Kubernetes Architecture - Kubelet in control plane?
Hello,
I just started the The Linux Foundation's LFS258 Kubernetes Fundamentals class and I am a little confused regarding the kubelets and the control plane.
Because in the official Kubernetes overview (https://kubernetes.io/docs/concepts/overview/components/) there are no kubelets in the control plane, but in the course architecture overview, there is a kubelet in the control plane.
I thought kubelets would only be necessary on worker nodes.
So, what is correct?
Is it possible to have kubelets in the control plane, because the control plane is also a node? Or is there another reason?
Thank you for your help!
Best Answer
-
Hi @foehlschlaeger,
I agree that the documentation's ambiguous description of the
kubelet
node agent does not quite clarify its purpose.Both
kubelet
andkube-proxy
node agents are found on every node of the cluster, that includes control-plane node(s) and worker nodes. One ofkubelet
's tasks is to coordinate with the container runtime the container lifecycle events: to start, stop, delete... Since the control-plane agents (api server, scheduler, controller-manager, etcd) are in fact containers, it is easy to see the need for akubelet
agent on a control-plane node as well.Regards,
-Chris1
Answers
-
Looks like you have inconsistent documentation in
Kubernetes Architecture / Worker Nodes
"All worker nodes run the kubelet and kube-proxy, as well as the container engine, such as containerd or cri-o".
The kubelet interacts with the underlying Docker Engine also installed on all the nodes, and makes sure that the containers that need to run are actually running.
Should it be Container Engine? As Docker just one (most popular) of the containers that could be used with Kubernetes?
0 -
@foehlschlaeger
I'm posting a reply to try to communicate my current understanding of the architecture.
It's to be interpreted as me wanting to participate in discussions about Kubernetes architecture rather than me believing I have the right answer to your question.I consider
kube-apiserver
,kube-controller-manager
andkube-scheduler
to be the control plane agents. I considerkubelet
andkube-proxy
to be worker node agents.When we deploy with
kubeadm init
, the control plane agents are run as pods, and so they need the worker node agents and a container runtime on the node.But that depends on how we're running the agents. We could just as well download the binaries and run the control plane agents as systemd services. That's the approach taken by Kelsey Hightower in Kubernetes the hard way: https://github.com/kelseyhightower/kubernetes-the-hard-way/
When following Kelseys approach, the control plane nodes will not show up in
kubectl get nodes
. Only the worker nodes runkubelet
and register as nodes. Only the worker nodes have a container runtime. Public access to applications will be going via the worker nodes. Perhaps a reverse proxy with all worker nodes as upstream servers.I wonder what a typical Kubernetes setup looks like in the wild. Is
kubeadm
heavily used as a deployment tool? Is it uncommon to see the agents run as systemd services? Is it common to put "application" pods running on control plane nodes?0
Categories
- All Categories
- 206 LFX Mentorship
- 206 LFX Mentorship: Linux Kernel
- 732 Linux Foundation IT Professional Programs
- 339 Cloud Engineer IT Professional Program
- 165 Advanced Cloud Engineer IT Professional Program
