Nodejs 12回调不适用于mysql连接？

Question

我正在编写此代码以在 AWS Lambda 中运行。

查看mysql连接文档

如果一切正常且没有错误，我希望收到消息“数据库连接成功！” 然后得到一条消息“connected as id”，但这不是它发生的顺序。这是我的代码。

'use strict';

let mysql = require('mysql');

const connection = mysql.createConnection({
  dateStrings: true,
  host     : process.env.rds_host,
  user     : process.env.rds_user,
  password : process.env.rds_password,
  database : process.env.rds_database,
  port     : process.env.rds_port
});

exports.handler = (event, context, callback) => {
  //prevent timeout from waiting event loop
  context.callbackWaitsForEmptyEventLoop = false;

  let sql = 'SELECT * FROM company ORDER BY points DESC, name ASC';
  let data = null;
  console.log('\nGetCompanies SQL: ', sql);
  let responseBody = "";
  let statusCode   = 0;

  connection.connect(function(err) {
  if (err) {
      statusCode = 500;
      responseBody = err;
    }
    else{
      console.log("Database connected successfully!");
      statusCode = 200;
      responseBody = "Database connected successfully!";
    }
  });
  console.log('connected as id ' + connection.threadId);

  connection.query(sql, data, function(queryError, results) {
    if(queryError) {
      console.error(queryError.message);
      callback(queryError);
    }
    else
    {
      console.log('\nGetCompanies Results: ', results[0]);
      callback(null, results);
    }
  });
};

这是记录的输出：

信息
GetCompanies SQL: SELECT * FROM company ORDER BY points DESC, name ASC 2020-01-01T11:52:57.813Z

INFO connected as id null 2020-01-01T11:52:57.952Z

INFO 数据库连接成功！ 2020-01-01T11:52:57.974Z

我的想法是我提供的功能：

connection.connect(function(err) {

将在connection.connect之后的任何代码之前执行。我这样想有错吗？

还有一个问题：为什么是

连接为 id null？我直接从mysql 文档中获得了该代码（connection.threadId）。

我知道这没什么大不了的，如果它影响的只是日志消息的顺序，但我还有其他功能我无法进行查询，因为连接还不存在。

当它吹过时会很混乱

if (err) {
  statusCode = 500;
  responseBody = err;
}
else{
  <do some connection.query code here>
}

我把这些日志信息放在这里是因为这个方法有效，而我的其他方法无效。

Answer 1

您出现这种行为是因为 console.log('connected as id ' + connection.threadId); 没有等待 connection.connect 完成连接。

你需要在这里使用promises。

您可以尝试使用async/await。

exports.handler = async (event, context, callback) => {
    //prevent timeout from waiting event loop
    context.callbackWaitsForEmptyEventLoop = false;

    let sql = 'SELECT * FROM company ORDER BY points DESC, name ASC';
    let data = null;
    console.log('\nGetCompanies SQL: ', sql);
    let responseBody = "";
    let statusCode   = 0;

    try {
        await connection.connect();

        console.log("Database connected successfully!");
        console.log('connected as id ' + connection.threadId);

        const queryResult = await connection.query(sql, data);

        // do something with queryResult
    } catch (e) {
        // handle error
    }
};

编辑

您链接到的资源建议在回调上使用承诺：

等待任何承诺而不是使用回调

Answer 2

would execute before any code after connection.connect. Am I wrong to think that?

• 您传递给connection.connect 的函数称为 回调函数。这意味着，它只会在之后被调用 成功连接（或连接尝试出错后）。所以答案是否定的。 connection.connect 被调用，然后它会立即调用下一条语句。它不会等待当前语句，因为 javascript 是事件驱动且非阻塞的。

连接为 id null？我直接从 mysql 文档中获得了该代码（connection.threadId）。

• 这是因为前面的语句，您在获取连接之前记录了 connectionId。

使用回调

您的代码应如下所示。我没有做太多修改，只有在调用了connect事件的回调后才建立与数据库的连接。

exports.handler = (event, context, callback) => {
  //prevent timeout from waiting event loop
  context.callbackWaitsForEmptyEventLoop = false;

  let sql = 'SELECT * FROM company ORDER BY points DESC, name ASC';
  let data = null;
  console.log('\nGetCompanies SQL: ', sql);
  let responseBody = "";
  let statusCode = 0;

  connection.connect(function (err) {

    // we are inside the callback function a successful connection has been obtained , or error connecting to database
    if (err) {
      statusCode = 500;
      responseBody = err;
    }
    else {
      console.log("Database connected successfully!");
      statusCode = 200;
      responseBody = "Database connected successfully!";
    }

    // connection exists
    console.log('connected as id ' + connection.threadId);

    connection.query(sql, data, function (queryError, results) {
      if (queryError) {
        console.error(queryError.message);
        callback(queryError);
      }
      else {
        console.log('\nGetCompanies Results: ', results[0]);
        callback(null, results);
      }
    });
  });
};

使用承诺

如果您可以使用 Promise 使您的代码具有可读性和可理解性。

import { promisify } from 'util';
exports.handler = async (event, context) => {
  //prevent timeout from waiting event loop
  context.callbackWaitsForEmptyEventLoop = false;

  let sql = 'SELECT * FROM company ORDER BY points DESC, name ASC';
  let data = null;
  console.log('\nGetCompanies SQL: ', sql);

  const connect = promisify(connection.connect);
  const query = promisify(connection.query);

  try {
    const connection = await connect();

    // connection exists
    console.log('connected as id ' + connection.threadId);
    const results= await query(sql);

    return {
      statusCode: 200,
      responseBody: JSON.stringify(results)
    }

  } catch (err) {
    return {
      statusCode: 500,
      responseBody: err.message
    }
  }
};

Answer 3

您应该将您的 connection.query 语句移动到 connection.connect 范围内，因为即使您无法连接到数据库，您也会尝试进行无法正常工作的查询。

exports.handler = (event, context, callback) => {
  //prevent timeout from waiting event loop
  context.callbackWaitsForEmptyEventLoop = false;

  let sql = 'SELECT * FROM company ORDER BY points DESC, name ASC';
  let data = null;
  console.log('\nGetCompanies SQL: ', sql);
  let responseBody = "";
  let statusCode   = 0;

  connection.connect(function(err) {
  if (err) {
      statusCode = 500;
      responseBody = err;
    }
    else{
      console.log("Database connected successfully!");
      statusCode = 200;
      responseBody = "Database connected successfully!";
      connection.query(sql, data, function(queryError, results) {
        if(queryError) {
          console.error(queryError.message);
          callback(queryError);
        }
        else
        {
          console.log('\nGetCompanies Results: ', results[0]);
          callback(null, results);
        }
      });
    }
  });
  console.log('connected as id ' + connection.threadId);

};