Understanding Distributed Scrapy Limitations

Standard Scrapy lacks native distributed capabilities for two primary reasons:

1. Each Scrapy instance operates with its own scheduler, preventing URL distribution across multiple machines (no shared scheduler)
2. Crawled data cannot be processed through a unified pipeline for centralized storage (no shared pipeline)

Implementing Distributed Crawling with Scrapy-Redis

The scrapy-redis package provides pre-built shared schedulers and pipelines that enable distributed web scraping. Two implementation approaches are available:

• Using the RedisSpider class
• Using the RedisCrawlSpider class

The configuration process remains consistent for both approaches.

Installation and Redis Configuration

First, install the necessary component:

pip install scrapy-redis

Modify the Redis configuration file (redis.conf):

# Comment out this line to allow external connections: bind 127.0.0.1
# Change this to allow external operations: protected-mode no

Spider File Modifications

Update your spider class by:

• Changing the parent class to RedisSpider (for Spider-based crawlers) or RedisCrawlSpider (for CrawlSpider-based crawlers)
• Removing or commenting the start_urls list
• Adding a redis_key attribute to specify the scheduler queue name

Scrapy Configuration Settings

Enable the scrapy-redis pipeline in settings.py:

ITEM_PIPELINES = {
    'scrapy_redis.pipelines.RedisPipeline': 400
}

Configure the scrapy-redis scheduler components:

# Enable scrapy-redis duplicate filter
DUPEFILTER_CLASS = "scrapy_redis.dupefilter.RFPDupeFilter"
# Use scrapy-redis scheduler
SCHEDULER = "scrapy_redis.scheduler.Scheduler"
# Enable persistence for pausing/resuming
SCHEDULER_PERSIST = True

Set Redis connection parameters:

REDIS_HOST = 'redis_server_ip'
REDIS_PORT = 6379
REDIS_ENCODING = 'utf-8'
REDIS_PARAMS = {'password': 'your_password'}

Execution Process

1. Start Redis server: redis-server redis.conf
2. Launch Redis client: redis-cli
3. Run the spider: scrapy runspider SpiderFile
4. Push initial URL to Redis queue: lpush redis_key_value http://example.com