1. James Gallagher OPeNDAP 1/10/14
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:
The ‘test’ servers for 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