Question

我确信这从根本上误解了流的工作原理，但我的头撞在墙上。

我有一些json格式的传感器数据，我想使用csv-write-stream包附加到csv文件中。数据作为发布请求发送到节点服务器，目的是将其附加到csv文件中。第一次将行写到csv文件中，但是如果我尝试发送另一个发布请求，则会收到“错误：结束后写入”错误。

function write_csv(obj) {
    writer.pipe(fs.createWriteStream('tides.csv', { flags: 'a' }));
    writer.write(obj);
    writer.end();
};

如果我注释掉“ writer.end（）”，它可以正常工作，但是这最终不会引发内存错误吗？如果是这样，什么是附加到csv文件并避免此错误的正确方法？

编辑：这是整个server.js文件

const express = require('express');
const bodyParser = require('body-parser');
const path = require('path');
const exphbs = require('express-handlebars');
const fs = require('fs');
const csvWriter = require('csv-write-stream');

const writer = csvWriter({ sendHeaders: false });
const app = express();

app.set('views', path.join(__dirname, 'views'));
app.engine('handlebars', exphbs({ defaultLayout: 'main' }));
app.set('view engine', 'handlebars');

app.set('port', (process.env.PORT || 3000));

app.use(express.static(path.join(__dirname, 'public')));

app.use(bodyParser.urlencoded({ extended: false }))
app.use(bodyParser.json())

app.get('/', function (req, res) {
    res.render('home')
})

app.post('/test', function (req, res, next) {
    // console.log("post received");
    distance = req.body.distance;
    let result = test(distance);
    let result_str = JSON.stringify(result);
    res.end(result_str)
});

function write_csv(obj) {
    writer.pipe(fs.createWriteStream('out.csv', { flags: 'a' }));
    writer.write(obj);
    writer.end();
};

function test(dist) {
    let d = new Date();
    let YYYY = d.getFullYear();
    let MM = d.getMonth();
    let DD = d.getDate();
    let HH = d.getHours();
    let mm = d.getMinutes();
    let ss = d.getSeconds();
    let date = YYYY + ':' + (MM + 1) + ':' + DD + ':' + HH + ':' + mm + ':' + ss;
    let time_distance = { 'time': date, 'distance': distance };
    console.log(time_distance);
    write_csv(time_distance);
    return time_distance;
};


app.listen(app.get('port'), function () {
    console.log('Sever started on port ' + app.get('port'));
})

Answer 1

没有看到完整的代码，我可以想象您多次调用write_csv，因为您试图将多个对象写入该文件。

问题在于，第一次调用write_csv会结束writer，这就是为什么第二次调用它会得到：

Error [ERR_STREAM_WRITE_AFTER_END]: write after end

function write_csv(obj) {
    writer.pipe(fs.createWriteStream('out.csv', { flags: 'a' }))
    writer.write()
    writer.end();
}
write_csv({ hello: 'world', foo: 'bar', baz: 'taco'});
// When you call it again, writer.end(); is already closed
// The following line will trigger the error
write_csv({ hello: 'world', foo: 'bar', baz: 'taco'});

您应该做的是，仅在写完后才关闭编写器。

const writer = csvWriter(); // Create new writer
// open file
writer.pipe(fs.createWriteStream('out.csv', { flags: 'a' }));

for(const obj of objects) // Write as many times as you wish
   writer.write(obj);

writer.end(); // I'm done writing.

现在的问题是，如果您尝试执行多个.writes，则将达到内存限制，因为您没有正确处理backpressure。

我建议阅读以下问题：

why does attempting to write large a large file cause js heap to run out of memory

要解决此问题，您将需要等待drain事件的发出。

这里是csvWriter的包装材料，可以处理背压。

const fs = require('fs');
const csvWriter = require('csv-write-stream');

class Writer {

    constructor(file) {
        this.writer = csvWriter();
        this.writer.pipe(fs.createWriteStream(file, { flags: 'a' }));
    }

    write(obj) {
        // if .write returns false we have to wait until `drain` is emitted
        if(!this.writer.write(obj))
            return new Promise(resolve => this.writer.once('drain', resolve))

        return true;
    }

    end() {
        // Wrap it in a promise if you wish to wait for the callback.
        this.writer.end(); 
    }

}

(async() => {
    const writer = new Writer('out.csv');

    for(let i = 0; i < 1e8; i++) {
        const res = writer.write({ hello: 'world', foo: 'bar', baz: 'taco' });
        if(res instanceof Promise) {
            // You can remove this if, and leave just: await writer.write...
            // but the code will be slower
            await res; // This will wait for the stream to emit the drain event
        }
    }

    writer.end();

})();

更新：现在，使用实际代码，上述答案仍然有效，但是由于您在接收请求时正在写入文件。您可以选择是否打开文件一次，并在每次请求时写入文件，然后在服务器关闭时（或选择时）关闭文件。或只是打开文件，对其进行写入，然后在每次请求时将其关闭，

对于前者，您应该使用上面的答案，对于后者，您需要做的就是每次调用write_csv时都创建一个新的编写器，而不是拥有一个全局编写器。

function write_csv(obj) {
    // Create a new writer every time
    const writer = csvWriter({ sendHeaders: false }); 
    writer.pipe(fs.createWriteStream('out.csv', { flags: 'a' }));
    writer.write(obj);
    writer.end();
};

Answer 2

您可以改用此代码，我认为它可以解决您的问题。在您的路线中使用此功能并传递json数据。

首先安装此软件包：npm i --save json2csv

const Json2csvParser = require("json2csv").Parser;
const fs = require("fs");

function csvConverter(jsonData, cb) {

  const json2csvParser = new Json2csvParser();
  const csv_data = json2csvParser.parse(jsonData);
  const file_name = "report";

  // TODO: Change file path accordingly
  const file_path = `/Users/public/csv_files/${file_name}.csv`;

  fs.appendFile(file_path, csv_data, err => {
    if (err) {
      console.log(err);
      cb(err, null);
      return;
    }
    const response = {
    msg: "successful",
    file_address: file_path
  };
    cb(null, response);
    return;
  });
}

csv-write-stream

2 个答案: