Deploying Postgres based Yesod web application to Kubernetes using Helm

Yesod is one of the most popular web frameworks in the Haskell land. This post will explore creating a sample Postgres based Yesod web application and then deploying it to Kubernetes cluster. We will create a Helm chart for doing our kubernetes release. Note that the entire source lives here:

• Sample yesod application
• Helm chart for the application

Creating Yesod application

The first step would be to create a sample Yesod application using the Stack build tool:

$ stack new yesod-demo yesod-postgres

Now, I will add an additional model to the existing scaffolding site:

Person json
    name Text
    age Int
    deriving Show

And also add four new routes to the application:

/person PersonR GET POST

/health/liveness LivenessR GET
/health/readiness ReadinessR GET

The /health based handlers are for the basic health checking of your application. Every kubelet in the worker node will perform a periodic diagnostic check to determine the health of your web application. Based on the result, it will perform an appropriate action (such as restarting your failed container).

Now let’s define some Yesod handlers for the above routes:

postPersonR :: Handler RepJson
postPersonR = do
  person :: Person <- requireJsonBody
  runDB $ insert_ person
  return $ repJson $ toJSON person

getPersonR :: Handler RepJson
getPersonR = do
  persons :: [Entity Person] <- runDB $ selectList [] []
  return $ repJson $ toJSON persons

getLivenessR :: Handler ()
getLivenessR = return ()

getReadinessR :: Handler RepJson
getReadinessR = do
  person :: [Entity Person] <- runDB $ selectList [] [LimitTo 1]
  return $ repJson $ toJSON person

The postPersonR function will insert a Person object in the database and the getPersonR function will return the list of all Person in the database. The other two handlers are for the probe checks which are performed by the kubelet. I have provided simplified implementation for them. The liveness probe is to tell the kubelet if the container is running. If it isn’t running, then the container is killed and based on the restart policy, an appropriate action is taken. The readiness probe on the otherhand informs the kubelet if the container is ready to service requests. You can read more about them here.

Building Docker Image

Now, I will update the stack.yaml file to build a docker image for our application:

image:
  container:
    name: psibi/yesod-demo:3.0
    base: fpco/stack-build
    add:
      static: /app
    
    entrypoints:
    - yesod-demo

docker:
  enable: true

Note that the name of the docker image is specified as psibi/yesod-demo:3.0. Also, I’m adding the static directory inside the container so that it can be served properly by the server. Now, doing stack image container will build the docker image. You can verify it using the docker tool:

 $ docker images
REPOSITORY                 TAG                 IMAGE ID            CREATED             SIZE
psibi/yesod-demo           3.0-yesod-demo      8e32c1329557        46 hours ago        7.92GB

Now, I will push it to the docker registry:

$ docker push psibi/yesod-demo:3.0-yesod-demo

Deployment to Kubernetes

Given that our application is available in the docker hub now, you can move on to create Kubernetes manifests and get ready for deploying our yesod application. Since our application has PostgreSQL as database backend, we will make use of the existing helm postgres chart for it. Note that for a mission critical workloads, something like RDS would be a better choice. Let’s create a helm chart for our application:

$ helm create yesod-postgres-chart
Creating yesod-postgres-chart
$ tree yesod-postgres-chart/
yesod-postgres-chart/
├── charts
├── Chart.yaml
├── templates
│   ├── deployment.yaml
│   ├── _helpers.tpl
│   ├── ingress.yaml
│   ├── NOTES.txt
│   └── service.yaml
└── values.yaml

2 directories, 7 files

Now I would create a requirements.txt file inside the folder and define our postgresql dependency there:

dependencies:
  - name: postgresql
    version: 0.15.0
    repository: https://kubernetes-charts.storage.googleapis.com

Now, let’s configure the postgresql helm chart from our values.yaml file:

postgresql:
  postgresUser: postgres
  postgresPassword: your-postgresql-password
  postgresDatabase: yesod-test
  persistence:
    storageClass: ssd-slow

Note that we are also providing a storageClass named ssd-slow in the above file. So, you would have to provision that in the k8s cluster. Using the StorageClass, the postgres helm chart will create volumes on demand. All the k8s manifests goes inside the templates directory. Let’s first create the above storageClass resource:

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: ssd-slow
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2

