goget/docs/recursive-mirror.md

# Recursive Downloading & Website Mirroring

goget can recursively download linked pages and mirror entire websites for offline viewing. This document explains the two modes, their configuration, and the underlying architecture.

## Quick Comparison

| Feature | Recursive (`--recursive`) | Mirror (`--mirror`) |
|---|---|---|
| Depth limit | By default (`--max-depth N`) | Infinite |
| Link conversion | Optional (`--convert-links`) | Automatic |
| Page requisites | Manual (`--page-requisites`) | Automatic |
| robots.txt | Respected by default | Respected by default |
| Use case | Download a subtree of a site | Full offline archive |

## Recursive Mode

Recursive mode follows links from HTML pages and downloads linked resources.

```bash
# Basic recursive download, depth 3
goget --url https://example.com/docs/ --recursive --max-depth 3

# Only PDF files
goget --recursive --accept "*.pdf" --url https://example.com/docs/

# Only images and HTML
goget --recursive --accept "image/*,text/html" --url https://example.com/
```

### Link Filtering

```mermaid
flowchart TD
    HTML[Parse HTML page]
    Extract["Extract all links\n(a, img, link, script)"]
    FilterByDomain{"Domain match?"}
    FilterByPattern{"Accept pattern\nmatch?"}
    FilterByDepth{"Depth ≤ max?"}
    FilterByParent{"No-parent\ncheck?"}
    ExcludeFilter{"Not in exclude\nlist?"}
    Enqueue["Add to download queue"]
    Skip["Skip"]

    HTML --> Extract --> FilterByDomain
    FilterByDomain -->|Yes| FilterByPattern
    FilterByDomain -->|No, --span-hosts| FilterByPattern
    FilterByDomain -->|No| Skip
    FilterByPattern -->|Yes| FilterByDepth
    FilterByPattern -->|No| Skip
    FilterByDepth -->|Yes| FilterByParent
    FilterByDepth -->|No| Skip
    FilterByParent -->|Pass| ExcludeFilter
    FilterByParent -->|Fail| Skip
    ExcludeFilter -->|Pass| Enqueue
    ExcludeFilter -->|Fail| Skip
```

### Accept Patterns

The `--accept` flag supports two types:

| Type | Example | Matches |
|---|---|---|
| **Glob** | `*.pdf` | Filenames ending in `.pdf` |
| **Glob** | `*.html,*.css` | Multiple patterns (comma-separated) |
| **MIME** | `text/html` | Exact MIME type |
| **MIME** | `image/*` | Any MIME in the `image/` category |

### Domain Control

| Flag | Effect |
|---|---|
| (default) | Only follow links within the starting domain |
| `--span-hosts` | Follow links to any domain |
| `--domains a.com,b.com` | Restrict to specific domains |
| `--follow-external` | Follow external links (aliases for `--span-hosts`) |
| `--no-parent` | Don't ascendant above the starting URL path |
| `--recursive-parallel` | `<N>` | Number of concurrent file downloads in recursive mode across all protocols (`0` = sequential). HTTP, WebDAV, FTP, and SFTP each get a worker pool bounded by a semaphore; subdirectory recursion propagates the limit. Connection pooling (FTP, SFTP) opens N independent control connections |

### Request Throttling

```bash
# Fixed 2-second delay between requests
goget --recursive --wait 2s --url https://example.com/docs/

# Randomize delay (0.5x – 1.5x of --wait)
goget --recursive --wait 2s --random-wait --url https://example.com/docs/
```

### Page Requisites

`--page-requisites` downloads CSS, JavaScript, and images needed to render each HTML page:

```bash
goget --recursive --page-requisites --url https://example.com/page.html
```

This parses `<link>`, `<script>`, `<img>`, `<source>`, and `<video>` tags and downloads their `src`/`href` targets.

