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Tokenizers
The tokenizer is the component of Nominatim that is responsible for analysing names of OSM objects and queries. Nominatim provides different tokenizers that use different strategies for normalisation. This page describes how tokenizers are expected to work and the public API that needs to be implemented when creating a new tokenizer. For information on how to configure a specific tokenizer for a database see the tokenizer chapter in the Customization Guide.
Generic Architecture
About Search Tokens
Search in Nominatim is organised around search tokens. Such a token represents string that can be part of the search query. Tokens are used so that the search index does not need to be organised around strings. Instead the database saves for each place which tokens match this place's name, address, house number etc. To be able to distinguish between these different types of information stored with the place, a search token also always has a certain type: name, house number, postcode etc.
During search an incoming query is transformed into a ordered list of such search tokens (or rather many lists, see below) and this list is then converted into a database query to find the right place.
It is the core task of the tokenizer to create, manage and assign the search tokens. The tokenizer is involved in two distinct operations:
- at import time: scanning names of OSM objects, normalizing them and building up the list of search tokens.
- at query time: scanning the query and returning the appropriate search tokens.
Importing
The indexer is responsible to enrich an OSM object (or place) with all data
required for geocoding. It is split into two parts: the controller collects
the places that require updating, enriches the place information as required
and hands the place to Postgresql. The collector is part of the Nominatim
library written in Python. Within Postgresql, the placex_update
trigger is responsible to fill out all secondary tables with extra geocoding
information. This part is written in PL/pgSQL.
The tokenizer is involved in both parts. When the indexer prepares a place,
it hands it over to the tokenizer to inspect the names and create all the
search tokens applicable for the place. This usually involves updating the
tokenizer's internal token lists and creating a list of all token IDs for
the specific place. This list is later needed in the PL/pgSQL part where the
indexer needs to add the token IDs to the appropriate search tables. To be
able to communicate the list between the Python part and the pl/pgSQL trigger,
the placex table contains a special JSONB column token_info which is there
for the exclusive use of the tokenizer.
The Python part of the tokenizer returns a structured information about the
tokens of a place to the indexer which converts it to JSON and inserts it into
the token_info column. The content of the column is then handed to the PL/pqSQL
callbacks of the tokenizer which extracts the required information. Usually
the tokenizer then removes all information from the token_info structure,
so that no information is ever persistently saved in the table. All information
that went in should have been processed after all and put into secondary tables.
This is however not a hard requirement. If the tokenizer needs to store
additional information about a place permanently, it may do so in the
token_info column. It just may never execute searches over it and
consequently not create any special indexes on it.
Querying
At query time, Nominatim builds up multiple interpretations of the search query. Each of these interpretations is tried against the database in order of the likelihood with which they match to the search query. The first interpretation that yields results wins.
The interpretations are encapsulated in the SearchDescription class. An
instance of this class is created by applying a sequence of
search tokens to an initially empty SearchDescription. It is the
responsibility of the tokenizer to parse the search query and derive all
possible sequences of search tokens. To that end the tokenizer needs to parse
the search query and look up matching words in its own data structures.
Tokenizer API
The following section describes the functions that need to be implemented for a custom tokenizer implementation.
!!! warning This API is currently in early alpha status. While this API is meant to be a public API on which other tokenizers may be implemented, the API is far away from being stable at the moment.
Directory Structure
Nominatim expects a single file src/nominatim_db/tokenizer/<NAME>_tokenizer.py
containing the Python part of the implementation.
<NAME> is a unique name for the tokenizer consisting of only lower-case
letters, digits and underscore. A tokenizer also needs to install some SQL
functions. By convention, these should be placed in lib-sql/tokenizer.
If the tokenizer has a default configuration file, this should be saved in
the settings/<NAME>_tokenizer.<SUFFIX>.
Configuration and Persistence
Tokenizers may define custom settings for their configuration. All settings
must be prefixed with NOMINATIM_TOKENIZER_. Settings may be transient or
persistent. Transient settings are loaded from the configuration file when
Nominatim is started and may thus be changed at any time. Persistent settings
are tied to a database installation and must only be read during installation
time. If they are needed for the runtime then they must be saved into the
nominatim_properties table and later loaded from there.
The Python module
The Python module is expect to export a single factory function:
def create(dsn: str, data_dir: Path) -> AbstractTokenizer
The dsn parameter contains the DSN of the Nominatim database. The data_dir
is a directory in the project directory that the tokenizer may use to save
database-specific data. The function must return the instance of the tokenizer
class as defined below.
