Working alone, the purpose of the
filter is to route search requests to one of a selection of
back-end databases. In this way, a single Z39.50 endpoint
(running Metaproxy) can provide access to several different
underlying services, including those that would otherwise be
inaccessible due to firewalls. In many useful configurations, the
back-end databases are local to the Metaproxy installation, but
the software does not enforce this, and any valid Z39.50 servers
may be used as back-ends.
For example, a
filter could be set up so that searches in the virtual database
"lc" are forwarded to the Library of Congress bibliographic
catalogue server, and searches in the virtual database "marc"
are forwarded to the toy database of MARC records that Index Data
hosts for testing purposes. A
configuration to make this switch would look like this:
<filter type="virt_db"> <virtual> <database>lc</database> <target>lx2.loc.gov:210/LCDB_MARC8</target> </virtual> <virtual> <database>marc</database> <target>indexdata.com/marc</target> </virtual> </filter>
As well as being useful in it own right, this filter also provides the foundation for multi-database searching.