extend documentation for updating database

Explains the different modes and adds hints for
setting up a systemd job.

docs/admin/Update.md

@@ -10,18 +10,21 @@ For a list of other methods to add or update data see the output of
     If you have configured a flatnode file for the import, then you
     need to keep this flatnode file around for updates.
 
-#### Installing the newest version of Pyosmium
+### Installing the newest version of Pyosmium
 
-It is recommended to install Pyosmium via pip. Make sure to use python3.
+The replication process uses
+[Pyosmium](https://docs.osmcode.org/pyosmium/latest/updating_osm_data.html)
+to download update data from the server.
+It is recommended to install Pyosmium via pip.
 Run (as the same user who will later run the updates):
 
 ```sh
 pip3 install --user osmium
 ```
 
-#### Setting up the update process
+### Setting up the update process
 
-Next the update needs to be initialised. By default Nominatim is configured
+Next the update process needs to be initialised. By default Nominatim is configured
 to update using the global minutely diffs.
 
 If you want a different update source you will need to add some settings
@@ -45,12 +48,119 @@ what you expect.
 The `replication --init` command needs to be rerun whenever the replication
 service is changed.
 
-#### Updating Nominatim
+### Updating Nominatim
 
-The following command will keep your database constantly up to date:
+Nominatim supports different modes how to retrieve the update data from the
+server. Which one you want to use depends on your exact setup and how often you
+want to retrieve updates.
+
+These instructions are for using a single source of updates. If you have
+imported multiple country extracts and want to keep them
+up-to-date, [Advanced installations section](Advanced-Installations.md)
+contains instructions to set up and update multiple country extracts.
+
+#### Continuous updates
+
+This is the easiest mode. Simply run the replication command without any
+parameters:
 
     nominatim replication
 
-If you have imported multiple country extracts and want to keep them
-up-to-date, [Advanced installations section](Advanced-Installations.md) contains instructions 
-to set up and update multiple country extracts.
+The update application keeps running forever and retrieves and applies
+new updates from the server as they are published.
+
+You can run this command as a simple systemd service. Create a service
+description like that in `/etc/systemd/system/nominatim-update.service`:
+
+```
+[Unit]
+Description=Continuous updates of Nominatim
+
+[Service]
+WorkingDirectory=/srv/nominatim
+ExecStart=nominatim replication
+StandardOutput=append:/var/log/nominatim-updates.log
+StandardError=append:/var/log/nominatim-updates.error.log
+User=nominatim
+Group=nominatim
+Type=simple
+
+[Install]
+WantedBy=multi-user.target
+```
+
+Replace the `WorkingDirectory` with your project directory. Also adapt user
+and group names as required.
+
+Now activate the service and start the updates:
+
+```
+sudo systemctl daemon-reload
+sudo systemctl enable nominatim-updates
+sudo systemctl start nominatim-updates
+```
+
+#### One-time mode
+
+When the `--once` parameter is given, then Nominatim will download exactly one
+batch of updates and then exit. This one-time mode still respects the
+`NOMINATIM_REPLICATION_UPDATE_INTERVAL` that you have set. If according to
+the update interval no new data has been published yet, it will go to sleep
+until the next expected update and only then attempt to download the next batch.
+
+The one-time mode is particularly useful if you want to run updates continuously
+but need to schedule other work in between updates. For example, the main
+service at osm.org uses it, to regularly recompute postcodes -- a process that
+must not be run while updates are in progress. Its update script
+looks like this:
+
+```sh
+#!/bin/bash
+
+# Switch to your project directory.
+cd /srv/nominatim
+
+while true; do
+  nominatim replication --once
+  if [ -f "/srv/nominatim/schedule-mainenance" ]; then
+    rm /srv/nominatim/schedule-mainenance
+    nominatim refresh --postcodes
+  fi
+done
+```
+
+A cron job then creates the file `/srv/nominatim/need-mainenance` once per night.
+
+
+#### Catch-up mode
+
+With the `--catch-up` parameter, Nominatim will immediately try to download
+all changes from the server until the database is up-to-date. The catch-up mode
+still respects the parameter `NOMINATIM_REPLICATION_MAX_DIFF`. It downloads and
+applies the changes in appropriate batches until all is done.
+
+The catch-up mode is foremost useful to bring the database up to speed after the
+initial import. Give that the service usually is not in production at this
+point, you can temporarily be a bit more generous with the batch size and
+number of threads you use for the updates by running catch-up like this:
+
+```
+cd /srv/nominatim
+NOMINATIM_REPLICATION_MAX_DIFF=5000 nominatim replication --catch-up --threads 15
+```
+
+The catch-up mode is also useful when you want to apply updates at a lower
+frequency than what the source publishes. You can set up a cron job to run
+replication catch-up at whatever interval you desire.
+
+!!! hint
+    When running scheduled updates with catch-up, it is a good idea to choose
+    a replication source with an update frequency that is an order of magnitude
+    lower. For example, if you want to update once a day, use an hourly updated
+    source. This makes sure that you don't miss an entire day of updates when
+    the source is unexpectely late to publish its update.
+
+    If you want to use the source with the same update frequency (e.g. a daily
+    updated source with daily updates), use the
+    continuous update mode. It ensures to re-request the newest update until it
+    is published.