返回承诺不适用于 AWS 开发工具包

Question

我创建了一个 API，它调用 get cloudWatch AWS API 并返回可以在我的应用程序上绘制的数据点。我对每个包都有单独的路由（如下面的路由代码所示）。此 API 使用 REST MVC 方法。所以我正在用我的功能做几件事。从 SQLite3 数据库中读取 EC2 实例数据以获取有关正在运行的实例（IP、instance_id、instance_launchtime）的信息，以便我可以将其放入 AWS 开发工具包中 getMetricStatistics API 所需的参数中。然后将来自 step1 的数据放入参数数组中（3 个以 3 个不同的度量数据点响应）。这会循环遍历每个参数，将其插入到 getMetricStatistics API（一个接一个，因为 getMetricStatistics 不会一次接受多个指标）以获取该实例的数据点并将它们推送到数组中。因为我相信数据库是异步的，这就是为什么我对它附加了一个承诺。当我将端点加载到浏览器中时，它只会继续加载并且不会显示任何数据。但是，当我刷新页面时，它会正确显示所有结果...API的模型：// Call AWS Cost Explorer APIconst AWS = require('aws-sdk');const config = require('./AWSconfig');const database = require('./db');const insightdb = database.insightdb;AWS.config.update({  accessKeyId: config.accessKeyId,  secretAccessKey: config.secretAccessKey,  region: config.region,});//Linking AWS CloudWatch Servicevar cloudWatch = new AWS.CloudWatch();const packageById = (packageId, callback) => {  insightdb.all(    'SELECT * FROM ec2Instance WHERE package_id == ?',    packageId,    (err, rows) => {      if (err) {        callback(err, null);      } else {        callback(null, rows);      }    }  );};// Parameter class to feed into the CloudWatch getMetricStatistics functionconst CWParams = function(reqDate, metricName,service,launchTime,instanceId) {  (this.EndTime = reqDate) /* required */,    (this.MetricName = metricName) /* required */,    (this.Namespace = service) /* required */,    (this.Period = 3600) /* required */,    (this.StartTime = launchTime) /* ${createDate}`, required */,    (this.Dimensions = [      {        Name: 'InstanceId' /* required */,        Value: instanceId /* required */,      },    ]),    (this.Statistics = ['Maximum']);};//Exports variables to the controller (so they can be re-used)module.exports = { cloudWatch, CWParams, packageById };API 的路径：module.exports = app => {  const insight = require('../controllers/insight.controller.js');  app.get('/insights/aws/:packageKey', insight.getAnalytics);};

Answer 1

就目前而言，在第二个 Promise 构造函数中，insightParams 保证尚未组合，因为insightParams = [.....]它位于异步调用的回调中。因此，程序流程需要确保所有的“promise2”内容仅在“promise1”完成后才发生。

如果异步函数在尽可能低的级别被“承诺”，那么高级代码中的事情就会变得简单得多。所以在模型中做两件事：

• 承诺cloudWatch.getMetricStatistics()

• 编写 packageById() 以返回Promise而不是接受回调。

模型因此变为：

const AWS = require('aws-sdk'); // no change

const config = require('./AWSconfig'); // no change

const database = require('./db'); // no change

const insightdb = database.insightdb; // no change

AWS.config.update({

    accessKeyId: config.accessKeyId,

    secretAccessKey: config.secretAccessKey,

    region: config.region

}); // no change

var cloudWatch = new AWS.CloudWatch(); // no change

// Promisify cloudWatch.getMetricStatistics() as  cloudWatch.getMetricStatisticsAsync().

cloudWatch.getMetricStatisticsAsync = (metric) => {

    return new Promise((resolve, reject) => {

        cloudWatch.getMetricStatistics(metric, function(err, data) {

            if (err) {

                if(!err.message) { // Probably not necessary but here goes ...

                    err.message = 'Error occured while running cloudWatch getMetricStatistcs API: ';

                }

                reject(err); // (very necessary)

            } else {

                resolve(data);

            }

        });

    });

};

// Ensure that packageById() returns Promise rather than accepting a callback.

const packageById = (packageId) => {

    return new Promise((resolve, reject) => {

        insightdb.all('SELECT * FROM ec2Instance WHERE package_id == ?', packageId, (err, rows) => {

            if (err) {

                reject(err);

            } else {

                resolve(rows);

            }

        });

    });

};

现在getAnalytics()可以这样写：

exports.getAnalytics = (req, res) => {

    packageById(req.params.packageKey)

    .then(data => {

        const currentDate = new Date().toISOString();

        let insightParams = [

            new CWParams(currentDate, 'CPUUtilization', 'AWS/EC2', data[0].launch_time, data[0].instance_id),

            new CWParams(currentDate, 'CPUCreditBalance', 'AWS/EC2', data[0].launch_time, data[0].instance_id),

            new CWParams(currentDate, 'CPUCreditUsage', 'AWS/EC2', data[0].launch_time, data[0].instance_id)

        ];

        // Composition of `insightParams` is synchronous so you can continue 

        // with the `cloudWatch.getMetricStatisticsAsync()` stuff inside the same .then().

        return Promise.all(insightParams.map(metric => cloudWatch.getMetricStatisticsAsync(metric))); // Simple because of the Promisification above.

    }, err => {

        // This callback handles error from packageById() only,

        // and is probably unnecessary but here goes ...

        if(!err.message) {

            err.message = 'Error while getting the insight configuration data.';

        }

        throw err;

    })

    .then(metricResults => {

        res.send(metricResults);

        console.log('AWS CW API successful');

    })

    .catch(err => {

        // Any async error arising above will drop through to here.

        res.status(500).send({

            'message': err.message

        }));

    });

};

请注意，多个 catchres.status(500).send()不是必须的。沿着 Promise 链向下传播的错误允许单个终端.catch()