NodeJS MySQL 转储

Posted 2023-02-24

【中文标题】NodeJS MySQL 转储

【英文标题】：NodeJS MySQL Dump

【发布时间】：2014-04-02 07:13:12

【问题描述】：

我尝试编写一个基本的 cron 脚本来运行和“转储”一个 mysql 数据库。出于某种原因，当它“成功保存文件”时，它确实创建了文件，但它是空的。如果我不保存文件，而是执行 console.log，它会打印一个空字符串。有什么想法我可能做错了吗？

提前致谢。

var mysql_backup = function()

    this.backup = '';
    this.mysql = require('mysql'),

    this.init = function()
        this.connection = this.mysql.createConnection(
            user: 'root',
            password: 'root',
            database: 'test'
        );
    

    this.query = function(sql, callback) 
        this.connection.query(sql, function (error, results, fields) 
            if (error) 
                throw error;
            
            if (results.length  > 0) 
                callback(results);
            
        );
    

    this.get_tables = function(callback)
        var me = this;
        me.query('SHOW TABLES',
            function(tables) 
                for (var table in tables)
                    me.query(
                        'SHOW CREATE TABLE ' + tables[table].Tables_in_test,
                        function(r)
                            for (var t in r) 
                                me.backup += "DROP TABLE " + r[t].Table + "\n\n";
                                me.backup += r[t]["Create Table"] + "\n\n";
                            
                        
                    )
                
                me.save_backup();
            );
    

    this.save_backup = function()
        var fs = require('fs');
        fs.writeFile("./backup_test.txt", this.backup, function(err) 
            if(err) 
                console.log(err);
             else 
                console.log("The file was saved!");
            
        );
    

;

var db = new mysql_backup;
db.init();
db.get_tables();
db.connection.destroy();

【参考方案1】：

编写的代码甚至没有为我保存文件。好像有几个问题。不确定这是实际代码还是复制粘贴中丢失了某些内容。但是，根据您所拥有的：

一个重要的问题是您永远不会在代码中使用 connection.connect() 连接到数据库。

连接后要运行的代码应该在 connection.connect() 回调中。例如

connection.connect(function (err, empty) 
    if (err)
        throw new Error ('Panic');

    // if no error, we are off to the races...

但是，即使您快速重构代码以将最后几行包装在 get 连接回调中，您仍然会遇到问题，因为您在进行各种 SQL 调用之前就破坏了连接，所以您需要将代码移动到某种最终回调中。

即使你这样做了，你仍然会有一个空文件，因为你是从你的“SHOW TABLES”回调中调用 save_backup，而不是在你通过内部回调实际填充它之后获得 CREATE TABLE 语句并填充备份属性。

这是对代码的最小重写，它将按照您的意图进行。需要注意的重要一点是“计数器”，它管理何时写入文件和关闭连接。如果是我的，我会做出其他改变，包括：

使用“自我”而不是“我” 使用数字 for 循环而不是 for (... in ...) 语法 我自己的回调符合（err, stuff）的节点约定 一个更实质性的变化是我将重写它以使用 Promise，因为这样做可以让您免于对深度嵌套回调固有的混乱感到痛苦。我个人喜欢 Q 库，但这里有几个选项。

希望这会有所帮助。

var mysql_backup = function()
    this.backup = '';
    this.mysql = require('mysql');

    this.init = function()
        this.connection = this.mysql.createConnection(
            user     : 'root',
            password : 'root',
            database : 'test'
        );

    ;

    this.query = function(sql, callback) 
        this.connection.query(sql, function (error, results, fields) 
            if (error) 
                throw error;
            
            if (results.length  > 0) 
                callback(results);
            
        );
    ;

    this.get_tables = function(callback)
        var counter = 0;
        var me = this;
        this.query('SHOW TABLES',
            function(tables) 
                for (table in tables)
                    counter++;
                    me.query(
                        'SHOW CREATE TABLE ' + tables[table].Tables_in_mvc,
                        function(r)
                            for (t in r) 
                                me.backup += "DROP TABLE " + r[t].Table + "\n\n";
                                me.backup += r[t]["Create Table"] + "\n\n";
                            
                            counter--;
                            if (counter === 0)
                                me.save_backup();
                                me.connection.destroy();

                            
                        
                    )
                
            );
    ;

    this.save_backup = function()
        var fs = require('fs');
        fs.writeFile("./backup_test.txt", this.backup, function(err) 
            if(err) 
                console.log(err);
             else 
                console.log("The file was saved!");
            
