Resume Downloads

Continue interrupted downloads using HTTP Range headers.

Enabling Resume

Resume is enabled by default:

var config = downloader.Config.default();
config.resume_downloads = true;  // Default

How It Works

1. Client checks for existing partial file
2. Sends HEAD request to verify server support
3. If supported, sends GET with Range header
4. Server responds with 206 Partial Content
5. Download continues from last byte

Requirements

Resume requires:

• Server support for Range requests (Accept-Ranges: bytes)
• Existing partial file on disk
• File unchanged on server since partial download

File Exists Actions

Control behavior when file exists:

var config = downloader.Config.default();

// Create file (1), file (2), etc. (default)
config.file_exists_action = .rename_with_number;

// Try resume, otherwise overwrite
config.file_exists_action = .resume_or_overwrite;

// Always overwrite
config.file_exists_action = .overwrite;

// Don't download if exists
config.file_exists_action = .skip;

// Return error if exists
config.file_exists_action = .fail;

Detecting Resume

Check if download was resumed via progress callback:

fn progressCallback(p: downloader.Progress) bool {
    if (p.is_resumed) {
        std.debug.print("Resuming from byte {d}\n", .{p.start_offset});
    }
    return true;
}

Example

var config = downloader.Config.default();
config.resume_downloads = true;
config.file_exists_action = .resume_or_overwrite;

var client = try downloader.Client.init(allocator, config);
defer client.deinit();

const bytes = try client.download(url, "large-file.iso", callback);
std.debug.print("Downloaded {d} bytes this session\n", .{bytes});

Fallback Behavior

If resume fails:

• Server doesn't support Range: starts fresh download
• File modified on server: starts fresh download
• Partial file deleted: starts fresh download

Disabling Resume

var config = downloader.Config.noResume();
// Sets resume_downloads = false
// Sets file_exists_action = .overwrite

Next Steps

• Retry Logic - Handle transient failures
• Configuration - All options