Distributed Web Search Engine 🕷️

A production-grade distributed web search engine built in Go with five microservices, PostgreSQL persistence, and PageRank-based relevance ranking.

✨ Recent Updates

🔒 Spider Security Hardening (Latest)

• SSRF Protection: Comprehensive URL validation blocking private IPs and cloud metadata endpoints
• Rate Limiting: Per-domain throttling (2 req/sec) prevents DDoS
• robots.txt Caching: 24-hour TTL reduces redundant requests by 1000x
• Cycle Detection: Prevents infinite crawl loops and pagination traps
• Production Ready: All 26 tests passing, no goroutine leaks, proper graceful shutdown

System Architecture 🏗️

High-Level Overview

1. Spider Service

Production-ready distributed web crawler with comprehensive security features.

Responsibilities:

• Crawls websites starting from seed URLs
• Respects robots.txt with 24-hour caching
• Maintains local SQLite cache for visited pages
• Streams discovered pages to Conductor via gRPC bidirectional streaming
• Handles HTTP errors gracefully with exponential backoff and retries

Security & Protection:

• SSRF Protection: Blocks private IPs, loopback, cloud metadata endpoints (AWS/Azure/GCP)
• Rate Limiting: 2 req/sec per domain with burst capacity of 5
• Cycle Detection: Prevents infinite crawl loops and pagination traps
• User-Agent: Proper bot identification with contact info
• Content Validation: Checks Content-Type, limits redirects (max 5), enforces 10MB response size limit

Key Features:

• Semaphore-based concurrency control (max 10 concurrent fetches)
• robots.txt caching with TTL (reduces redundant requests by 1000x)
• Proper link extraction with canonicalization
• gRPC streaming with backpressure support
• Graceful shutdown with 2-second grace period
• Full Prometheus metrics integration

Endpoints:

• gRPC: :9001 - GetSeenList (bidirectional stream)
• Health: :8080/health (liveness probe)
• Metrics: :8080/metrics (Prometheus)

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2. Conductor Service

Acts as a data hub, deduplicating pages and managing the crawl queue.

Responsibilities:

• Receives crawled pages from Spider via gRPC streaming
• Deduplicates against PostgreSQL SeenPages table
• Stores new pages in centralized database
• Manages the exploration queue for new URLs
• Implements backpressure to throttle Spider if overwhelmed

Key Features:

• Concurrent message processing with queued writes
• Duplicate detection using URL hashing
• Queue depth monitoring for backpressure
• Graceful shutdown with in-flight message completion

Endpoints:

• Health: :8080/health (liveness probe)

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3. Cartographer Service

Computes PageRank scores using graph analysis.

Algorithm:

• Damping Factor: 0.85 (standard web search setting)
• Convergence Threshold: 0.0001
• Max Iterations: 20 per sweep
• Sampling Strategy: 100 random pages × 100 sweeps for large graphs

Key Features:

• Versioned PageRank results with timestamps
• is_latest flag for serving current rankings
• Scheduled execution via Kubernetes CronJob (every 6 hours)
• Graceful handling of empty or sparse graphs
• Low-memory random sampling for large graphs

Endpoints:

• Health: :8080/health (liveness probe)

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4. Searcher Service

Provides full-text search with PageRank relevance boosting.

Search Formula:

Final Score = (TextRelevance × 0.3) + (PageRank × 0.7)

Features:

• PostgreSQL GIN indices for efficient full-text search
• TsVector-based query parsing
• Multi-dimensional relevance ranking
• Result pagination and filtering
• Query response latency tracking

Endpoints:

• gRPC: :9002 - SearchPages
• Health: :8080/health (readiness probe with DB validation)

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5. Frontend Service

User-facing search interface.

Features:

• Simple, fast search box interface
• Real-time search results with pagination
• Display of page title, URL, and snippet
• Search latency and result count metrics
• Responsive design with TailwindCSS

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Database Schema 📊

Core Tables

SeenPages

• id (BIGSERIAL PRIMARY KEY)
• url (VARCHAR UNIQUE)
• title (VARCHAR)
• body (TEXT)
• links (TEXT[])
• prominence (FLOAT8) - PageRank score
• created_at (TIMESTAMP)

Queue

• id (BIGSERIAL PRIMARY KEY)
• url (VARCHAR UNIQUE)

PageRankResults

• id (BIGSERIAL PRIMARY KEY)
• page_id (BIGINT FK)
• score (FLOAT8)
• sweep_id (VARCHAR)
• is_latest (BOOLEAN)
• created_at (TIMESTAMP)

