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"""
URL Frontier implementation for web crawler
The URL Frontier maintains URLs to be crawled with two main goals:
1. Prioritization - Important URLs are crawled first
2. Politeness - Avoid overloading web servers with too many requests
"""
import time
import logging
import heapq
import pickle
import threading
import random
from typing import Dict, List, Tuple, Optional, Any, Set
from collections import deque
import redis
from redis.exceptions import RedisError
import mmh3
import os
import json
from models import URL, Priority, URLStatus
import config
# Import local configuration if available
try:
import local_config
# Override config settings with local settings
for key in dir(local_config):
if key.isupper():
setattr(config, key, getattr(local_config, key))
logging.info("Loaded local configuration")
except ImportError:
pass
# Configure logging
logging.basicConfig(
level=getattr(logging, config.LOG_LEVEL),
format=config.LOG_FORMAT
)
logger = logging.getLogger(__name__)
class URLFrontier:
"""
URL Frontier implementation with prioritization and politeness
Architecture:
- Front queues: Priority-based queues
- Back queues: Host-based queues for politeness
This uses Redis for persistent storage to handle large number of URLs
and enable distributed crawling. In deployment mode, it can also use
in-memory storage.
"""
def __init__(self, redis_client: Optional[redis.Redis] = None, use_memory: bool = False):
"""Initialize the URL Frontier"""
self.use_memory = use_memory
if use_memory:
# Initialize in-memory storage
self.memory_storage = {
'seen_urls': set(),
'priority_queues': [[] for _ in range(config.PRIORITY_QUEUE_NUM)],
'host_queues': [[] for _ in range(config.HOST_QUEUE_NUM)]
}
else:
# Use Redis
self.redis = redis_client or redis.from_url(config.REDIS_URI)
self.priority_count = config.PRIORITY_QUEUE_NUM # Number of priority queues
self.host_count = config.HOST_QUEUE_NUM # Number of host queues
self.url_seen_key = "webcrawler:url_seen" # Bloom filter for seen URLs
self.priority_queue_key_prefix = "webcrawler:priority_queue:"
self.host_queue_key_prefix = "webcrawler:host_queue:"
self.lock = threading.RLock() # Thread-safe operations
# Simple mode uses Redis Set instead of Bloom filter
self.use_simple_mode = getattr(config, 'USE_SIMPLE_URL_SEEN', False)
logger.info(f"URLFrontier using simple mode: {self.use_simple_mode}")
# Ensure directory for checkpoint exists
if not os.path.exists(config.STORAGE_PATH):
os.makedirs(config.STORAGE_PATH)
# Initialize URL seen storage
if not self.use_memory:
self._init_url_seen()
def _init_url_seen(self):
"""Initialize URL seen storage based on configuration"""
try:
# If using simple mode, just use a Redis set
if self.use_simple_mode:
if not self.redis.exists(self.url_seen_key):
logger.info("Initializing URL seen set")
self.redis.sadd(self.url_seen_key, "initialized")
return
# Try to use Bloom filter
if not self.redis.exists(self.url_seen_key):
logger.info("Initializing URL seen bloom filter")
try:
# Use a bloom filter with 100 million items and 0.01 false positive rate
# This requires approximately 119.5 MB of memory
self.redis.execute_command("BF.RESERVE", self.url_seen_key, 0.01, 100000000)
except RedisError as e:
logger.error(f"Failed to initialize bloom filter: {e}")
logger.info("Falling back to simple set for URL seen detection")
self.use_simple_mode = True
# Initialize a set instead
if not self.redis.exists(self.url_seen_key):
self.redis.sadd(self.url_seen_key, "initialized")
except RedisError as e:
logger.error(f"Error initializing URL seen: {e}")
# Fallback to set if bloom filter is not available
self.use_simple_mode = True
if not self.redis.exists(self.url_seen_key):
self.redis.sadd(self.url_seen_key, "initialized")
def add_url(self, url_obj: URL) -> bool:
"""Add a URL to the frontier"""
with self.lock:
url = url_obj.url
# Check if URL has been seen
if self.use_memory:
if url in self.memory_storage['seen_urls']:
return False
self.memory_storage['seen_urls'].add(url)
else:
if self.use_simple_mode:
if self.redis.sismember(self.url_seen_key, url):
return False
self.redis.sadd(self.url_seen_key, url)
else:
if self._check_url_seen(url):
return False
self._mark_url_seen(url)
# Add to priority queue
priority_index = url_obj.priority.value % self.priority_count
if self.use_memory:
self.memory_storage['priority_queues'][priority_index].append(url_obj)
else:
