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<div class="document" id="remote-backend">
<h1 class="title">Remote Backend</h1>
<p>This document describes how to make use of the facilities in Xapian for
distributed searches.</p>
<div class="section" id="overview">
<h1>Overview</h1>
<p>There are two sides to the distributed searching. The client end is the
program initiating the search on behalf of a user, and the server end is
the program which provides a searching interface over a set of databases
for the client. There can be many servers, with many clients sharing
them. In theory, a server can also be a client to other servers, but
this may not be very useful or efficient.</p>
<p>The client runs queries in the same way that it would on local databases
- the difference is in how the database is opened. Once the database is
opened, the query process is identical to any other. Using a stub
database with "auto" backend is a good way to wrap up access to a remote
database in a neat way.</p>
<p>The remote backend currently support two client/server methods: prog and
tcp. They both use the same protocol, although different means to
contact the server.</p>
</div>
<div class="section" id="the-prog-method">
<h1>The Prog Method</h1>
<p>The prog method spawns a program when the database is opened, and
communicates with it over a pipe. This can be used to connect to a
remote Xapian database across an SSH tunnel for example, providing
authentication and encryption. The xapian-progsrv program is designed to
be the program at the far end of the connection.</p>
<p>From the client end, create the database with
<tt class="docutils literal"><span class="pre">Xapian::Database</span> <span class="pre">database(Xapian::Remote::open(program,</span> <span class="pre">args));</span></tt> -
for example:</p>
<pre class="literal-block">
Xapian::Database database(Xapian::Remote::open("ssh", "search.example.com xapian-progsrv /var/lib/xapian/data/db1"));
</pre>
<p>If the program has no path, the PATH environment variable is used.</p>
</div>
<div class="section" id="the-tcp-method">
<h1>The TCP Method</h1>
<p>The tcp method uses TCP/IP sockets to connect to a running server on a
remote machine (or indeed a local one, but that's rather pointless!)</p>
<p>From the client end, create the database with
<tt class="docutils literal"><span class="pre">Xapian::Database</span> <span class="pre">database(Xapian::Remote::open(host,</span> <span class="pre">port));</span></tt> - for
example:</p>
<pre class="literal-block">
Xapian::Database database(Xapian::Remote::open("searchserver", 33333));
</pre>
<p>The host is the server's hostname, the port is the tcp port on the
server to use.</p>
<p>The server is xapian-tcpsrv, which is installed by xapian-core's
"<tt class="docutils literal"><span class="pre">make</span> <span class="pre">install</span></tt>". This should be started and left running in the
background before searches are performed.</p>
<p>One or more databases need to be specified by listing their paths.</p>
<p>There's also one required command line option for xapian-tcpsrv: <tt class="docutils literal"><span class="pre">--port</span>
<span class="pre">PORTNUM</span></tt>, which specifies the port to listen on. For the full list of
accepted command line options, run <tt class="docutils literal"><span class="pre">xapian-tcpsrv</span> <span class="pre">--help</span></tt> or see the
xapian-tcpsrv man page.</p>
<p>Once started, the server will run and listen for connections on the
specified port. Each connection is handled by a forked child process
(or a new thread under Windows), so concurrent read access is supported.</p>
</div>
<div class="section" id="notes">
<h1>Notes</h1>
<p>A remote search should behave just like the equivalent local one, except
a few features aren't currently implemented (e.g. spelling, synonyms,
user metadata).</p>
<p>Exceptions are propagated across the link and thrown again at the client
end.</p>
<p>The remote backend now support writable databases. Just start
<tt class="docutils literal"><span class="pre">xapian-progsrv</span></tt> or <tt class="docutils literal"><span class="pre">xapian-tcpsrv</span></tt> with the option <tt class="docutils literal"><span class="pre">--writable</span></tt>.
Only one database may be specified when <tt class="docutils literal"><span class="pre">--writable</span></tt> is used.</p>
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