nodejs sql process control

When Node.js uses a relational database, it often needs to perform asynchronous operations, such as obtaining data from the database and processing it. The processing results will affect the next operation. At this time, we need to use process control.

Process control is used to control code in different states, such as sequential execution, parallel execution, exception handling, etc. In Node.js, common process controls include callback functions, Promise, async/await, and generators. When using Node.js to connect to a relational database, callback functions, Promise and async/await are particularly important. The usage of the three process controls will be introduced in detail below.

1. Callback function

The callback function is the classic process control method in Node.js. The callback function is a function that can be executed after the current function is executed, and the result is passed as its parameter to achieve asynchronous control. Some function libraries in Node.js and some database drivers provide callback functions to implement asynchronous calls. For example, in the native file module fs in Node.js, you can read the file content through the callback function:

const fs = require('fs');
fs.readFile('./file.txt', 'utf8', (err, data) => {
  if (err) return console.error(err);
  console.log(data);
});

When the readFile operation is completed, the callback function will be called and the result will be Passed as a parameter to the callback function. The first parameter in the callback function usually represents the error message, and the second parameter usually represents the operation result.

However, when encountering nested asynchronous operations, the nesting of callback functions can become very cumbersome. At this time, Promise can be used to optimize the code.

2. Promise

Promise is a way to avoid nesting of callback functions. Promise represents the final result of an asynchronous operation. You can know whether the operation succeeded or failed through its status. Promise supports chained calls, which can handle exceptions in very convenient situations, thereby avoiding nesting of callback functions.

In Node.js, you can use the third-party Promise library bluebird to manage Promise objects. For example, we can use Promise to read the file content:

const Promise = require('bluebird');
const fs = Promise.promisifyAll(require('fs'));

fs.readFileAsync('./file.txt', 'utf8')
  .then(data => {
    console.log(data);
  })
  .catch(err => {
    console.error(err);
  });

After using bluebird, you can use the promisifyAll method to turn the methods in the native fs module into Promise objects, so you can use Promise's chain calling method operates.

In the above example, use the then method to obtain the result of a successful asynchronous operation, and use the catch method to obtain the result of a failed asynchronous operation.

When using Promise, if you encounter multiple asynchronous operations that need to be connected in series, using chain calls can make the code more concise:

fs.readFileAsync('./file.txt', 'utf8')
  .then(data => {
    console.log(data);
    return fs.writeFileAsync('./file-copy.txt', data);
  })
  .then(() => {
    console.log('文件写入成功');
  })
  .catch(err => {
    console.error(err);
  });

In the above code, the file is read first, Then write the file contents into a new file, and finally output the file writing success message.

3. async/await

async/await is the asynchronous operation syntax introduced in ES2017. It can make asynchronous code look like synchronous code without blocking the process. When the async function is executed, it will automatically return a Promise object. The await keyword is used inside the async function to wait for the Promise object to return the result. If the operation is successful, the result of the asynchronous operation will be returned, otherwise an error will be thrown.

When using Node.js to connect to the database, we often need to perform asynchronous operations, such as obtaining data from the database and processing it. The following is an example based on the MySQL module:

const mysql = require('mysql');
const connection = mysql.createConnection({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'test'
});

const query = async (sql) => {
  return new Promise((resolve, reject) => {
    connection.query(sql, (err, rows) => {
      if (err) reject(err);
      else resolve(rows);
    });
  });
};

(async () => {
  try {
    const users = await query('SELECT * FROM users WHERE age > 18');
    console.log(users);
  } catch (err) {
    console.error(err);
  }
})();

In the above code, async/await is used to query users older than 18 years old from the MySQL database and output the results. In the query function, Promise is used to wrap the MySQL query operation, so that the await keyword can be used to perform asynchronous operations in sequence.

Summary

Process control is a very important concept in Node.js. Reasonable process control can improve the readability and maintainability of the code. In Node.js, common process control methods include callback functions, Promise, async/await and generators. When using relational databases, callback functions, Promise and async/await are the most commonly used process control methods. We can flexibly use these methods to manage asynchronous operations and improve the readability and maintainability of the code.

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