Neo4j databases and graphs

This section describes databases and graphs in Neo4j.

Cypher queries are executed against a Neo4j database, but normally apply to specific graphs. It is important to understand the meaning of these terms and exactly when a graph is not a database.

DBMS

A Neo4j Database Management System is capable of containing and managing multiple graphs contained in databases. Client applications will connect to the DBMS and open sessions against it. A client session provides access to any graph in the DBMS.

Graph

This is a data model within a database. Normally there is only one graph within each database, and many administrative commands that refer to a specific graph do so using the database name.

Cypher queries executed in a session may declare which graph they apply to, or use a default, given by the session.

Composite databases can contain multiple graphs, by means of aliases to other databases. Queries submitted to composite databases may refer to multiple graphs within the same query.

For more information, see Operations manual → Composite databases.

Database

A database is a storage and retrieval mechanism for collecting data in a defined space on disk and in memory.

Most of the time Cypher queries are reading or updating queries, which are run against a graph. There are also administrative commands that apply to a database, or to the entire DBMS. Administrative commands cannot be run in a session connected to a normal user database, but instead need to be run within a session connected to the system database. Administrative commands execute on the system database. If an administrative command is submitted to a user database, it is rerouted to the system database.

The system database and the default database

All Neo4j servers contain a built-in database called system, which behaves differently than all other databases. The system database stores system data and you can not perform graph queries against it.

A fresh installation of Neo4j includes two databases:

  • system - the system database described above, containing meta-data on the DBMS and security configuration.

  • neo4j - the default database, named using the config option dbms.default_database=neo4j.

For more information about the system database, see the sections on Database management and Access control.

Different editions of Neo4j

Neo4j has two editions, a commercial Enterprise Edition with additional performance and administrative features, and an open-source Community Edition. Cypher works almost identically between the two editions, and as such most of this manual will not differentiate between them. In the few cases where there is a difference in Cypher language support or behaviour between editions, these are highlighted as described below in Limited Support Features.

However, it is worth listing up-front the key areas that are not supported in the open-source edition:

Feature Enterprise Community

Multi-database

Any number of user databases.

Only system and one user database.

Role-based security

User, role, and privilege management for flexible access control and sub-graph access control.

Multi-user management. All users have full access rights.

Constraints

All constraints: node existence constraints, relationship existence constraints, node property uniqueness constraints, and node key constraints.

Only node property uniqueness constraints.

Limited Support Features

Some elements of Cypher do not work in all deployments of Neo4j.

Specific labels are added to the documentation to highlight these cases.

Description Label

This feature has been deprecated and will be removed or replaced in the future.

Deprecated

This feature only works in the enterprise edition of Neo4j.

Enterprise Edition