How Can Async/Await Solve Concurrency Issues When Querying MySQL in Node.js?

Asynchronous Execution in Node.js: Using Async/Await with MySQL

As a newcomer to Node.js, you may encounter difficulties adapting its modern syntax, especially when working with asynchronous operations. Let's explore how to use the async/await keywords to achieve synchronized results when querying a MySQL database.

In your code, you wish to execute multiple queries and concatenate the results into a single string. However, since Node.js executes JavaScript code in a non-blocking manner, the below code does not guarantee that the variables string1 to string4 will contain the expected results:

// Original Code
var string1 = '';
var string2 = '';
var string3 = '';
var string4 = '';

DatabasePool.getConnection((err, connection) => {
  // Query 1
  connection.query(query, (err, result) => {
    string1 = result;
  });

  // Query 2
  connection.query(query, (err, result) => {
    string2 = result;
  });

  // Query 3
  connection.query(query, (err, result) => {
    string3 = result;
  });

  // Query 4
  connection.query(query, (err, result) => {
    string4 = result;
  });

  // Attempt to concatenate results
  var appended_text = string1 + string2 + string3 + string4;
});

Solution with Async/Await

To resolve this issue and allow for synchronous execution, Node.js 8 provides the util.promisify() function. Here's how to integrate it with the MySQL library:

// Improved Code
const mysql = require('mysql');
const util = require('util');
const conn = mysql.createConnection({ yourHOST, yourUSER, yourPW, yourDB });

// Promisify the query method
const query = util.promisify(conn.query).bind(conn);

(async () => {
  try {
    // Execute queries concurrently
    const [query1Result, query2Result, query3Result, query4Result] = await Promise.all([
      query('select count(*) as count from file_managed'),
      query('another query'),
      query('another query'),
      query('another query'),
    ]);

    // Concatenate results
    const appended_text = query1Result[0].count + query2Result[0].count + query3Result[0].count + query4Result[0].count;
  } finally {
    // Close the connection
    conn.end();
  }
})();

In this revised code:

• The query method is converted to a Promise using util.promisify().
• The bind() function ensures that this still refers to the original object.
• The Promise.all() method executes all four queries concurrently and returns an array of results.
• The results are extracted and concatenated into appended_text.
• Finally, the connection is closed.

This approach allows you to seamlessly synchronize your queries and obtain the desired output in a reliable manner, making async/await a powerful tool for handling asynchronous operations in Node.js.

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