Express中的响应流在Azure App Service中不起作用

问题描述

我正在尝试使用通过Azure App Service托管的NodeJS Express服务器将响应流式传输到客户端.但是，我注意到它不是真正的流式传输，而是尝试整体发送响应.当响应大小很大(> 50MB)时，客户端会收到内部服务器错误，但服务器不会引发错误.

I am trying to stream responses to my client using a NodeJS Express server hosted using Azure App Service. However, I noticed that it is not really streaming but tries to send the response as a whole. When the response size is huge (>50MB), the client gets an Internal Server Error, but the server does not throw an error.

此外，当我在Docker内部运行服务器(节点映像: 10.22.0-alpine3.9 )时，我看到即使是巨大的响应，客户端也将响应作为流获得.(这是我真正需要的行为)

Further, when I run the server inside a Docker (Node Image: 10.22.0-alpine3.9), I see that the client gets the response as a stream even for huge responses. (This is the behavior I actually need)

我的 web.config 文件如下.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <add name="iisnode" path="server.js" verb="*" modules="iisnode" responseBufferLimit="0"/>
    </handlers>
    <iisnode flushResponse="true" />
    ...
  </system.webServer>
</configuration>

这只是我程序的简单解释.

This is a small explanation as to what my program does.

我有一个外部API，该API返回类似于以下内容的对象.

I have an external API that returns an object similar to the following.

{
  "title":"Test Title",
  "lastBuildDate":"1597981114347",
  "items":[
    {
      id: 'item1',
      value: 'value1'
    },
    {
      id: 'item2',
      value: 'value2'
    },
    ...
  [
}

我只想过滤 items 数组中的元素，然后将它们发送给客户端.客户应收到如下响应.

I want to filter only the elements in items array and send those to the client. The client should get a response like below.

[
   {
     id: 'item1',
     value: 'value1'
   },
   {
     id: 'item2',
     value: 'value2'
   },
   ...
[

有时该对象太大(大于50MB)，因此，我将响应作为流发送，以避免在服务器中使用过多的缓冲内存.下面是我用于流式传输响应的代码.

Sometimes this object is too large (>50MB) and because of that, I am sending the response as a stream to avoid using too much buffer memory in my server. Below here is the code I used to stream the response.

const https = require('https');
const { withParser } = require('stream-json/filters/Pick');
const { streamArray } = require('stream-json/streamers/StreamArray');
const { chain } = require('stream-chain');

exports.getStreamResponse = async function (req, res) {
  const options = {
    hostname,
    port,
    path,
    method: 'GET',
  };

  return new Promise((resolve, reject) => {
    https.request(options, (dataStream) => {
      const pipeline = chain([
        dataStream,
        withParser({ filter: 'items' }),
        streamArray()
      ]);
  
      res.write("[");
  
      let separator = '';
  
      pipeline.on('data', data => {
        res.write(separator + JSON.stringify(data.value));
        if (!separator) {
          separator = ',';
        }
      });
  
      pipeline.on('end', () => {
        res.write("]");
        res.end();
        resolve();
      });

      pipeline.on('error', (error) => {
        reject(error);
      });
    });
  })
};
            

我还注意到，如果我像下面那样编写代码，我总是会得到一个流响应.但是，响应的格式不正确.

I also noticed that if I write the code like below, I always get a stream response. However, the response is not in the correct format as needed.

https.request(options, (dataStream) => {
  dataStream.pipe(res);
});

推荐答案

就像我在问题的后半部分描述的那样，直接将 res (我对客户端的响应)传递给dataStream (我从外部API获取的数据流)允许流传输而没有任何问题.

Like I described in the latter part of my question, directly piping the res (my response to the client) to dataStream (the data stream I got from the external API) allowed to stream without any issues.

扩展相同的行为，我创建了一个 Readable 流，该流等效于应发送给我的客户端的响应.然后，我将其通过管道传输到 res ，并且可以正常工作.

Extending the same behavior, I created a Readable stream which is equivalent to the response I should send to my client. Then I piped it to res and it worked.

这是我的解决方法.

const https = require('https');
const { withParser } = require('stream-json/filters/Pick');
const { streamArray } = require('stream-json/streamers/StreamArray');
const { chain } = require('stream-chain');
const { Readable } = require('stream');

exports.getStreamResponse = async function (req, res) {
  const options = {
    hostname,
    port,
    path,
    method: 'GET',
  };

  return new Promise((resolve, reject) => {
    https.request(options, (dataStream) => {
      const pipeline = chain([
        dataStream,
        withParser({ filter: 'items' }),
        streamArray()
      ]);
  
      // create a readable stream to collect data from response 
      const readable = new Readable({
        // this empty method is to avoid 'ERR_METHOD_NOT_IMPLEMENTED'
        // error when read method is called while there is no data in the
        // readable stream
        read(size) { }
      });
  
      let separator = '';
  
      readable.pipe(res);
      readable.push("[");

      pipeline.on('data', data => {
        readable.push(separator + JSON.stringify(data.value));
        if (!separator) {
          separator = ',';
        }
      });

      pipeline.on('end', () => {
        readable.push("]");
        readable.push(null);
        resolve();
      });
            
      pipeline.on('error', reject);
    });
  })
};

但是，我注意到此解决方案比我遇到的解决方案需要更多的内存.可能是因为我正在创建一个多余的可读流.

However, I noticed this solution requires more memory than the solution I had issues with. Probably because I am creating a readable stream that is redundant.

这篇关于Express中的响应流在Azure App Service中不起作用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！