Guarantee application availability with OpenShift readiness probes

Red Hat OpenShift allows you to perform rolling updates of existing running applications. This feature, which is native to Kubernetes, means you can spin up a different version of an application—newer or older—and automatically route traffic to different versions as appropriate. When done correctly, this can be used to create a zero downtime deployment.

That last part—done correctly—is the focus of this sandbox activity. We'll guide you through the process of deploying an application, adding a Readiness Check to the application, actively using the application, and then initiating a rolling update of the application.

Expected time to completion: Five minutes

Prerequisites

You must first complete the OpenShift Sandbox Activity entitled Code deployment with Kubernetes rolling update.

Working environment for this activity

This activity will require you to work at the command line, using either bash or PowerShell.

What you'll be doing

1. Log in to your OpenShift Sandbox at the command line
2. Create a readiness probe for an existing deployment
3. Run a curl command loop to view the application output
4. Change deployment image to version 3
5. Observe rolling deployment results

Part 1: Log in to your OpenShift Sandbox at the command line

Step 1.1

Log in to your OpenShift Sandbox by following these instructions.

Create a readiness probe for an existing deployment

Step 1.1

Run the following command to create the readiness probe:

oc set probe deploy/myboot --readiness --get-url=http://:8080/health

Part 2: Run a curl command loop to view the application output

Step 2.1

Run the following command to see the URL of the application:

oc get routes

You will see output similar to the following:

NAME     HOST/PORT                                                                    PATH   SERVICES   PORT   TERMINATION   WILDCARD
myboot   myboot-rhn-engineering-dschenck-dev.apps.sandbox.x8i5.p1.openshiftapps.com          myboot     http                 None

Step 2.2

We will now run a curl command loop against this URL to see the application output. Replace the URL in the following command based on the results of the previous command (oc get routes) and enter it at a command line.

If you are using bash:

for ((i=1;i<=10000;i++)); do curl http://myboot-rhn-engineering-foo-dev.apps.sandbox.x8i5.p1.openshiftapps.com; sleep .01; done;

If you are using PowerShell:

while ($true) { (curl myboot-rhn-engineering-foo-dev.apps.sandbox.x8i5.p1.openshiftapps.com).content;start-sleep -Milliseconds 200; }

Figure 1 shows an example of the kind of output you can expect.

[myboot_curl_powershell_output.png] Figure 1: curl command loop output.

Part 3: Change deployment image to version 3

Step 3.1

Run the following command to switch to the version 3 image for the myboot application:

oc set image deploy/myboot myboot=quay.io/rhdevelopers/myboot:v3

Part 4: Observe rolling deployment results

Step 4.1

As the command in step 3.1 is processed, you will see the output of the curl loop change. This is a result of the rolling update with zero downtime.

Learn more

Learn more about Kubernetes probes from these Red Hat resources:

• You (probably) need liveness and readiness probes
• Eliminate downtime during OpenShift rolling updates
• Monitoring application health by using health checks
• Liveness and readiness probes