How to Install GraphHopper on NixOS Latest

GraphHopper is an open-source routing engine that is used to calculate road or path routes. This tutorial explains the steps to install GraphHopper on NixOS Latest.

Prerequisites

NixOS Latest OS installed on your system.
Java 8 or higher installed on your system.

Installation

Open a terminal and navigate to the GraphHopper website https://graphhopper.com/.
Click on the Download option, a page with the download links will appear. Choose the latest version that supports the NixOS platform.
Once the download is complete open the terminal.
Now, navigate to the directory where you downloaded the GraphHopper application.
```
$ cd ~/Downloads
```
Extract the downloaded file
```
$ tar -xvf graphhopper-1.4.0.tar.gz
```
Move the extracted directory to a new location
```
$ sudo mv graphhopper-1.4.0 /opt
```

Set the environment variable for GraphHopper

$ export GRAPHHOPPER_DIR=/opt/graphhopper-1.4.0

Now, create a GraphHopper service to start GraphHopper on boot
```
$ sudo vim /etc/nixos/configuration.nix
```
Add the following code in the configuration file
```
...
services.graphhopper = {
  enable = true;
  dataDir = "/opt/graphhopper-1.4.0/data";
  configFile = "/opt/graphhopper-1.4.0/config-example.yml";
  cmd = "${GRAPHHOPPER_DIR}/graphhopper.sh start";
};
...
```
(Note: Ensure all of these files/subdirectories exist, otherwise you may encounter errors. If user doesn't have permission to create this directory, it's best to symlink to a directory the user owns. For example, /mnt/data/graphhopper-1.4.0/data)

Now, create a symlink for the GraphHopper service

$ sudo ln -s /etc/nixos/configuration.nix /etc/nixos/$(uname --nodename).nix

Redeploy NixOS
```
$ sudo nixos-rebuild switch
```
Start the GraphHopper service
```
$ sudo systemctl start graphhopper
```
Test the GraphHopper service
```
http://localhost:8989
```

You have successfully installed and configured GraphHopper on NixOS Latest OS. Now, you can use it to calculate road or path routes for your maps.