Chapter 7. Virtual databases and multi-database searching
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# Chapter 7. Virtual databases and multi-database searching

## 1. Introductory notes

Two of Metaproxy's filters are concerned with multiple-database operations. Of these, `virt_db` can work alone to control the routing of searches to one of a number of servers, while `multi` can work together with `virt_db` to perform multi-database searching, merging the results into a unified result-set - "metasearch in a box".

The interaction between these two filters is necessarily complex: it reflects the real, irreducible complexity of multi-database searching in a protocol such as Z39.50 that separates initialization from searching, and in which the database to be searched is not known at initialization time.

It's possible to use these filters without understanding the details of their functioning and the interaction between them; the next two sections of this chapter are "HOW-TO" guides for doing just that. However, debugging complex configurations will require a deeper understanding, which the last two sections of this chapter attempts to provide.

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5. Config file syntax checking | Home | 2. Virtual databases with the `virt_db` filter