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Learn the best practices for sending streams from forked child processes in Node.js using Fastify and streams.
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How to Properly Send a Stream from a Forked Child Process in Node.js?

Sending streams from a forked child process in Node.js can be a bit tricky, especially when working with frameworks like Fastify. You might find yourself running into issues, such as only receiving the last piece of data and encountering errors like "Reply already sent". In this guide, we will walk through the steps to properly handle a cursor stream from a forked child process, ensuring your application runs smoothly and efficiently.

The Problem

In your case, you've set up a mechanism to send data from a child process back to the parent, but you're facing a few hurdles:

You're only seeing the last data piece printed from your cursor stream.

You have encountered the Fastify error: "Reply already sent".

This suggests that the way you are handling the communication between the parent and child processes requires some adjustments.

The Solution

To address this issue, we need to leverage Node.js streams effectively. Below, I'll break down the solution into clear steps so you can understand how to implement it in your application.

Step 1: Set Up Your Fastify Server

First, you'll need to create a Fastify server if you haven't already. Here's a simple setup to get you started:

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Step 2: Create a Readable Stream

Within your Fastify route handler, create a Readable stream. This stream will be used to push data from your child process back to the client.

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Step 3: Fork the Child Process

Utilize Node.js's child_process module to fork your child process. Ensure you have the correct file path.

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Step 4: Handle Messages from the Child Process

Listen for messages from the child process. When messages are received, push them into the stream.

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Step 5: Close the Stream

Don’t forget to handle the closure of the child process, which signals that there are no more messages to send. This tells the readable stream to end its lifecycle.

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Step 6: Send the Stream in the Response

Finally, send the readable stream in the response of your Fastify route.

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Example Code

Here is the complete example to put it all together:

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Conclusion

In summary, the process of streaming data from a forked child process in Node.js requires the right setup of streams and proper communication handling between the parent and child processes. Following the steps outlined above, you can ensure that your Fastify server can efficiently handle and respond to streaming data without encountering common errors.

If you need to send a stream from a forked child process, make sure to implement these practices to enhance the performance and reliability of your application.