Health Check

 

What is HEALTHCHECK

  • The HEALTHCHECK instruction tells Docker how to test a container to check that it is still working.
  • There could be a scenario where our application process is still running, but may not respond to our request.
  • HEALTHCHECK instruction helps us to identify this issue.
  • We can have multiple HEALTHCHECK instruction in our Dockerfile but only last one will take an effect.
  • When a container has a HEALTHCHECK specified, it has a health status in addition to its normal status.

Note: Kubernetes doesn't support this HEALTHCHECK instruction. Kubernetes has its own way to checking if containers are healthy.

HEALTHCHECK

  • The HEALTHCHECK instruction can be used in two forms.
    • HEALTHCHECK [OPTIONS] CMD command. Which checks container health by running a command inside the container.
    • HEALTHCHECK NONE. This will disable any health check inherited from the base image.
  • If you are using the app from Simple Spring App, make sure you are in dockerfile-initial branch.
  • We have couple of REST endpoints in the app
    • /test/ which kind of hello-world endpoint for us
    • /test/status-5xx which will return status 200 for first 5 times, but after that it will throw exception. Our app will still be running.
    • /test/exit-1 to exit the app with non-zero status code.
    • /test/exit-0 to exit the app with zero status code.
  • Let's add HEALTHCHECK in our image.
  • HEALTHCHECK CMD curl --fail http://localhost:8080/test || exit 1
  • We are making a curl request to our app and if it fails we are exiting our app with status code 1.
FROM openjdk:8-jre-alpine
WORKDIR /myApp
EXPOSE 8080
RUN apk add --no-cache curl
COPY ["onlyweb/target/*.jar", "./app.jar"]
CMD ["java", "-jar", "app.jar"]
HEALTHCHECK CMD curl --fail http://localhost:8080/test || exit 1
  • Let's build and run this image.
IMAGE                                 STATUS                    
amantuladhar/doc-kub-ws:web-basic     Up XX seconds (healthy)
  • We will play around with HEALTHCHECK more, but first let's learn about different options HEALTHCHECK supports.

--interval

  • --interval=@duration
  • Default duration time is 30s
  • This configures time HEALTHCHECK needs to wait before it runs next HEALTHCHECK CMD.

--timeout

  • --timeout=@duration
  • Default timeout is 30s
  • This configures the time it should take a HEALTHCHECK CMD to finish.
  • If a single CMD run takes longer than timeout seconds then the check is considered to have failed.

--retries

  • --retries=@numberOfRetries
  • Default value for retry is 3.
  • This configures how many times HEALTHCHECK needs to fail before it determines out container is unhealthy.

--start-period

  • --start-period=@duration
  • Default value for start period duration is 0s.
  • With is option we can configure time it will take for app to run.
  • If we run pre-mature HEALTHCHECK our app might not have initialized and it may result in our container be in unhealthy state.
  • Basically it will run our first HEALTHCHECK CMD after time specified.

 

Playing with HEALTHCHECK

  • Let's update our HEALTHCHECK instruction.
FROM openjdk:8-jre-alpine
WORKDIR myapp/
COPY ["onlyweb/target/*.jar", "app.jar"]
EXPOSE 8080
RUN apk add --no-cache curl
CMD ["java", "-jar", "app.jar"]
HEALTHCHECK  --start-period=10s \
             --interval=5s \
             --timeout=2s  \
             --retries=3 \
             CMD curl --fail http://localhost:8080/test/status-5xx || exit 1
  • Build you image and run it. And keep an eye on health status.
  • Remember status-5xx will return status 200 for first 5 calls
  • So for a minute or so our container health will be healthy.
  • But because the HEALTHCHECK is called frequently, and for our app at exactly 6th call it will start to fail.
  • On a 6th call it won't flag our container as unhealthy as we defined retries.
  • Docker makes sure that three consecutive failure happens before it flags the container as unhealthy.
IMAGE                                  CREATED             STATUS                          
amantuladhar/doc-kub-ws:web-basic      10 seconds ago      Up 10 seconds (health: starting)

IMAGE                                 CREATED             STATUS                 
amantuladhar/doc-kub-ws:web-basic     24 seconds ago      Up 24 seconds (healthy)

IMAGE                                 CREATED             STATUS                  
amantuladhar/doc-kub-ws:web-basic     2 minutes ago       Up 2 minutes (unhealthy)

Complete dockerfile for with health check web-healthcheck web-healthcheck