priority_key = f"{self.priority_queue_key_prefix}{priority_index}"
self.redis.rpush(priority_key, url_obj.json())
return True
def get_next_url(self) -> Optional[URL]:
"""Get the next URL to crawl"""
with self.lock:
# Try each priority queue
for i in range(self.priority_count):
if self.use_memory:
queue = self.memory_storage['priority_queues'][i]
if queue:
return queue.pop(0)
else:
priority_key = f"{self.priority_queue_key_prefix}{i}"
url_data = self.redis.lpop(priority_key)
if url_data:
return URL.parse_raw(url_data)
return None
def _check_url_seen(self, url: str) -> bool:
"""Check if URL has been seen"""
if self.use_memory:
return url in self.memory_storage['seen_urls']
elif self.use_simple_mode:
return self.redis.sismember(self.url_seen_key, url)
else:
# Using Redis Bloom filter
return bool(self.redis.getbit(self.url_seen_key, self._hash_url(url)))
def _mark_url_seen(self, url: str) -> None:
"""Mark URL as seen"""
if self.use_memory:
self.memory_storage['seen_urls'].add(url)
elif self.use_simple_mode:
self.redis.sadd(self.url_seen_key, url)
else:
# Using Redis Bloom filter
self.redis.setbit(self.url_seen_key, self._hash_url(url), 1)
def _hash_url(self, url: str) -> int:
"""Hash URL for Bloom filter"""
return hash(url) % (1 << 32) # 32-bit hash
def size(self) -> int:
"""Get the total size of all queues"""
if self.use_memory:
return sum(len(q) for q in self.memory_storage['priority_queues'])
else:
total = 0
for i in range(self.priority_count):
priority_key = f"{self.priority_queue_key_prefix}{i}"
total += self.redis.llen(priority_key)
return total
def get_stats(self) -> Dict[str, Any]:
"""Get frontier statistics"""
stats = {
"size": self.size(),
"priority_queues": {},
"host_queues": {},
}
try:
# Get priority queue stats
for priority in range(1, self.priority_count + 1):
queue_key = f"{self.priority_queue_key_prefix}{priority}"
stats["priority_queues"][f"priority_{priority}"] = self.redis.llen(queue_key)
# Get host queue stats (just count total host queues with items)
host_queue_count = 0
for host_id in range(self.host_count):
queue_key = f"{self.host_queue_key_prefix}{host_id}"
if self.redis.llen(queue_key) > 0:
host_queue_count += 1
stats["host_queues"]["count_with_items"] = host_queue_count
# Add URLs seen count if using simple mode
if self.use_simple_mode:
stats["urls_seen"] = self.redis.scard(self.url_seen_key)
return stats
except RedisError as e:
logger.error(f"Error getting frontier stats: {e}")
return stats
def checkpoint(self) -> bool:
"""Save frontier state"""
if self.use_memory:
# No need to checkpoint in-memory storage
return True
try:
# Save priority queues
for i in range(self.priority_count):
priority_key = f"{self.priority_queue_key_prefix}{i}"
queue_data = []
while True:
url_data = self.redis.lpop(priority_key)
if not url_data:
break
queue_data.append(url_data)
# Save to file
checkpoint_file = os.path.join(config.STORAGE_PATH, f"priority_queue_{i}.json")
with open(checkpoint_file, 'w') as f:
json.dump(queue_data, f)
# Restore queue
for url_data in reversed(queue_data):
self.redis.rpush(priority_key, url_data)
return True
except Exception as e:
logger.error(f"Error creating frontier checkpoint: {e}")
return False
def restore(self) -> bool:
"""Restore frontier state"""
if self.use_memory:
# No need to restore in-memory storage
return True
try:
# Restore priority queues
for i in range(self.priority_count):
checkpoint_file = os.path.join(config.STORAGE_PATH, f"priority_queue_{i}.json")
if os.path.exists(checkpoint_file):
with open(checkpoint_file, 'r') as f:
queue_data = json.load(f)
# Clear existing queue
priority_key = f"{self.priority_queue_key_prefix}{i}"
self.redis.delete(priority_key)
# Restore queue
for url_data in queue_data:
self.redis.rpush(priority_key, url_data)
return True
except Exception as e:
logger.error(f"Error restoring frontier checkpoint: {e}")
return False
def clear(self) -> bool:
"""
Clear all queues in the frontier
Returns:
bool: True if successful, False otherwise
"""
try:
# Delete all queue keys
keys = []
for priority in range(1, self.priority_count + 1):
keys.append(f"{self.priority_queue_key_prefix}{priority}")
for host_id in range(self.host_count):
keys.append(f"{self.host_queue_key_prefix}{host_id}")
if keys:
self.redis.delete(*keys)
# Reset URL seen filter (optional)
self.redis.delete(self.url_seen_key)
logger.info("Frontier cleared")
return True
except Exception as e:
logger.error(f"Error clearing frontier: {e}")
return False |