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Overview
The unstructured grid services prototype will be capable of delivering unstructured grid numerical model output in a standard form through a dap service which will allow for subsetting and time averaging of the data.

UW, OpenDAP, and UCSD are building a pilot demonstration of interoperability between Gridfields and Hyrax via a subsetting operation over an unstructured grid data source returning a UGRID-compliant dataset.  All code and supporting documentation will be deposited on the OOI Confluence website.

The demo will service an OpenDAP request over unstructured grids.  The request will perform simple subsetting.  The returned stream will be compliant with the UGRID model being developed by David Stuebe, Rich Signell, and Bill Howe.  The request url may include a custom dispatch handler call of the form

...&subset(expr)

where expr is a conditional expression involving the attributes of the source grid.  For example, a bounding box expression "x between 29000 and 31000 and y between 28000 and 31000" can be encoded as follows:

...&subset(x<31000%26x>29000%26y<31000%2628000>y)

The Hyrax server will translate this expression into an equivalent GridField query, evaluate it, format the result, and resturn the result using the UGRID format.
 

Technology

1) GridFields is a library for manipulating unstructured grid datasets that are difficult to handle with conventional tools such as NetCDF (see appendix).  In order to be useful within the OOI framework, we need to show that the GridFields library can be integrated with the Hyrax Server using the Custom Dispatch Handler.

2) Hyrax Data Server is a framework to support multiple transport-level communication and interprocesscommunication protocols. The OLFS component is responsible for implementation of DAP over HTTP. This component receives DAP requests over HTTP, parses the request and makes a series of requests to the BES component that result in a serialized DAP object. The OLFS then packages that serialized DAP object as specified by the DAP over HTTP specification and returns the result. Important features of this architecture are that the OLFS and BES need not run on the same computer, can employ secure (SSL) communications, and need not be written in the same programming language (in fact, in Hyrax 4.x the OLFS is written in Java while the BES is written in C++). Finally, the protocol used to communicate between the OLFS and the BES has been designed so that one OLFS can 'talk' to several BES instances.

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