As the name implies, DaemonSets are the ideal choice for logging agents and monitoring tools you have running on your cluster. Introduced in Kubernetes 1.1, these objects function almost identically to the ReplicationControllers that you’re familiar with, with the notable difference that each node will receive exactly one pod based on your template.
ReplicationControllers and Deployments are not designed to run cluster-wide daemons, such as log collectors. The pods they create are scheduled intelligently onto each minion node, meaning there is no guarantee that each node receive exactly one pod.
Ideally, DaemonSets solve this problem. However, the kube-up scripts for AWS do not register the master’s kubelet with the Kubernetes API server, meaning DaemonSets won’t be spun up on your master node. I believe this is an odd default, but fortunately Kubernetes has a few flags you can enable on your master kubelet to support DaemonSets.
Registering your master with the Kube API server
Note: I’m assuming you use the kube-up script included in Kubernetes to spin up your cluster on AWS or a similar non-GCE cloud provider with SSH access to your master node. I’m using Debian Jessie but any systemd-based OS should work the same way.
Start off by SSHing into your master node using the same PEM file you used to spin up the cluster:
$ ssh -i ~/.ssh/kubernetes.pem email@example.com
Now on your master node, you’re going to need the cluster-local IP address of your master node so we can attach the kubelet to the correct API server:
$ hostname -i
We can’t use localhost because the API server’s certificate isn’t signed for local requests.
Next, we’re going to edit the master kubelet’s configuration file, adding a few new flags:
$ sudo vim /etc/sysconfig/kubelet
The last line will look something like this:
These are the configuration flags passed to the master node’s kubelet. We’re going to add 3 flags to this list, described below. Modify the line to look like this:
--api-servers=MASTER_LOCAL_IPtells the kubelet where the Kubernetes API server is located, in this case on the local instance
--register-node=truewill ask the kubelet to register as a node with the API server on startup
--register-schedulable=falsemakes the master node unschedulable for any pods except for DaemonSets (awesome!)
Go ahead and save the kubelet configuration file.
There’s one more modification we’ll need to make to the kubelet so that it can register with the API server: give it the Kubernetes API certificates via a kubeconfig file. The easiest way to get the correct kubeconfig settings for your master node is by SSHing into any of your minion nodes. For the default AWS setup:
$ sudo cat /mnt/ephemeral/kubernetes/kubelet/kubeconfig
- name: kubelet
Copy the contents of that file from a minion node to the same location on your master node, save, and you’re good to go.
Finally, restart your master node’s kubelet:
$ sudo systemctl restart kubelet
You can see if it worked by running:
$ sudo journalctl -f -n 100 -u kubelet
As long as you don’t see an error message saying that your kubelet crashed, you’re all set! If there was an issue, double-check your configuration flags and restart the kubelet again.
Nodes take a little while to register, so wait about 5 minutes or so. On your local computer, you can list all of the nodes using kubectl to make sure it worked:
$ kubectl get nodes
NAME STATUS AGE
172–20–0–221.us-west-2.compute.internal Ready 5d
172–20–0–222.us-west-2.compute.internal Ready 5d
172–20–0–223.us-west-2.compute.internal Ready 5d
172–20–0–224.us-west-2.compute.internal Ready 5d
172–20–0–74.us-west-2.compute.internal Ready 5d
172–20–0–9.us-west-2.compute.internal Ready,SchedulingDisabled 5d
Notice the final node with scheduling disabled. That’s your master kubelet registered as a Kubernetes node! You can get started scheduling DaemonSets right away and they’ll automatically be provisioned on the master node as well as each of the minions.
Bonus: Some fun DaemonSets for you to use
We’ve got some utilities containerized over at the Pavlov GitHub. In particular, we’ve open sourced our fluentd container that handles all our log collection and sends it to Amazon S3. We’ve included an example Kubernetes object file to make it really easy to get up and running. Give it a whirl and let us know what you think!
I’m Alex Kern, co-founder and CTO of Pavlov. We make an intelligent warehouse for your customer data. We’re currently in a limited release while we on-board companies and developers. If you found this helpful, recommend it to a friend. 🙂