crawl4ai/docs/md_v2/api/async-webcrawler.md

AsyncWebCrawler

The AsyncWebCrawler is the core class for asynchronous web crawling in Crawl4AI. You typically create it once, optionally customize it with a BrowserConfig (e.g., headless, user agent), then run multiple arun() calls with different CrawlerRunConfig objects.

Recommended usage:

1. Create a BrowserConfig for global browser settings. 

2. Instantiate AsyncWebCrawler(config=browser_config). 

3. Use the crawler in an async context manager (async with) or manage start/close manually. 

4. Call arun(url, config=crawler_run_config) for each page you want.

---

1. Constructor Overview

class AsyncWebCrawler:
    def __init__(
        self,
        crawler_strategy: Optional[AsyncCrawlerStrategy] = None,
        config: Optional[BrowserConfig] = None,
        always_bypass_cache: bool = False,           # deprecated
        always_by_pass_cache: Optional[bool] = None, # also deprecated
        base_directory: str = ...,
        thread_safe: bool = False,
        **kwargs,
    ):
        """
        Create an AsyncWebCrawler instance.

        Args:
            crawler_strategy: 
                (Advanced) Provide a custom crawler strategy if needed.
            config: 
                A BrowserConfig object specifying how the browser is set up.
            always_bypass_cache: 
                (Deprecated) Use CrawlerRunConfig.cache_mode instead.
            base_directory:     
                Folder for storing caches/logs (if relevant).
            thread_safe: 
                If True, attempts some concurrency safeguards. Usually False.
            **kwargs: 
                Additional legacy or debugging parameters.
        """
    )

### Typical Initialization

```python
from crawl4ai import AsyncWebCrawler, BrowserConfig

browser_cfg = BrowserConfig(
    browser_type="chromium",
    headless=True,
    verbose=True
)

crawler = AsyncWebCrawler(config=browser_cfg)

Notes:

• Legacy parameters like always_bypass_cache remain for backward compatibility, but prefer to set caching in CrawlerRunConfig.

---

2. Lifecycle: Start/Close or Context Manager

2.1 Context Manager (Recommended)

async with AsyncWebCrawler(config=browser_cfg) as crawler:
    result = await crawler.arun("https://example.com")
    # The crawler automatically starts/closes resources

When the async with block ends, the crawler cleans up (closes the browser, etc.).

2.2 Manual Start & Close

crawler = AsyncWebCrawler(config=browser_cfg)
await crawler.start()

result1 = await crawler.arun("https://example.com")
result2 = await crawler.arun("https://another.com")

await crawler.close()

Use this style if you have a long-running application or need full control of the crawler’s lifecycle.

---

3. Primary Method: arun()

async def arun(
    self,
    url: str,
    config: Optional[CrawlerRunConfig] = None,
    # Legacy parameters for backward compatibility...
) -> CrawlResult:
    ...

3.1 New Approach

You pass a CrawlerRunConfig object that sets up everything about a crawl—content filtering, caching, session reuse, JS code, screenshots, etc.

import asyncio
from crawl4ai import CrawlerRunConfig, CacheMode

run_cfg = CrawlerRunConfig(
    cache_mode=CacheMode.BYPASS,
    css_selector="main.article",
    word_count_threshold=10,
    screenshot=True
)

async with AsyncWebCrawler(config=browser_cfg) as crawler:
    result = await crawler.arun("https://example.com/news", config=run_cfg)
    print("Crawled HTML length:", len(result.cleaned_html))
    if result.screenshot:
        print("Screenshot base64 length:", len(result.screenshot))

3.2 Legacy Parameters Still Accepted

For backward compatibility, arun() can still accept direct arguments like css_selector=..., word_count_threshold=..., etc., but we strongly advise migrating them into a CrawlerRunConfig.

---

4. Batch Processing: arun_many()

async def arun_many(
    self,
    urls: List[str],
    config: Optional[CrawlerRunConfig] = None,
    # Legacy parameters maintained for backwards compatibility...
) -> List[CrawlResult]:
    """
    Process multiple URLs with intelligent rate limiting and resource monitoring.
    """

4.1 Resource-Aware Crawling

The arun_many() method now uses an intelligent dispatcher that:

• Monitors system memory usage
• Implements adaptive rate limiting
• Provides detailed progress monitoring
• Manages concurrent crawls efficiently

4.2 Example Usage

Check page Multi-url Crawling for a detailed example of how to use arun_many().

### 4.3 Key Features

1. **Rate Limiting**
   
   - Automatic delay between requests
   - Exponential backoff on rate limit detection
   - Domain-specific rate limiting
   - Configurable retry strategy

2. **Resource Monitoring**

   - Memory usage tracking
   - Adaptive concurrency based on system load
   - Automatic pausing when resources are constrained

3. **Progress Monitoring**

   - Detailed or aggregated progress display
   - Real-time status updates
   - Memory usage statistics

4. **Error Handling**

   - Graceful handling of rate limits
   - Automatic retries with backoff
   - Detailed error reporting

---

## 5. `CrawlResult` Output

Each `arun()` returns a **`CrawlResult`** containing:

- `url`: Final URL (if redirected).
- `html`: Original HTML.
- `cleaned_html`: Sanitized HTML.
- `markdown_v2` (or future `markdown`): Markdown outputs (raw, fit, etc.).
- `extracted_content`: If an extraction strategy was used (JSON for CSS/LLM strategies).
- `screenshot`, `pdf`: If screenshots/PDF requested.
- `media`, `links`: Information about discovered images/links.
- `success`, `error_message`: Status info.

For details, see [CrawlResult doc](./crawl-result.md).

---

## 6. Quick Example

Below is an example hooking it all together:

