Скачать или смотреть Resolving Async/Await Issues in Node.js Application with PM2

Discover how to tackle `async/await` issues in your Node.js application, particularly when using PDF generation and PM2. This guide covers the root cause and provides a simple solution to ensure smooth execution.
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Troubleshooting Async/Await Issues in Node.js with PM2

If you are working with a Node.js application that uses express for routing and pdfmake for PDF generation, you might encounter an unexpected issue where your async/await functions stop working after creating a write stream with fs.createWriteStream(path). This problem can be frustrating, especially when it doesn't appear locally but manifests on a production server. In this blog, we will explain the potential causes and provide a clear solution.

Understanding the Problem

You have a node application that retrieves data, generates a PDF document, and saves it to disk upon a button click. However, you notice that while debugging locally, all your async/await functions are behaving as expected. Yet, once you deploy the application using PM2 on a production server, some async/await calls appear ignored after a write stream is created.

Example Scenario

Here is a simplified version of the express route in question:

[[See Video to Reveal this Text or Code Snippet]]

The problem arises particularly in this part of the code, where pdf.generateInvoice is called after creating the PDF document and directing it to a file using a write stream.

Pinpointing the Cause

The source of the issue is often related to how you run your application with PM2. In this case, the problem was tied to the usage of the --watch flag when starting the application. Here’s how it works:

PM2 Watch Mode: When you use pm2 start appName --watch, PM2 monitors the changes in your codebase. If any file changes (including runtime-generated files like PDFs), it automatically restarts the Node.js application.

Write Stream Impact: In your instance, while the app was trying to write the PDF to disk, PM2 detected the file creation as a change, triggering a restart. This abrupt interruption in the application flow caused unpredictable behavior with your async/await functions.

Solution Overview

To resolve this issue, you can simply run your application without the --watch flag. Here’s the clear course of action:

Stop the current PM2 process:

[[See Video to Reveal this Text or Code Snippet]]

Restart your application without the watch mode:

[[See Video to Reveal this Text or Code Snippet]]

By avoiding the automagic restarts on detected changes, you allow your application's async/await functionality to execute correctly, ensuring all parts of your PDF generation and saving logic run as intended.

Conclusion

If you find your async/await functions behaving inconsistently in a Node.js application deployed with PM2, remember to check your start command for the --watch flag. Removing it can resolve confusing issues around writing files and executing asynchronous code.

By understanding how PM2 manages your application and tweaking its settings, you can ensure a smoother operation in your production environment. Don't forget to keep your packages and versions updated, as improvements and bug fixes can aid in preventing unforeseen issues in the future.

Keep coding, and happy debugging!