict view of REST • Not all of DAP4 fits; DAP4 is not REST • The DAP4 web services are REST • The DAP4 data/constraint model is not

  8. Dataset Metadata (DMR) • This response holds all of the variables and • Attributes • This defines the environment in which the Constraint will be evaluated

  9. Data and Constraints • As with DAP2, Data is returned in a two-part response • Can request one or more variables • Can ‘slice’ the dimensions of array variables • Can filter both Tabular and Array data • (because a client must understand the data model and that cannot be expressed by links, this part of the ‘web API’ is not REST).

  10. Examples… • The current draft specification has lots of examples: • http://docs.opendap.org/index.php/DAP4:_Specification_Volume_1 • The ‘test’ servers for DAP4: • http://54.204.231.163:8080/opendap/data/reader/dap4/

  11. JSON • Simple responses for single atoms - Data • Metadata (DMR) encoding

  12. Summary • DAP (DAP2 and DAP4) is based on datasets built of variables that share the characteristics of programming languages • Constraints are used to subset data on the server • DAP4 is a REST API • DAP4 specifies ‘modern’ web services

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