Catalog Organization

Query optimization and automatic code deployment (Plug-And-Play) are driven by the metadata in the catalog. The catalog contains the definitions of  schemas for virtual tables defined over the data sources.  The catalog also contains the descriptions of the user-defined types and operators (MOCHLETS), and other relevant information such as the selectivity of various operators. All these virtual tables, types and operators are referred to as "resources", and each one is uniquely identified by a Uniform Resource Identifier (URI). In MOCHA, URIs are of the form:

mocha://<host>/<source|repository>/<name>.

Keyword mocha identifies the URI as one used by MOCHA. The <host> portion identifies the host for a data source or code repository, while <source|repository> indicates the specific name of a data source (e.g. database name) or code repository. Finally the <name> part gives the particular name for the resource. For example, the URI for a user-defined type named Raster, used to represent satellite images, can be as follows:

mocha://cs1.umd.edu/EarthSci/Raster

The metadata for each resource is specified in a document encoded with the Resource Description Framework (RDF), which is an XML-based technology used to specify metadata for resources available in networked environments. In MOCHA, for each resource there is catalog entry in the form (URI, RDF File), and this is used by the system to understand the behavior and proper utilization of each resource. Figure 7 depicts the RDF description for the Raster data type.  As we can see, the document specifies information about the  URI, type name, Java class implementing the type, code repository where the Java class can be found, typical size of the objects of this type, and the e-mail address of the developer who implemented the data type.

Figure 7: XML-based metadata in MOCHA catalog

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