NodeJS MySQL转储

Question

我试图编写一个基本的cron脚本来运行和“转储”一个mysql数据库。由于某种原因，当“成功保存文件”时，它会创建文件，但是它为空。如果执行保存console.log而不是保存文件，它将打印一个空字符串。对我可能做错的事情有任何想法吗？

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();

Answer 1

编写的代码甚至都没有保存到我的文件中。似乎有一些问题。不知道这是实际的代码还是某些内容丢失在复制粘贴中。但是，根据您所拥有的：

一个很大的问题是，您永远不会使用connection.connect（）连接到代码中的数据库。

连接后要运行的代码应位于connection.connect（）回调内部。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换行到该get连接回调中，您仍然会遇到问题，因为在进行各种SQL调用之前您正在销毁连接，因此您将需要移动代码进入某种最终回调。

即使这样做了，您仍然会有一个空文件，因为您是从“ SHOW
TABLES”回调中调用save_backup，而不是通过内部回调实际填充它之后，在内部回调中您会获得CREATE TABLE语句并填充备份属性。

这是对代码的最小重写，可以完成您的预期工作。要注意的重要事项是“计数器”，它管理何时写入文件和关闭连接。如果属于我，我会进行其他更改，包括：

• 用“自我”代替“我”
• 使用数字for循环而不是for（… in …）语法
• 我自己的回调属于（err，stuff）的节点约定
• 更实质性的变化是，我将使用Promise重写此代码，因为这样做可以使您深深地陷入嵌套回调的内在困惑中。我个人喜欢Q库，但是这里有几种选择。

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：如果您感到好奇，这是一个使用Promise进行了广泛评论的实现。请注意，由于没有说明Q承诺库功能的注释，它比原始版本要短一些，并且还提供了更全面的错误处理。

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');