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Serverless applications - Develop | Serverless | OpenShift Container Platform 4.6
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Serverless applications

Serverless applications are created and deployed as Kubernetes services, defined by a route and a configuration, and contained in a YAML file. To deploy a serverless application using OpenShift Serverless, you must create a Knative Service object.

Example Knative Service object YAML file
apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: hello (1)
  namespace: default (2)
spec:
  template:
    spec:
      containers:
        - image: docker.io/openshift/hello-openshift (3)
          env:
            - name: RESPONSE (4)
              value: "Hello Serverless!"
1 The name of the application.
2 The namespace the application uses.
3 The image of the application.
4 The environment variable printed out by the sample application.

You can create a serverless application by using one of the following methods:

  • Create a Knative service from the OpenShift Container Platform web console. See the documentation about Creating applications using the Developer perspective.

  • Create a Knative service by using the Knative (kn) CLI.

  • Create and apply a Knative Service object as a YAML file, by using the oc CLI.

Creating serverless applications by using the Knative CLI

Using the Knative (kn) CLI to create serverless applications provides a more streamlined and intuitive user interface over modifying YAML files directly. You can use the kn service create command to create a basic serverless application.

Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • You have installed the Knative (kn) CLI.

  • You have created a project or have access to a project with the appropriate roles and permissions to create applications and other workloads in OpenShift Container Platform.

Procedure

  • Create a Knative service:

    $ kn service create <service_name> --image <image> --tag <tag-value>

    Where:

    • --image is the URI of the image for the application.

    • --tag is an optional flag that can be used to add a tag to the initial revision that is created with the service.

      Example command
      $ kn service create event-display \
    --image quay.io/openshift-knative/knative-eventing-sources-event-display:latest
      Example output
      Creating service 'event-display' in namespace 'default':

  0.271s The Route is still working to reflect the latest desired specification.
  0.580s Configuration "event-display" is waiting for a Revision to become ready.
  3.857s ...
  3.861s Ingress has not yet been reconciled.
  4.270s Ready to serve.

Service 'event-display' created with latest revision 'event-display-bxshg-1' and URL:
http://event-display-default.apps-crc.testing

Creating a service using offline mode

You can execute kn service commands in offline mode, so that no changes happen on the cluster, and instead the service descriptor file is created on your local machine. After the descriptor file is created, you can modify the file before propagating changes to the cluster.

The offline mode of the Knative CLI is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.

For more information about the support scope of Red Hat Technology Preview features, see https://access.redhat.com/support/offerings/techpreview/.
Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • You have installed the Knative (kn) CLI.

Procedure

  1. In offline mode, create a local Knative service descriptor file:

    $ kn service create event-display \
    --image quay.io/openshift-knative/knative-eventing-sources-event-display:latest \
    --target ./ \
    --namespace test
    Example output
    Service 'event-display' created in namespace 'test'.

    • The --target ./ flag enables offline mode and specifies ./ as the directory for storing the new directory tree.

      If you do not specify an existing directory, but use a filename, such as --target my-service.yaml, then no directory tree is created. Instead, only the service descriptor file my-service.yaml is created in the current directory.

      The filename can have the .yaml, .yml, or .json extension. Choosing .json creates the service descriptor file in the JSON format.

    • The --namespace test option places the new service in the test namespace.

      If you do not use --namespace, and you are logged in to an OpenShift cluster, the descriptor file is created in the current namespace. Otherwise, the descriptor file is created in the default namespace.

  2. Examine the created directory structure:

    $ tree ./
    Example output
    ./
└── test
    └── ksvc
        └── event-display.yaml

2 directories, 1 file

    • The current ./ directory specified with --target contains the new test/ directory that is named after the specified namespace.

    • The test/ directory contains the ksvc directory, named after the resource type.

    • The ksvc directory contains the descriptor file event-display.yaml, named according to the specified service name.

  3. Examine the generated service descriptor file:

    $ cat test/ksvc/event-display.yaml
    Example output
    apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  creationTimestamp: null
  name: event-display
  namespace: test
spec:
  template:
    metadata:
      annotations:
        client.knative.dev/user-image: quay.io/openshift-knative/knative-eventing-sources-event-display:latest
      creationTimestamp: null
    spec:
      containers:
      - image: quay.io/openshift-knative/knative-eventing-sources-event-display:latest
        name: ""
        resources: {}
status: {}

  4. List information about the new service:

    $ kn service describe event-display --target ./ --namespace test
    Example output
    Name:       event-display
Namespace:  test
Age:
URL:

Revisions:

Conditions:
  OK TYPE    AGE REASON

    • The --target ./ option specifies the root directory for the directory structure containing namespace subdirectories.

      Alternatively, you can directly specify a YAML or JSON filename with the --target option. The accepted file extensions are .yaml, .yml, and .json.

    • The --namespace option specifies the namespace, which communicates to kn the subdirectory that contains the necessary service descriptor file.