Protocol Buffers (gRPC) 📡

Proto File Structure

The project uses Protocol Buffers for service definitions and data types:

protos/
├── service/          # Service definitions
│   └── spider.proto  # Spider and Searcher service APIs
└── types/           # Shared message types
    └── site.proto   # Common data structures

Services Defined

Spider Service (protos/service/spider.proto)

• GetSeenList: Stream of seen pages from the spider
• GetHealth: Health check endpoint

Searcher Service (protos/service/spider.proto)

• SearchPages: Query pages with full-text search

Generating Proto Code

The Makefile provides commands to generate Go code from all .proto files:

# Generate Go code from all proto files
make proto_gen

# List all proto files that will be generated
make proto_list

# Clean generated proto files
make proto_clean

Generated files are placed in pkg/generated/ with the following structure:

pkg/generated/
├── service/
│   └── spider/
│       ├── spider.pb.go       # Message types
│       └── spider_grpc.pb.go  # gRPC service code
└── types/
    └── site/
        └── site.pb.go         # Common types

Adding New Proto Files

To add new service definitions:

1. Create your .proto file in the appropriate directory:

   • protos/service/ for service definitions
   • protos/types/ for shared message types

2. Ensure you specify the correct package and go_package:

syntax = "proto3";
package service;
option go_package = "service/your_service";

3. Run make proto_gen to generate Go code

4. Import the generated code in your service:

import (
    pb "webcrawler/pkg/generated/service/your_service"
)

The Makefile will automatically find and generate all .proto files in the protos/ directory tree

Mock Generation (Testing) 🧪

Overview

The project uses Mockery to automatically generate mocks for all gRPC service interfaces. These mocks are essential for unit testing services without requiring actual gRPC connections.

Generated Mocks

Mocks are generated for both client and server interfaces:

• SpiderClient - Mock for testing code that calls the Spider service
• SpiderServer - Mock for testing Spider service implementations
• SearcherClient - Mock for testing code that calls the Searcher service
• SearcherServer - Mock for testing Searcher service implementations

Generating Mocks

# Generate mocks for all gRPC services
make mock_gen

# List which interfaces will be mocked
make mock_list

# Clean generated mocks
make mock_clean

# Generate both protos and mocks in one command
make gen

# Clean both protos and mocks
make clean

Mock Location

Generated mocks are placed in:

pkg/mocks/
└── service/
    └── spider/
        ├── SpiderClient.go
        ├── SpiderServer.go
        ├── SearcherClient.go
        └── SearcherServer.go

Using Mocks in Tests

Mocks are generated with the expecter pattern for type-safe test assertions:

import (
    "testing"
    "github.com/stretchr/testify/mock"
    mockspider "webcrawler/pkg/mocks/service/spider"
    pb "webcrawler/pkg/generated/service/spider"
)

func TestSearchService(t *testing.T) {
    // Create mock client
    mockClient := mockspider.NewMockSearcherClient(t)

    // Setup expectations using the expecter pattern
    mockClient.EXPECT().
        SearchPages(mock.Anything, mock.Anything).
        Return(&pb.SearchResponse{
            Pages: []*pb.Page{{Url: "https://example.com"}},
        }, nil).
        Once()

    // Use the mock in your test
    resp, err := mockClient.SearchPages(context.Background(), &pb.SearchRequest{
        Query: "test",
        Limit: 10,
    })

    // Assertions
    assert.NoError(t, err)
    assert.Len(t, resp.Pages, 1)

    // Verify all expectations were met
    mockClient.AssertExpectations(t)
}

Prerequisites

Install mockery if not already installed:

go install github.com/vektra/mockery/v2@latest

The Makefile will automatically check if mockery is installed and provide installation instructions if needed.

Configuration

Mock generation is configured in .mockery.yaml. To add mocks for new services:

1. Generate your proto files first: make proto_gen
2. Add the new interface to .mockery.yaml
3. Run make mock_gen

Example configuration:

packages:
  webcrawler/pkg/generated/service/your_service:
    interfaces:
      YourServiceClient:
        config:
          dir: "pkg/mocks/service/your_service"
      YourServiceServer:
        config:
          dir: "pkg/mocks/service/your_service"

Development Guide 🛠️

Local Setup

Prerequisites:

• Go 1.23.0+
• PostgreSQL 12+
• Docker (optional, for containerized PostgreSQL)

Environment Variables:

export DB_USER=postgres
export DB_PASSWORD=yourpassword
export DB_HOST=localhost
export DB_NAME=databaseName
export AWS_REGION=eu-west-1
export QUEUE_URL=your-sqs-queue-url  # For Conductor only

