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Discover the best practices for sending multiple emails simultaneously using `nodemailer` in Node.js, and avoid common errors like SQLITE transaction issues.
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How to Send Multiple Emails at Once with nodemailer

When working on a Node.js application that requires sending multiple emails (for instance, notifications, newsletters, or reminders), many developers encounter issues related to handling asynchronous operations and database transactions. One common error that developers face is the UnhandledPromiseRejectionWarning: Error: Message failed: 450 SQLITE_ERROR: cannot start a transaction within a transaction.

This error often occurs when attempts are made to concurrently use transactions while interacting with a SQLite database, especially when the email-sending process is wrapped in a transaction.

In this guide, we will break down how to efficiently send multiple emails using nodemailer, while highlighting common pitfalls and how to avoid them.

Understanding the Problem

In your case, you're trying to fetch data from a SQLite database using sequelize and then send emails based on that data using nodemailer. Here's a simplified flow of what you're trying to achieve:

Fetch email details from the database.

Send emails based on the fetched details.

Handle any potential errors that arise, particularly with transactions in SQLite.

The main issue arises when the asynchronous nature of the email-sending process conflicts with the SQLite transactions, resulting in errors.

A Step-By-Step Solution

Step 1: Create the Transporter

First, set up your nodemailer transporter. This transporter will handle the connection to your email service (e.g., Gmail):

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Step 2: Fetch Emails from the Database

Using sequelize, fetch the email entries you need to send. Make sure you handle this fetching in a way that does not wrap the email-sending function inside an active database transaction.

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Step 3: Send Emails Concurrently

This is a crucial part. Use Promise.all combined with map to send out emails concurrently, ensuring that you're creating the promises correctly:

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Step 4: Comprehensive Error Handling

It's vital to handle potential errors gracefully. In the code above, a try-catch block is used to catch any exceptions that might be thrown during the email-sending process.

Example of Sending Multiple Emails

Finally, here's a simple example of how to send multiple emails in parallel using Promise.all:

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Conclusion

Sending multiple emails using nodemailer can be a straightforward process if you manage your promises and database transactions correctly. By following the steps outlined above, you can avoid common pitfalls and ensure your emails are sent without issues.

If you maintain proper separation of concerns between your email-sending logic and database transactions, you will find that errors like 450 SQLITE_ERROR become less frequent.

Feel free to share your thoughts or ask any follow-up questions in the comments below! Happy coding!