Kubernetes has a concept of ConfigMap and Secret to store special configuration and secret (duh!) data. Let’s define the ConfigMap resource for our yesod application:

apiVersion: v1
data:
  yesod_static_path: /app
  yesod_db_name: yesod-test
  yesod_db_port: "5432"
  yesod_host_name: "{{ .Release.Name  }}-postgresql"
kind: ConfigMap
metadata:
  name: {{ template "yesod-postgres-chart.full_name" . }}-configmap

We have previously seen in our stack.yaml that our static folder is added under /app, so corresponding we give the proper path to the key yesod_static_path. I have defined some helper functions in the file _helpers.tpl which I’m using now for the generation of metadata.name. We will be using these helper function throughout our template. Now, let’s define the Secret manifest for your application:

apiVersion: v1
kind: Secret
metadata:
  name: {{ template "yesod-postgres-chart.full_name" . }}-secret
type: Opaque
data:
  yesod-db-password: {{ .Values.postgresql.postgresPassword | b64enc | quote }}
  yesod-db-user: {{ .Values.postgresql.postgresUser | b64enc | quote }}

Now comes the important part – Deployment manifest. They basically are a controller which will manager your pods. This manifest for our application is slightly bigger:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: {{ template "yesod-postgres-chart.full_name" . }}
spec:
  replicas: 1
  template:
    metadata:
      labels:
        {{- include "yesod-postgres-chart.release_labels" . | indent 8 }}
    spec:
      containers:
        - name: yesod-demo
          image: {{ .Values.image.repository }}:{{ .Values.image.tag }}
          resources: 
{{ toYaml .Values.yesodapp.resources | indent 12 }}
          livenessProbe:
            httpGet:
              path: /health/liveness
              port: 3000
              scheme: HTTP
              initialDelaySeconds: 30
              periodSeconds: 15
              timeoutSeconds: 5              
          readinessProbe:
            httpGet:
              path: /health/readiness
              port: 3000
              scheme: HTTP
            initialDelaySeconds: 30
            timeoutSeconds: 1
          env:
            - name: YESOD_STATIC_DIR
              valueFrom:
                configMapKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-configmap
                  key: yesod_static_path
            - name: YESOD_PGPORT
              valueFrom:
                configMapKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-configmap
                  key: yesod_db_port
            - name: YESOD_PGHOST
              valueFrom:
                configMapKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-configmap
                  key: yesod_host_name
            - name: YESOD_PGDATABASE
              valueFrom:
                configMapKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-configmap
                  key: yesod_db_name
            - name: YESOD_PGUSER
              valueFrom:
                secretKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-secret
                  key: yesod-db-user
            - name: YESOD_PGPASS
              valueFrom:
                secretKeyRef:
                  name: {{ template "yesod-postgres-chart.full_name" . }}-secret
                  key: yesod-db-password

While the above file may look complex, it’s actually quite simple. We are specifying the docker image in spec.containers.image and the compute resources in spec.containers.resources. Then we specify the liveness and readiness probe with the proper path which we defined earlier. After that we define the environment variables which are required for proper running of this application. We fetch the variables from the configmaps and secrets we defined earlier.

The final step is to create a Service manifest which will act as a persistent endpoints for the pods created out of deployment. Services use labels to select a particular pod. This is our Service definition:

kind: Service
apiVersion: v1
metadata:
  name: {{ template "yesod-postgres-chart.full_name" . }}
  labels:
    {{- include "yesod-postgres-chart.release_labels" . | indent 4 }}
spec:
  selector:
    app: {{ template "yesod-postgres-chart.full_name" . }}
  ports:
    - protocol: "TCP"
      port: 3100
      targetPort: 3000
  type: ClusterIP

In the above file, you have defined the service type as ClusterIP. That makes our service only reachable within the cluster. Through spec.ports.targetPort we indicate the actual port on the POD where our service is running. Since our yesod application runs on port 3000 on the container, we specify it appropriately. The spec.ports.port indicates the port in which it will be available to other services in the cluster. Now, I will add nginx deployment on top of this which will route the requests to our yesod application. I won’t show it’s Deployment manifest as it’s quite similar to the previous one we saw, but it’s Service object is interesting:

kind: Service
apiVersion: v1
metadata:
  name: {{ template "yesod-nginx-resource.full_name" . }}
spec:
  selector:
    {{- include "yesod-nginx-resource.release_labels" . | indent 4 }}
  ports:
    - protocol: "TCP"
      port: 80
      targetPort: 80
  type: LoadBalancer

The type LoadBalancer will expose the service using the underlying cloud’s load balancer. Once you have the entire thing ready, you can install the chart:

helm install . --name="yesod-demo"