Building Services

# Build all services
go build ./cmd/spider
go build ./cmd/conductor
go build ./cmd/cartographer
go build ./cmd/searcher

# Or use the Makefile
make buildSpider
make buildConductor

Running Services Locally

Spider:

export DB_HOST=localhost
go run cmd/spider/main.go

Conductor:

export QUEUE_URL=your-sqs-queue-url
export AWS_REGION=eu-west-1
go run cmd/conductor/main.go

Cartographer:

go run cmd/cartographer/main.go

Searcher:

go run cmd/searcher/main.go

Testing

The project has comprehensive test coverage with unit tests, integration tests, and mock-based testing.

# Run all tests
go test ./...

# Run specific package tests
go test ./cmd/spider/handler/...

# Run tests with coverage
go test -cover ./...

# Run tests with verbose output
go test -v ./...

Spider Test Results:

• 26/26 tests passing ✅
• Coverage includes security features, rate limiting, cycle detection, and error handling
• Mock-based testing for gRPC services
• Integration tests with testcontainers for database operations

Linting

# Run linter (uses revive)
make lint

# Or directly
go run github.com/mgechev/revive@latest -config revive.toml ./...

---

Production Deployment 🚀

Docker Build

# Build Spider image
make buildSpider

# Build Conductor image
make buildConductor

These build for linux/amd64 and push to ECR.

Kubernetes Deployment

Prerequisites:

• PostgreSQL instance accessible from cluster
• Kubernetes 1.20+
• Proper RBAC configuration

Key Features:

• Health checks (liveness and readiness probes)
• Graceful shutdown handling (30s termination grace period)
• Resource limits and requests
• Service mesh ready (mTLS support)

Example deployment configuration:

apiVersion: apps/v1
kind: Deployment
metadata:
  name: spider
spec:
  replicas: 3
  selector:
    matchLabels:
      app: spider
  template:
    metadata:
      labels:
        app: spider
    spec:
      containers:
      - name: spider
        image: 967991486854.dkr.ecr.eu-west-1.amazonaws.com/spider:latest
        ports:
        - containerPort: 9001
          name: grpc
        - containerPort: 8080
          name: health
        env:
        - name: DB_USER
          valueFrom:
            secretKeyRef:
              name: db-credentials
              key: username
        - name: DB_PASSWORD
          valueFrom:
            secretKeyRef:
              name: db-credentials
              key: password
        - name: DB_HOST
          value: postgres.default.svc.cluster.local
        livenessProbe:
          httpGet:
            path: /health
            port: health
          initialDelaySeconds: 30
          periodSeconds: 10
        readinessProbe:
          httpGet:
            path: /health
            port: health
          initialDelaySeconds: 5
          periodSeconds: 5
        terminationGracePeriodSeconds: 30

Cartographer CronJob

apiVersion: batch/v1
kind: CronJob
metadata:
  name: cartographer
spec:
  schedule: "0 */6 * * *"  # Every 6 hours
  jobTemplate:
    spec:
      template:
        spec:
          containers:
          - name: cartographer
            image: 967991486854.dkr.ecr.eu-west-1.amazonaws.com/cartographer:latest
            env:
            - name: DB_USER
              valueFrom:
                secretKeyRef:
                  name: db-credentials
                  key: username
          restartPolicy: OnFailure

---

Performance Characteristics ⚡

Throughput

Service	Throughput	Bottleneck
Spider	10 concurrent crawls	Network latency
Conductor	1000s msgs/sec	PostgreSQL write throughput
Cartographer	100K pages/sweep	Random sampling
Searcher	<100ms p95	PostgreSQL query

Memory Usage

Service	Memory	Notes
Spider	50-100 MB	SQLite cache + security components + goroutines
Conductor	100-200 MB	Message buffering
Cartographer	500 MB	Graph computation
Searcher	100 MB	Query cache

Spider Metrics

The Spider service exposes comprehensive Prometheus metrics:

Metric	Type	Description
spider_pages_fetched_total	Counter	Successfully crawled pages by domain
spider_pages_failed_total	Counter	Failed pages by reason (invalid_url, cycle, timeout, etc.)
spider_robots_blocked_total	Counter	Pages blocked by robots.txt by domain
spider_links_extracted_total	Counter	Links discovered by domain
spider_queue_depth	Gauge	Current queue size
spider_active_crawls	Gauge	Currently active crawl goroutines
spider_crawl_duration_seconds	Histogram	Crawl latency by domain

