Apache Solr FTS Plugin (fts-solr)

Solr is a Lucene indexing server. Dovecot communicates to it using HTTP/XML queries.

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This documentation is written for Solr 7.7.0. You will need to adapt if using a different version.

Installation and operation of a Solr system is outside the scope of this documentation. Refer to the Solr documentation for further information.

This documentation focuses on Dovecot-specific tasks that need to be done to configure the system for FTS support.

Compiling

Dovecot is not compiled with Solr FTS support by default. To enable it, you need to add the --with-solr parameter to your invocation of the configure script. You will also need to have libexpat installed, including development headers (typically from a separate development package). Configuration will fail if --with-solr is enabled while libexpat headers cannot be found.

Server Setup

Solr Installation

Using this FTS engine requires Solr to be installed. Most OS distributions have packages for this.

To use Solr with Dovecot, it needs to configured specifically for use with Dovecot.

sudo -u solr /opt/solr/bin/solr create -c dovecot

Once the instance is created, you can start the Solr service.

By default, the Solr administration page for the newly created instance is located at https://localhost:8983/solr/#/~cores/dovecot. It can be used to check the status of the Solr instance. Configuration errors are often most conveniently viewed here. Solr also writes log files, which can be used for debugging.

Solr Configuration

There are three primary configuration files that need to be changed to accommodate the Dovecot FTS needs: the instance configuration file solrconfig.xml and the schema files schema.xml and managed-schema used by the instance.

These files are both located in the conf directory of the Solr instance (e.g., /var/solr/data/dovecot/conf/).

Remove default core configuration files

rm -f /var/solr/data/dovecot/conf/schema.xml
rm -f /var/solr/data/dovecot/conf/managed-schema
rm -f /var/solr/data/dovecot/conf/solrconfig.xml

Install schema.xml and solrconfig.xml

Copy doc/solr-config-7.7.0.xml and doc/solr-schema-7.7.0.xml to /var/solr/data/dovecot/conf/ as solrconfig.xml and schema.xml. The managed-schema file is generated based on schema.xml.

Dovecot Setup

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This plugin requires the fts plugin to be activated and configured

Dovecot Settings

fts_solr_batch_size

Default	1000
Value	unsigned integer

Configures the number of mails sent to Solr in a single request.

• With fts_autoindex = yes each new mail gets separately indexed on arrival, so fts_solr_batch_size only matters during the initial indexing of a mailbox.
• With fts_autoindex = no new mails don't get indexed on arrival, so fts_solr_batch_size is used when indexing is triggered.

fts_solr_soft_commit

Default	yes
Value	boolean
See Also	FTS Solr: Soft Commits

Controls whether new mails are immediately searchable via Solr.

fts_solr_url

Default	[None]
Value	string

Required base URL for Solr.

INFO

Remember to add your core name if using solr 7+: /solr/dovecot.

Example:

fts_solr_url = http://solr.example.org:8983/solr/dovecot/
fts_solr_batch_size = 1000

Example Configuration

mail_plugins {
  fts = yes
  fts_solr = yes
}

fts solr {
  url = url=https://solr.example.org:8983/solr/
  batch_size = 1000
}

Important notes:

• Some mail clients will not submit any search requests for certain fields if they index things locally eg. Thunderbird will not send any requests for fields such as sender/recipients/subject when Body is not included as this data is contained within the local index.

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.

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.

Optimize should be run somewhat rarely, e.g. once a day
curl https://<hostname/ip>:<port|default 8983>/solr/dovecot/update?optimize=true

# Commit should be run pretty often, e.g. every minute
curl https://<hostname/ip>:<port|default 8983>/solr/dovecot/update?commit=true

You may not need those if you are using a recent Solr (7+) or SolrCloud. The default configuration of Solr is to auto-commit every once in a while (~15sec) so commit is not necessary. Also, the default TieredMergePolicy in Solr will automatically purge removed documents later, so optimize is not necessary.

Soft Commits

If soft commits are enabled, dovecot will perform a soft commit to Solr at the end of transaction. This has the benefit that search results are immediately available. You can also enable automatic commits in SOLR config, with

<autoSoftCommit>
  <maxTime>60000</maxTime>
</autoSoftCommit>

or setting it in solrconfig.xml with

${solr.autoSoftCommit.maxTime:60000}

Re-index Mailbox

If you require Dovecot to reindex a whole mailbox, you can run doveadm fts rescan. This command will only take action when a search is done and will apply to the whole mailbox.

If you want to index a single mailbox/all mailboxes you can run doveadm index.This command will happen immediately and will block until the indexing is completed.

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>

Solr 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, or empty for public namespaces

• 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/Solr 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 Solr/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.

Testing

# telnet localhost imap
* OK [CAPABILITY IMAP4rev1 LITERAL+ SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS MULTIAPPEND UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS STARTTLS AUTH=PLAIN AUTH=LOGIN] I am ready.
1 login username password
2 select Inbox
3 SEARCH text "test"

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_url setting using userdb: Extra Fields.

  • MySQL: query = SELECT concat('https://', solr_host, ':8983/solr/dovecot/') AS fts_solr_url, ...

                   ... concat('https://', solr_host, ':8983/solr/dovecot/') AS fts_solr_url, ...
    

  • LDAP:

dovecot.conf

userdb ldap {
  ...
  fields {
    fts_solr_url = https://%{ldap:solrHost}:8983/solr/dovecot/
  }
}

You can also use SolrCloud, the clustered version of Solr, that allows you to scale up, and adds failover / high availability to your FTS system. Dovecot-solr works fine with a SolrCloud cluster as long as the solr schema is the right one.

External Tutorials

External sites with tutorials on using Solr under Dovecot

• FreeBSD

• Substring searches with ngrams

Tips

Some additional things which might help you configuring Solr search:

• If you are using Tomcat: Set maxHttpHeaderSize="65536" (connector definition for port 8080 in /etc/tomcat7/server.xml) to accept long search query strings (iPhones tend to send multi-kilobyte-sized queries)

• Set df to hdr in /etc/solr/conf/solrconfig.xml (/select request handler) to avoid strange undefined field text errors.

• Please keep in mind that you will have to change the Solr URL to include the core name (ie: dovecot: https://localhost:8939/solr/dovecot).

• To troubleshoot the settings you can enable debug logs using:

  log_debug = category=fts-solr

  and rawlogs using:

  fts_solr {
    http_client_rawlog_dir = /path/to/writable/directory/solr-rawlogs
  }