Document lean mode

README.md

@@ -171,6 +171,8 @@ document the usage here.
 
 ## Import OpenStreetMap data
 
+**Hint:** This step may be skipped if you are using [Lean mode](./docs/lean-mode.md).
+
 You need to import road information from OpenStreetMap for the portal to work.
 This information is stored in your PostgreSQL database and used when processing
 tracks (instead of querying the Overpass API), as well as for vector tile

docs/lean-mode.md

@@ -0,0 +1,57 @@
+# Lean mode
+
+The application can be configured in "lean mode" through the `LEAN_MODE`
+setting in `config.py`. A lean installation is easier to set up, as a few steps
+can be skipped. However, the performance of the application will degrade in
+lean mode, and lots of advanced features will not be available.
+
+Lean mode is meant as an entrypoint to get started with collecting data,
+without the hassle of importing and maintaining OpenStreetMap data.
+
+## Disabled features in lean mode
+
+* No map tiles are generated.
+* The frontend will not show an overview map, only per-track maps.
+* The `roads` database table is not used, neither for processing tracks, nor
+  for generating map tiles.
+* The API will not generate auxiliary information for display on the
+  (nonexistent) map, such as per-road statistics.
+
+## Switch to/from lean mode 
+
+To enable lean mode, set the following in your `config.py` (or in
+`config.overrides.py`, especially in development setups):
+
+```python
+LEAN_MODE = True
+```
+
+To disable lean mode, set it to `False` instead.
+
+For lean mode, it is important that the config variable `OBS_FACE_CACHE_DIR` is
+properly set, or that you are happy with its default value of using
+`$DATA_DIR/obs-face-cache`.
+
+When turning off lean mode, make sure to fill your `roads` table properly, as
+otherwise the track processing will not work. When turning on lean mode, you
+may truncate the `roads` table to save space, but you don't need to, it simply
+becomes unused.
+
+## Benefits
+
+* When using lean mode, you can skip the import of OpenStreetMap data during
+  setup, and you also do not need to keep it updated. 
+* People can already start uploading data and the data is also processed,
+  giving you as a maintainer more time to set up the full application, if you
+  want to.
+
+## Drawbacks
+
+* Lean mode is less performant when processing tracks.
+* Lean mode track processing depends on the Overpass API data source, which may
+  be slow, unavailable, or rate limiting the requests, so processing may fail.
+  We use caching to prevent some issues, but as we depend on a third party
+  service here that is accessed for free and that generates a lot of server
+  load, we really can't ask for much. If you frequently run into issues, the
+  best bet is to manage OSM data yourself and turn off lean mode.
+* Of course some features are missing.