Solr Full Text Search Indexing
==============================

Solr [http://lucene.apache.org/solr/] is a Lucene indexing server. Dovecot
communicates to it using HTTP/XML queries.

Compiling
---------

Give '--with-solr' parameter to 'configure' script. You'll also need to have
libexpat and libcurl installed.

Configuration
-------------

Replace Solr's existing 'solr/conf/schema.xml' using 'doc/solr-schema.xml' from
Dovecot. You may want to check if the file contains something you want to
modify. See Solr wiki [http://wiki.apache.org/solr/SchemaXml] for how to
configure it.

On Dovecot's side add:

---%<-------------------------------------------------------------------------
mail_plugins = $mail_plugins fts fts_solr

plugin {
  fts = solr
  fts_solr = url=http://solr.example.org:8983/solr/
}
---%<-------------------------------------------------------------------------

Fields listed in 'fts_solr' plugin setting are space separated. They can
contain:

 * url=<solr url> : Required base URL for Solr.
 * debug : Enable HTTP debugging. Writes to error log.
 * break-imap-search : Use Solr also for indexing TEXT and BODY searches. This
   makes your server non-IMAP-compliant. (This is always enabled in v2.1+)

Solr commits & optimization
---------------------------

Solr indexes should be optimized once in a while to make searches faster and to
remove space used by deleted mails. Dovecot never asks Solr to optimize, so you
should do this yourself. Perhaps a cronjob that sends the optimize-command to
Solr every n hours.

With v2.2.3+ Dovecot only does soft commits to the Solr index to improve
performance. You must run a hard commit once in a while or Solr will keep
increasing its transaction log sizes. For example send the commit command to
Solr every few minutes.

Sorting by relevancy
--------------------

Solr/Lucene supports returning a relevancy score for search results. If you
want to sort the search results by the score, use Dovecot's non-standard
X-SCORE sort key:

---%<-------------------------------------------------------------------------
1 SORT (X-SCORE) UTF-8 <search parameters>
---%<-------------------------------------------------------------------------

Indexes
-------

Dovecot creates the following fields:

 * id: Unique ID consisting of uid/uidv/user/box.
    * Note that your user names really shouldn't contain '/' character.
 * uid: Message's IMAP UID.
 * uidv: Mailbox's UIDVALIDITY. This changes if mailbox gets recreated.
 * box: Mailbox name
 * user: User name who owns the mailbox (indexing shared/public mailboxes is
   probably broken currently)
 * hdr: Indexed message headers
 * body: Indexed message body
 * any: "Copy field" from hdr and body, i.e. searching based on this will
   search from both headers and bodies.

Lucene does duplicate suppression based on the "id" field, so even if Dovecot
sends the same message multiple times to Solr it gets indexed only once. This
might happen currently if multiple searches are started at the same time.

You might want to build a cronjob to go through the Lucene indexes once in a
while to delete indexed messages (or entire mailboxes) that no longer exist on
the filesystem. It shouldn't normally find any such messages though.

Sharding
--------

If you have more users than fit into a single Solr box, you can split users off
to different servers. A couple of different ways you could do it are:

 * Have some HTTP proxy redirecting the connections based on the URL
 * Configure Dovecot's userdb lookup to return a different host for 'fts_solr'
   setting using <extra fields> [UserDatabase.ExtraFields.txt].
    * LDAP: 'user_attrs = ..., solrHost=fts_solr=url=http://%$:8983/solr/'
    * MySQL: 'user_query = SELECT concat('url=http://', solr_host,
      ':8983/solr/') AS fts_solr, ...'

External Tutorials
------------------

External sites with tutorials on using Solr under Dovecot

 * Installing Apache Solr with Dovecot for fulltext search results
   [http://atmail.com/kb/2009/installing-apache-solr-with-dovecot-for-fulltext-search-results/]
   - Guide for installing all the dependencies for Solr to work under
   CentOS/Fedora. Step by step tutorial.

(This file was created from the wiki on 2013-08-05 04:42)
