- 1 OPULS Process
- 2 DAP4 Specification
- 3 DAP Use Cases
- 4 DAP4 Features Straw man
- 5 Proposals
- 6 Commentary on DAP4 Related Topics
- 7 OPULS Implementation Thoughts
- 8 OPULS references
- 9 Old DAP4 Design and Implementation
1 OPULS Process
Short version: Each proposal is in one of the following categories:
- Under Consideration;
- Under Revision; or
2 DAP4 Specification
This section is deprecated in favor of the top-level DAP4 Specification page.
The draft specification has two volumes.
- Overview: New Features Introduced in DAP4
- Volume 1: Data Model, Persistent Representation, and Constraints
- Volume 2: Web Services Specification
- Volume 3: DAP4 Extensions
- DAP4 Extensions
3 DAP Use Cases
As part of the DAP2 --> DAP4 transition process, we are soliciting use cases regarding DAP client application development from the community. If you'd like to submit a use case, you can send it to any of the OPULS developers (e.g., James Gallagher) or the OPeNDAP technical discussion list. If you'd like you use case to be anonymous, that's fine, just let us know.
We will use these use cases to spot likely problems that may be encountered by members of the community who have developed specific client applications that interact with DAP servers.
3.1 Use cases from GSFC
These use cases were submitted by Chris Lynnes:
- Use Case #1
- Spatial/Variable Subsetter. Our Simple Subset Wizard (SSW) works by constructing URLs to download spatial / variable subsets from the server as netCDF files (http://disc.gsfc.nasa.gov/SSW). This has allowed us to stop writing custom subsetter programs for each dataset or group of datasets.
- Use Case #2
- Giovanni (http://giovanni.gsfc.nasa.gov/giovanni/) provides data exploration capabilities for datasets from GES DISC as well as a few select other providers. Data are acquired from servers via OPeNDAP, saved as netCDF/CF1, which is the Giovanni internal standard. This allows the deployment of many netCDF-capable tools for analysis and data manipulation purposes.
- Use Case #3
- MapServer provides WMS services for GES DISC data. Rather than site individual instances on each data server, we use the .vrt (Virtual files) capability in MapServer to set up the OPeNDAP connections needed to acquire data for the MapServer.
- Use Case #4
- We are currently experimenting with the NcML aggregation capability to see if it is feasible to offer the ability to generate time series at a point using just Hyrax and the NcML handler.
4 DAP4 Features Straw man
This is a straw man design, broken down into sections. It is incomplete and its content is subject to revision. This was our foil; don't use it. The two documents about under 'DAP Specification' are the current state of the DAP4 specification.
Versions Checksum DAP Service Terminus Data Model and a XML Schema that provides one representation for the data model. Requests Responses Web Services (Starting Point)
5.1 Data model
|Grids Ideas||Accepted||Dennis, et al.||(Cleaned up 4/10/2012)|
|XML Use in DAP4||Accepted||Dennis||2/25/2012|
|Top Level Group||Accepted||Dennis||4/12/2012|
|Proposal for Supporting Keys in Sequences||Revised||Dennis||5/28/2012|
|Constraints and Shared Dimensions||Under Consideration, closely related to Grids Ideas||James & Dennis||Modified 04/12/2012|
|Ideas for Defining VLens (and Sequences)||Under Consideration||Dennis||02/25/2012|
|Path Specification in DAP4||New||Dennis||3/30/2012|
|Coordinate Systems Element||New||Dennis||4/10/2012|
|Subsetting Arrays and Grids By Value||New||Nathan||4/27/2012|
|Proposal for a Constraint Notation||New||Dennis||4/30/2012|
|Proposal for an Abstract Constraint Specification||New||James, ed.||5/22/2012|
|Proposal for Structure Projection||New||Dennis||5/1/2012|
|Old Version of Grids Proposal||Obsolete||Dennis||4/10/2012|
