IB Computer Science C.2.5 Parallel Web Crawling Notes

1. Traditional web crawlers operate sequentially, processing one web page at a time.
2. However, as the web has grown exponentially, sequential crawling has become inefficient.
3. To address this, parallel web crawling techniques have been developed, allowing multiple crawlers to work simultaneously.

How Parallel Web Crawling Works

Multiple Crawlers

1. In parallel web crawling, multiple crawler instances run concurrently, each responsible for a subset of the web.
2. These crawlers can operate on different machines or within a distributed system, allowing for greater scalability.

Task Distribution

1. The web is divided into segments, and each crawler is assigned a specific segment to process.
2. This division can be based on various criteria, such as domain, URL patterns, or geographical location.

Coordination and Communication

1. Crawlers must coordinate to avoid duplicating efforts and ensure comprehensive coverage.
2. This coordination is achieved through shared data structures, such as distributed hash tables, and communication protocols.

Load Balancing

1. To maximize efficiency, the workload is balanced among crawlers.
2. If one crawler finishes its segment early, it can be reassigned to assist with other segments.

Benefits of Parallel Web Crawling

1. Increased Speed: By processing multiple web pages simultaneously, parallel crawlers can index the web much faster than sequential crawlers.
2. Scalability: Parallel crawling systems can easily scale by adding more crawlers, making them suitable for large-scale web indexing.
3. Fault Tolerance: If one crawler fails, others can continue working, ensuring the crawling process is not disrupted.

Challenges of Parallel Web Crawling

1. Coordination Complexity: Managing multiple crawlers requires sophisticated coordination mechanisms to prevent duplication and ensure comprehensive coverage.
2. Resource Management: Parallel crawlers consume more resources, such as bandwidth and storage, requiring efficient resource management.
3. Politeness and Throttling: Crawlers must respect website policies, such as robots.txt, and avoid overloading servers by throttling requests.