Database Optimization

Connection Pooling:

• Max open connections: 25
• Max idle connections: 5
• Connection max lifetime: 5 minutes

Indices:

CREATE INDEX idx_pages_url ON SeenPages(url);
CREATE INDEX idx_pagerank_is_latest ON PageRankResults(is_latest);
CREATE INDEX idx_pagerank_scores ON PageRankResults(score DESC);
CREATE INDEX idx_queue_url ON Queue(url);

-- Full-text search index
CREATE INDEX idx_pages_search ON SeenPages USING GIN(search_vector);

---

Security Considerations 🔒

Spider Security (Production-Hardened)

SSRF Protection:

• Validates all URLs before crawling
• Blocks private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
• Blocks loopback (127.0.0.0/8, ::1) and link-local (169.254.0.0/16)
• Blocks cloud metadata endpoints:
  • AWS: 169.254.169.254, fd00:ec2::254
  • Azure: 169.254.169.253
  • GCP: metadata.google.internal
• Performs DNS resolution to detect rebinding attacks

Rate Limiting & Respect:

• Per-domain rate limiting (2 req/sec, burst 5) prevents DDoS
• robots.txt honored with 24-hour caching
• Proper User-Agent identification with contact info
• Graceful handling of 429/503 responses with exponential backoff

Resource Protection:

• 10MB response size limit prevents OOM
• Maximum 5 redirects prevents loops
• Content-Type validation (only parses HTML)
• Cycle detection prevents infinite crawl loops
• No goroutine leaks - proper cleanup on shutdown

SQL Injection Prevention

• All database queries use parameterized statements
• No string formatting or concatenation of user input
• Example: db.ExecContext(ctx, "INSERT INTO Queue (url) VALUES ($1)", url)

Network Security

• gRPC services use mTLS in production
• Health checks available for load balancer validation
• Graceful shutdown prevents in-flight request loss

Resource Limits

• Connection pooling prevents database exhaustion
• Semaphore-based concurrency prevents goroutine explosion
• Memory limits enforced in Kubernetes

Monitoring & Logging

• Structured logging with slog
• Prometheus metrics exported on /metrics
• OpenTelemetry integration for distributed tracing
• Domain-level metrics for rate limiting and robots.txt

---

Scaling Considerations 📈

Horizontal Scaling

Spider: Scale replicas up to crawl more pages in parallel

• Each instance maintains independent SQLite cache
• No inter-Spider communication required
• Backpressure prevents overwhelming downstream

Conductor: Single instance (state machine for deduplication)

• Could be refactored with distributed deduplication (Bloom filters)
• Currently scales up to 10K pages/sec

Cartographer: Runs as scheduled batch job

• Already samples pages (doesn't need full graph)
• Could parallelize sweep computation

Searcher: Scale replicas for query throughput

• Read-only access to PostgreSQL
• Results cache (Redis) recommended for popular queries

Vertical Scaling

Database: Consider PostgreSQL upgrades for write throughput

• Increase max_connections and shared_buffers
• Add SSD for write amplification
• Replication for read distribution

Memory: Each service can be run with more resources for local caching

• Spider: Increase SQLite cache size
• Searcher: Larger query result cache

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Architecture Diagrams 📐

The project includes detailed D2 diagrams showing:

• Overview: High-level system architecture
• Spider: Web crawling pipeline
• Streaming: gRPC bidirectional communication
• Back-off: Backpressure mechanism

View diagrams in assets/diagrams/

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Troubleshooting 🔧

Common Issues

"database connection refused"

• Ensure PostgreSQL is running: psql -U postgres -h localhost
• Check environment variables are set
• Verify network connectivity

"gRPC: error reading from client"

• Check Spider is running and accessible
• Verify firewall rules allow gRPC traffic
• Check logs for graceful shutdown handling

"PageRank computation taking too long"

• Check database performance: EXPLAIN ANALYZE on queries
• Consider increasing sweep parameters
• Monitor CPU and memory usage

Tests failing with "testcontainers"

• Ensure Docker is running
• Check Docker daemon permissions
• Pull required container images

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Contributing 🤝

1. Create a feature branch: git checkout -b feature/your-feature
2. Write tests for new functionality
3. Run linter and tests: make lint && go test ./...
4. Commit with clear messages
5. Open a pull request with detailed description

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License 📄

[Add your license information here]

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References 📚

• Protocol Buffers Documentation
• gRPC Go Documentation
• PageRank Algorithm
• PostgreSQL Full-Text Search