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The YAZ proxy is a transparent Z39.50-to-Z39.50 gateway. That is, it is a Z39.50 server which has as its back-end a Z39.50 client that forwards requests on to another server (known as the backend target.)
The YAZ Proxy is useful for debugging Z39.50 software, logging APDUs, redirecting Z39.50 packages through firewalls, etc. Furthermore, it offers facilities that often boost performance for connectionless Z39.50 clients such as web gateways.
Unlike most other server software, the proxy runs single-threaded, single-process. Every I/O operation is non-blocking so it is very lightweight and extremely fast. It does not store any state information on the hard drive, except any log files you ask for.
Suppose you use a commercial Z39.50 client for which you do not have source code, and it's not behaving how you think it should when running against some specific server that you have no control over. One way to diagnose the problem is to find out what packets (APDUs) are being sent and received, but not all client applications have facilities to do APDU logging.
No problem. Run the proxy on a friendly machine, get it to log APDUs, and point the errant client at the proxy instead of directly at the server that's causing it problems.
Suppose the server is running on foo.bar.com, port 18398. Run the proxy on the machine of your choice, say your.company.com like this:
(The -a - option requests APDU logging on standard output, -t tcp:foo.bar.com:18398 specifies where the backend target is, and tcp:@:9000 tells the proxy to listen on port 9000 and accept connections from any machine.)
Now change your client application's configuration so that instead of connecting to foo.bar.com port 18398, it connects to your.company.com port 9000, and start it up. It will work exactly as usual, but all the packets will be sent via the proxy, which will generate a log like this: