memgraph-database-configuration

Database Configuration in Memgraph

Triggers

Syntax

CREATE TRIGGER name
  [SECURITY DEFINER|INVOKER]
  [ON [() | -->] CREATE|UPDATE|DELETE]
  [BEFORE|AFTER COMMIT]
  EXECUTE <cypher statements>

Default security: DEFINER. Triggers are persisted to disk.

Predefined variables by event

Event	Available variables
ON () CREATE	createdVertices
ON --> CREATE	createdEdges
ON CREATE	createdVertices, createdEdges, createdObjects
ON () UPDATE	setVertexProperties, removedVertexProperties, setVertexLabels, removedVertexLabels, updatedVertices
ON --> UPDATE	setEdgeProperties, removedEdgeProperties, updatedEdges
ON UPDATE	All update vars + updatedObjects
ON () DELETE	deletedVertices
ON --> DELETE	deletedEdges
ON DELETE	deletedVertices, deletedEdges, deletedObjects
No event	All variables available

Examples

Auto-set creation timestamp:

CREATE TRIGGER setCreatedAt
ON () CREATE AFTER COMMIT EXECUTE
UNWIND createdVertices AS v SET v.created_at = timestamp();

Auto-set updated_at:

CREATE TRIGGER setUpdatedAt
ON UPDATE AFTER COMMIT EXECUTE
UNWIND updatedObjects AS obj
WITH CASE
  WHEN obj.vertex IS NOT null THEN obj.vertex
  WHEN obj.edge IS NOT null THEN obj.edge
END AS entity
SET entity.updated_at = timestamp();

Dynamic PageRank via trigger:

CREATE TRIGGER pagerankTrigger
BEFORE COMMIT EXECUTE
CALL pagerank_online.update(createdVertices, createdEdges, deletedVertices, deletedEdges)
YIELD node, rank SET node.rank = rank;

Manage

DROP TRIGGER name;
SHOW TRIGGERS;

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Transactions

Explicit transactions

BEGIN;
MATCH (n:Account {id: 1}) SET n.balance = n.balance - 100;
MATCH (n:Account {id: 2}) SET n.balance = n.balance + 100;
COMMIT;

ROLLBACK; to discard. If any query fails, the transaction cannot be committed.

Show and terminate

SHOW TRANSACTIONS;
SHOW RUNNING TRANSACTIONS;
TERMINATE TRANSACTIONS "tid1", "tid2";

Requires TRANSACTION_MANAGEMENT privilege to see/terminate other users' transactions.

Isolation levels

SET GLOBAL TRANSACTION ISOLATION LEVEL SNAPSHOT ISOLATION;
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET NEXT TRANSACTION ISOLATION LEVEL READ UNCOMMITTED;
SHOW STORAGE INFO;

Level	Default	Protects against
SNAPSHOT ISOLATION	Yes (IN_MEMORY_TRANSACTIONAL)	Dirty read, non-repeatable read, phantom
READ COMMITTED		Dirty read
READ UNCOMMITTED		Nothing (read-only access)

IN_MEMORY_ANALYTICAL has no isolation levels. ON_DISK_TRANSACTIONAL supports only SNAPSHOT ISOLATION.

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Data types

Property types

Type	Notes
Null	Same as property absent
String
Boolean
Integer
Float	Resolution via --storage-floating-point-resolution-bits
List	Homogeneous for storage as node property
Map	Must replace entirely
Duration	duration("P2DT3H")
Date	date("2024-01-15")
LocalTime	localTime("14:30:00")
LocalDateTime	localDateTime("2024-01-15T14:30:00")
ZonedDateTime	datetime("2024-01-15T14:30:00Z")
Enum	Must be defined first (see memgraph-indexes-and-constraints)
Point	2D/3D, Cartesian or WGS-84

Lists and Maps cannot be mutated element-by-element - replace the whole value.

Temporal arithmetic

Duration +/- Duration = Duration. Date +/- Duration = Date. Date - Date = Duration. Same for LocalTime, LocalDateTime, ZonedDateTime.

Point types

Type	SRID	Constructor
WGS-84 2D	4326	point({longitude: -73.93, latitude: 40.73})
WGS-84 3D	4979	point({longitude: -73.93, latitude: 40.73, height: 10})
Cartesian 2D	7203	point({x: 0, y: 1})
Cartesian 3D	9157	point({x: 0, y: 1, z: 2})

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Storage modes

STORAGE MODE IN_MEMORY_TRANSACTIONAL;
STORAGE MODE IN_MEMORY_ANALYTICAL;
STORAGE MODE ON_DISK_TRANSACTIONAL;
SHOW STORAGE INFO;

Mode	ACID	WAL	Periodic snapshots	Use case
IN_MEMORY_TRANSACTIONAL	Full	Yes	Yes	Default - concurrent reads/writes
IN_MEMORY_ANALYTICAL	No	No	Manual only	Bulk import, analytics (up to 6x faster)
ON_DISK_TRANSACTIONAL	Snapshot isolation	RocksDB	-	Experimental, larger-than-memory

Cannot switch in-memory to on-disk with data present. Cannot switch with active transactions.

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Data durability

Snapshots

CREATE SNAPSHOT;
SHOW SNAPSHOTS;

Periodic interval (runtime):

SET DATABASE SETTING "storage.snapshot.interval" TO "1200";
SET DATABASE SETTING "storage.snapshot.interval" TO "* * 12 * * *";

Flags: --storage-snapshot-interval, --storage-snapshot-on-exit, --storage-parallel-snapshot-creation.

WAL

Enabled by default (--storage-wal-enabled). Cannot use WAL without snapshots. Older WAL files are deleted after each snapshot.

Data directory management

LOCK DATA DIRECTORY;
UNLOCK DATA DIRECTORY;
DATA DIRECTORY LOCK STATUS;

Memory management

MATCH (n) RETURN n QUERY MEMORY LIMIT 50 MB;
CALL proc() PROCEDURE MEMORY LIMIT 100 MB YIELD *;
FREE MEMORY;
SHOW STORAGE INFO;

Instance limit: --memory-limit (MiB). Property compression: --storage-property-store-compression-enabled.

Memory estimation

RAM ≈ nodes × 204B + edges × 154B + properties + indexes

For 50+ indexes add ~20% overhead.

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Key rules

1. Snapshots and WAL are not compatible across Memgraph versions
2. Cannot use WAL without snapshots enabled
3. Cannot switch in-memory to on-disk storage with data present
4. IN_MEMORY_ANALYTICAL has no isolation levels or ACID guarantees
5. Descending indexes and automatic index creation only work in IN_MEMORY_TRANSACTIONAL (see memgraph-indexes-and-constraints)