节点读取指定块大小的文件

Question

目标：将大文件上传到 AWS Glacier，而不会将整个文件保存在内存中。

我目前正在使用 fs.readFileSync() 上传到冰川，一切正常。但是，我需要处理大于 4GB 的文件，并且我想并行上传多个块。这意味着转向分段上传。 我可以选择块大小，但冰川需要每个块的大小相同（最后一个除外）

This 线程建议我可以在读取流上设置块大小，但我实际上并不能保证得到它。

关于如何在不将整个文件读入内存并手动拆分的情况下获得一致的部分的任何信息？

假设我能做到这一点，我只是要使用集群，其中有几个进程以尽可能快的速度拉出流，因为它们可以上传到 AWS。 如果这似乎是并行工作的错误方式，我会喜欢那里的建议。

Answer 1

如果不出意外，您可以手动使用fs.open()、fs.read() 和fs.close()。示例：

var CHUNK_SIZE = 10 * 1024 * 1024, // 10MB
    buffer = Buffer.alloc(CHUNK_SIZE),
    filePath = '/tmp/foo';

fs.open(filePath, 'r', function(err, fd) {
  if (err) throw err;
  function readNextChunk() {
    fs.read(fd, buffer, 0, CHUNK_SIZE, null, function(err, nread) {
      if (err) throw err;

      if (nread === 0) {
        // done reading file, do any necessary finalization steps

        fs.close(fd, function(err) {
          if (err) throw err;
        });
        return;
      }

      var data;
      if (nread < CHUNK_SIZE)
        data = buffer.slice(0, nread);
      else
        data = buffer;

      // do something with `data`, then call `readNextChunk();`
    });
  }
  readNextChunk();
});

Answer 2

您可以考虑使用下面的 sn-p，我们以 1024 字节的块读取文件

var fs = require('fs');

var data = '';

var readStream = fs.createReadStream('/tmp/foo.txt',{ highWaterMark: 1 * 1024, encoding: 'utf8' });

readStream.on('data', function(chunk) {
    data += chunk;
    console.log('chunk Data : ')
    console.log(chunk);// your processing chunk logic will go here

}).on('end', function() {
    console.log('###################');
    console.log(data); 
// here you see all data processed at end of file
    });

请注意：highWaterMark 是用于块大小的参数 希望这会有所帮助！

网页参考：https://stackabuse.com/read-files-with-node-js/Changing readstream chunksize

Answer 3

基于mscdex's answer，这是一个使用同步替代方案和 StringDecoder 来正确解析 UTF-8 的模块

readableStream 的问题在于，为了使用它，您必须将整个项目转换为使用异步发射器和回调。如果你在编写一些简单的代码，比如 nodejs 中的一个小 CLI，那就没有意义了。

//usage
let file = new UTF8FileReader()
file.open('./myfile.txt', 1024) 
while ( file.isOpen ) {
    let stringData=file.readChunk()
    console.log(stringData)
}


//--------------------
// UTF8FileReader.ts
//--------------------
import * as fs from 'fs';
import { StringDecoder, NodeStringDecoder } from "string_decoder";

export class UTF8FileReader {

    filename: string;
    isOpen: boolean = false;
    private chunkSize: number;
    private fd: number; //file handle from fs.OpenFileSync
    private readFilePos: number;
    private readBuffer: Buffer;

    private utf8decoder: NodeStringDecoder

    /**
     * open the file | throw
     * @param filename
     */
    open(filename, chunkSize: number = 16 * 1024) {

        this.chunkSize = chunkSize;

        try {
            this.fd = fs.openSync(filename, 'r');
        }
        catch (e) {
            throw new Error("opening " + filename + ", error:" + e.toString());
        }

        this.filename = filename;
        this.isOpen = true;

        this.readBuffer = Buffer.alloc(this.chunkSize);
        this.readFilePos = 0;

        //a StringDecoder is a buffered object that ensures complete UTF-8 multibyte decoding from a byte buffer
        this.utf8decoder = new StringDecoder('utf8')

    }

    /**
     * read another chunk from the file 
     * return the decoded UTF8 into a string
     * (or throw)
     * */
    readChunk(): string {

        let decodedString = '' //return '' by default

        if (!this.isOpen) {
            return decodedString;
        }

        let readByteCount: number;
        try {
            readByteCount = fs.readSync(this.fd, this.readBuffer, 0, this.chunkSize, this.readFilePos);
        }
        catch (e) {
            throw new Error("reading " + this.filename + ", error:" + e.toString());
        }

        if (readByteCount) {
            //some data read, advance readFilePos 
            this.readFilePos += readByteCount;
            //get only the read bytes (if we reached the end of the file)
            const onlyReadBytesBuf = this.readBuffer.slice(0, readByteCount);
            //correctly decode as utf8, and store in decodedString
            //yes, the api is called "write", but it decodes a string - it's a write-decode-and-return the string kind-of-thing :)
            decodedString = this.utf8decoder.write(onlyReadBytesBuf); 
        }
        else {
            //read returns 0 => all bytes read
            this.close();
        }
        return decodedString 
    }

    close() {
        if (!this.isOpen) {
            return;
        }
        fs.closeSync(this.fd);
        this.isOpen = false;
        this.utf8decoder.end();
    }

}

如果你还没有 typescript，这里是 .js 转译的代码：

// UTF8FileReader.js
"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.UTF8FileReader = void 0;
//--------------------
// UTF8FileReader
//--------------------
const fs = require("fs");
const string_decoder_1 = require("string_decoder");
class UTF8FileReader {
    constructor() {
        this.isOpen = false;
    }
    /**
     * open the file | throw
     * @param filename
     */
    open(filename, chunkSize = 16 * 1024) {
        this.chunkSize = chunkSize;
        try {
            this.fd = fs.openSync(filename, 'r');
        }
        catch (e) {
            throw new Error("opening " + filename + ", error:" + e.toString());
        }
        this.filename = filename;
        this.isOpen = true;
        this.readBuffer = Buffer.alloc(this.chunkSize);
        this.readFilePos = 0;
        //a StringDecoder is a buffered object that ensures complete UTF-8 multibyte decoding from a byte buffer
        this.utf8decoder = new string_decoder_1.StringDecoder('utf8');
    }
    /**
     * read another chunk from the file
     * return the decoded UTF8 into a string
     * (or throw)
     * */
    readChunk() {
        let decodedString = ''; //return '' by default
        if (!this.isOpen) {
            return decodedString;
        }
        let readByteCount;
        try {
            readByteCount = fs.readSync(this.fd, this.readBuffer, 0, this.chunkSize, this.readFilePos);
        }
        catch (e) {
            throw new Error("reading " + this.filename + ", error:" + e.toString());
        }
        if (readByteCount) {
            //some data read, advance readFilePos 
            this.readFilePos += readByteCount;
            //get only the read bytes (if we reached the end of the file)
            const onlyReadBytesBuf = this.readBuffer.slice(0, readByteCount);
            //correctly decode as utf8, and store in decodedString
            //yes, the api is called "write", but it decodes a string - it's a write-decode-and-return the string kind-of-thing :)
            decodedString = this.utf8decoder.write(onlyReadBytesBuf);
        }
        else {
            //read returns 0 => all bytes read
            this.close();
        }
        return decodedString;
    }
    close() {
        if (!this.isOpen) {
            return;
        }
        fs.closeSync(this.fd);
        this.isOpen = false;
        this.utf8decoder.end();
    }
}
exports.UTF8FileReader = UTF8FileReader;