-<!-- $Id: book.xml,v 1.29 2006-05-03 13:33:22 mike Exp $ -->
+<!-- $Id: book.xml,v 1.32 2006-05-16 10:34:48 mike Exp $ -->
<bookinfo>
<title>Metaproxy - User's Guide and Reference</title>
<author>
be metasearched in this way: issues of resource usage and
administrative complexity dictate the practical limits.
</para>
+ <para>
+ What happens when one of the databases doesn't respond? By default,
+ the entire multi-database search fails, and the appropriate
+ diagnostic is returned to the client. This is usually appropriate
+ during development, when technicians need maximum information, but
+ can be inconvenient in deployment, when users typically don't want
+ to be bothered with problems of this kind and prefer just to get
+ the records from the databases that are available. To obtain this
+ latter behaviour add an empty
+ <literal><hideunavailable></literal>
+ element inside the
+ <literal>multi</literal> filter:
+ </para>
+ <screen><![CDATA[ <filter type="multi">
+ <hideunavailable/>
+ </filter>]]></screen>
+ <para>
+ Under this regime, an error is reported to the client only if
+ <emphasis>all</emphasis> the databases in a multi-database search
+ are unavailable.
+ </para>
</section>
>the HTTP 1.1 specification</ulink>.
</para>
<para>
- The role of the <literal>virt_db</literal> filter is to rewrite
- this otherInfo packet dependent on the virtual database that the
- client wants to search.
+ Within Metaproxy, Search requests that are part of the same
+ session as an Init request that carries a
+ <literal>VAL_PROXY</literal> otherInfo are also annotated with the
+ same information. The role of the <literal>virt_db</literal>
+ filter is to rewrite this otherInfo packet dependent on the
+ virtual database that the client wants to search.
</para>
<para>
When Metaproxy receives a Z39.50 Init request from a client, it
doesn't immediately forward that request to the back-end server.
Why not? Because it doesn't know <emphasis>which</emphasis>
- back-end server to forward it to until the client sends a search
+ back-end server to forward it to until the client sends a Search
request that specifies the database that it wants to search in.
Instead, it just treasures the Init request up in its heart; and,
later, the first time the client does a search on one of the
<literal><target></literal>
elements. What does this mean? Only that the filter will add
multiple <literal>VAL_PROXY</literal> otherInfo packets to the
- search requests that pass through it. That's because the virtual
+ Search requests that pass through it. That's because the virtual
DB filter is dumb, and does exactly what it's told - no more, no
less.
- </para>
- <para>
- If a search request with multiple <literal>VAL_PROXY</literal>
+ If a Search request with multiple <literal>VAL_PROXY</literal>
otherInfo packets reaches a <literal>z3950_client</literal>
filter, this is an error. That filter doesn't know how to deal
with multiple targets, so it will either just pick one and search
The <literal>multi</literal> filter comes to the rescue! This is
the only filter that knows how to deal with multiple
<literal>VAL_PROXY</literal> otherInfo packets, and it does so by
- making multiple copies of the entire search-request: one for each
+ making multiple copies of the entire Search request: one for each
<literal>VAL_PROXY</literal>. Each of these new copies is then
- passed down through the remaining filters in the route, instead of
- the original one. (The copies are handled in parallel though the
- spawning of new threads.) Since the copies each have ony one
+ passed down through the remaining filters in the route. (The
+ copies are handled in parallel though the
+ spawning of new threads.) Since the copies each have only one
<literal>VAL_PROXY</literal> otherInfo, they can be handled by the
<literal>z3950_client</literal> filter, which happily deals with
each one individually. When the results of the individual
searches come back up to the <literal>multi</literal> filter, it
- merges them into a single search-response, which is what
+ merges them into a single Search response, which is what
eventually makes it back to the client.
</para>
</section>