# Virtual Databases

Virtual databases in DAH control mappings between the DAH and specific databases in child servers. When you query the DAH, you can use the `DatabaseMatch` parameter to match a specific DAH virtual database (VDB), which in turn maps to databases in the child servers.

A VDB configuration might be useful in a complicated DAH environment, where you have mirror and non-mirror servers, and databases in different child servers might have different names.

## Map Databases

When you create a VDB, you configure a name, and a list of databases that the VDB maps to. You can map a VDB to multiple databases from a single child server, and to databases from different child servers. See Example: Virtual Databases

## VDB Types

The type of VDB defines whether DAH must treat the mapped databases as mirrors. `Combinator` VDBs act in non-mirror mode, while `Distributor` VDBs act in mirror mode.

## Other VDB Configuration

You can configure VDBs to map to other VDBs in the same DAH. These are called recursive VDBs. It is sometimes better to replace recursive VDBs with actual DAH servers in a multiple DAH structure. See Advanced DAH Structures.

Related Topics

Example: Virtual Databases

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