Add the NcML handler to the BES

From OPeNDAP Documentation

Back: AIS Using NcML or BES Aggregation using NcML

1 Description

Give a short descriptive name for the use case to serve as a unique identifier. Consider goal-driven use case name.

NcML_Handler_Configuration_00

2 Goal

The goal briefly describes what the user intends to achieve with this use case.

A person who is configuring a BES daemon adds the NcML handler and viola, the AIS and whatever else we do with NcML works.

3 Summary

Give a summary of the use case to capture the essence of the use case (no longer than a page). It provides a quick overview and includes the goal and principal actor.

The NcML handler creates a synthetic data set using some extra information, written using NcML, and an existing data set. The new information is added to, or replaces, the existing information in the data set.

NcML has the syntax to add to, supplant or remove attributes from a data set. It can also add new variables to a data set. In the design that goes with this use case we are proposing to implement only the attribute part of the NcML syntax initially and then add the capability to add variables later.

This use case covers just how the NcML handler is added to the BES and how it treats the .ncml files to make the synthetic data sets.

4 Actors

List actors, people or things outside the system that either acts on the system (primary actors) or is acted on by the system (secondary actors). Primary actors are ones that invoke the use case and benefit from the result. Identify sensors, models, portals and relevant data resources. Identify the primary actor and briefly describe role.

A data provider or system administrator.

5 Preconditions

Here we state any assumptions about the state of the system that must be met for the trigger (below) to initiate the use case. Any assumptions about other systems can also be stated here, for example, weather conditions. List all preconditions.

  1. The BES is installed and configured, except for the NcML handler
  2. There are data sets
  3. There is at least one NcML file under/within the BES' DocumentRoot

6 Triggers

Here we describe in detail the event or events that brings about the execution of this use case. Triggers can be external, temporal, or internal. They can be single events or when a set of conditions are met, List all triggers and relationships.

The trigger is the need to add this feature to the Hyrax data server. In the case of the REAP or IOOS projects, the NcML Handler will implement the AIS that will be used to build documents these projects need. Satisfying the requirements for those projects will be the triggers. In general, the trigger will be the need to use the features of the AIS with one or more data sets.

If the NcML handler supports remote data sets, then a second type of trigger might be the desire to provide additional information (using the AIS) for a remote data set.

7 Basic Flow

Often referred to as the primary scenario or course of events. In the basic flow we describe the flow that would be followed if the use case where to follow its main plot from start to end. Error states or alternate states that might be highlighted are not included here. This gives any browser of the document a quick view of how the system will work. Here the flow can be documented as a list, a conversation or as a story.(as much as required)

  1. User (installer/configurer) installs (builds and installs or installs a binary) of the NcML handler code
  2. The user edits the BES.conf file so that any .ncml files are routed to the NcML handler
    1. To do this, the NcML handler is bound to files matching a certain regular expression int eh bes.conf file just as is the case with other handlers.
    2. The NcML file(s) are stored in a directory under the BES's DocumentRoot.
  3. The user (re)starts the BES

Here are two relevant lines from the bes.conf file before adding the handler to the server:

BES.modules=dap,cmd,ascii,usage,www,nc,h4
BES.Catalog.catalog.TypeMatch=nc:.*\.(nc|NC)(\.gz|\.bz2|\.Z)?$;h4.*\.(hdf|HDF|eos)(\.gz|\.bz2|\.Z)?$;

We would add a ncml handler to the BES.modules line and a regular expression for it to the Typematch line like this:

BES.modules=dap,cmd,ascii,usage,www,nc,h4,ncml
BES.Catalog.catalog.TypeMatch=nc:.*\.(nc|NC)(\.gz|\.bz2|\.Z)?$;h4.*\.(hdf|HDF|eos)(\.gz|\.bz2|\.Z)?$;ncml:.*\.ncml

Thus any request with file that ends in .ncml (i.e., that matches teh regex .*\.ncml) as its target will be handled by the NcML handler.

8 Alternate Flow

Here we give any alternate flows that might occur. May include flows that involve error conditions. Or flows that fall outside of the basic flow.

9 Post Conditions

Here we give any conditions that will be true of the state of the system after the use case has been completed.

10 Activity Diagram

Here a diagram is given to show the flow of events that surrounds the use case.

11 Notes

There is always some piece of information that is required that has no other place to go. This is the place for that information.

The basic system design so far assumes that the NcML files will reference data sets using file:/ URLs and that may introduce a problem with resources early on because that will mean that the data must be local to whomever is writing the NcML files. This will mean that either we have to serve all the data to experiment with the system or we have to rely on others to write that metadata for us. Either option seems doomed to fail. If we provide the NcML handler with the capability to work with remote data, even if it's horribly inefficient, we can use the full data sets and experiment with different metadata microdocuments, then have installed at the server sites ones we're fairly confident are good. The extra time spent adding this feature will almost certainly be a sound investment.

The downside is that we have to be able to remove this from anything we distribute unless we put the entire client-side of libdap through a security audit.

12 Resources

In order to support the capabilities described in this Use Case, a set of resources must be available and/or configured. These resources include data and services, and the systems that offer them. This section will call out examples of these resources.

12.1 Other Resources

Resource Owner Description Availability Source System
Name Organization that owns/ manages resource Short description of the resource How often the resource is available Name of system which provides resource
BES & the NcML handler Data Provider The data server Once installed, it can be hard to get the admin to make changes N/a
Data set The data provider This is the thing that we're adding information to Depends on how easy it is for the metadata writer to get access to the server's/data's file system Hardware running the server and/or staging the data.
Metadata writer Hopefully not owned, since this is a person ;-) This is the person who figures out which XML micro documents are needed for a particular data set Hard to tell at this stage. The institution that hosts the data
In-House (opendap) metadata writer opendap Like the on-site metadata writers, this person figures out what metadata are needed and adds that to files served by the NcML handler In this case, the data would be local or the NcML handler would be configured to work with remote resources (using http:// URLs in addition to file:/ URLs). opendap/IOOS/REAP