      If you do not use --namespace, and you are logged in to an OpenShift cluster, kn searches for the service in the subdirectory that is named after the current namespace. Otherwise, kn searches in the default/ subdirectory.

  5. Use the service descriptor file to create the service on the cluster:

    $ kn service create -f test/ksvc/event-display.yaml
    Example output
    Creating service 'event-display' in namespace 'test':

  0.058s The Route is still working to reflect the latest desired specification.
  0.098s ...
  0.168s Configuration "event-display" is waiting for a Revision to become ready.
 23.377s ...
 23.419s Ingress has not yet been reconciled.
 23.534s Waiting for load balancer to be ready
 23.723s Ready to serve.

Service 'event-display' created to latest revision 'event-display-00001' is available at URL:
http://event-display-test.apps.example.com

Creating serverless applications using YAML

Creating Knative resources by using YAML files uses a declarative API, which enables you to describe applications declaratively and in a reproducible manner. To create a serverless application by using YAML, you must create a YAML file that defines a Knative Service object, then apply it by using oc apply.

After the service is created and the application is deployed, Knative creates an immutable revision for this version of the application. Knative also performs network programming to create a route, ingress, service, and load balancer for your application and automatically scales your pods up and down based on traffic.

Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • You have created a project or have access to a project with the appropriate roles and permissions to create applications and other workloads in OpenShift Container Platform.

  • Install the OpenShift CLI (oc).

Procedure

  1. Create a YAML file containing the following sample code:

    apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: event-delivery
  namespace: default
spec:
  template:
    spec:
      containers:
        - image: quay.io/openshift-knative/knative-eventing-sources-event-display:latest
          env:
            - name: RESPONSE
              value: "Hello Serverless!"

  2. Navigate to the directory where the YAML file is contained, and deploy the application by applying the YAML file:

    $ oc apply -f <filename>

Verifying your serverless application deployment

To verify that your serverless application has been deployed successfully, you must get the application URL created by Knative, and then send a request to that URL and observe the output. OpenShift Serverless supports the use of both HTTP and HTTPS URLs, however the output from oc get ksvc always prints URLs using the http:// format.

Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • You have installed the oc CLI.

  • You have created a Knative service.

Prerequisites

  • Install the OpenShift CLI (oc).

Procedure

  1. Find the application URL:

    $ oc get ksvc <service_name>
    Example command
    $ oc get ksvc event-delivery
    Example output
    NAME            URL                                        LATESTCREATED         LATESTREADY           READY   REASON
event-delivery   http://event-delivery-default.example.com   event-delivery-4wsd2   event-delivery-4wsd2   True

  2. Make a request to your cluster and observe the output.

    Example HTTP request
    $ curl http://event-delivery-default.example.com
    Example HTTPS request
    $ curl https://event-delivery-default.example.com
    Example output
    Hello Serverless!

  3. Optional. If you receive an error relating to a self-signed certificate in the certificate chain, you can add the --insecure flag to the curl command to ignore the error:

    $ curl https://event-delivery-default.example.com --insecure
    Example output
    Hello Serverless!

    Self-signed certificates must not be used in a production deployment. This method is only for testing purposes.

  4. Optional. If your OpenShift Container Platform cluster is configured with a certificate that is signed by a certificate authority (CA) but not yet globally configured for your system, you can specify this with the curl command. The path to the certificate can be passed to the curl command by using the --cacert flag:

    $ curl https://event-delivery-default.example.com --cacert <file>
    Example output
    Hello Serverless!

Interacting with a serverless application using HTTP2 and gRPC

OpenShift Serverless supports only insecure or edge-terminated routes. Insecure or edge-terminated routes do not support HTTP2 on OpenShift Container Platform. These routes also do not support gRPC because gRPC is transported by HTTP2. If you use these protocols in your application, you must call the application using the ingress gateway directly. To do this you must find the ingress gateway’s public address and the application’s specific host.

This method needs to expose Kourier Gateway using the LoadBalancer service type. You can configure this by adding the following YAML to your KnativeServing custom resource definition (CRD):

...
spec:
  ingress:
    kourier:
      service-type: LoadBalancer
...
Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • Install the OpenShift CLI (oc).

  • You have created a Knative service.

Procedure

  1. Find the application host. See the instructions in Verifying your serverless application deployment.

  2. Find the ingress gateway’s public address:

    $ oc -n knative-serving-ingress get svc kourier
    Example output
    NAME                   TYPE           CLUSTER-IP      EXTERNAL-IP                                                             PORT(S)                                                                                                                                      AGE
kourier   LoadBalancer   172.30.51.103   a83e86291bcdd11e993af02b7a65e514-33544245.us-east-1.elb.amazonaws.com   80:31380/TCP,443:31390/TCP   67m

    The public address is surfaced in the EXTERNAL-IP field, and in this case is a83e86291bcdd11e993af02b7a65e514-33544245.us-east-1.elb.amazonaws.com.

