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Hello World - Haskell
A simple web app written in Haskell that you can use for testing. It reads in an
env variable TARGET
and prints “Hello ${TARGET}!". If TARGET is not
specified, it will use “World” as the TARGET.
Prerequisites
- A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions if you need to create one.
- Docker installed and running on your local machine, and a Docker Hub account configured (we'll use it for a container registry).
Recreating the sample code
While you can clone all of the code from this directory, hello world apps are generally more useful if you build them step-by-step. The following instructions recreate the source files from this folder.
-
Create a new file named
stack.yaml
and paste the following code:flags: {} packages: - . extra-deps: [] resolver: lts-10.7
-
Create a new file named
package.yaml
and paste the following codename: helloworld-haskell version: 0.1.0.0 dependencies: - base >= 4.7 && < 5 - scotty - text executables: helloworld-haskell-exe: main: Main.hs source-dirs: app ghc-options: - -threaded - -rtsopts - -with-rtsopts=-N
-
Create a
app
folder, then create a new file namedMain.hs
in that folder and paste the following code. This code creates a basic web server which listens on port 8080:{-# LANGUAGE OverloadedStrings #-} import Data.Maybe import Data.Monoid ((<>)) import Data.Text.Lazy (Text) import Data.Text.Lazy import System.Environment (lookupEnv) import Web.Scotty (ActionM, ScottyM, scotty) import Web.Scotty.Trans main :: IO () main = do t <- fromMaybe "World" <$> lookupEnv "TARGET" pStr <- fromMaybe "8080" <$> lookupEnv "PORT" let p = read pStr :: Int scotty p (route t) route :: String -> ScottyM() route t = get "/" $ hello t hello :: String -> ActionM() hello t = text $ pack ("Hello " ++ t)
-
In your project directory, create a file named
Dockerfile
and copy the code block below into it.# Use the official Haskell image to create a build artifact. # https://hub.docker.com/_/haskell/ FROM haskell:8.2.2 as builder # Copy local code to the container image. WORKDIR /app COPY . . # Build and test our code, then build the “helloworld-haskell-exe” executable. RUN stack setup RUN stack build --copy-bins # Use a Docker multi-stage build to create a lean production image. # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds FROM fpco/haskell-scratch:integer-gmp # Copy the "helloworld-haskell-exe" executable from the builder stage to the production image. WORKDIR /root/ COPY --from=builder /root/.local/bin/helloworld-haskell-exe . # Run the web service on container startup. CMD ["./helloworld-haskell-exe"]
-
Create a new file,
service.yaml
and copy the following service definition into the file. Make sure to replace{username}
with your Docker Hub username.apiVersion: serving.knative.dev/v1 kind: Service metadata: name: helloworld-haskell namespace: default spec: template: spec: containers: - image: docker.io/{username}/helloworld-haskell env: - name: TARGET value: "Haskell Sample v1"
Build and deploy this sample
Once you have recreated the sample code files (or used the files in the sample folder) you're ready to build and deploy the sample app.
-
Use Docker to build the sample code into a container. To build and push with Docker Hub, enter these commands replacing
{username}
with your Docker Hub username:# Build the container on your local machine docker build -t {username}/helloworld-haskell . # Push the container to docker registry docker push {username}/helloworld-haskell
-
After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster. Ensure that the container image value in
service.yaml
matches the container you built in the previous step. Apply the configuration usingkubectl
:kubectl apply --filename service.yaml
-
Now that your service is created, Knative will perform the following steps:
- Create a new immutable revision for this version of the app.
- Network programming to create a route, ingress, service, and load balance for your app.
- Automatically scale your pods up and down (including to zero active pods).
-
To find the URL for your service, enter:
kubectl get ksvc helloworld-haskell --output=custom-columns=NAME:.metadata.name,URL:.status.url NAME URL helloworld-haskell http://helloworld-haskell.default.1.2.3.4.xip.io
-
Now you can make a request to your app and see the result. Replace the URL below with the URL returned in the previous command.
curl http://helloworld-haskell.default.1.2.3.4.xip.io Hello world: Haskell Sample v1
Removing the sample app deployment
To remove the sample app from your cluster, delete the service record:
kubectl delete --filename service.yaml
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