Explain what the Health Check mechanism is in the application deployment process.

The Health Check mechanism is a process where a system or orchestrator (for example, Kubernetes) regularly verifies whether the application is functioning correctly and is available to handle requests.

Health Check helps to:

• Automatically detect application failures or hangs.
• Restart or replace non-working instances.
• Ensure high availability and reliability of the service.

Typically, Health Check is implemented through special HTTP requests to a specific endpoint (for example, /health), which returns the application's status. If the response does not match the expected (for example, status code 200), it is considered that the application is unhealthy.

An example of a simple health check in Node.js:

const http = require('http');

const server = http.createServer((req, res) => {
  if (req.url === '/health') {
    res.writeHead(200, {'Content-Type': 'application/json'});
    res.end(JSON.stringify({ status: 'ok' }));
  } else {
    res.writeHead(404);
    res.end();
  }
});

server.listen(3000);