Error: Can't add new command when connection is in closed state QueryFailedError (NestJS + MySQL2 + TypeORM): A Comprehensive Guide to Resolving the Issue

Are you tired of encountering the frustrating “Error: Can't add new command when connection is in closed state QueryFailedError” while working with NestJS, MySQL2, and TypeORM? This error can be a major roadblock in your development process, but fear not, dear reader, for we’ve got you covered! In this article, we’ll delve into the depths of this error, explore its causes, and provide you with a step-by-step guide to resolving it once and for all.

What is the “Error: Can't add new command when connection is in closed state QueryFailedError”?

The “Error: Can't add new command when connection is in closed state QueryFailedError” is a common error that occurs when your application tries to execute a query on a closed database connection. This error is often thrown by TypeORM, a popular Object-Relational Mapping (ORM) tool for TypeScript, when it’s unable to establish a connection to the database.

Causes of the Error

There are several reasons why you might encounter this error. Some of the most common causes include:

• Incorrect database configuration: Make sure your database credentials, such as username, password, and hostname, are correct and match the ones specified in your TypeORM configuration.
• Database connection timeout: If the database connection takes too long to establish, it may time out, causing the error.
• TypeORM configuration issues: Ensure that your TypeORM configuration is correct and matches the requirements of your application.
• MySQL2 connection issues: Verify that your MySQL2 connection is established correctly and is not closed prematurely.

Resolving the Error: A Step-by-Step Guide

To resolve the “Error: Can't add new command when connection is in closed state QueryFailedError”, follow these steps:

Step 1: Verify Your Database Configuration

First things first, double-check your database configuration. Make sure your database credentials are correct and match the ones specified in your TypeORM configuration.

// typeorm.config.ts
import { TypeOrmModuleOptions } from '@nestjs/typeorm';

export const typeOrmConfig: TypeOrmModuleOptions = {
  type: 'mysql',
  host: 'localhost',
  port: 3306,
  username: 'root',
  password: 'password',
  database: 'database',
};

Step 2: Check Your TypeORM Configuration

Review your TypeORM configuration to ensure it’s correct and matches the requirements of your application. Pay attention to the `connectTimeout` and `acquireTimeout` options, as these can affect the database connection.

// typeorm.config.ts
import { TypeOrmModuleOptions } from '@nestjs/typeorm';

export const typeOrmConfig: TypeOrmModuleOptions = {
  type: 'mysql',
  host: 'localhost',
  port: 3306,
  username: 'root',
  password: 'password',
  database: 'database',
  connectTimeout: 10000, // adjust this value according to your needs
  acquireTimeout: 30000, // adjust this value according to your needs
};

Step 3: Establish a Stable MySQL2 Connection

Verify that your MySQL2 connection is established correctly and is not closed prematurely. You can do this by creating a separate connection to the database using the `mysql2` package.

// mysql-connection.ts
import mysql from 'mysql2/promise';

const mysqlConnection = mysql.createConnection({
  host: 'localhost',
  port: 3306,
  user: 'root',
  password: 'password',
  database: 'database',
});

mysqlConnection.connect((err) => {
  if (err) {
    console.error('Error connecting to MySQL:', err);
    return;
  }
  console.log('MySQL connection established successfully!');
});

Step 4: Integrate the MySQL2 Connection with TypeORM

Integrate the MySQL2 connection with TypeORM by creating a custom database connection using the `createConnection` method from TypeORM.

// typeorm.config.ts
import { createConnection } from 'typeorm';
import mysql from 'mysql2/promise';

const mysqlConnection = mysql.createConnection({
  host: 'localhost',
  port: 3306,
  user: 'root',
  password: 'password',
  database: 'database',
});

const typeOrmConnection = createConnection({
  type: 'mysql',
  connection: mysqlConnection,
  entities: [__dirname + '/../**/*.entity{.ts,.js}'],
  synchronize: true,
});

export default typeOrmConnection;

Step 5: Implement Connection Retries and Timeout

To ensure that your application can recover from connection timeouts and errors, implement connection retries and timeouts using the `retry` package.

// typeorm.config.ts
import { createConnection } from 'typeorm';
import mysql from 'mysql2/promise';
import { retry } from 'retry';

const mysqlConnection = mysql.createConnection({
  host: 'localhost',
  port: 3306,
  user: 'root',
  password: 'password',
  database: 'database',
});

const retryOptions = {
  retries: 3,
  factor: 2,
  minTimeout: 1000,
  maxTimeout: 3000,
};

const typeOrmConnection = createConnection({
  type: 'mysql',
  connection: mysqlConnection,
  entities: [__dirname + '/../**/*.entity{.ts,.js}'],
  synchronize: true,
  retry: retryOptions,
});

export default typeOrmConnection;

Conclusion

In conclusion, the “Error: Can't add new command when connection is in closed state QueryFailedError” can be a frustrating error to encounter, but with the right approach, you can resolve it and ensure that your application runs smoothly. By following the steps outlined in this guide, you’ll be able to:

• Verify your database configuration
• Check your TypeORM configuration
• Establish a stable MySQL2 connection
• Integrate the MySQL2 connection with TypeORM
• Implement connection retries and timeouts

Remember to stay vigilant and monitor your application’s performance to ensure that the error does not resurface. Happy coding!

Keyword	Description
Error: Can't add new command when connection is in closed state QueryFailedError	A common error that occurs when TypeORM is unable to establish a connection to the database.
NestJS	A popular Node.js framework for building server-side applications.
MySQL2	A popular database driver for MySQL.
TypeORM	An Object-Relational Mapping (ORM) tool for TypeScript.

Frequently Asked Question

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