- 66 DevOps Engineer IT Professional Program
- 131 Cloud Native Developer IT Professional Program
- 119 Express Training Courses
- 119 Express Courses - Discussion Forum
- 5.9K Training Courses
- 40 LFC110 Class Forum - Discontinued
- 66 LFC131 Class Forum
- 39 LFD102 Class Forum
- 219 LFD103 Class Forum
- 17 LFD110 Class Forum
- 32 LFD121 Class Forum
- 17 LFD133 Class Forum
- 6 LFD134 Class Forum
- 17 LFD137 Class Forum
- 70 LFD201 Class Forum
- 3 LFD210 Class Forum
- 2 LFD210-CN Class Forum
- 2 LFD213 Class Forum - Discontinued
- 128 LFD232 Class Forum - Discontinued
- 1 LFD233 Class Forum
- 2 LFD237 Class Forum
- 23 LFD254 Class Forum
- 683 LFD259 Class Forum
- 109 LFD272 Class Forum
- 3 LFD272-JP クラス フォーラム
- 10 LFD273 Class Forum
- 93 LFS101 Class Forum
- LFS111 Class Forum
- 2 LFS112 Class Forum
- 1 LFS116 Class Forum
- 3 LFS118 Class Forum
- 2 LFS142 Class Forum
- 3 LFS144 Class Forum
- 3 LFS145 Class Forum
- 1 LFS146 Class Forum
- 2 LFS147 Class Forum
- 8 LFS151 Class Forum
- 1 LFS157 Class Forum
- 10 LFS158 Class Forum
- 4 LFS162 Class Forum
- 1 LFS166 Class Forum
- 3 LFS167 Class Forum
- 1 LFS170 Class Forum
- 1 LFS171 Class Forum
- 2 LFS178 Class Forum
- 2 LFS180 Class Forum
- 1 LFS182 Class Forum
- 4 LFS183 Class Forum
- 30 LFS200 Class Forum
- 737 LFS201 Class Forum - Discontinued
- 2 LFS201-JP クラス フォーラム
- 17 LFS203 Class Forum
- 111 LFS207 Class Forum
- 1 LFS207-DE-Klassenforum
- LFS207-JP クラス フォーラム
- 301 LFS211 Class Forum
- 55 LFS216 Class Forum
- 49 LFS241 Class Forum
- 43 LFS242 Class Forum
- 37 LFS243 Class Forum
- 13 LFS244 Class Forum
- 1 LFS245 Class Forum
- 44 LFS250 Class Forum
- 1 LFS250-JP クラス フォーラム
- LFS251 Class Forum
- 143 LFS253 Class Forum
- LFS254 Class Forum
- LFS255 Class Forum
- 6 LFS256 Class Forum
- LFS257 Class Forum
- 1.2K LFS258 Class Forum
- 9 LFS258-JP クラス フォーラム
- 114 LFS260 Class Forum
- 151 LFS261 Class Forum
- 41 LFS262 Class Forum
- 82 LFS263 Class Forum - Discontinued
- 15 LFS264 Class Forum - Discontinued
- 11 LFS266 Class Forum - Discontinued
- 23 LFS267 Class Forum
- 18 LFS268 Class Forum
- 29 LFS269 Class Forum
- 199 LFS272 Class Forum
- 1 LFS272-JP クラス フォーラム
- LFS274 Class Forum
- 3 LFS281 Class Forum
- 2 LFW111 Class Forum
- 257 LFW211 Class Forum
- 176 LFW212 Class Forum
- 12 SKF100 Class Forum
- SKF200 Class Forum
- 791 Hardware
- 199 Drivers
- 68 I/O Devices
- 37 Monitors
- 98 Multimedia
- 174 Networking
- 91 Printers & Scanners
- 85 Storage
- 753 Linux Distributions
- 82 Debian
- 67 Fedora
- 16 Linux Mint
- 13 Mageia
- 23 openSUSE
- 147 Red Hat Enterprise
- 31 Slackware
- 13 SUSE Enterprise
- 350 Ubuntu
- 464 Linux System Administration
- 39 Cloud Computing
- 70 Command Line/Scripting
- Github systems admin projects
- 91 Linux Security
- 78 Network Management
- 101 System Management
- 47 Web Management
- 56 Mobile Computing
- 17 Android
- 28 Development
- 1.2K New to Linux
- 1K Getting Started with Linux
- 365 Off Topic
- 113 Introductions
- 171 Small Talk
- 20 Study Material
- 522 Programming and Development
- 291 Kernel Development
- 213 Software Development
- 1.1K Software
- 211 Applications
- 180 Command Line
- 3 Compiling/Installing
- 405 Games
- 311 Installation
- 79 All In Program
- 79 All In Forum
Upcoming Training
-
August 20, 2018
Kubernetes Administration (LFS458)
-
August 20, 2018
Linux System Administration (LFS301)
-
August 27, 2018
Open Source Virtualization (LFS462)
-
August 27, 2018
Linux Kernel Debugging and Security (LFD440)