Docker / Kubernetes - DaemonSet

DaemonSets try to adhere to a one-Pod-per-node model, either across the entire cluster or a subset of nodes. So, when we add nodes to a node pool, DaemonSets automatically add Pods to the new nodes as needed.

DaemonSets are useful for deploying ongoing background tasks that we need to run on all or certain nodes, and which do not require user intervention.

Examples of such tasks include storage daemons like ceph or glusterd, log collection daemons like fluentd or logstash, and node monitoring daemons like collectd, Prometheus (node_exporter), or Datadog agent.

The following is an example of a DaemonSet manifest file (fluentd-daemonset.yaml):

# fluentd-daemonset.yaml
apiVersion: apps/v1
kind: DaemonSet
metadata:
  name: fluentd-daemonset
spec:
  template:
    metadata:
      labels:
        name: fluentd # Pod template's label selector
    spec:
      containers:
      - name: fluentd
        image: gcr.io/google-containers/fluentd-elasticsearch:1.20
  selector:
      matchLabels:
        name: fluentd # Label selector that determines which Pods belong to the DaemonSet

In the manifest:

1. A DaemonSet named fluentd-daemonset is created, indicated by the metadata: name field.
2. DaemonSet's Pod is labelled fluentd in the .spec.template which is a pod template that requires to have labels in place.
3. The Pod's container pulls the fluentd-elasticsearch image at version 1.20. The container image is hosted by Container Registry.
4. The .spec.selector field is a label selector. These are the labels used to identify potential Pods to acquire.

On GKE, we have 3 nodes running in the cluster:

$ kubectl get node
NAME                                        STATUS    ROLES     AGE       VERSION
gke-bogotobogo-default-pool-abee3b7e-cbgv   Ready     <none>    2h        v1.11.7-gke.12
gke-bogotobogo-default-pool-abee3b7e-jjnt   Ready     <none>    2h        v1.11.7-gke.12
gke-bogotobogo-default-pool-abee3b7e-qp86   Ready     <none>    2h        v1.11.7-gke.12

Let's deploy the DaemeonSet controller to the cluster:

$ kubectl create -f fluentd-daemonset.yaml
daemonset.apps/fluentd-daemonset created

Let's check our pods, especially, in which node they are running on:

$ kubectl get po -o wide
NAME                      READY     STATUS    RESTARTS   AGE       IP           NODE                                        NOMINATED NODE
fluentd-daemonset-dzp2n   1/1       Running   0          5m        10.40.2.8    gke-bogotobogo-default-pool-abee3b7e-qp86   <none>
fluentd-daemonset-rllk5   1/1       Running   0          5m        10.40.0.4    gke-bogotobogo-default-pool-abee3b7e-cbgv   <none>
fluentd-daemonset-swhzg   1/1       Running   0          5m        10.40.1.11   gke-bogotobogo-default-pool-abee3b7e-jjnt   <none>

Currently, 3 pods are running, one per node.

Let's display DaemonSet:

$ kubectl get ds
NAME                DESIRED   CURRENT   READY     UP-TO-DATE   AVAILABLE   NODE SELECTOR   AGE
fluentd-daemonset   3         3         3         3            3           <none>          7m

$ kubectl describe ds fluentd-daemonset
Name:           fluentd-daemonset
Selector:       name=fluentd
Node-Selector:  <none>
Labels:         name=fluentd
Annotations:    <none>
Desired Number of Nodes Scheduled: 3
Current Number of Nodes Scheduled: 3
Number of Nodes Scheduled with Up-to-date Pods: 3
Number of Nodes Scheduled with Available Pods: 3
Number of Nodes Misscheduled: 0
Pods Status:  3 Running / 0 Waiting / 0 Succeeded / 0 Failed
Pod Template:
  Labels:  name=fluentd
  Containers:
   fluentd:
    Image:        gcr.io/google-containers/fluentd-elasticsearch:1.20
    Port:         <none>
    Host Port:    <none>
    Environment:  <none>
    Mounts:       <none>
  Volumes:        <none>
Events:
  Type    Reason            Age   From                  Message
  ----    ------            ----  ----                  -------
  Normal  SuccessfulCreate  9m    daemonset-controller  Created pod: fluentd-daemonset-dzp2n
  Normal  SuccessfulCreate  9m    daemonset-controller  Created pod: fluentd-daemonset-swhzg
  Normal  SuccessfulCreate  9m    daemonset-controller  Created pod: fluentd-daemonset-rllk5

We created one object which is DeamonSet which in turn created the pods based on the pod manifest. Let's delete the DaemonSet:

$ kubectl delete ds fluentd-daemonset
daemonset.extensions "fluentd-daemonset" deleted

$ kubectl get ds
No resources found.

So, the DeamonSet has been deleted. But what happened to the pods it created?

$ kubectl get po
No resources found.

No pods!

Note that if we delete the pods while keeping the DaemonSet, our pods will be created automatically by the DeamonSet!

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4. Docker install on Ubuntu 14.04
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6. Nginx image - share/copy files, Dockerfile
7. Working with Docker images : brief introduction
8. Docker image and container via docker commands (search, pull, run, ps, restart, attach, and rm)
9. More on docker run command (docker run -it, docker run --rm, etc.)
10. Docker Networks - Bridge Driver Network
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12. File sharing between host and container (docker run -d -p -v)
13. Linking containers and volume for datastore
14. Dockerfile - Build Docker images automatically I - FROM, MAINTAINER, and build context
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16. Dockerfile - Build Docker images automatically III - RUN
17. Dockerfile - Build Docker images automatically IV - CMD
18. Dockerfile - Build Docker images automatically V - WORKDIR, ENV, ADD, and ENTRYPOINT
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25. Docker - Deploying a Java EE JBoss/WildFly Application on AWS Elastic Beanstalk Using Docker Containers
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37. Docker Compose - A gentle introduction with WordPress
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39. MEAN Stack app on Docker containers : micro services
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41. Docker Compose - Hashicorp's Vault and Consul Part A (install vault, unsealing, static secrets, and policies)
42. Docker Compose - Hashicorp's Vault and Consul Part B (EaaS, dynamic secrets, leases, and revocation)
43. Docker Compose - Hashicorp's Vault and Consul Part C (Consul)
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48. Docker & Kubernetes : Ambassador - Envoy API Gateway on Kubernetes
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50. Docker Cheat Sheet
51. Docker Q & A #1
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53. Kubernetes Q & A - Part II
54. Docker - Run a React app in a docker
55. Docker - Run a React app in a docker II (snapshot app with nginx)
56. Docker - NodeJS and MySQL app with React in a docker
57. Docker - Step by Step NodeJS and MySQL app with React - I
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77. Docker & Kubernetes : Persistent Volumes - Dynamic volume provisioning
78. Docker & Kubernetes : DaemonSet
79. Docker & Kubernetes : Secrets
80. Docker & Kubernetes : kubectl command
81. Docker & Kubernetes : Assign a Kubernetes Pod to a particular node in a Kubernetes cluster
82. Docker & Kubernetes : Configure a Pod to Use a ConfigMap
83. AWS : EKS (Elastic Container Service for Kubernetes)
84. Docker & Kubernetes : Run a React app in a minikube
85. Docker & Kubernetes : Minikube install on AWS EC2
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