使用Node.js进行同步数据库查询 [英] Synchronous database queries with Node.js

问题描述

我有一个Node.js / Express应用程序查询路由中的MySQL数据库，并将结果显示给用户。我的问题是我如何运行查询和阻塞，直到两个查询完成之前重定向用户到他们要求的页面？

I have a Node.js/Express app that queries a MySQL db within the route and displays the result to the user. My problem is how do I run the queries and block until both queries are done before redirecting the user to the page they requested?

在我的示例中，我有2个查询需要在我呈现页面之前完成。如果我在查询1的'result'回调中嵌套查询2，我可以得到查询同步运行。然而，当查询数量增加时，这将变得非常复杂。

In my example I have 2 queries that need to finish before I render the page. I can get the queries to run synchronously if i nest query 2 inside the 'result' callback of query 1. This however will become very convoluted when the number of queries increase.

我如何在同步运行多个（在这种情况下是2个）数据库查询，而不在先前的查询的'result'回调中嵌套后续查询？

How do I go about running multiple (in this case 2) database queries synchronously without nesting the subsequent query in the prior query's 'result' callback?

在Node模块中的Flow control / Async goodies，并尝试了flow-js，但是我无法使用异步查询。

I've looked at the 'Flow control / Async goodies' in the Node modules and tried flow-js but I can't get it to work with the async queries.

我尝试从'/ home'路由执行的2个查询。 Node的专家可以解释正确的方法。

Listed below are the 2 queries that I'm attempting to execute from the '/home' route. Can the Node experts explain the 'right' way to do this.

app.get('/home', function (req,res) {
    var user_array = [];
    var title_array = [];

    // first query
    var sql = 'select user_name from users';
    db.execute(sql)
        .addListener('row', function(r) {
            user_array.push( { user_name: r.user_name } );
        })
        .addListener('result', function(r) {
            req.session.user_array = user_array;
        });

    // second query
    var sql = 'select title from code_samples';
    db.execute(sql)
        .addListener('row', function(r) {
            title_array.push( { title: r.title } );
        })
        .addListener('result', function(r) {
            req.session.title_array = title_array;
        });

        // because the queries are async no data is returned to the user
        res.render('home.ejs', {layout: false, locals: { user_name: user_array, title: title_array }});
});

推荐答案

节点的目标不在乎什么顺序发生在这。这可能使一些情况复杂化。嵌套回调没有耻辱。一旦你习惯了它的外观，你可能会发现你实际喜欢那种风格。我做;它是非常清楚什么订单回调将触发。

The goal with node is not to care what order things happen in. This can complicate some scenarios. There is no shame in nesting callbacks. Once you are used to how it looks, you may find that you actually prefer that style. I do; it is very clear what order callbacks will fire. You can forgo the anonymous functions to make it less verbose if you have to.

如果你愿意重组你的代码，你可以使用典型嵌套回调方法。如果你想避免回调，有许多异步框架，将尝试和帮助你这样做。你可能想检查的是async.js（https://github.com/fjakobs/async.js）。每个例子：

If you are willing to restructure your code a bit, you can use the "typical" nested callback method. If you want to avoid callbacks, there are numerous async frameworks that will try and help you do this. One that you might want to check out is async.js (https://github.com/fjakobs/async.js). Example of each:

app.get('/home', function (req,res) {
    var lock = 2;
    var result = {};
    result.user_array = [];
    result.title_array = [];

    var finishRequest = function(result) {
        req.session.title_array = result.title_array;
        req.session.user_array = result.user_array;
        res.render('home.ejs', {layout: false, locals: { user_name: result.user_array, title: result.title_array }});
    };

    // first query
    var q1 = function(fn) {
      var sql = 'select user_name from users';
      db.execute(sql)
          .addListener('row', function(r) {
              result.user_array.push( { user_name: r.user_name } );
          })
          .addListener('result', function(r) {
              return fn && fn(null, result);
        });
    };

    // second query
    var q2 = function(fn) {
      var sql = 'select title from code_samples';
      db.execute(sql)
          .addListener('row', function(r) {
              result.title_array.push( { title: r.title } );
          })
          .addListener('result', function(r) {
              return fn && fn(null, result);
          });
    }

    //Standard nested callbacks
    q1(function (err, result) {
      if (err) { return; //do something}

      q2(function (err, result) {
        if (err) { return; //do something}

        finishRequest(result);
      });
    });

    //Using async.js
    async.list([
        q1,
        q2,
    ]).call().end(function(err, result) {
      finishRequest(result);
    });

});

对于一次性，我可能只使用引用计数类型方法。

For a one-off, I would probably just use a reference counting type approach. Simply keep track of how many queries you want to execute and render the response when they have all finished.

app.get('/home', function (req,res) {
    var lock = 2;
    var user_array = [];
    var title_array = [];

    var finishRequest = function() {
        res.render('home.ejs', {layout: false, locals: { user_name: user_array, title: title_array }});
    }

    // first query
    var sql = 'select user_name from users';
    db.execute(sql)
        .addListener('row', function(r) {
            user_array.push( { user_name: r.user_name } );
        })
        .addListener('result', function(r) {
            req.session.user_array = user_array;
            lock -= 1;

            if (lock === 0) {
              finishRequest();
            }
        });

    // second query
    var sql = 'select title from code_samples';
    db.execute(sql)
        .addListener('row', function(r) {
            title_array.push( { title: r.title } );
        })
        .addListener('result', function(r) {
            req.session.title_array = title_array;
            lock -= 1;

            if (lock === 0) {
              finishRequest();
            }
        });
});

更好的方法是在每个result回调中调用finishRequest非空数组，然后再呈现响应。在您的情况下是否会工作取决于您的要求。

An even nicer approach would be to simply call finishRequest() in each 'result' callback an check for non-empty arrays before you render the response. Whether that will work in your case depends on your requirements.

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