Add documentation about the user directory search algorithm (#16320)
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Patrick Cloke
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Improve documentation of the user directory search algorithm.
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User Directory API Implementation
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=================================
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# User Directory API Implementation
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The user directory is currently maintained based on the 'visible' users
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on this particular server - i.e. ones which your account shares a room with, or
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who are present in a publicly viewable room present on the server.
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The user directory is maintained based on users that are 'visible' to the homeserver -
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i.e. ones which are local to the server and ones which any local user shares a
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room with.
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The directory info is stored in various tables, which can (typically after
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DB corruption) get stale or out of sync. If this happens, for now the
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The directory info is stored in various tables, which can sometimes get out of
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sync (although this is considered a bug). If this happens, for now the
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solution to fix it is to use the [admin API](usage/administration/admin_api/background_updates.md#run)
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and execute the job `regenerate_directory`. This should then start a background task to
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flush the current tables and regenerate the directory.
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flush the current tables and regenerate the directory. Depending on the size
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of your homeserver (number of users and rooms) this can take a while.
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Data model
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----------
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## Data model
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There are five relevant tables that collectively form the "user directory".
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Three of them track a master list of all the users we could search for.
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The last two (collectively called the "search tables") track who can
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see who.
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Three of them track a list of all known users. The last two (collectively called
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the "search tables") track which users are visible to each other.
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From all of these tables we exclude three types of local user:
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- support users
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- appservice users
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- deactivated users
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* `user_directory`. This contains the user_id, display name and avatar we'll
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return when you search the directory.
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- Because there's only one directory entry per user, it's important that we only
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ever put publicly visible names here. Otherwise we might leak a private
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A description of each table follows:
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* `user_directory`. This contains the user ID, display name and avatar of each user.
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- Because there is only one directory entry per user, it is important that it
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only contain publicly visible information. Otherwise, this will leak the
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nickname or avatar used in a private room.
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- Indexed on rooms. Indexed on users.
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* `user_directory_search`. To be joined to `user_directory`. It contains an extra
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column that enables full text search based on user ids and display names.
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Different schemas for SQLite and Postgres with different code paths to match.
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column that enables full text search based on user IDs and display names.
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Different schemas for SQLite and Postgres are used.
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- Indexed on the full text search data. Indexed on users.
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* `user_directory_stream_pos`. When the initial background update to populate
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the directory is complete, we record a stream position here. This indicates
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that synapse should now listen for room changes and incrementally update
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the directory where necessary.
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the directory where necessary. (See [stream positions](development/synapse_architecture/streams.html).)
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* `users_in_public_rooms`. Contains associations between users and the public rooms they're in.
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Used to determine which users are in public rooms and should be publicly visible in the directory.
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* `users_in_public_rooms`. Contains associations between users and the public
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rooms they're in. Used to determine which users are in public rooms and should
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be publicly visible in the directory. Both local and remote users are tracked.
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* `users_who_share_private_rooms`. Rows are triples `(L, M, room id)` where `L`
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is a local user and `M` is a local or remote user. `L` and `M` should be
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different, but this isn't enforced by a constraint.
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Note that if two local users share a room then there will be two entries:
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`(user1, user2, !room_id)` and `(user2, user1, !room_id)`.
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## Configuration options
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The exact way user search works can be tweaked via some server-level
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[configuration options](usage/configuration/config_documentation.md#user_directory).
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The information is not repeated here, but the options are mentioned below.
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## Search algorithm
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If `search_all_users` is `false`, then results are limited to users who:
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1. Are found in the `users_in_public_rooms` table, or
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2. Are found in the `users_who_share_private_rooms` where `L` is the requesting
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user and `M` is the search result.
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Otherwise, if `search_all_users` is `true`, no such limits are placed and all
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users known to the server (matching the search query) will be returned.
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By default, locked users are not returned. If `show_locked_users` is `true` then
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no filtering on the locked status of a user is done.
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The user provided search term is lowercased and normalized using [NFKC](https://en.wikipedia.org/wiki/Unicode_equivalence#Normalization),
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this treats the string as case-insensitive, canonicalizes different forms of the
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same text, and maps some "roughly equivalent" characters together.
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The search term is then split into words:
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* If [ICU](https://en.wikipedia.org/wiki/International_Components_for_Unicode) is
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available, then the system's [default locale](https://unicode-org.github.io/icu/userguide/locale/#default-locales)
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will be used to break the search term into words. (See the
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[installation instructions](setup/installation.md) for how to install ICU.)
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* If unavailable, then runs of ASCII characters, numbers, underscores, and hypens
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are considered words.
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The queries for PostgreSQL and SQLite are detailed below, by their overall goal
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is to find matching users, preferring users who are "real" (e.g. not bots,
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not deactivated). It is assumed that real users will have an display name and
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avatar set.
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### PostgreSQL
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The above words are then transformed into two queries:
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1. "exact" which matches the parsed words exactly (using [`to_tsquery`](https://www.postgresql.org/docs/current/textsearch-controls.html#TEXTSEARCH-PARSING-QUERIES));
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2. "prefix" which matches the parsed words as prefixes (using `to_tsquery`).
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Results are composed of all rows in the `user_directory_search` table whose information
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matches one (or both) of these queries. Results are ordered by calculating a weighted
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score for each result, higher scores are returned first:
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* 4x if a user ID exists.
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* 1.2x if the user has a display name set.
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* 1.2x if the user has an avatar set.
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* 0x-3x by the full text search results using the [`ts_rank_cd` function](https://www.postgresql.org/docs/current/textsearch-controls.html#TEXTSEARCH-RANKING)
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against the "exact" search query; this has four variables with the following weightings:
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* `D`: 0.1 for the user ID's domain
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* `C`: 0.1 for unused
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* `B`: 0.9 for the user's display name (or an empty string if it is not set)
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* `A`: 0.1 for the user ID's localpart
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* 0x-1x by the full text search results using the `ts_rank_cd` function against the
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"prefix" search query. (Using the same weightings as above.)
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* If `prefer_local_users` is `true`, then 2x if the user is local to the homeserver.
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Note that `ts_rank_cd` returns a weight between 0 and 1. The initial weighting of
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all results is 1.
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### SQLite
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Results are composed of all rows in the `user_directory_search` whose information
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matches the query. Results are ordered by the following information, with each
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subsequent column used as a tiebreaker, for each result:
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1. By the [`rank`](https://www.sqlite.org/windowfunctions.html#built_in_window_functions)
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of the full text search results using the [`matchinfo` function](https://www.sqlite.org/fts3.html#matchinfo). Higher
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ranks are returned first.
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2. If `prefer_local_users` is `true`, then users local to the homeserver are
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returned first.
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3. Users with a display name set are returned first.
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4. Users with an avatar set are returned first.
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