The YAZ Proxy is highly configurable and can be used in a number of different
applications, ranging from debugging Z39.50-based applications and protecting
overworked servers, to improving the performance of stateless WWW/Z39.50
gateways.  Among other features, it includes:

 * SRW/SRU server function, to allow any Z39.50 server to also
   support the ZiNG protocols
 * Load balancing across multiple backend servers
 * Session-sharing and pre-initialization to improve performance in
   servers with expensive session initialization
 * Configurable request filtering, to keep bad requests from reaching
   the server
 * XML support -- MARC records can be converted to MARCXML, and
   XSLT-transformations allow the proxy to support arbitrary
   retrieval schemas in XML
 * Load governor function limits requests from aggressive batch-mode clients
 * Configurable logging
 * Efficient multiplexing software enables small memory footprint and
   very high performance

WWW: http://www.indexdata.dk/yazproxy/