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HLDsystem_design~12 mins

Design a web crawler in HLD - Architecture Diagram

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System Overview - Design a web crawler

A web crawler automatically browses the internet to collect and index web pages. It must efficiently fetch pages, avoid duplicate visits, and handle large-scale data with fault tolerance.

Architecture Diagram
User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Scheduler ---> URL Frontier (Queue) ---> Fetcher Pool ---> Parser ---> Storage (Database)
                                         |                              |
                                         v                              v
                                      Cache                         Message Queue
Components
User
client
Initiates crawl requests and views crawl status
Load Balancer
load_balancer
Distributes incoming crawl requests evenly to API Gateway
API Gateway
api_gateway
Receives requests, handles authentication, and routes to Scheduler
Scheduler
service
Manages crawl jobs and schedules URLs to be fetched
URL Frontier (Queue)
queue
Stores URLs to be crawled in order and avoids duplicates
Fetcher Pool
service
Fetches web pages from URLs concurrently
Parser
service
Extracts links and content from fetched pages
Storage (Database)
database
Stores crawled page data and metadata
Cache
cache
Caches recently fetched pages to reduce duplicate fetches
Message Queue
queue
Handles asynchronous communication between Parser and Scheduler
Request Flow - 10 Hops
User → Load Balancer
Load Balancer → API Gateway
API Gateway → Scheduler
Scheduler → URL Frontier (Queue)
Fetcher Pool → Cache
Fetcher Pool → Web Servers (Internet)
Fetcher Pool → Parser
Parser → Storage (Database)
Parser → Message Queue
Message Queue → Scheduler
Failure Scenario
Component Fails:Database
Impact:New page data cannot be stored; crawl continues but data loss occurs
Mitigation:Use database replication and failover; cache recent pages to reduce data loss
Architecture Quiz - 3 Questions
Test your understanding
Which component ensures URLs are not crawled multiple times?
AAPI Gateway
BFetcher Pool
CURL Frontier (Queue)
DLoad Balancer
Design Principle
This design uses a modular pipeline with queues and caches to handle large-scale crawling efficiently. It separates concerns: scheduling, fetching, parsing, and storage, enabling scalability and fault tolerance.