How to set up triggers

Memgraph supports database triggers that can be executed if a particular type of event occurs. Events are related to changes in data, such as created, updated, and deleted nodes or relationships.

How to create a trigger?

You can create a trigger by executing Cypher clauses. Creating a trigger will ensure that some procedural code is executed on a certain type of event. All created triggers are persisted on the disk, which means they will be active on database reruns and no information is ever lost.

Trigger execution upon node creation

Node creation is the most common event your Memgraph database can react to. For example, you need to update some values on a created node. If you need a trigger after the node (vertex) has been created, you can set up the following trigger:

CREATE TRIGGER createVertex
ON () CREATE AFTER COMMIT EXECUTE
UNWIND createdVertices AS createdVertex
SET createdVertex.created = timestamp()

Here the trigger's name is createdVertex and should be unique. Cypher clause ON CREATE defines on what event will trigger be executed. AFTER COMMIT EXECUTE means the trigger will be executed after changes have been committed to the database. For the sake of ease of use, triggers have a set of predefined variables. One of them is createdVertices, a list of all created nodes (vertices). In this example, the list is unwound by a Cypher clause UNWIND. To find a complete list of predefined variables, supported operations, and configuration details, look at triggers reference guide.

In this trigger, a node is getting a timestamp upon creation via the SET createdVertex.created = timestamp() Cypher clause.

Trigger execution upon node update

Node property updates are common in graphs, in order to react to them, you can create a trigger for that type of event:

CREATE TRIGGER updateVertex
ON () UPDATE AFTER COMMIT EXECUTE
UNWIND updatedVertices AS updatedVertex
SET updatedVertex.updated_at = timestamp()

The trigger for node updates is almost the same as node creation. Notice different predefined variable updatedVertices and ON UPDATE Cypher clause.

In this trigger, a node is getting a new updated timestamp via SET updatedVertex.updated_at = timestamp() Cypher clause.

Trigger execution upon node or relationship creation

You can also set up a trigger for multiple events, such as node or relationship creation. It doesn't matter what will be created, a node or relationship, but the event will execute the trigger. A sample query for that kind of trigger:

CREATE TRIGGER exampleTrigger
ON CREATE AFTER COMMIT EXECUTE
UNWIND createdObjects AS createdObject
WITH CASE
        WHEN createdObject.vertex IS NOT null THEN createdObject.vertex
        WHEN createdObject.edge IS NOT null THEN createdObject.edge
    END AS object
SET object.created_at = timestamp();

A predefined variable createdObjects is a list of dictionaries. Each dictionary contains information about the created object, which can be either a node or a relationship. The object's key event_type is set based on the dictionary and information within it, and the value of the key vertex or edge (depending on the type of object) is set to that created object.

In this trigger, the node or relationships property created_at is set to the current timestamp value via SET object.created_at = timestamp(); Cypher clause.

How to create a trigger for Python query module?

If you want a trigger to be activated by executing code from a Python query module, you can call the query module from the trigger. In the example below, the trigger will call query_module.new_edge(edge) each time a new relationship (edge) is created:

CREATE TRIGGER newEdge
ON CREATE BEFORE COMMIT EXECUTE
UNWIND createdEdges AS edge
CALL query_module.new_edge(edge) YIELD *;

Make sure your function accepts the proper Memgraph type, mgp.Edge in this case.

def new_edge(
    context: mgp.ProcCtx,
    edge: mgp.Edge
)

Memgraph Python API is defined by mgp.py script, and in it, you can find all supported types such as mgp.Edge, mgp.Vertex etc. If you want to explore the API further, feel free to check the reference guide on Python API.

How to create a trigger for dynamic algorithms?

Dynamic algorithms are often designed for dataset updates. With a trigger, you can ensure that any dataset is up to date and consistent. In the sample code below, a trigger is set to use MAGE pagerank_online algorithm. For more details on dynamic algorithms, visit MAGE docs. In this case, all created or deleted objects are passed from the database transaction to the trigger. After each transaction that has created or deleted objects, the trigger will automatically execute the PageRank algorithm and update the rank property. This will ensure data consistency and lead to performance benefits.

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