PART# 1 - Efficient File Transfer System Using HTTP for Large Datasets - Chunked Uploads

The Range Requests feature allows clients to download specific portions of a file rather than the entire file. This is useful when:

• Resuming interrupted downloads.
• Downloading only a portion of a large file (e.g., video streaming).
• Reducing bandwidth usage by downloading files in chunks.

The given code sets up a system to handle file downloads with support for range requests, ensuring that large files can be downloaded in chunks or resumed from where they left off.

HTML & JavaScript Code Explanation

• HTML Structure: This file creates a basic webpage with Bootstrap styling. It contains a card titled "Downloads" with a list (<ul>) for displaying available files for download.
• CSS & JS Files: External CSS (Bootstrap) and JS (jQuery, Bootstrap) files are linked for styling and interactivity.
• JavaScript File: The chunked-file-upload.js script will handle the logic for fetching the available files and managing download requests.

PHP Code: Handling File Downloads (Range Requests)

• Purpose: This script handles file downloads with support for partial downloads (i.e., range requests).

• Main Flow:

  1. Receiving the File: It checks if a file parameter is present in the URL ($_GET['file']). If it exists, it extracts the file name using basename() and constructs the file path in ../assets/uploads/.
  2. File Existence: It checks if the file exists using file_exists(). If not, a 404 error is returned.
  3. Range Handling:
     • The script checks if the HTTP Range header is present. This header is sent when the client requests only part of the file (i.e., for resuming interrupted downloads).
     • Using a regular expression, it extracts the start and optional end positions of the byte range.
     • If a range is provided, it sends a 206 Partial Content response and adjusts headers accordingly (Content-Range).
  4. File Streaming:
     • Using fseek(), the script moves the file pointer to the correct byte offset.
     • The file is read and sent to the client in chunks (8KB by default), ensuring efficient memory usage.
  5. Headers: It sends appropriate headers to indicate that the file is downloadable (Content-Disposition) and supports range requests (Accept-Ranges).

Why Use It?

• Efficient Large File Handling: It allows the server to send only the required parts of a file, making it possible to resume interrupted downloads or request specific parts.
• Client-Side Control: Modern browsers can handle range requests, allowing users to restart downloads without fetching the entire file again.

PHP Code: Fetching Files for Download

• Purpose: This script returns a list of available files in the upload directory as a JSON array. The files can then be listed on the client-side for selection.

• Main Flow:

  1. Directory Scanning: It scans the ../assets/uploads/ directory using scandir() to get all files.
  2. File Filtering: It removes . (current directory), .. (parent directory), and .gitignore from the list using array_diff().
  3. Return as JSON: The cleaned file list is returned as a JSON-encoded array for easy use by JavaScript on the front-end.

Why Use It?

• Dynamic File Listing: This script dynamically retrieves files, allowing users to download any file available in the upload directory without manually updating the HTML.
• Simplified Client-Side Interaction: The JSON response is easy to work with using JavaScript, making it simple to display files in a user-friendly format (e.g., a list of clickable links).

Final Summary

• HTML/JS: Provides a user interface to display downloadable files. The list of files is fetched dynamically from the server.
• PHP (download.php): Manages the actual file download process, supporting range requests for partial downloads. It efficiently streams files in chunks to avoid high memory usage.
• PHP (get_files.php): Retrieves the list of available files in the upload directory and returns it as a JSON array for the front-end to display.

The range request functionality ensures efficient and reliable downloads, especially for large files or interrupted downloads, enhancing the user experience.

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Source Code