docs: add developer doc page for Tokenizer

docs/develop/Tokenizers.md

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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 administration guide](../admin/Tokenizers.md).
+
+## 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
+
+