In the articles Kuberentes at an OrangePi and Setting up the OrangePi it was described how I build my toy cluster. Meanwhile the cluster received some updates. Two more nodes were added, a Raspberry Pi 3 B+ and a Orange Pi One Plus. Outside temperature and humidity is now measured with a DHT22 sensor. As an NFS storage for persistent volumes I also added a 500GB HDD (leftover from replacing the HDD in my PS4 with an SSD). Finally a 12cm cooler running at 5 V now keeps the cluster temperature low. In addition I added a ST7789 Display to show the cluster status as well as node health and temperature. This article will cover how to connect the display to the cluster and how to manage the communication between the cluster nodes using DeamonSets.
1. Connecting the Display
To display the current status of the cluster a waveshare ST7789 with 2 inch size is chosen. This display is then connected to the master-node using the following wiring:
Communication with the display is handled using python and the spidev package.
2. Using a simple REST Endpoint to gather data
Communication within the cluster is easy. At the master-node a container with a REST API will be deployed which will trigger the display. At each node we will deploy a small container that frequently sends the current temperature to the REST endpoint. For the sake of simplicity this example is kept as simple as possible, but can easily be extended if necessary. The code can be found in the corresponding git repos (see links) while the compiled images are available due to dockerhub here (sender) and here (reciever).
To build the images it is useful to have a development PI, since the images have to be build using arm architecture. Alternatively emulation, for example with Qemu, can be used. Pushing the Images to dockerhub is straightforward
docker tag git_reciever:latest <your-repo>/temperaturemonitor_reciever:latest docker push <your-repo>/temperaturemonitor_reciever:latest docker tag git_sender:latest <your-repo>/temperaturemonitor_sender:latest docker push <your-repo>/temperaturemonitor_sender:latest
3. Setup a DeamonSet to measure node temperature
First we will label the node where the display was connected. Labels are useful to control where certain pods are deployed. Since we want the plot controlling the display to be deployed at the pod where the display is installed, labels are necessary for our purpose. To see all labels assigned to far use
kubectl get nodes --show-labels
To add a new label to a node
kubectl label nodes <your-node-name> display=true
Next we create a namespace. In fact creating an own namespace is optional. However namespaces help to structure the cluster and also add a bit of security since we can control which pods have access to the ClusterIP Service used to communicate between the pods. We will create a namespace with the name “disp-monitoring” with
kubectl create namespace disp-monitoring
To switch to this namespace type
kubectl config set-context --current --namespace disp-monitoring
With all the infrastructure set up we can deploy our sensor-display deployment using this deployment yaml
#apiVersion: v1 #kind: Namespace #metadata: # name: disp-monitoring # labels: # apps: web-based # annotations: # type: monitor #--- apiVersion: v1 kind: Service metadata: name: svc-monitor namespace: disp-monitoring spec: selector: env: display ports: - protocol: TCP port: 65432 targetPort: 65432 --- apiVersion: v1 kind: Pod metadata: name: receiver namespace: disp-monitoring labels: env: display spec: containers: - image: <your-repo>/temperaturemonitor_receiver:latest securityContext: privileged: true name: receiver ports: - containerPort: 65432 nodeSelector: display: "true" --- apiVersion: apps/v1 kind: DaemonSet metadata: name: sender namespace: disp-monitoring spec: selector: matchLabels: name: sender template: metadata: labels: name: sender spec: tolerations: # this toleration is to have the daemonset runnable on master nodes # remove it if your masters can't run pods - key: node-role.kubernetes.io/master effect: NoSchedule containers: - name: sender image: <your-repo>/temperaturemonitor_sender:latest env: - name: RECEIVERIP value: svc-monitor
To deploy the app directly out of my git repo, simply execute
kubectl apply -f https://raw.githubusercontent.com/heikowagner/temperaturemonitor_reciever/master/deployment.yaml