Updated the StatefulSet second exercise

[n0rq1]

The second exercise provides a Dockerfile + code for a simple web app, the rest of the implementation is up to the user.

As opposed to what I originally had (Helm chart and some k8s manifests provided).

[windsurf-bot]

Other comments (6)

• examples/ch9/statefulsets/counterapp/server.js (39-41) Similar to the previous comment, errors in `writeCounter()` should be logged:

    } catch (e) {
      console.error('Error writing counter:', e);
      return false;
    }
  

• examples/ch9/statefulsets/counterapp/server.js (18-33) The application uses synchronous file operations (`fs.existsSync`, `fs.mkdirSync`, `fs.writeFileSync`, `fs.readFileSync`) which can block the Node.js event loop. For a production application, consider using the asynchronous versions of these functions to improve performance under load.
• examples/ch9/statefulsets/counterapp/server.js (27-29) The `JSON.parse()` call could throw an exception if the file contains invalid JSON, but it's not specifically handled. Consider adding a try-catch specifically for the parsing operation:

      const raw = fs.readFileSync(DATA_FILE, 'utf8');
      try {
        const obj = JSON.parse(raw);
        return typeof obj.count === 'number' ? obj.count : 0;
      } catch (parseError) {
        console.error('Error parsing counter file:', parseError);
        return 0;
      }
  

• examples/ch9/statefulsets/counterapp/src/app.js (1-7) The fetchState function should provide error feedback to the user when the API call fails. Consider adding error handling to display a message when the API is unavailable.

  async function fetchState() {
    try {
      const res = await fetch('/api/state');
      if (!res.ok) {
        document.getElementById('count').textContent = 'Error';
        document.getElementById('podName').textContent = 'API unavailable';
        return;
      }
      const data = await res.json();
      document.getElementById('count').textContent = data.count;
      document.getElementById('podName').textContent = `pod: ${data.podName}`;
    } catch (error) {
      document.getElementById('count').textContent = 'Error';
      document.getElementById('podName').textContent = 'API unavailable';
      console.error('Failed to fetch state:', error);
    }
  }
  

• examples/ch9/statefulsets/counterapp/src/app.js (9-14) The increment function should provide error feedback when the API call fails. Consider adding error handling similar to fetchState.

  async function increment() {
    try {
      const res = await fetch('/api/increment', { method: 'POST' });
      if (!res.ok) {
        document.getElementById('count').textContent = 'Error';
        return;
      }
      const data = await res.json();
      document.getElementById('count').textContent = data.count;
    } catch (error) {
      document.getElementById('count').textContent = 'Error';
      console.error('Failed to increment count:', error);
    }
  }
  

• examples/ch9/statefulsets/counterapp/src/app.js (16-19) Consider adding a periodic refresh to automatically update the counter state. This would be particularly useful in a StatefulSet demonstration to show real-time updates from other pods.

  window.addEventListener('DOMContentLoaded', () => {
    document.getElementById('incrementBtn').addEventListener('click', increment);
    fetchState();
    
    // Refresh state every 5 seconds
    setInterval(fetchState, 5000);
  });
  

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[windsurf-bot]

The shell command in the args array needs to be properly quoted to be executed as a single command:

Suggested change

	`args: ["shuf -i 0-100 -n 1 >> /opt/number.out; sleep 10000"]`.
	`args: ["/bin/sh", "-c", "shuf -i 0-100 -n 1 >> /opt/number.out; sleep 10000"]`.

[windsurf-bot]

The exercise should explicitly instruct students to configure persistent storage for the counter app StatefulSet using volumeClaimTemplates. Without this, the counter state won't persist when pods are deleted, which is a key learning objective for StatefulSets.

[windsurf-bot]

The catch blocks in both readCounter() and writeCounter() functions silently swallow errors. Consider logging the actual error message to help with debugging:

Suggested change

	} catch (e) {
	return 0;
	}
	} catch (e) {
	console.error('Error reading counter:', e);
	return 0;
	}