## Mirror Mode

| `--dry-run` | — | List all files that would be downloaded in recursive/mirror mode without saving to disk |

`--mirror` is a shorthand for `--recursive --convert-links --page-requisites --infinite-depth`:

```bash
# Full site mirror
goget --url https://example.com --mirror --output ./mirror

# With link conversion for offline viewing
goget --url https://example.com --mirror --convert-links --output ./mirror
```

### Link Conversion

When `--convert-links` is enabled, HTML links are rewritten for local offline viewing:

```
Before: <a href="https://example.com/about/">About</a>
After:  <a href="./about/index.html">About</a>

Before: <img src="https://example.com/img/logo.png">
After:  <img src="./img/logo.png">
```

The link rewriter handles:
- Absolute URLs → relative paths
- Protocol-relative URLs (`//example.com/...`)
- Root-relative URLs (`/about/`)
- CSS `url()` references

### Robots.txt

goget respects `robots.txt` by default. Use `--no-robots` to ignore it:

```bash
goget --mirror --no-robots --url https://example.com
```

### Asset Control

```bash
# Mirror but skip CSS/JS/images
goget --mirror --no-mirror-assets --url https://example.com

# Only mirror specific file types
goget --mirror --accept "*.html,*.jpg" --url https://example.com
```

## Output Structure

Mirrored sites preserve the URL path structure:

```
mirror/
├── index.html
├── about/
│   └── index.html
├── blog/
│   ├── index.html
│   └── post-1.html
├── css/
│   └── style.css
└── img/
    └── logo.png
```

Use `--cut-dirs N` to strip directory components:

```bash
# Original: https://example.com/pub/docs/file.html
# Without cut: ./mirror/pub/docs/file.html
# With --cut-dirs 2: ./mirror/doc/file.html
goget --mirror --cut-dirs 2 --url https://example.com/pub/docs/
```

## Architecture

### Crawler (`internal/recursive`)

```go
type CrawlerConfig struct {
    MaxDepth        int
    FollowExternal  bool
    ExcludePatterns []string
    IncludePatterns []string
    AcceptPatterns  []string
    NoParent        bool
    PageRequisites  bool
    ConvertLinks    bool
    OutputDir       string
    Delay           time.Duration
    RandomWait      bool
    MaxWorkers      int  // concurrent download workers (0 = sequential)
}
```

The crawler:
1. Downloads an HTML page
2. Parses it with `golang.org/x/net/html`
3. Extracts all links (`a[href]`, `img[src]`, `link[href]`, `script[src]`)
4. Filters against accept/reject patterns, domain rules, and depth limits
5. Enqueues matching URLs for download
6. Applies configurable delay between requests

### Mirror (`internal/mirror`)

The mirror wraps the crawler with additional configuration:
- Infinite depth (or configurable via `MirrorConfig.MaxDepth`)
- Automatic page requisites
- Automatic link conversion
- robots.txt compliance

### Link Rewriting (`internal/linkrewrite`)

After the download completes, links are rewritten in-place:
1. Read each HTML file
2. Parse with `golang.org/x/net/html`
3. Identify all `href` and `src` attributes
4. Convert absolute URLs to relative paths
5. Write back the modified HTML (backup `.orig` if `--backup-converted`)

## Timestamping

`--timestamping` downloads a file only if the server copy is newer than the local copy:

```bash
goget --timestamping --url https://example.com/file.zip
```

goget compares the `Last-Modified` header against the local file's modification time.

## WARC Archiving

For legal and archival compliance, output can be written in WARC format:

```bash
goget --warc-file archive.warc --url https://example.com/page.html
```

Each downloaded resource gets a separate WARC record with:
- Request metadata (URL, timestamp, headers)
- Response metadata (status code, content type, headers)
- Raw response body

## Performance Tips

- **Limit depth** — `--max-depth 3` prevents runaway recursion on large sites
- **Filter aggressively** — Use `--accept "*.pdf,*.html"` and `--reject "*.zip"` to narrow scope
- **Use delay** — `--wait 500ms` prevents server overload and IP blocks
- **Limit domains** — `--domains example.com` prevents crawling external CDNs
- **Skip assets** — `--no-mirror-assets` for text-only archives