Last time, I said that in the next entry in this series I would do something fun (and likely impractical) with kubernetes. I lied; sorry. Since then, I received several great questions that I didn’t manage to cover, so I thought that I’d address a couple of them now. This entry will be rather piecemeal, but hopefully much shorter than the prior ones!
The start of accidental PVP harm
Minecraft uses a simple configuration file to store many of its default settings. This is called the
server.properties file. Let’s take a look at the one running in our Docker container:
ftb-server $ sudo docker exec -ti <container name> /bin/bash root@baea2dfbd18f:/opt/ftb# more server.properties
#Minecraft server properties < snip! > server-port=25565 level-type=DEFAULT enable-rcon=false level-seed= force-gamemode=false server-ip= max-build-height=256 spawn-npcs=true white-list=false spawn-animals=true hardcore=false snooper-enabled=true online-mode=true resource-pack= pvp=true difficulty=1 < snip! >
We can see some pretty interesting settings in there. Some have to do with gameplay (e.g.
difficulty), and some have to do with server management (e.g.
server-port). What if we want to change it up a bit? That configuration file came from the zip file that we unpacked in our
Dockerfile. We have a few different options:
server.propertiesfile when running the container
The first option is going to be our winner. What we really want is to modify the config file, and have that change persist in the image. We already have a command prompt inside the container, so let’s modify the
server.properties file such that the difficulty is normal, and different players on the server can’t kill each other:
root@baea2dfbd18f:/opt/ftb# echo "<full modified content of server.properties>" \ > server.properties
Now, we need to exit the container and commit the changes with a message and author:
root@baea2dfbd18f:/opt/ftb# exit ftb-server $ sudo docker commit -m "customized the server.properties" \ -a "Jane Doe" \ <container id> <docker user>/ftb:v2
You can verify that the image has been updated by listing all the images:
ftb-server $ sudo docker images
REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE <docker user>/ftb v2 9ead6660604b 7 seconds ago 1.013 GB
Great! Now the different players on the server can’t kill each other. But if we start another server with the image in the registry, `pvp` will become true again. How do we persist it?
Once we have the change committed, we push the changes to Google Container Registry so that we can pull them down onto new servers:
ftb-server $ sudo docker tag <docker user>/ftb:v2 gcr.io/<project id>/ftb-v2 ftb-server $ sudo gcloud docker push gcr.io/<project id>/ftb-v2
We’re done! If we want to test it, we just start a new container like we have before, using this updated image (make sure to stop the old one, since we’re using the same port mapping for both).
Creeping up on a Creeper
However, we should be asking some questions about our updated image. What does it actually include? And didn’t we commit all of the progress we’ve made in the game?
The image now contains all changes to the container we committed. That includes the changes we made to
server.properties, as well as all the extracted files that the Minecraft server requires to run. It does not mean that the world files have been committed for one simple reason: our world is stored on a data volume, and data volumes are exempt when you commit a container.
One of the most common questions about running a Minecraft server is that of cost. How much is it going to set us back? Unfortunately, answering that question is not as straightforward as if we’d bought physical hardware to use as a server. Still, while our monthly bill will depend on a number of factors, we can get a rough estimate.
I run my server for just a few people, and shut it down when we’re not playing (though anyone can start it for a little solitary game time). Both of these factors keep my costs down. In fact, my costs are so low that I’ve gotten bills for approximately $0.05 on more than one occasion. Here’s a more typical monthly bill for me, from March 2015:
Network Internet Egress: 1982.657 Mebibytes $0.23 $0.23 Standard Intel N1 1 VCPU: 2990 Minutes $3.14 $3.37 Storage Pd Capacity: 7430 Gibibyte-hours $0.40 $3.77 Storage Pd Snapshot: 2192.33 Gibibyte-hours $0.08 $3.85 Storage: 167.94 Gibibyte-hours $0.01 $3.86 Sales tax (on $3.37) $0.32 $4.18 "Totals for Mar 1, 2015 - Mar 31, 2015" $4.18 $4.18
As you can see, I’m running an
n1-standard1 instance, and I played for almost 50 hours that month. This was after the point that I started taking semi-regular snapshots, which accounts for a fraction of the total bill. However, my usage isn’t necessarily representative of everyone’s, which is why there are tools to help us estimate cost. To get a really detailed estimate of costs, we can use the pricing calculator.
Or, we can make a rough estimate by looking at the big ticket items (in our case, the virtual machine) to understand what our pricing options are. If our
n1-standard1 runs for more than 25% of the month, we can take advantage of a sustained use discount. The discount starts at 20% and goes up the more we use the instance. If we run the instance for an entire month, the hourly price is discounted by 30%.
Interlude over, now back to our regularly scheduled programming. We’ll tackle Minecraft, Kubernetes, and absurdity in the conclusion (?!) to this series.
You can see the files we used in this blog entry on GitHub.