Open Source Graph Database
Relationships are cheap to store and brutally expensive to query once they are flattened into rows and reassembled with deep joins - and the moment your real question is how things connect rather than what they are, that mismatch is the whole problem. The open source graph databases here treat edges as first-class data with native traversal, so the way you walk connections is open to inspect and tune rather than buried inside a proprietary engine you cannot see into.
Dgraph
Horizontally scalable distributed graph database with GraphQL, ACID transactions, and linearizable reads
Cognee
Open-source AI memory platform for agents with self-hosted knowledge graph memory across sessions
Neo4j
Graph database with ACID transactions, Cypher queries, and managed or self-managed deployment
NebulaGraph
Distributed graph database with horizontal scalability, high availability, and OpenCypher-compatible queries
JanusGraph
Distributed graph database for large graphs, with transactional queries across multi-machine clusters
Apache AGE
PostgreSQL extension that adds graph database support with openCypher and SQL
Cartography
Python tool that maps infrastructure assets and relationships into a Neo4j graph database
Apache HugeGraph
Graph database for large-scale vertex and edge storage with OLTP, Gremlin, and Cypher access
ArcadeDB
Open-source multi-model database with SQL, graph, document, key-value, vector, and time-series support