Storage

Durability and data recovery

Memgraph uses two mechanisms to ensure the durability of the stored data:

• write-ahead logging (WAL)
• periodic snapshots

In write-ahead logging, all database modifications are recorded in a log file before being applied to the database. WAL ensures that all operations are done atomically and provides steps needed to reconstruct the database state.

Snapshots are taken periodically during the entire runtime of Memgraph. When a snapshot is triggered, the whole data storage is written to disk. The snapshot file provides a quicker way to restore the database state.

Database recovery is done on startup from the most recent snapshot file. Since the snapshot may be older than the most recent update logged in the WAL file, the recovery process will apply the remaining state changes found in the WAL file.

Behavior of these mechanisms can be tweaked in the configuration file, usually found in /etc/memgraph/memgraph.conf.

Check the reference guide on Configuration to see the possible configuration settings for storage.

caution

Snapshot and WAL files are (currently) not compatible between Memgraph versions.

Storable data types

Since Memgraph is a graph database management system, data is stored in the form of graph elements: nodes and relationships. Each graph element can contain various types of data. This chapter describes which data types are supported in Memgraph.

Node labels & relationship types

Nodes can have labels that are used to label or group nodes. A label is a text value, and each node can have any number of labels, even none. Labels can be changed at any time.

Relationships have a type, also represented as text. Unlike nodes, relationships must have exactly one relationship type and once it is set upon creation, it can never be modified again.

Properties

Nodes and relationships can store various properties. Properties are similar to mappings or tables containing property names and their accompanying values. Property names are represented as text, while values can be of different types. Each property can store a single value, and it is not possible to have multiple properties with the same name on a single graph element. But, the same property names can be found across multiple graph elements. Also, there are no restrictions on the number of properties that can be stored in a single graph element. The only restriction is that the values must be of the supported types. Below is a table of supported data types.

Type	Description
Null	Property has no value, which is the same as if the property does not exist.
String	A character string (text).
Boolean	A boolean value, either true or false.
Integer	An integer number.
Float	A floating-point number (real number).
List	A list containing any number of property values of any supported type under a single property name.
Map	A mapping of string keys to values of any supported type.
Duration	A period of time.
Date	A date with year, month, and day.
LocalTime	Time within a day without timezone.
LocalDateTime	Date and local time.

Check the reference guide to read more about temporal types Duration, Date, LocalTime and LocalDateTime.

note

Even though it's possible to store List and Map property values, it is impossible to modify them. It is, however, possible to replace them entirely. So, the following queries are valid:

CREATE (:Node {property: [1, 2, 3]});
CREATE (:Node {property: {key: "value"}});

But these are not:

MATCH (n:Node) SET n.property[0] = 0;
MATCH (n:Node) SET n.property.key = "other value";

Disabling properties on relationships

If you have a use-case that doesn't use properties on relationships, you can specify a flag in the Memgraph configuration file to disable them and reduce memory usage.

--storage-properties-on-edges=false

You can disable properties on relationships with a non-empty database, just make sure the relationships are without properties.

Storage statistics

You can retrieve information and statistics about the storage of a running Memgraph instance by using the following query.

SHOW STORAGE INFO;

Example results:

storage info	value
average_degree	2.872567
disk_usage	38028
edge_count	90674
memory_usage	88842240
vertex_count	63131

All of the *_usage results are expressed in bytes unless explicitly stated otherwise.