```python
import asyncio
from crawl4ai import AsyncWebCrawler, BrowserConfig, CrawlerRunConfig, CacheMode
from crawl4ai.extraction_strategy import JsonCssExtractionStrategy
import json

async def main():
    # 1. Browser config
    browser_cfg = BrowserConfig(
        browser_type="firefox",
        headless=False,
        verbose=True
    )

    # 2. Run config
    schema = {
        "name": "Articles",
        "baseSelector": "article.post",
        "fields": [
            {
                "name": "title", 
                "selector": "h2", 
                "type": "text"
            },
            {
                "name": "url", 
                "selector": "a", 
                "type": "attribute", 
                "attribute": "href"
            }
        ]
    }

    run_cfg = CrawlerRunConfig(
        cache_mode=CacheMode.BYPASS,
        extraction_strategy=JsonCssExtractionStrategy(schema),
        word_count_threshold=15,
        remove_overlay_elements=True,
        wait_for="css:.post"  # Wait for posts to appear
    )

    async with AsyncWebCrawler(config=browser_cfg) as crawler:
        result = await crawler.arun(
            url="https://example.com/blog",
            config=run_cfg
        )

        if result.success:
            print("Cleaned HTML length:", len(result.cleaned_html))
            if result.extracted_content:
                articles = json.loads(result.extracted_content)
                print("Extracted articles:", articles[:2])
        else:
            print("Error:", result.error_message)

asyncio.run(main())

Explanation:

• We define a BrowserConfig with Firefox, no headless, and verbose=True. 
• We define a CrawlerRunConfig that bypasses cache, uses a CSS extraction schema, has a word_count_threshold=15, etc. 
• We pass them to AsyncWebCrawler(config=...) and arun(url=..., config=...).

---

7. Best Practices & Migration Notes

1. Use BrowserConfig for global settings about the browser’s environment.  2. Use CrawlerRunConfig for per-crawl logic (caching, content filtering, extraction strategies, wait conditions).  3. Avoid legacy parameters like css_selector or word_count_threshold directly in arun(). Instead:

run_cfg = CrawlerRunConfig(css_selector=".main-content", word_count_threshold=20)
result = await crawler.arun(url="...", config=run_cfg)

4. Context Manager usage is simplest unless you want a persistent crawler across many calls.

---

8. Summary

AsyncWebCrawler is your entry point to asynchronous crawling:

• Constructor accepts BrowserConfig (or defaults). 
• arun(url, config=CrawlerRunConfig) is the main method for single-page crawls. 
• arun_many(urls, config=CrawlerRunConfig) handles concurrency across multiple URLs. 
• For advanced lifecycle control, use start() and close() explicitly. 

Migration:

• If you used AsyncWebCrawler(browser_type="chromium", css_selector="..."), move browser settings to BrowserConfig(...) and content/crawl logic to CrawlerRunConfig(...).

This modular approach ensures your code is clean, scalable, and easy to maintain. For any advanced or rarely used parameters, see the BrowserConfig docs.