differences between DAP2 and DAP4

DAP4 Introduction
James Gallagher
Input on a Developer’s meeting
All about DAP4
The CDM, too
Comparison and discussion of DAP4 and CDM
• DAP2: DDS, DAS, Data
• DAP4: DSR plus DMR, Data
• This looks like the ‘name game’ but it’s a real
• 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
• 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
• However, a server must implement Content
• i.e., a browser asking for the DMR winds up
displaying a (HTML) web interface while a
smart client parses XML
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
• 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).
• The current draft specification has lots of
• http://docs.opendap.org/index.php/DAP4:_Specification_Volume_1
• The ‘test’ servers for DAP4:
• Simple responses for single atoms - Data
• Metadata (DMR) encoding
• 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
• DAP4 is a REST API
• DAP4 specifies ‘modern’ web services

similar documents