Dovecot Flatcurve FTS Plugin

Added: 3.0.0

This is a Dovecot FTS plugin to enable message indexing using the Xapian Open Source Search Engine Library.

Note

Requires Xapian 1.4+.

The plugin relies on Dovecot to do the necessary stemming. It is intended to act as a simple interface to the Xapian storage/search query functionality.

This driver supports match scoring and substring matches (on by default), which means it is RFC 3501 (IMAP4rev1) compliant. This driver does not support fuzzy searches.

The driver passes all of the ImapTest search tests.

Note

This plugin requires the fts plugin to be activated and configured

Enabling flatcurve is designed to be as easy as adding this to configuration:

mail_plugins = $mail_plugins fts fts_flatcurve

plugin {
  fts = flatcurve
}

Settings

Note

The default settings should be fine in most scenarios.

fts_flatcurve_commit_limit

Default	500
Value	unsigned integer
Advanced Setting; this should not normally be changed.

Commit database changes after this many documents are updated. Higher commit limits will result in faster indexing for large transactions (i.e. indexing a large mailbox) at the expense of high memory usage. The default value should be sufficient to allow indexing in a 256 MB maximum size process.

Set to 0 to use the Xapian default.

fts_flatcurve_max_term_size

Default	30
Value	unsigned integer
Advanced Setting; this should not normally be changed.

The maximum number of characters in a term to index.

The maximum value for this setting is 200.

fts_flatcurve_min_term_size

Default	2
Value	unsigned integer
Advanced Setting; this should not normally be changed.

The minimum number of characters in a term to index.

fts_flatcurve_optimize_limit

Default	10
Value	unsigned integer
Advanced Setting; this should not normally be changed.

Once the database reaches this number of shards, automatically optimize the DB at shutdown.

Set to 0 to disable auto-optimization.

fts_flatcurve_rotate_count

Default	5000
Value	unsigned integer
Advanced Setting; this should not normally be changed.

When the "current" fts database reaches this number of messages, it is rotated to a read-only database and replaced by a new write DB. Most people should not change this setting.

Set to 0 to disable rotation.

fts_flatcurve_rotate_time

Default	5000
Value	time (milliseconds)
Advanced Setting; this should not normally be changed.

When the "current" fts database exceeds this length of time (in msecs) to commit changes, it is rotated to a read-only database and replaced by a new write DB. Most people should not change this setting.

Set to 0 to disable rotation.

fts_flatcurve_substring_search

Default	no
Value	boolean

If enabled, allows substring searches (RFC 3501 compliant). However, this requires significant additional storage space. Many users today expect "Google-like" behavior, which is prefix searching, so substring searching is arguably not the modern expected behavior anyway. Therefore, even though it is not strictly RFC compliant, prefix (non-substring) searching is enabled by default.

Configuration Example

mail_plugins = $mail_plugins fts fts_flatcurve

plugin {
  fts = flatcurve

  # Maximum email-address token size (254) is larger than Xapian can handle,
  # so we will need to truncate at some level. It is doubtful that large
  # email-addresses are useful for search purposes, so this optional config
  # will prevent these large addresses (more than 100 bytes) from being
  # stored.
  #fts_tokenizers = generic email-address
  #fts_tokenizer_email_address = maxlen=100

  # All of these are optional, and indicate the default values.
  # They are listed here for documentation purposes; most people should
  # not need to define/override in their config.
  fts_flatcurve_commit_limit = 500
  fts_flatcurve_max_term_size = 30
  fts_flatcurve_min_term_size = 2
  fts_flatcurve_optimize_limit = 10
  fts_flatcurve_rotate_count = 5000
  fts_flatcurve_rotate_time = 5000
  fts_flatcurve_substring_search = no
}

Data Storage

Xapian search data is stored separately for each mailbox.

The data is stored under a 'fts-flatcurve' directory in the Dovecot index file location for the mailbox. The Xapian library is responsible for all data stored in that directory - no Dovecot code directly writes to any file.

Logging/Events

INFO

This plugin emits with category fts-flatcurve, a child of the category fts (see events design).