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How to set up a Windward server on GCP and some insight into systemd

Windward is an action-filled multiplayer sandbox game that puts you in control of a ship sailing the high seas of a large procedurally-generated world.

Thanks to the recent Steam Pirate Sale and my friend Alex, I started playing Windward. For technical reasons, we had to set up a dedicated server which turned out to be fairly simple and has the added benefit of asynchronous play that since then lured in more players (shout out to Patrick!).

This article touches on three important aspects:

The third section explains the details behind some choices in the second section and is not required reading if you just want to set up your own server. If you want to understand why the service makes use of tmux in such a weird way, read until the end.

Set up a GCP Compute Engine VM

For this, you can follow the excellent, official tutorial at https://cloud.google.com/community/tutorials/setup-arma-server-compute-engine almost step by step until right before the section “Set up the Arma server”.

The key differences are:

Also make sure to chose a zone (both for the instance and the external IP) that anticipates where your players will come from. E.g. on my server, all players are from Germany, so I chose europe-west3 (Frankfurt).

Configure Linux to run the Windward server

Once you can connect to the machine via SSH (see https://cloud.google.com/compute/docs/instances/connecting-to-instance#gcetools if you’re unsure how), as a first order of business, create a dedicated user. This is to avoid running the server binary with the (sudo) privileges of your account. Also, in order to allow the server to start automatically when the machine starts, enable lingering for the newly created user.

sudo adduser wward
sudo usermod -L wward
sudo loginctl enable-linger wward

Install mono which is needed to run the (.NET) binary and other tools we’ll need long the way. Updating the server also shouldn’t hurt…

sudo apt update
sudo apt upgrade -y
sudo apt install -y mono-complete tmux unzip

Switch to the user we just created via sudo su wward and set the following environment variables so you’re able to use systemctl as wward.

export XDG_RUNTIME_DIR="/run/user/$UID"
export DBUS_SESSION_BUS_ADDRESS="unix:path=${XDG_RUNTIME_DIR}/bus"

Download the server binary from the official source. (Alternatively, you can also find it in your Steam folder under Steam/steamapps/common/Windward/.)

curl -O http://www.tasharen.com/windward/WWServer.zip
unzip WWServer.zip
rm WWServer.zip

In order to be able to actually start a campaign, you’ll have to create it manually, by running the game on your local machine and starting a new campaign. Once the corresponding .dat and .dat.config files show up under Documents\Windward\Worlds\, copy them into ~/Windward/Worlds/ on your server. The rest of the tutorial will assume that your campaign is called “MyCampaign”. You might have to replace that name with whatever you chose in the following steps.

Then, create a script called ~/start-server.sh with the following contents

#!/bin/bash

/usr/bin/mono ~/WWServer.exe \
        -name "Foo Server" \
        -world "MyCampaign" \
        -tcp 5127

Make it executable via chmod +x ~/start-server.sh. If you want your server to appear on the public list of available game servers, you can also specify the -public option.

Then, create ~/.config/systemd/user/windward.service with the following contents

[Unit]
Description=Windward Dedicated Server

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/tmux new-session -d -s Windward %h/start-server.sh
ExecStop=/usr/bin/tmux send-keys -t Windward q Enter Enter

[Install]
WantedBy=default.target

Enable and start the service via

systemctl --user enable windward.service
systemctl --user start windward.service

You should be all good to go. All the best and may you always have enough water under your keel.

Epilogue: Why tmux?

The avid reader might be confused about the use of tmux here. Why not just start the server directly in the service and be done with it? That is exactly what I did when I first migrated the server from a tmux session to a “proper” systemd user service. On the same day, we noticed larger latencies in game but I didn’t attribute them to the change. Only when I saw the CPU usage on the GCP dashboard the next day, I noticed that it had jumped from below 20% to over 80% the moment I moved the server to a systemd service.

Very quickly, I found this thread on Stackexchange where someone describes a very similar problem. Incidentally, they are also using mono. As it turns out, mono is not the problem, though.

As one commenter notes,

Systemd runs process without stdin (=/dev/null). All syscalls to read() are finished immediately (with normal stdin, read() is blocked until new data arrive).

Unfortunately, the Windward server waits for input on stdin. Running it in the aforementioned fashion will result in the server spamming the output and continously reading from /dev/null/. If you try to run the server as a systemd user service directly, you’ll see many lines similar to this one in journalctl:

Sep 24 13:55:59 windward-server start-server.sh[631]: [2020/09/24 13:55:59] \
Press 'q' followed by ENTER when you want to quit.

Another commenter suggests using the StandardInput=tty option. Unfortunately, this does not work here, as our user can’t interact with the TTY. I’m not sure if this is because of the VM or because of insufficient privileges, but I couldn’t get it to work so I started looking for alternatives. I also tried files and named pipes but none would reliably bring the load down.

The author of the aforementioned thread discovered that they could get the load down by running their executable inside screen. After knowing about the problem with reading from stdin, this is no longer surprising, as the process inside screen will have their own stdin and stdout. Using tmux instead of screen, and some insight from another discussion on Serverfault, I even found a nice way to terminate the server in the intended way: tmux allows us to send keystrokes to the running program, so we can configure the service to use that to bring down the server:

ExecStop=/usr/bin/tmux send-keys -t Windward q Enter Enter

Very nice indeed.

Go to the corresponding issue on GitHub, in order to discuss this article.