Relational Database Handler

In order to provide support for a Sensor Observation Service (SOS) implementation Hyrax will need a handler that allows it to access data in a Relational Database Management System (RDBMS). (This requirement stems from the observation that many of our stake holders store their in situ measurement data in RDBMSs)

It is anticipated RDBMS handler will have a much broader application than just SOS related data sets.

Use Cases

1. Adding the RDH to the BES
2. RDH handles bes:showCatalog request
3. RDH handles a DDX request
4. RDH handles a DDS request
5. RDH handles a DAS request
6. RDH handles a DAP2 data request

Definitions

row set

A row set is an object which encapsulates a set of rows. Database tables are row sets. A database view is a row set. SQL queries return row sets. SQL JOIN operations take row sets as input and produce row sets.

Background

In the past a Java servlet called the DODS Relational Database Server (DRDS) was used to provide DAP access to RDBMS holdings. However this older implementation has a number of shortcomings that preclude it's direct use in our current server architecture:

• No longer supported.
• Uses the Java DAP implementation
• Not a BES module
• Significant memory limitations
• Difficult to configure/localize

Although there has been continuing interest in a DRDS replacement within the OPeNDAP community, no funding has been available to develop a soution until recently. The IOOS project has a need to provide a Sensor Observation Service (SOS) interface for Hyrax. Since much of the sensor data is already held in RDBMSs it is a natural and necessary time to develop a DRDS replacement.

Design

The RDH will be a BES module/plug-in. It will use an implementation of the ODBC (most likely unixODBC) to access the RDMS(s). ODBC Data Sources will be defined at the system level as usual. [1][2] [3] [4] [5]

ODBC API

Using ODBC (or SQL) introspection methods in the ODBC API the RDH will identify the collection of tables and views available in the Data Source. Using a custom class factory to build instances of DAP objects that can read ODBC the RDH will traverse the available tables/views and convert them into DAP Sequence objects where the columns in the tables/views become variables in the Sequence. The types generated by the custom factory will have implemented read methods that will be able to read the row set returned through the ODBC API. Each Sequence requested will be associated with a separate SQL query. As the DDS is serialized, each Sequence will use the ODBC API to send the query to the Data Source and then use the returned row set as the data content for the serialization of it's (the Sequence's) variables.

Details:

• Each simple DAP type implementation will need to be able to:
  • read from the Result Set
  • move the Cursor to the next Column

(The implementer will have to decide the order of these two operations)

• The Sequence type will have to manage moving the Result Set Cursor from one row to the next.
• Some helper class ConstraintEvaluator? will have to convert DAP constraint expressions to SQL queries.