Python Tokenizer Class
All tokenizers must inherit from nominatim_db.tokenizer.base.AbstractTokenizer
and implement the abstract functions defined there.
::: nominatim_db.tokenizer.base.AbstractTokenizer options: heading_level: 6
Python Analyzer Class
::: nominatim_db.tokenizer.base.AbstractAnalyzer options: heading_level: 6
PL/pgSQL Functions
The tokenizer must provide access functions for the token_info column
to the indexer which extracts the necessary information for the global
search tables. If the tokenizer needs additional SQL functions for private
use, then these functions must be prefixed with token_ in order to ensure
that there are no naming conflicts with the SQL indexer code.
The following functions are expected:
FUNCTION token_get_name_search_tokens(info JSONB) RETURNS INTEGER[]
Return an array of token IDs of search terms that should match the name(s) for the given place. These tokens are used to look up the place by name and, where the place functions as part of an address for another place, by address. Must return NULL when the place has no name.
FUNCTION token_get_name_match_tokens(info JSONB) RETURNS INTEGER[]
Return an array of token IDs of full names of the place that should be used to match addresses. The list of match tokens is usually more strict than search tokens as it is used to find a match between two OSM tag values which are expected to contain matching full names. Partial terms should not be used for match tokens. Must return NULL when the place has no name.
FUNCTION token_get_housenumber_search_tokens(info JSONB) RETURNS INTEGER[]
Return an array of token IDs of house number tokens that apply to the place. Note that a place may have multiple house numbers, for example when apartments each have their own number. Must be NULL when the place has no house numbers.
FUNCTION token_normalized_housenumber(info JSONB) RETURNS TEXT
Return the house number(s) in the normalized form that can be matched against a house number token text. If a place has multiple house numbers they must be listed with a semicolon as delimiter. Must be NULL when the place has no house numbers.
FUNCTION token_is_street_address(info JSONB) RETURNS BOOLEAN
Return true if this is an object that should be parented against a street. Only relevant for objects with address rank 30.
FUNCTION token_has_addr_street(info JSONB) RETURNS BOOLEAN
Return true if there are street names to match against for finding the parent of the object.
FUNCTION token_has_addr_place(info JSONB) RETURNS BOOLEAN
Return true if there are place names to match against for finding the parent of the object.
FUNCTION token_matches_street(info JSONB, street_tokens INTEGER[]) RETURNS BOOLEAN
Check if the given tokens (previously saved from token_get_name_match_tokens())
match against the addr:street tag name. Must return either NULL or FALSE
when the place has no addr:street tag.
FUNCTION token_matches_place(info JSONB, place_tokens INTEGER[]) RETURNS BOOLEAN
Check if the given tokens (previously saved from token_get_name_match_tokens())
match against the addr:place tag name. Must return either NULL or FALSE
when the place has no addr:place tag.
FUNCTION token_addr_place_search_tokens(info JSONB) RETURNS INTEGER[]
Return the search token IDs extracted from the addr:place tag. These tokens
are used for searches by address when no matching place can be found in the
database. Must be NULL when the place has no addr:place tag.
FUNCTION token_get_address_keys(info JSONB) RETURNS SETOF TEXT
Return the set of keys for which address information is provided. This
should correspond to the list of (relevant) addr:* tags with the addr:
prefix removed or the keys used in the address dictionary of the place info.
FUNCTION token_get_address_search_tokens(info JSONB, key TEXT) RETURNS INTEGER[]
Return the array of search tokens for the given address part. key can be
expected to be one of those returned with token_get_address_keys(). The
search tokens are added to the address search vector of the place, when no
corresponding OSM object could be found for the given address part from which
to copy the name information.
FUNCTION token_matches_address(info JSONB, key TEXT, tokens INTEGER[])
Check if the given tokens match against the address part key.
Warning: the tokens that are handed in are the lists previously saved
from token_get_name_search_tokens(), not from the match token list. This
is an historical oddity which will be fixed at some point in the future.
Currently, tokenizers are encouraged to make sure that matching works against
both the search token list and the match token list.
FUNCTION token_get_postcode(info JSONB) RETURNS TEXT
Return the postcode for the object, if any exists. The postcode must be in the form that should also be presented to the end-user.
FUNCTION token_strip_info(info JSONB) RETURNS JSONB
Return the part of the token_info field that should be stored in the database
permanently. The indexer calls this function when all processing is done and
replaces the content of the token_info column with the returned value before
the trigger stores the information in the database. May return NULL if no
information should be stored permanently.