javascript - p2p文件下载中node.js写入文件流无序

Question

伙计们。我尝试使用node.js 创建一个p2p 文件共享应用程序。下载文件时，会逐 block 下载文件。

以下代码中使用的 block 大小为 1KB。但它有一个问题，当可用套接字数 MAX_SOCKET_CNT 设置为 30 时，它将不起作用。

如何运行代码:

首先，运行node server.js，然后运行node client.js，这会将名为fav.mp3的文件下载到fav-local.mp3。

下载完成后，尝试运行diff fav.mp3 fav-local.mp3检查文件是否下载完整。

能帮我看看问题出在哪里吗？

欢迎任何答案或建议。提前致谢。

这是源代码:

服务器.js

var http = require("http");
var url = require("url");
var fs = require('fs');

function downloadBlock(request, response){
  var urlParts = url.parse(request.url, true);
  var query = urlParts.query;

  if('block_id' in query){
      response.writeHead(200, {"Content-Type": "audio/mpeg"});
      var startHere=parseInt(query["block_id"]);
      var BLOCK_SIZE=1024;
      var currentPosition=startHere*BLOCK_SIZE;
      var readStream = fs.createReadStream('fav.mp3',{start: startHere*BLOCK_SIZE, end:startHere*BLOCK_SIZE+BLOCK_SIZE-1});
      readStream.pipe(response);
  }else{
      response.writeHead(200, {"Content-Type": "text/html"});
      response.write("refused");
      response.end();
      console.log("Warning: not a file block download request...");
  }
}


http.createServer(downloadBlock).listen(8801);
console.log("Server has started. please ensure that the fav.mp3 file(size=439355B) is here.");

<小时/>

client.js

var http = require('http');
var fs = require('fs');

var remoteFile='fav.mp3';
var fileSize=439355;
var localFile='fav-local.mp3';
var totalBlocks=Math.floor((fileSize+1023)/1024);
/**************************************************/
//KEY POINT
var MAX_SOCKET_CNT=totalBlocks; //worked
//var MAX_SOCKET_CNT=30;//not work //????because of  recursive  downloadBlock function????
/*************************************************/
for(var i=0;i<MAX_SOCKET_CNT;++i){
        downloadBlock('127.0.0.1',8801,remoteFile, localFile,i,totalBlocks);
}

function downloadBlock(IP,PORT,remoteFile,localFile,blockID,totalBlocksNum){
        if(blockID >= totalBlocksNum) return;
        var BLOCK_SIZE=1024;
        var file = fs.createWriteStream(localFile,{start: blockID*BLOCK_SIZE});
        var request = http.get("http://localhost:"+PORT+"/download_block?block_id="+blockID, function(response) {
          response.pipe(file);
          file.on('finish', function() {
                var callback=function downloadBlockOver(){
                        console.log("compelete download blockID:"+blockID);
                        var nextBlockID=blockID+MAX_SOCKET_CNT;
                        if(nextBlockID<totalBlocksNum){
                                downloadBlock(IP,PORT,remoteFile,localFile,nextBlockID,totalBlocksNum); //why not this work if MAX_SOCKET_CNT=30???    
                        }
                }
                file.close(callback);
           });
        });
}

Answer 1

啊哈。最后，我找到了解决方案。您必须先创建该文件。然后在下载之前用“r+”更改写入流的标志。

function touchFile(fileName,fileSize){
    var BLOCK_SIZE=1024;
    var totalBlocks=Math.floor((fileSize+BLOCK_SIZE-1)/BLOCK_SIZE);
    var file = fs.createWriteStream(fileName);
    var blockBuffer=new Buffer(BLOCK_SIZE);
    for(var i=0;i<totalBlocks-1;++i){
            file.write(blockBuffer,0, BLOCK_SIZE);
    }
    var leftToFill=new Buffer(fileSize-(totalBlocks-1)*BLOCK_SIZE);
    file.write(leftToFill,0, leftToFill.Length);
    file.end();
}

添加函数“touchFile(localFile,fileSize);”在下载 block 之前。然后更改:

var file = fs.createWriteStream(localFile,{start: blockID*BLOCK_SIZE});

至

var file = fs.createWriteStream(localFile,{start: blockID*BLOCK_SIZE,flags:'r+',autoClose: true});

现在就可以了。