using-graph-databases

# Graph Databases

## Purpose

This skill guides selection and implementation of graph databases for applications where relationships between entities are first-class citizens. Unlike relational databases that model relationships through foreign keys and joins, graph databases natively represent connections as properties, enabling efficient traversal-heavy queries.

## When to Use This Skill

Use graph databases when:
- **Deep relationship traversals** (4+ hops): "Friends of friends of friends"
- **Variable/evolving relationships**: Schema changes don't break existing queries
- **Path finding**: Shortest route, network analysis, dependency chains
- **Pattern matching**: Fraud detection, recommendation engines, access control

**Do NOT use graph databases when**:
- Fixed schema with shallow joins (2-3 tables) → Use PostgreSQL
- Primarily aggregations/analytics → Use columnar databases
- Key-value lookups only → Use Redis/DynamoDB

## Quick Decision Framework

```
DATA CHARACTERISTICS?
├── Fixed schema, shallow joins (≤3 hops)
│   └─ PostgreSQL (relational)
│
├── Already on PostgreSQL + simple graphs
│   └─ Apache AGE (PostgreSQL extension)
│
├── Deep traversals (4+ hops) + general purpose
│   └─ Neo4j (battle-tested, largest ecosystem)
│
├── Multi-model (documents + graph)
│   └─ ArangoDB
│
├── AWS-native, serverless
│   └─ Amazon Neptune
│
└── Real-time streaming, in-memory
    └─ Memgraph
```

## Core Concepts

### Property Graph Model

Graph databases store data as:
- **Nodes** (vertices): Entities with labels and properties
- **Relationships** (edges): Typed connections with properties
- **Properties**: Key-value pairs on nodes and relationships

```
(Person {name: "Alice", age: 28})-[:FRIEND {since: "2020-01-15"}]->(Person {name: "Bob"})
```

### Query Languages

| Language | Databases | Readability | Best For |
|----------|-----------|-------------|----------|
| **Cypher** | Neo4j, Memgraph, AGE | ⭐⭐⭐⭐⭐ SQL-like | General purpose |
| **Gremlin** | Neptune, JanusGraph | ⭐⭐⭐ Functional | Cross-database |
| **AQL** | ArangoDB | ⭐⭐⭐⭐ SQL-like | Multi-model |
| **SPARQL** | Neptune, RDF stores | ⭐⭐⭐ W3C standard | Semantic web |

## Common Cypher Patterns

Reference `references/cypher-patterns.md` for comprehensive examples.

### Pattern 1: Basic Matching
```cypher
// Find all users at a company
MATCH (u:User)-[:WORKS_AT]->(c:Company {name: 'Acme Corp'})
RETURN u.name, u.title
```

### Pattern 2: Variable-Length Paths
```cypher
// Find friends up to 3 degrees away
MATCH (u:User {name: 'Alice'})-[:FRIEND*1..3]->(friend)
WHERE u <> friend
RETURN DISTINCT friend.name
LIMIT 100
```

### Pattern 3: Shortest Path
```cypher
// Find shortest connection between two users
MATCH path = shortestPath(
  (a:User {name: 'Alice'})-[*]-(b:User {name: 'Bob'})
)
RETURN path, length(path) AS distance
```

### Pattern 4: Recommendations
```cypher
// Collaborative filtering: Products liked by similar users
MATCH (u:User {id: $userId})-[:PURCHASED]->(p:Product)<-[:PURCHASED]-(similar)
MATCH (similar)-[:PURCHASED]->(rec:Product)
WHERE NOT exists((u)-[:PURCHASED]->(rec))
RETURN rec.name, count(*) AS score
ORDER BY score DESC
LIMIT 10
```

### Pattern 5: Fraud Detection
```cypher
// Detect circular money flows
MATCH path = (a:Account)-[:SENT*3..6]->(a)
WHERE all(r IN relationships(path) WHERE r.amount > 1000)
RETURN path, [r IN relationships(path) | r.amount] AS amounts
```

## Database Selection Guide

### Neo4j (Primary Recommendation)

**Use for**: General-purpose graph applications

**Strengths**:
- Most mature (2007), largest community (2M+ developers)
- 65+ graph algorithms (GDS library): PageRank, Louvain, Dijkstra
- Best tooling: Neo4j Browser, Bloom visualization
- Comprehensive Cypher support

**Installation**:
```bash
# Python driver
pip install neo4j

# TypeScript driver
npm install neo4j-driver

# Rust driver
cargo add neo4rs
```

Reference: `references/neo4j.md`

### ArangoDB

**Use for**: Multi-model applications (documents + graph)

**Strengths**:
- Store documents AND graph in one database
- AQL combines document and graph queries
- Schema flexibility with relationships

Reference: `references/arangodb.md`

### Apache AGE

**Use for**: Adding graph capabilities to existing PostgreSQL

**Strengths**:
- Extend PostgreSQL with graph queries
- No new infrastructure needed
- Query both relational and graph data

Reference: Implementation details in examples/

### Amazon Neptune

**Use for**: AWS-native, serverless deployments

**Strengths**:
- Fully managed, auto-scaling
- Supports Gremlin AND SPARQL
- AWS ecosystem integration

## Graph Data Modeling Patterns

Reference `references/graph-modeling.md` for comprehensive patterns.

### Best Practice 1: Relationships as First-Class Citizens

**Anti-pattern** (storing relationships in node properties):
```cypher
// BAD
(:Person {name: 'Alice', friend_ids: ['b123', 'c456']})
```

**Pattern** (explicit relationships):
```cypher
// GOOD
(:Person {name: 'Alice'})-[:FRIEND]->(:Person {id: 'b123'})
(:Person {name: 'Alice'})-[:FRIEND]->(:Person {id: 'c456'})
```

### Best Practice 2: Relationship Properties for Metadata

```cypher
// Track interaction details on relationships
(:Person)-[:FRIEND {
  since: '2020-01-15',
  strength: 0.85,
  last_interaction: datetime()
}]->(:Person)
```

### Best Practice 3: Bounded Traversals for Performance

```cypher
// SLOW: Unbounded traversal
MATCH (a)-[:FRIEND*]->(distant)
RETURN distant

// FAST: Bounded depth with index
MATCH (a)-[:FRIEND*1..4]->(distant)
WHERE distant.active = true
RETURN distant
LIMIT 100
```

### Best Practice 4: Avoid Supernodes

**Problem**: Nodes with thousands of relationships slow traversals.

**Solution**: Intermediate aggregation nodes
```cypher
// Instead of: (:User)-[:POSTED]->(:Post) [1M relationships]

// Use time partitioning:
(:User)-[:POSTED_IN]->(:Year {year: 2025})
       -[:HAS_MONTH]->(:Month {month: 12})
       -[:HAS_POST]->(:Post)
```

## Use Case Examples

### Social Network

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