Neo4j Graph Database

Neo4j is a free and open-source graph database management system that stores data as nodes and relationships (edges) rather than in tables or documents, enabling efficient querying of connected data. Developed by Neo4j Inc. (founded in 2007 by Emil Eifrem, Johan Svensson, and Peter Neubauer in Sweden, now headquartered in San Mateo, California), Neo4j is the most widely deployed graph database in the world, used by over 700 commercial customers including Cisco, eBay, LinkedIn, and NASA. Key features: property graph model: data is stored as nodes (entities), relationships (connections between nodes, always directed), and properties (key-value pairs on both nodes and relationships). This model naturally represents social networks, recommendation engines, fraud detection graphs, and knowledge graphs. Cypher Query Language: Neo4j's declarative graph query language, designed to be intuitive and human-readable using ASCII-art-like patterns to express graph traversals. For example, MATCH (a:Person)-[:KNOWS]->(b:Person) RETURN a, b. ACID transactions: full ACID compliance with deadlock detection and retry. Index-free adjacency: relationships are stored as direct physical pointers, making graph traversals O(1) per hop regardless of graph size. This is the key architectural advantage over relational databases for connected-data queries. Indexing: B-tree, Lucene full-text, and lookup indexes. Clustering: Neo4j Enterprise supports causal clustering and Raft-based consensus for high availability. Graph data science library: 65+ graph algorithms (PageRank, Louvain, Dijkstra, A*). REST API and Bolt binary protocol. Java-based. Cross-platform. GPLv3 (Community) / commercial (Enterprise).