How to install Xapian for Cyrus 3.0 or later
============================================
Please follow the installation guide for Cyrus dependencies in the
main README. Then see the section for Xapian configuration.

How to install Xapian for Cyrus 2.5
===================================
To build xapian support into Cyrus, you need to build your own xapian
library as follows:

# choose your own adventures here
export XAPIAN_DIR=/opt/xapian
export CYRUS_DIR=/opt/cyrus
export SRC_DIR=/opt/src

cd $SRC_DIR
wget http://www.cyrusimap.org/releases/cyrus-imapd-3.0.0-beta1.tar.gz
wget http://oligarchy.co.uk/xapian/1.2.21/xapian-core-1.2.21.tar.xz
tar -xf xapian-core-1.2.21.tar.xz
cd xapian-core-1.2.21
tar -xf $cyrusdir/contrib/xapian_quilt.tar.gz
QUILT_PATCHES=xapian_quilt quilt push -a
autoreconf -v -i
./configure --prefix=$XAPIAN_DIR
make
make install
cd $SRC_DIR
tar -xf cyrus-imapd-3.0.0-beta1.tar.gz
cd cyrus-imapd-3.0.0-beta1
XAPIAN_CONFIG=$XAPIAN_DIR/bin/xapian-config ./configure --enable-xapian --prefix=$CYRUS_DIR
make
make install

And you have a Xapian-supporting Cyrus IMAPd!

How to configure Cyrus to use Xapian search
===========================================
You'll need to set up sync log to a channel called squatter, and
at least one search tier.

search_engine: xapian
search_index_headers: no
search_batchsize: 8192
defaultpartition: default
defaultsearchtier: t1
partition-default: /var/cyrus/spool
t1searchpartition-default: /var/cyrus/search

And then you'll need to arrange for a rolling squatter to run on
startup:

cyrus.conf:

DAEMON {
  # run a rolling squatter
  squatter cmd="squatter -R"
}

If you want to do more complex search tiers and repacking and stuff,
you'll probably want to read:

http://lists.tartarus.org/pipermail/xapian-discuss/2014-October/009112.html

And see how we do it at FastMail.