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

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;

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;

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

Data types

Property types

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

Storage modes

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

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

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.

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)