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O Pe NDAP Actor Use Case 02

Locating, subsetting, and importing DAP data into Kepler: Actor Implements Sub-Setting Interface


1.0.0 Initial Draft (ndp)

1.0.1 Minor edits (jhrg)

1.0.2 Imported into REAP wiki.


A user uses a Kepler Actor to locate and subset DAP data, using a graphical interface provided by the Actor. The user may subset by asking for specific variables and by decimating arrays. Relational subsetting is not supoorted. The data becomes available in the Kepler data model for further processing.


  • User.
  • Kepler software application
  • OPeNDAP Kepler "Actor"
  • A server on the web with DAP accessible data.


  • A list of OPeNDAP data sources, in HTML, is available via HTTP.


Basic course of events:

  1. The user starts the Kelper application.
  2. The user creates a new Kepler Workflow.
  3. The user adds an instance of the OpendapDataSource Actor to the Workflow.
  4. The user configures the OpendapDataSource Actor by:
    1. Right-clicking on it and selecting "Configure Actor" from the Actor's pop-up menu. This will cause a window to appear that contains 2 panes.

      The first pane contains an embedded browser. The default page is the OPeNDAP Data Sources page at

      The second pane contains an exandable/collapsable check box Tree.
    2. The user navigates in the browser pane to a DAP server that contains data that they wish to use in Kepler.
    3. The user selects a data set to access. The browser intercepts outgoing requests for the OPeNDAP HTML data request form, or any DAP data product. It will display the DAP "info" response.
    4. In the second pane the DDS of the selected data set will be displayed as a check-box Tree. Container variables (Structures, Grids, and Sequences) may be expanded and some or all of their children may be selected by clicking the check-box associated with the variable. If the variable is an array, then the array subsetting interface will appear in the tree as part of the array node.
    5. The user navigates the check-box Tree selecting variables and specifying array subsetting parameters. When satisfied the user presses the "done" button. The user SHOULD subset array variables such that they are reduced to 1 or 2 effective (see Note 1) dimensions.
  5. The Actor constructs a DAP constraint expression (CE) by querying the Tree's state. (See Note 02) If no variables are selected then the CE is empty, which is valid.
  6. The OpendapDataSource Actor will reconfigure it's output terminals to reflect the variables specified in the CE. If the CE is null then the

Alternative Paths:

Previous use (if the OPeNDAP "Actor" has been used before):
4.1 If the Actor has been previously configured it is the last page viewed. The remaining steps are the same.

Existing Workflow:
2 The user opens a Workflow stored in a file.
3 Goto Step 4. (Main Path)


  1. DAP data is now available to Kepler actors by connecting to the data source terminals on the configured OpendapDataSource Actor.
  2. The User will need to connect one or more of these data source terminals to additional Kepler Actors for data processing and display.

Business Rules:

Proposed: Have the OPeNDAP Actor enforce array subsetting rules such that arrays must be subset to 1 or 2 effective dimensions (See Note 01)


  1. Multidimensional arrays can subset so that their "effective" dimensionality is reduced by specifying (int the DAP constraint expression) a single value (row?) for one or more of the array's dimensions.
  2. The CE is made up of a "projection" and a "selection". The "projection" specifies which variables in the data set that the user wishes to access. The "selection" is a series of clauses that specify which conditions data must meet. If the user requests array variables then they SHOULD provide a CE that subsets them in order to reduce their effective dimensions to 1 or 2. Array's of dimensions 1 or 2 can be represented efficintly in the Kepler data model as a MatrixToken.


Nathan Potter, 12/11/2007

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This page last changed on 10-Dec-2007 14:13:27 PST by uid=potter,o=unaffiliated.