        );
    

;

var db = new mysql_backup;
db.init();
db.connection.connect(function (err)
    if (err) console.log(err);
    db.get_tables(function(x););

);

更新：如果您好奇，这里有一个使用 Promise 的大量注释实现。请注意，没有 cmets 解释 Q Promise 库函数，它比原始版本要短一些，并且还提供更全面的错误处理。

var MysqlBackup = function(connectionInfo, filename)

    var Q = require('q');
    var self = this;
    this.backup = '';
    // my personal preference is to simply require() inline if I am only
    // going to use something a single time. I am certain some will find
    // this a terrible practice
    this.connection = require('mysql').createConnection(connectionInfo);

    function getTables()
        //  return a promise from invoking the node-style 'query' method
        //  of self.connection with parameter 'SHOW TABLES'.
        return Q.ninvoke(self.connection,'query', 'SHOW TABLES');
    ;

    function doTableEntries(theResults)

        // note that because promises only pass a single parameter around,
        // if the 'denodeify-ed' callback has more than two parameters (the
        // first being the err param), the parameters will be stuffed into
        // an array. In this case, the content of the 'fields' param of the
        // mysql callback is in theResults[1]

        var tables = theResults[0];
        // create an array of promises resulting from another Q.ninvoke()
        // query call, chained to .then(). Note that then() expects a function,
        // so recordEntry() in fact builds and returns a new one-off function
        // for actually recording the entry (see recordEntry() impl. below)

        var tableDefinitionGetters = [];
        for (var i = 0; i < tables.length ; i++)
            //  I noticed in your original code that your Tables_in_[] did not
            //  match your connection details ('mvc' vs 'test'), but the below
            //  should work and is a more generalized solution
            var tableName = tables[i]['Tables_in_'+connectionInfo.database];

            tableDefinitionGetters.push(Q.ninvoke(self.connection, 'query', 'SHOW CREATE TABLE ' + tableName)
                                        .then(recordEntry(tableName)) );
        

        // now that you have an array of promises, you can use Q.allSettled
        // to return a promise which will be settled (resolved or rejected)
        // when all of the promises in the array are settled. Q.all is similar,
        // but its promise will be rejected (immediately) if any promise in the
        // array is rejected. I tend to use allSettled() in most cases.

        return Q.allSettled(tableDefinitionGetters);
    ;

    function recordEntry (tableName)
        return function(createTableQryResult)
            self.backup += "DROP TABLE " + tableName + "\n\n";
            self.backup += createTableQryResult[0][0]["Create Table"] + "\n\n";
        ;
    ;

    function saveFile()
        // Q.denodeify return a promise-enabled version of a node-style function
        // the below is probably excessively terse with its immediate invocation
        return (Q.denodeify(require('fs').writeFile))(filename, self.backup);
    

    // with the above all done, now you can actually make the magic happen,
    // starting with the promise-return Q.ninvoke to connect to the DB
    // note that the successive .then()s will be executed iff (if and only
    // if) the preceding item resolves successfully, .catch() will get
    // executed in the event of any upstream error, and finally() will
    // get executed no matter what.

    Q.ninvoke(this.connection, 'connect')
    .then(getTables)
    .then(doTableEntries)
    .then(saveFile)
    .then( function() console.log('Success');  )
    .catch( function(err) console.log('Something went awry', err);  )
    .finally( function() self.connection.destroy();  );
;

var myConnection = 
    host     : '127.0.0.1',
    user     : 'root',
    password : 'root',
    database : 'test'
;

// I have left this as constructor-based calling approach, but the
// constructor just does it all so I just ignore the return value

new MysqlBackup(myConnection,'./backup_test.txt');

【讨论】：

完美！谢谢你。对 NodeJS 来说还是相当新的。看来我需要开始研究承诺了。

不客气。当我发现它们时我很高兴，因为一旦事情变得超过几个层次的回调，我发现代码变得有点难以遵循，我也喜欢错误传播的“免费赠品”。

感谢编辑！我要看看，看看我能从你的例子中学到什么。我开始深入研究 Promise，它与我以前见过的任何东西都不一样（我主要习惯于用 php 编程）。再次感谢！

希望评论的例子能帮助你理解，虽然我不是一个承诺专家。在 Youtube 上，您可以找到 Q 的主要作者之一 Dominic Denicola 的会议演讲，您可能会觉得这很有帮助。正如我所提到的，还有其他方法可以处理涉及节点异步性质的回调***。