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docs/develop/overview.md

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+# Basic Architecture
+
+Nominatim provides geocoding based on OpenStreetMap data. It uses a Postgresql
+database as a backend for storing the data.
+
+There are three basic parts to Nominatim's architecture: the data import,
+the address computation and the search frontend.
+
+The __data import__ stage reads the raw OSM data and extracts all information
+that is useful for geocoding. This part is done by osm2pgsql, the same tool
+that can also be used to import a rendering database. It uses the special
+gazetteer output plugin in `osm2pgsql/output-gazetter.[ch]pp`. The result of
+the import can be found in the database table `place`.
+
+The __address computation__ or __indexing__ stage takes the data from `place`
+and adds additional information needed for geocoding. It ranks the places by
+importance, links objects that belong together and computes addresses and
+the search index. Most of this work is done in Pl/pqSQL via database triggers
+and can be found in the file `sql/functions.sql`.
+
+The __search frontend__ implements the actual API. It takes queries for
+search and reverse geocoding queries from the user, looks up the data and
+returns the results in the requested format. This part is written in PHP
+and can be found in the `lib/` and `website/` directories.