# Updating OpenStreetMap data from change files

OpenStreetMap is a database that is constantly extended and updated. When you download the planet or an extract of it, you only get a snapshot of the database at a given point in time. To keep up-to-date with the development of OSM, you either need to download a new snapshot or you can update your existing data from change files published along with the planet file. Pyosmium ships with two tools that help you to process change files: pyosmium-get-changes and pyosmium-up-to-date.

This section explains the basics of OSM change files and how to use Pyosmium’s tools to keep your data up to date.

## About change files

Regular change files are published for the planet and also by some extract services. These change files are special OSM data files containing all changes to the database in a regular interval. Change files are not referentially complete. That means that they only contain OSM objects that have changed but not necessarily all the objects that are referenced by the changed objects. The result is that change file are rarely useful on their own. But they can be used to update an existing snapshot of OSM data.

## Getting change files

There are multiple sources for OSM change files available:

> - https://planet.openstreetmap.org/replication is the official source for planet-wide updates. There are change files for minutely, hourly and daily intervals available.

> - Geofabrik offers daily change files for all its updates. See the extract page for a link to the replication URL. Note that change files go only about 3 months back. Older files are deleted.

> - download.openstreetmap.fr offers minutely change files for all its extracts.

For other services also check out the list of providers on the OSM wiki.

## Updating a planet or extract

If you have downloaded the planet or an extract with a replication service, then updating your OSM file can be as easy as:

    pyosmium-up-to-date <osmfile.osm.pbf>

This finds the right replication source and file to start with, downloads changes and updates the given file with the data. You can repeat this command whenever you want to have newer data. The command automatically picks up at the same point where it left off after the previous update.

### Choosing the replication source

OSM files in PBF format are able to save the replication source and the current status on their own. If you want to switch the replication source or have a file that does not have the information, you need to bootstrap the update process and manually point pyosmium-up-to-date to the right service:

    pyosmium-up-to-date --ignore-osmosis-headers --server <replication URL> <osmfile.osm.pbf>

pyosmium-up-to-date automatically finds the right sequence ID to use by looking at the age of the data in your OSM file. It updates the file and stores the new replication source in the file. The additional parameters are then not necessary anymore for subsequent updates.

Attention

Always use the PBF format to store your data. Other format do not support to save the replication information. pyosmium-up-to-date is still able to update these kind of files if you manually point to the replication server but the process is always more costly because it needs to find the right starting point for updates first.

### Updating larger amounts of data

When used without any parameters, pyosmium downloads at a maximum about 1GB of changes. That corresponds to about 3 days of planet-wide changes. You can increase the amount using the additional –size parameter:

    pyosmium-up-to-date --size=10000 planet.osm.pbf

This would download about 10GB or 30 days of change data. If your OSM data file is older than that, downloading the full file anew is likely going to be faster.

pyosmium-up-to-date uses return codes to signal if it has downloaded all available updates. A return code of 0 means that it has downloaded and applied all available data. A return code of 1 indicates that it has applied some updates but more are available.

A minimal script that updates a file until it is really up-to-date with the replcaition source would look like this:

    status=1  # we wnat more data
    while [ $status -eq 1 ]; do
      pyosmium-up-to-date planet.osm.pbf
      # save the return code
      status=$?
    done

## Creating change files for updating databases

There are quite a few tools that can import OSM data into databases, for example osm2pgsql, imposm or Nominatim. These tools often can use change files to keep their database up-to-date. pyosmium can be used to create the appropriate change files. This is slightly more involved than updating a file.

### Preparing the state file

Before downloading the updates, you need to find out, with which sequence number to start. The easiest way to remember your current status is to save the number in a file. Pyosmium can then read and update the file for you.

#### Method 1: Starting from the import file

If you still have the OSM file you used to set up your database, then create a state file as follows:

    pyosmium-get-changes -O <osmfile.osm.pbf> -f sequence.state -v

Note that there is no output file yet. This creates a new file sequence.state with the sequence ID where updates should start and prints the URL of the replication service to use.

#### Method 2: Starting from a date

If you do not have the original OSM file anymore, then a good strategy is to look for the date of the newest node in the database to find the snapshot date of your database. Find the highest node ID, then look up the date for version 1 on the OSM website. For example the date for node 2367234 can be found at https://www.openstreetmap.org/api/0.6/node/23672341/1 Find and copy the timestamp field. Then create a state file using this date:

    pyosmium-get-changes -D 2007-01-01T14:16:21Z -f sequence.state -v

Also here, this creates a new file sequence.state with the sequence ID where updates should start and prints the URL of the replication service to use.

### Creating a change file

Now you can create change files using the state:

    pyosmium-get-changes --server <replication server> -f sequence.state -o newchange.osc.gz

This downloads the latest changes from the server, saves them in the file newchange.osc.gz and updates your state file. <replication server> is the URL that was printed, when you set up the state file. The parameter can be omitted when you use minutely change files from openstreetmap.org. This simplifies multiple edits of the same element into the final change. If you want to retain the full version history specify –no-deduplicate.

pyosmium-get-changes loads only about 100MB worth of updates at once (about 8 hours of planet updates). If you want more, then add a –size parameter.

### Continuously updating a database

pyosmium-get-changes emits special return codes that can be used to set up a script that continuously fetches updates and applies them to a database. The important error codes are:

> - 0 - changes successfully downloaded and new change file created

> - 3 - no new changes are available from the server

All other error codes indicate fatal errors.

A simple shell script can look like this:

    while true; do
      # pyosmium-get-changes would not overwrite an existing changes file
      rm -f newchange.osc.gz
      # get the next batch of changes
      pyosmium-get-changes -f sequence.state -o newchange.osc.gz
      # save the return code
      status=$?

      if [ $status -eq 0 ]; then
        # apply newchange.osc.gz here
        ....
      elif [ $status -eq 3 ]; then
        # No new data, so sleep for a bit
        sleep 60
      else
        echo "Fatal error, stopping updates."
        exit $status
    done