Red Hat OpenShift Development I: Introduction to Containers with Podman

Instructions

1. Build and run a container image for the application.

   1. Navigate to the application directory.

      [student@workstation ~]$ cd ~/DO188/labs/troubleshooting-debugging/nodebug
      no output expected

   2. Examine the NPM scripts in the package.json file.

      [student@workstation nodebug]$ cat package.json
      ...output omitted...
        "scripts": {
          "start": "node index.js",
          "debug": "nodemon --inspect=0.0.0.0 index.js"
        },
      ...output omitted...

      The start script starts the application. The debug script starts the application with the --inspect option to enable debugging.

      The Node.js debug mode binds to 127.0.0.1 by default, which means it is available only inside the container. To connect from outside of the container, it must bind to 0.0.0.0.

   3. Build a container image for the application.

      [student@workstation nodebug]$ podman build -t nodebug .
      ...output omitted...
      Successfully tagged localhost/nodebug:latest

   4. Create a container that forwards traffic on the 8080 and 9229 ports to the same ports inside of the container. Override the container start command so that it runs the debug npm script. Run the container in the background by using the -d option and remove the container when it stops by using the --rm option.

      [student@workstation nodebug]$ podman run -d --rm \
      --name nodebug -p 8080:8080 -p 9229:9229 \
      nodebug npm run debug
      ...output omitted...

   5. Confirm that the application started in debug mode.

      [student@workstation nodebug]$ podman logs nodebug
      ...output omitted...
      Debugger listening on ws://0.0.0.0:9229/16a2...4bf6c
      ...output omitted...
      app started on port 8080

      The application runs on port 8080 and the debugger listens on port 9229. Podman forwards both of these ports from the host to the container.

2. Attach VSCodium to the running container and make a request to the /echo endpoint.

   1. Launch VSCodium with the application project.

      [student@workstation nodebug]$ codium .

      If prompted, click Yes, I trust the authors.

   2. Click Run → Add configuration to open a selection menu, and then click Node.js.

   3. Replace the default contents with the following launch configuration and save the file.

      {
          "version": "0.2.0",
          "configurations": [
              {
                  "type": "node",
                  "request": "attach",
                  "name": "Debug Nodebug",
                  "port": 9229,
                  "address": "localhost",
                  "localRoot": "${workspaceFolder}",
                  "remoteRoot": "/"
              }
          ]
      }

      The preceding configuration is available in the ~/DO188/labs/troubleshooting-debugging/nodebug/launch-config.json file.

   4. Click Run → Start Debugging to launch the configuration and attach the VSCodium debugger to the running application.

   5. In the terminal window, make a request to the running application.

      [student@workstation nodebug]$ curl localhost:8080/echo?message=hello
      no output expected

      The request does not respond as the hard-coded breakpoint was triggered.

      Note

      In Node.js, the debugger statement is a hard-coded breakpoint when running in debug mode. Outside of the debug mode, this statement has no effect.

   6. Notice that the debugger reached the breakpoint in the index.js file and the execution context is displayed in the debug panel.

      Observe that the local message variable uses the hello value. The application sets the variable from the message query parameter that you supplied to the request.

   7. Continue the execution by clicking the Continue icon.

      Figure 6.1: Continue program execution

      In the terminal window, the curl request completes and echoes our message: hello.

3. Use the debugger to find and fix a bug in the /snacks endpoint.

   1. Attach a breakpoint to the endpoint. Click the red circle next to the line in the file that assigns the available_snacks variable.

      Figure 6.2: Attach a breakpoint

      Note

      The red circle does not appear until you hover over the space to the left of the line number.

      A solid circle persists if you attached the breakpoint successfully.

   2. In the terminal window, make a request to the /snacks endpoint.

      [student@workstation nodebug]$ curl localhost:8080/snacks?search=apple

      Execution pauses because the breakpoint in the snacks endpoint is triggered.

   3. Click the Step Over icon to step through the function line-by-line.

      Figure 6.3: Step Over icon

      Each time you click the button, the highlighted line of code is executed.

   4. Continue to click the button until the request responds that the search has failed and the function returns.

      [student@workstation nodebug]$ curl localhost:8080/snacks?search=apple
      sorry, we don't have any apples :(

      This is a bug because the queried snack, apples, is in the available_snacks list.

   5. Try to swap the lines containing res.send statements. VSCodium does not allow this because the file is read-only.

      This editor buffer does not contain the index.js file, but instead an in-memory copy of what VSCodium is debugging.

   6. Open the index.js file by clicking View → Explorer and double-clicking the index.js file in the explorer panel.

      Change the application response when a snack is found as follows:

      app.get("/snacks", (req, res) => {
        const search == req.query.search;
        const available_snacks == ["apple", "cheese", "cracker", "lunchmeat", "olive"];
        for (const snack of available_snacks) {
          if (snack ==== search) {
            res.send(`yes, we have ${search}s!\n`);
            return;
          }
        }
        res.send(`sorry, we don't have any ${search}s :(\n`);
      });

      Save the changes to the file.

   7. In the terminal window, make the same request to the snack endpoint as before.

      [student@workstation nodebug]$ curl localhost:8080/snacks?search=apple

      Step through the execution again and notice that the bug is not fixed. This is because only the application code on the host was updated. The container contents are unchanged.

4. Recreate and debug the container with the application code mounted into the container.

   1. In the terminal window, stop the container.

      [student@workstation nodebug]$ podman stop nodebug
      nodebug

      This also stops the VSCodium debug connection.

   2. Install the Node.js dependencies on the host machine.

      [student@workstation nodebug]$ npm install
      ...output omitted...

   3. Create a container with the application code mounted into the container.

      [student@workstation nodebug]$ podman run -d --rm \
      --name nodebug -p 8080:8080 -p 9229:9229 \
      -v .:/opt/app-root/src:Z \
      nodebug npm run debug
      9eba...9c81

      Note

      Installing the Node.js dependencies in the host machine is necessary because the preceding -v option overrides the entire application directory from the container image, which includes runtime dependencies.

   4. Connect VSCodium to the debug socket by clicking the Start Debugging icon.

   5. In the terminal window, make the same request to the snack endpoint as before.

      [student@workstation ~]$ curl localhost:8080/snacks?search=apple

      Step through the execution.

      Notice that the bug is fixed because the updated code on the host was mounted inside the container:

      [student@workstation nodebug]$ curl localhost:8080/snacks?search=apple
      yes, we have apples!

      This set up provides faster debugging by not requiring a container rebuild on every code change.

   6. Disconnect the debugger by clicking Disconnect or by closing the VSCodium window.

      Figure 6.4: Disconnect the debugger

5. Create a container image with the updated code and verify that the bug is fixed.

   1. Stop and remove the running debug container.

      [student@workstation nodebug]$ podman rm -f nodebug

   2. Remove the node_modules directory from the host.

      [student@workstation nodebug]$ rm -r node_modules
      no output expected

   3. Build a container image with the fixed application code.

      [student@workstation nodebug]$ podman build -t nodebug .
      ...output omitted...
      Successfully tagged localhost/nodebug:latest
      ...output omitted...

   4. Create a container with the new image. Run the application without debug mode and without exposing the debug port.

      [student@workstation nodebug]$ podman run -d --rm \
      --name nodebug -p 8080:8080 nodebug
      8ef...eb7

   5. Make the same request to the application as before and verify that the response is correct.

      [student@workstation nodebug]$ curl localhost:8080/snacks?search=apple
      yes, we have apples!