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Neo4j Internals: Transactions, Availability, Scale

Part of the Neo4j internals series: Native Graph Processing | Native Graph Storage | Transactions, Availability, Scale

Transactions

Neo4j supports full ACID transactions:

  • Every modification runs inside a transaction.
  • Write-ahead log ensures durability: Changes are logged before being applied to store files.
  • Transactions can span multiple nodes, relationships, and properties.

Recoverability

  • Write-ahead log for crash recovery.
  • On restart, Neo4j replays uncommitted transactions from the log.

Availability

  • Master-slave replication with automatic failover.
  • Slaves serve reads immediately. Writes go to master and propagate.
  • On master failure, a slave is elected as new master.

Scale

  • Reads: Scale horizontally by adding slaves.
  • Writes: Scale vertically (single master).
  • Cache sharding: Route requests to instances with relevant portions of the graph in memory.
  • Graph sharding (splitting the graph across machines): Remains an open research problem. Cutting a graph inevitably severs relationships, degrading traversal performance.