It will take some time for our pods to get up and start servicing the requests. We specify the release name as yesod-demo. You can find the load balancer DNS name by querying using kubectl:

$ kubectl get services
NAME                              TYPE           CLUSTER-IP       EXTERNAL-IP                                                               PORT(S)        AGE
kubernetes                        ClusterIP      172.20.0.1       <none>                                                                    443/TCP        12d
nginx-yesod-demo-yesod-postgres-chart   LoadBalancer   172.20.155.194   a0d96c1a5a14a11e88a68065a88681d5-2013621451.us-west-2.elb.amazonaws.com   80:31578/TCP   3d
yesod-demo-postgresql                   ClusterIP      172.20.173.246   <none>                                                                    5432/TCP       3d
yesod-demo-yesod-postgres-chart         ClusterIP      172.20.125.142   <none>                                                                    3100/TCP       3d

To see the log of the yesod application:

$ kubectl get pods
NAME                                            READY     STATUS    RESTARTS   AGE
nginx-yesod-demo-yesod-postgres-chart-f4f5979-2w6s9   1/1       Running   0          3d
yesod-demo-postgresql-786b4cc4-dwpph                  1/1       Running   0          3d
yesod-demo-yesod-postgres-chart-7bcb474796-xdbfd      1/1       Running   0          11h
$ kubectl logs -f yesod-demo-yesod-postgres-chart-7bcb474796-xdbfd
10.0.0.28 - - [19/Aug/2018:17:13:11 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:21 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:31 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:38 +0000] "GET /health/readiness HTTP/1.1" 200 33 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:41 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:48 +0000] "GET /health/readiness HTTP/1.1" 200 33 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:51 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:13:58 +0000] "GET /health/readiness HTTP/1.1" 200 33 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:14:01 +0000] "GET /health/liveness HTTP/1.1" 200 0 "" "kube-probe/1.10"
10.0.0.28 - - [19/Aug/2018:17:14:08 +0000] "GET /health/readiness HTTP/1.1" 200 33 "" "kube-probe/1.10"

You can see in the logs of our pod, that the web application is being continously probed by diagnostic checks. Let’s confirm that our web application is indeed working:

$ curl --header "Content-Type: application/json" --request POST --data '{"name":"Sibi","age":"32"}' https://a0d96c1a5a14a11e88a68065a88681d5-2013621451.us-west-2.elb.amazonaws.com
$ curl --header "Content-Type: application/json" --request POST --data '{"name":"Sibi","age":32}' https://a0d96c1a5a14a11e88a68065a88681d5-2013621451.us-west-2.elb.amazonaws.com/person
{"age":32,"name":"Sibi"}⏎                                                            

Now, if you do a curl request on the index page, then you will observe that the urls in the web application won’t be correct. Yesod, by default put’s the hostname in the url according to the configuration present in settings.yml file. You will be seeing the hostname of the pod in your linked urls. It will render improperly in your browser because of this:

There are three ways to tackle this problem:

• Override Approot in Yesod typeclass instance: This is the most straightforward approach and I would just define approot to ApprootRelative so that there is no root URL. You can see the other options here..
• Another option is to find out the load balancer’s DNS Name after kubernetes is deployed and pass new environment variable for your yesod application in accordance with your settings.yml file. But this would mean deploying again using helm upgrade. I dislike this approach as it involves multiple deployment using Helm. There is currently an open issue in Helm related to this.
• The third option is to call the Kubernetes API from your web application as a part of initialization step and get the DNS name dynamically and update your application settings accordingly. You have to use the Services API for achieving it.

For this demo application, let’s use the first approach and deploy a new image with tag 4.0:

$ docker push psibi/yesod-demo:4.0-yesod-demo

Now, let’s update the values.yaml file to the newest tag of the image and then upgrade the deployment using helm:

$ helm upgrade yesod-demo .

Now, you can indeed verify that the pod has updated to the new image

$ kubectl describe pod yesod-demo-yesod-postgres-chart-7bcb474796-xdbfd | grep Image
    Image:          psibi/yesod-demo:4.0-yesod-demo
    Image ID:       docker-pullable://psibi/yesod-demo@sha256:5d22e4dfd4ba4050f56f02f0b98eb7ae92d78ee4b5f2e6a0ec4fd9e6f9a068c7

Try visiting the homepage and you can observe that all the static resources of your yesod application would have properly loaded this time.

As you can see your application loads properly now. This concludes our post on deploying our Yesod application to Kubernetes. Let me know if you have any questions/queries through the comments section.

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