Section 1.1 What Are HTTP Range Requests?

HTTP range requests enable the server to send only a specific portion of an HTTP message to the client. This feature is particularly useful for transferring large media files and facilitating the resumption of interrupted downloads. If the server's response contains an Accept-Ranges header (with a value other than "none"), it indicates support for range requests.

Multiple parts can be requested simultaneously within a Range header, allowing the server to return them as a multipart file. A valid range response requires the server to use the 206 Partial Content status code. Conversely, if the requested range is invalid, a 416 Range Not Satisfiable status code will be returned, indicating a client-side error. Servers can also choose to disregard the Range header and send the entire file with a 200 status code.

Range Syntax Overview: Range: =- Range: =- Range: =-, - Range: =-, - - unit: Typically bytes. - range-start: An integer indicating the beginning of the requested range. - range-end: An optional integer indicating the end of the requested range. If omitted, the end of the document is assumed.

After grasping the syntax, let's look at a few examples: Single part ranges: $ curl http://i.imgur.com/z4d4kWk.jpg -i -H "Range: bytes=0-1023" Multipart ranges: $ curl http://www.example.com -i -H "Range: bytes=0-50, 100-150"

With this knowledge, let’s explore how to implement large file downloads.

Section 1.2 Implementing Large File Downloads

To understand the implementation better, let’s examine the overall workflow:

Now, let’s define some essential helper functions involved in this process.

1. getContentLength Function
   • This function retrieves the length of a file by sending a HEAD request and reading the Content-Length from the response header.
2. asyncPool Function
   • This function controls concurrency for asynchronous tasks. It takes three parameters:
     • concurrency: The limit for concurrent operations (>= 1).
     • iterable: An input object, such as a String or Array.
     • iteratorFn: A function that returns a promise or is async.
3. getBinaryContent Function
   • Initiates a range request based on specified parameters to download file data blocks.
4. concatenate Function
   • Since ArrayBuffer objects cannot be manipulated directly, this function merges downloaded file data blocks after converting them to Uint8Array objects.
5. saveAs Function
   • Implements client-side file saving. This basic implementation can be enhanced by using libraries like FileSaver.js. It employs Blob and Object URLs to facilitate saving.
6. download Function
   • Executes the download operation, accepting parameters for the resource URL, chunk size, and concurrency limit.
7. Complete Example for Concurrent Download
   • Utilizing the functions described above, we can construct a complete example for the concurrent download of large files.

This guide illustrates how to use the asyncPool function in JavaScript for concurrent downloads, retrieve file sizes via HEAD requests, initiate HTTP range requests, and save files on the client side.