  3. Manually set the host header of your HTTP request to the application’s host, but direct the request itself against the public address of the ingress gateway.

    $ curl -H "Host: hello-default.example.com" a83e86291bcdd11e993af02b7a65e514-33544245.us-east-1.elb.amazonaws.com
    Example output
    Hello Serverless!

    You can also make a gRPC request by setting the authority to the application’s host, while directing the request against the ingress gateway directly:

    grpc.Dial(
    "a83e86291bcdd11e993af02b7a65e514-33544245.us-east-1.elb.amazonaws.com:80",
    grpc.WithAuthority("hello-default.example.com:80"),
    grpc.WithInsecure(),
)

    Ensure that you append the respective port, 80 by default, to both hosts as shown in the previous example.

Enabling communication with Knative applications on a cluster with restrictive network policies

If you are using a cluster that multiple users have access to, your cluster might use network policies to control which pods, services, and namespaces can communicate with each other over the network. If your cluster uses restrictive network policies, it is possible that Knative system pods are not able to access your Knative application. For example, if your namespace has the following network policy, which denies all requests, Knative system pods cannot access your Knative application:

Example NetworkPolicy object that denies all requests to the namespace
kind: NetworkPolicy
apiVersion: networking.k8s.io/v1
metadata:
  name: deny-by-default
  namespace: example-namespace
spec:
  podSelector:
  ingress: []

To allow access to your applications from Knative system pods, you must add a label to each of the Knative system namespaces, and then create a NetworkPolicy object in your application namespace that allows access to the namespace for other namespaces that have this label.

A network policy that denies requests to non-Knative services on your cluster still prevents access to these services. However, by allowing access from Knative system namespaces to your Knative application, you are allowing access to your Knative application from all namespaces in the cluster.

If you do not want to allow access to your Knative application from all namespaces on the cluster, you might want to use JSON Web Token authentication for Knative services instead. JSON Web Token authentication for Knative services requires Service Mesh.
Prerequisites

  • Install the OpenShift CLI (oc).

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

Procedure

  1. Add the knative.openshift.io/system-namespace=true label to each Knative system namespace that requires access to your application:

    1. Label the knative-serving namespace:

      $ oc label namespace knative-serving knative.openshift.io/system-namespace=true

    2. Label the knative-serving-ingress namespace:

      $ oc label namespace knative-serving-ingress knative.openshift.io/system-namespace=true

    3. Label the knative-eventing namespace:

      $ oc label namespace knative-eventing knative.openshift.io/system-namespace=true

    4. Label the knative-kafka namespace:

      $ oc label namespace knative-kafka knative.openshift.io/system-namespace=true

  2. Create a NetworkPolicy object in your application namespace to allow access from namespaces with the knative.openshift.io/system-namespace label:

    Example NetworkPolicy object
    apiVersion: networking.k8s.io/v1
kind: NetworkPolicy
metadata:
  name: <network_policy_name> (1)
  namespace: <namespace> (2)
spec:
  ingress:
  - from:
    - namespaceSelector:
        matchLabels:
          knative.openshift.io/system-namespace: "true"
  podSelector: {}
  policyTypes:
  - Ingress
    1 Provide a name for your network policy.
    2 The namespace where your application exists.

Configuring init containers

Init containers are specialized containers that are run before application containers in a pod. They are generally used to implement initialization logic for an application, which may include running setup scripts or downloading required configurations.

Init containers may cause longer application start-up times and should be used with caution for serverless applications, which are expected to scale up and down frequently.

Multiple init containers are supported in a single Knative service spec. Knative provides a default, configurable naming template if a template name is not provided. The init containers template can be set by adding an appropriate value in a Knative Service object spec.

Prerequisites

  • OpenShift Serverless Operator and Knative Serving are installed on your cluster.

  • Before you can use init containers for Knative services, an administrator must add the kubernetes.podspec-init-containers flag to the KnativeServing custom resource (CR). See the OpenShift Serverless "Global configuration" documentation for more information.

Procedure

  • Add the initContainers spec to a Knative Service object:

    Example service spec
    apiVersion: serving.knative.dev/v1
kind: Service
...
spec:
  template:
    spec:
      initContainers:
        - imagePullPolicy: IfNotPresent (1)
          image: <image_uri> (2)
          volumeMounts: (3)
            - name: data
              mountPath: /data
...
    1 The image pull policy when the image is downloaded.
    2 The URI for the init container image.
    3 The location where volumes are mounted within the container file system.

HTTPS redirection per service

You can enable or disable HTTPS redirection for a service by configuring the networking.knative.dev/http-option annotation. The following example shows how you can use this annotation in a Knative Service YAML object:

apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: example
  namespace: default
  annotations:
    networking.knative.dev/http-option: "redirected"
spec:
  ...

Additional resources