**Course Title:** Mastering Express.js: Building Scalable Web Applications and APIs **Section Title:** Deployment and Continuous Integration **Topic:** Monitoring and maintaining deployed applications **Introduction:** In the previous topics, we have covered the setup and deployment of Express.js applications. However, monitoring and maintaining deployed applications is a crucial aspect of ensuring the scalability and reliability of our web applications. In this topic, we will explore the different tools and techniques used for monitoring and maintaining deployed applications, and provide practical examples of how to implement them. **Monitoring Applications:** Monitoring applications involves tracking their performance, identifying issues, and taking corrective actions to prevent downtime. There are several tools available for monitoring applications, including: 1. **New Relic:** A comprehensive monitoring tool that provides detailed insights into application performance, including CPU usage, memory usage, and database performance. (https://newrelic.com/) 2. **Datadog:** A monitoring tool that provides real-time insights into application performance, including metrics, logs, and alerts. (https://www.datadoghq.com/) 3. **Prometheus:** An open-source monitoring system that provides detailed insights into application performance, including metrics and alerts. (https://prometheus.io/) **Key Concepts:** 1. **Metrics:** Metrics are numerical values that describe application performance, such as CPU usage, memory usage, and response times. 2. **Logs:** Logs are text files that contain information about application events, such as errors, warnings, and information messages. 3. **Alerts:** Alerts are notifications that are sent when a threshold is exceeded, such as when CPU usage exceeds 80%. **Practical Takeaways:** 1. **Use a monitoring tool:** Choose a monitoring tool that fits your needs and implement it in your application. 2. **Collect metrics:** Collect metrics that describe application performance, such as CPU usage, memory usage, and response times. 3. **Log events:** Log events that occur in your application, such as errors, warnings, and information messages. 4. **Set alerts:** Set alerts that are triggered when a threshold is exceeded, such as when CPU usage exceeds 80%. **Example:** Let's say we have an Express.js application that uses the New Relic monitoring tool. We can collect metrics such as CPU usage, memory usage, and response times, and log events such as errors and warnings. We can also set alerts that are triggered when a threshold is exceeded, such as when CPU usage exceeds 80%. ```javascript const express = require('express'); const app = express(); const newRelic = require('new-relic'); newRelic.init({ app: app, metrics: { cpu: true, memory: true, responseTime: true } }); app.get('/', (req, res) => { // Code to handle GET request }); app.listen(3000, () => { console.log('Server listening on port 3000'); }); ``` **Conclusion:** Monitoring and maintaining deployed applications is a crucial aspect of ensuring the scalability and reliability of our web applications. By using a monitoring tool, collecting metrics, logging events, and setting alerts, we can identify issues and take corrective actions to prevent downtime. In the next topic, we will review advanced topics such as caching strategies and performance optimization. **Leave a comment or ask for help:** If you have any questions or need further clarification on any of the concepts covered in this topic, please leave a comment below.