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I just learned about graph databases as opposed to relational databases (RDBMS). I went through some resources on the neo4j website and read some chapters in the Oreilly Book on Graph Databases. However i cant get my head around how a graph database actually stores its data?
If i had to store a graph in a RDBMS i would create diffrent lists for vertices and nodes. How is the Graph Database diffrent? I really struggle to picture in my head how e.g. neo4j stores and links its data (the nodes and vertices) diffrently to conventional RDBMS.
If anyone could help me understand and visiulize how a graph database internally works, id be really thankful. If you dont quite understand my question, im happy to explain it more specifically.
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Most graph database systems store data in a structure similar to linked lists. They store direct links to data which is connected, rather than similar objects.
Data stored on disk is all linked lists of fixed size records. Properties are stored as a linked list of property records, each holding a key and value and pointing to the next property. Each node and relationship references its first property record.
The main difference between these two types of databases is in the way relationships between entities are stored. In a graph database, relationships are stored at the individual record level, while a relational database uses predefined structures, a.k.a. table definitions.
Graphs and graph databases provide graph models to represent relationships. They allow users to apply pattern recognition, classification, statistical analysis, and machine learning to these models, which enables more efficient analysis at scale against massive amounts of data.
A graph database is defined as a specialized, single-purpose platform for creating and manipulating graphs. Graphs contain nodes, edges, and properties, all of which are used to represent and store data in a way that relational databases are not equipped to do.
Your answer is the chapter 6 of the O’Reilly’s Graph Databases book about Graph Database Internals. This chapter describes how Neo4j works internally, including how the Native Graph Storage works.
Neo4j stores nodes, relationships, labels and properties in separated files.
Nodes are stored in the file neostore.nodestore.db. This file has a fixed-size by each new created node. For each node added to the database this file is increased by 9 bytes. This way a node with id 100 can be easily found in the 900 byte into the file (id 100 x 9 bytes per node = 900 byte). Node records have pointers to the first node relationship, first node property and for the node labels.
Relationships are stored in the file neotore.relationshipstore.db. This is a fixed-size file too. Each relationship has pointers to the start and end nodes, relationship type (in the neostore.relationshiptypestore.db file), next and previous relationship records for each of the start and end nodes, and a flag indicating if the relationship is the first in the relationship chain.
Properties of nodes and relationships are stored in the file neostore.propertystore.db. Each property record has a pointer to the next property and can holds a maximum of 4 properties. Each property has a pointer to the property name (neostore.propertystore.db.index file), the property type. The property value can be an inline value or a pointer to a dynamic file for large strings (neostore.propertystore.db.strings) and arrays (neostore.propertystore.db.arrays file).
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