(Node.js) HTTP Download in Parallel with Simultaneous Range Requests

Demonstrates how to download a large file with parallel simultaneous requests, where each request downloads a segment (range) of the remote file.

Install Chilkat for Node.js and Electron using npm at Chilkat npm packages for Node.js Chilkat npm packages for Electron on Windows, Linux, MacOSX, and ARM

var os = require('os');
if (os.platform() == 'win32') {  
    if (os.arch() == 'ia32') {
        var chilkat = require('@chilkat/ck-node21-win-ia32');
    } else {
        var chilkat = require('@chilkat/ck-node21-win64'); 
    }
} else if (os.platform() == 'linux') {
    if (os.arch() == 'arm') {
        var chilkat = require('@chilkat/ck-node21-arm');
    } else if (os.arch() == 'x86') {
        var chilkat = require('@chilkat/ck-node21-linux32');
    } else {
        var chilkat = require('@chilkat/ck-node21-linux64');
    }
} else if (os.platform() == 'darwin') {
    if (os.arch() == 'arm64') {
        var chilkat = require('@chilkat/ck-node21-mac-m1');
    } else {
        var chilkat = require('@chilkat/ck-node21-macosx');
    }
}

function chilkatExample() {

    // This requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    var http = new chilkat.Http();
    var success;

    // First get the size of the file to be downloaded.
    var url = "https://www.chilkatsoft.com/hamlet.xml";

    // resp: HttpResponse
    var resp = http.GetHead(url);
    if (http.LastMethodSuccess == false) {
        console.log(http.LastErrorText);
        return;
    }

    var remoteFileSize = resp.ContentLength;

    console.log("Downloading " + remoteFileSize + " bytes...");

    // Let's download in 4 chunks.
    // (the last chunk will be whatever remains after the 1st 3 equal sized chunks)
    var chunkSize = remoteFileSize / 4;

    // The Range header is used to download a range from a resource
    // Range: bytes=<range-start>-<range-end>
    // or
    // Range: bytes=<range-start>-

    // We're writing code this way for clarity..
    var http1 = new chilkat.Http();
    var http2 = new chilkat.Http();
    var http3 = new chilkat.Http();
    var http4 = new chilkat.Http();

    var sbRange = new chilkat.StringBuilder();
    sbRange.SetString("bytes=<range-start>-<range-end>");
    var numReplaced = sbRange.ReplaceI("<range-start>",0);
    numReplaced = sbRange.ReplaceI("<range-end>",chunkSize - 1);
    console.log(sbRange.GetAsString());
    http1.SetRequestHeader("Range",sbRange.GetAsString());

    sbRange.SetString("bytes=<range-start>-<range-end>");
    numReplaced = sbRange.ReplaceI("<range-start>",chunkSize);
    numReplaced = sbRange.ReplaceI("<range-end>",2 * chunkSize - 1);
    console.log(sbRange.GetAsString());
    http2.SetRequestHeader("Range",sbRange.GetAsString());

    sbRange.SetString("bytes=<range-start>-<range-end>");
    numReplaced = sbRange.ReplaceI("<range-start>",2 * chunkSize);
    numReplaced = sbRange.ReplaceI("<range-end>",3 * chunkSize - 1);
    console.log(sbRange.GetAsString());
    http3.SetRequestHeader("Range",sbRange.GetAsString());

    sbRange.SetString("bytes=<range-start>-");
    numReplaced = sbRange.ReplaceI("<range-start>",3 * chunkSize);
    console.log(sbRange.GetAsString());
    http4.SetRequestHeader("Range",sbRange.GetAsString());

    // Start each range download
    // task1: Task
    var task1 = http1.DownloadAsync(url,"qa_output/chunk1.dat");
    task1.Run();

    // task2: Task
    var task2 = http2.DownloadAsync(url,"qa_output/chunk2.dat");
    task2.Run();

    // task3: Task
    var task3 = http3.DownloadAsync(url,"qa_output/chunk3.dat");
    task3.Run();

    // task4: Task
    var task4 = http4.DownloadAsync(url,"qa_output/chunk4.dat");
    task4.Run();

    // Wait for the downloads to complete.
    var numLive = 4;
    while (numLive > 0) {
        numLive = 0;
        if (task1.Live == true) {
            numLive = numLive+1;
        }

        if (task2.Live == true) {
            numLive = numLive+1;
        }

        if (task3.Live == true) {
            numLive = numLive+1;
        }

        if (task4.Live == true) {
            numLive = numLive+1;
        }

        if (numLive > 0) {
            // SleepMs is a convenience method to cause the caller to sleep for N millisec.
            // It does not cause the given task to sleep..
            task1.SleepMs(10);
        }

    }

    // All should be downloaded now..
    // Examine the result of each Download.
    var numErrors = 0;
    if (task1.GetResultBool() == false) {
        console.log(task1.ResultErrorText);
        numErrors = numErrors+1;
    }

    if (task2.GetResultBool() == false) {
        console.log(task2.ResultErrorText);
        numErrors = numErrors+1;
    }

    if (task3.GetResultBool() == false) {
        console.log(task3.ResultErrorText);
        numErrors = numErrors+1;
    }

    if (task4.GetResultBool() == false) {
        console.log(task4.ResultErrorText);
        numErrors = numErrors+1;
    }

    if (numErrors > 0) {

        return;
    }

    // All downloads were successful.
    // Compose the file from the parts.
    var fac = new chilkat.FileAccess();
    success = fac.ReassembleFile("qa_output","chunk","dat","qa_output/hamlet.xml");
    if (success == false) {
        console.log(fac.LastErrorText);
    }
    else {
        console.log("Success.");
    }

    // Let's download in the regular way, and then compare files..
    success = http.Download(url,"qa_output/hamletRegular.xml");

    // Compare files.
    var bSame = fac.FileContentsEqual("qa_output/hamlet.xml","qa_output/hamletRegular.xml");
    console.log("bSame = " + bSame);

}

chilkatExample();