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

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

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

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

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

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

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

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

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).

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/

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

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