Add example systemd-files for running multiple instances

Author: msiebuhr

examples/systemd/README.md

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+System D on Ubuntu
+==================
+
+**IMPORTANT**: These files are written for Unity Cache Server 5.0.
+
+These files are for setting up per-project cache servers on the same machine
+using different ports. This way, the contents of a corrupted server can be
+removed without affecting all other builds.
+
+Setting up multiple instances of the same service with different configurations
+is called *template units* in systemd parlance.
+
+ * Fedora Magazine on [Template unit
+   files](https://fedoramagazine.org/systemd-template-unit-files/)
+ * Digital Ocean on [Template
+   specifiers](https://www.digitalocean.com/community/tutorials/understanding-systemd-units-and-unit-files#creating-instance-units-from-template-unit-files)
+ * System D [full unit
+   documentation](https://www.freedesktop.org/software/systemd/man/systemd.unit.html)
+
+Setup
+-----
+
+Install Node.js and git. Then check out the cache server itself and install
+it's dependencies:
+
+	# From https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions
+	curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
+	sudo apt-get install -y nodejs git
+	
+	# Add unitycache user + homedir
+	adduser --system unitycacheserver
+	
+	# Get server code
+	sudo -u unitycacheserver git clone https://github.com/Unity-Technologies/unity-cache-server ~unitycacheserver/unity-cache-server
+	cd ~unitycacheserver/unity-cache-server
+	sudo -u unitycacheserver npm --cache ~unitycacheserver/.npm install --production
+	cd -
+	
+Next up is a configuration file for each. Since The cache server does not
+actually accept a configuration file, we will write is as command-line
+arguments passed to each instance, eg. for *GAME NAME* on port 5002, we add a
+file named `5002-GAME-NAME.sh`:
+
+    OPTIONS=--port=5002
+
+The shell-file is read by systemd on service startup and the exported
+`$OPTIONS` is appended to the command-line arguments at startup. Only exception
+is the `--path` argument which is deduced from the base name of the file
+(eg. `5002-GAME-NAME` here) and set uses that as a sub-folder in
+unitycacheserver's home directory, eg. `--path
+/home/unitycacheserver/5002-GAME-NAME`, as it saves a lot of typing and
+copy/paste errors.
+
+Now, move all these files into the `unitycacheserver`-users' directory:
+	
+	sudo mv unitycacheserver/*.sh ~/unitycacheserver
+	sudo chown -R unitycacheserver ~/unitycacheserver/*.sh
+
+Finally, we copy over the systemd-scripts and set up a service instance per
+server we want running:
+
+	# Set up server + service-script for each
+	cp systemd-services/* /etc/systemd/system/
+	
+	# Set up a service instance per server we want
+	for i in `ls ~unitycacheserver/*.sh`; do systemctl enable unity-cache-server@`basename $i .sh`; done
+	
+	# Start all servers by starting the 'parent' service
+	systemctl start unity-cache-server
+

examples/systemd/unity-cache-server.service

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+[Unit]
+    Description=Start all unity-cache-server instances
+[Service]
+    Type=oneshot
+    ExecStart=/bin/true
+    RemainAfterExit=yes
+
+[Install]
+    WantedBy=multi-user.target

examples/systemd/unity-cache-server@.service

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+[Unit]
+    Description=Unity Cache Server listening on '%i'
+    Requires=unity-cache-server.service
+    Before=unity-cache-server.service
+    BindsTo=unity-cache-server.service
+
+[Service]
+    Type=simple
+    SyslogIdentifier=unity-cache-server-%i
+    TimeoutStartSec=300
+    ExecStart=/home/unitycacheserver/unity-cache-server/main.js --path /home/unitycacheserver/cache-%i $OPTIONS
+    EnvironmentFile=/home/unitycacheserver/%i.sh
+    Restart=on-failure
+    User=unitycacheserver
+
+[Install]
+     WantedBy=unity-cache-server.service