Node.js serverless functions on Red Hat OpenShift, Part 3: Debugging on a cluster

This article is the third in a series about running Node.js applications in containers in an orchestration environment such as Kubernetes or Red Hat OpenShift. The first article focused on control over logging, and the second article on debugging JavaScript programs on your local system. This time, we'll look at how to use the Chrome DevTools inspector to debug a Node.js function that is running inside a container on an OpenShift cluster.

Prerequisites

To follow along, you will need an OpenShift cluster with the Serverless Operator installed. See the article, Create your first serverless function with Red Hat OpenShift Serverless Functions to set up this environment.  

If you need an Openshift cluster to play around with that already has the serverless operators installed and setup, you can easily create an instance of the Openshift Sandbox.  This post won’t go into setting up the sandbox, but  the process is fairly straightforward.

This article also assumes some familiarity with the Chrome DevTools inspector. For an introduction, please see How to debug your Node.js application on OpenShift with Chrome DevTools.

Get the example code

The example that we are going to use can be retrieved from the Node.js HTTP function repository on GitHub. Similar to the example used in our previous article, this function application was scaffolded with the kn-func command-line tool, which we described in that article.

We'll have to make a few changes before using the code for this article. To start, note that the package.json file for this example defines three scripts:

"scripts": {

    "test": "node test/unit.js && node test/integration.js",

    "start": "faas-js-runtime ./index.js",

    "debug": "nodemon --inspect ./node_modules/faas-js-runtime/bin/cli.js ./index.js"

  }

These scripts are similar to the ones in the previous article, and we will make the same changes here that we did previously.

First, simply switch the debug script with the start script. We make this change because the kn-func command can’t specify which script to run. You can name the start script whatever you like.

Next, make a simple change to the start script. You need to tell it to listen on all available addresses because you are running as localhost (IP address 127.0.0.1) inside the container, which the debugger can’t access by default. Therefore, change the --inspect option in the start script to --inspect=0.0.0.0.

These changes should produce scripts similar to the following:

"scripts": {

    "test": "node test/unit.js && node test/integration.js",

    "debug": "faas-js-runtime ./index.js",

    "start": "nodemon --inspect=0.0.0.0 ./node_modules/faas-js-runtime/bin/cli.js ./index.js"

  }

Debugging Node.js functions in a container

Now, create a container using the kn-func build command. If this is the first time you are building the application, the command prompts you to add a registry and namespace for the containerized function. By default, the registry is Docker Hub. For the namespace, enter your Docker Hub ID.

Once the image is built, use the docker command to run the container and start debugging your Node.js functions. Since the debugger listens on port 9229, you need to expose that port as well as port 8080, which is the port to access your application:

$ docker run --rm  -p 8080:8080 -p 9229:9229 lholmquist/debugging-with-functions-on-cluster:latest

The output should be similar to:

Debugger listening on ws://0.0.0.0:9229/584eb679-4db1-4a40-9519-5bf5c42275f5

For help, see: https://nodejs.org/en/docs/inspector

The server has started. http://localhost:8080

Now that the container is running, you can open Chrome DevTools and navigate to the chrome://inspect URL, where you should see a link labeled inspect (Figure 1). Click this link to connect to your running container.

Now you should see the Chrome inspector, showing code similar to Figure 2.

Set a breakpoint at some point in the program, then navigate to http://localhost:8080 to run the application. The inspector stops at the breakpoint shown in Figure 3, allowing you to inspect variables and do other debugging tasks.

Debugging Node.js functions on an OpenShift cluster

Having debugged your program in your container, you can use a similar process to debug it on an OpenShift cluster. Make the same changes in the npm scripts and use the same command to build the container. To deploy the container to the cluster, use the kn-func command:

$ kn-func deploy

Once the container is deployed, navigate to the topology view in the OpenShift console, which should show you something like Figure 4.

Clicking the icon on the top-right corner of the function's box navigates to the application's route. You should then see the JSON output that the function sends when invoked.

To start debugging, you need the help of the oc port-forward command. This command was described in the article How to debug your Node.js application on OpenShift with Chrome DevTools, so I won't explain here what each part of the command does. For this example, your command should look something like this:

$ oc port-forward $(oc get po | grep debugging-with-functions | grep Running | awk '{print $1}') 8888:9229

The command should start forwarding requests to the debugger process. If it is successful, you'll see log messages similar to these:

Forwarding from 127.0.0.1:8888 -> 9229
Forwarding from [::1]:8888 -> 9229
Handling connection for 8888
Handling connection for 8888

With forwarding enabled, navigate again in your browser to the chrome://inspect URL and you should see something similar to Figure 5.

As in the previous example with the container, clicking the inspect link should show the debugger. Again add a breakpoint, then navigate to the route that OpenShift provides to access the application. The debugger should break on the point you added, as shown in Figure 6.

Conclusion

This article showed how to debug a Node.js application while running inside a container, as well as running on an OpenShift cluster.

Stay tuned for more posts related to running Node.js applications on Red Hat OpenShift Serverless. You can also check out the latest documentation at the About OpenShift Serverless Functions site.

If you want to learn more about what Red Hat is up to on the Node.js front, check out our Node.js landing page.