|Filter Constraints||new||Dennis, John, Ethan||9/17/2012|
|Constraint Expressions||new||Everyone; original note by Dennis, ed. by James||10/29/2012|
|DAP4 Projection Syntax||withdrawn||Dennis||2/8/2013|
|Proposed Changes to Checksumming||accepted with modifications 3/19/2013||Dennis||3/1/2013|
|Proposal for new VLEN representation||New||Dennis||7/21/2013|
|Proposal for a Comprehensive Constraint Expression Syntax||New||James, Dennis, et al.||7/29/2013|
|Alternate Proposal for a Constraint Expression Syntax||New||Dennis||10/16/2013|
|Type 1 Server Side Functions Proposal||New||Dennis||1/4/16|
5.2 Responses Specific to DAP4
|Proposal to use Multi-part Mime||new 3/19/2013||Dennis||3/1/2013|
|DAP4: Inclusion of response metadata in the DMR||New||James||7/23/2013|
|DAP4 DDX Grammar||Under Consideration||Dennis|
|DAP4: DDX Lexical Elements||Accepted||Dennis||2/28/2012|
|Character Escape Mechanisms in DAP4||Accepted||Dennis||4/5/2012|
|DAP4: Encoding for the Data Response||Accepted||James, based on Proposed DAP4 On-The-Wire Format by Dennis||6/5/2012, updated 8/31/12|
|DAP4: Chunked encoding||Accepted||James||6/8/2012, updated 8/31/12|
|Proposed DAP4 On-The-Wire Format||Obsolete||Dennis||4/8/2012|
|Proposed Multipart Mime Format||Obsolete||Dennis||4/8/2012|
|Proposed Error Response Format||Obsolete||Dennis||5/8/2012|
|Proposal to Replace Chunking||Obsolete||Dennis||8/23/2012|
5.3 Web & HTTP
|DAP4 Constraint expressions and query strings.||Under Consideration||Nathan Potter||09/24/2012|
|Possible Notation for Server Commands||New||Dennis||Modified 04/12/2012|
|DAP4 Asynchronous Request-Response Proposal (new new version)||Accepted||Ethan Davis and Nathan Potter and James Gallagher||6/15/2012; updated 1 Aug 2012|
|DAP4 Dataset Services Response||Accepted||Nathan Potter||6/19/2012|
6 Commentary on DAP4 Related Topics
These are not intended as specific proposal, but rather as commentary on some issues with DAP4 that need addressing with specific proposals.
- Characterization of URL Annotations (Modified: 2/26/2012)
- Constructing a DDX from a Query (new: 04/12/2012)
- Notes on Constraints (new: 04/29/2012)
- DAP4: An Essay on Domain Specific Models (new: 05/05/2012)
7 OPULS Implementation Thoughts
7.1 Unidata's Current Thoughts
1) DAP4 C client library for use in netCDF-C library 2) DAP4 Java client library for use in netCDF-Java library 3) DAP4 Java server library for use in TDS 4) Conformance test service against which a DAP4 client library can be tested 5) Conformance test service against which a DAP4 server can be tested
The current CDM and TDS use of opendap-java library has performance problems due to extra API layers and extra copying of data. We would prefer direct serialization between CDM objects and the DAP on-the-wire data and would recommend that the OPeNDAP Java library leverage the netCDF-Java libraries.
If there are needs that the DAP4 opendap-java library be independent of the netCDF-Java library, then in order for CDM and TDS to use the opendap-java library, we would need the performance concerns mentioned above addressed. A reasonable metric would be within some percentage of cdmremote, say within 20-25%.
Unidata's performance requirements for Java libraries:
1) The netCDF-Java library performance reading DAP4 through the DAP4 Java client library must perform within 20-25% of cdmremote . 2) TDS performance when handling DAP4 requests using the DAP4 Java server library should perform within 20-25% of cdmremote .
We believe this means that the DAP4 Java libraries must provide a sufficiently generic API that can be used directly with the CDM/TDS IOSP interface.
1) Write the DAP4 C client library. 2) ??? some part of Java server library 3) ??? some parts of conformance testing services
 cdmremote is an experimental CDM streaming service. It is basically a direct serialization of the CDM.