apoc.trigger.install

Procedure

apoc.trigger.install(databaseName STRING, name STRING, statement STRING, selector MAP<STRING, ANY>, config MAP<STRING, ANY>) - eventually adds a trigger for a given database which is invoked when a successful transaction occurs.

Signature

apoc.trigger.install(databaseName :: STRING, name :: STRING, statement :: STRING, selector :: MAP, config = {} :: MAP) :: (name :: STRING, query :: STRING, selector :: MAP, params :: MAP, installed :: BOOLEAN, paused :: BOOLEAN)

Input parameters

Name Type Default

databaseName

STRING

null

name

STRING

null

statement

STRING

null

selector

MAP

null

config

MAP

{}

Output parameters

Name Type

name

STRING

query

STRING

selector

MAP

params

MAP

installed

BOOLEAN

paused

BOOLEAN

Enable Triggers

By default triggers are disabled. We can enable them by setting the following property in apoc.conf:

apoc.conf
apoc.trigger.enabled=true
apoc.trigger.refresh=60000
Table 1. Description
Option Key Value Description

apoc.trigger.enabled

true/false, default false

Enable/Disable the feature

apoc.trigger.refresh

number, default 60000

Interval in ms after which a replication check is triggered across all cluster nodes

Usage Examples

The apoc.trigger.* procedures are intended to be executed in the system database, therefore they have to be executed by opening a system database session.

There are several ways of doing this:

  • When using Cypher-shell or Neo4j Browser, prefix Cypher queries with :use system

  • When using Fabric, prefix Cypher queries with USE system

  • When using the drivers, open a session directly against the system database

Moreover, the apoc.trigger procedures accept as first parameter the name of the database in which the triggers should be installed, updated, or removed.

Installing, updating, or removing a trigger is an eventually consistent operation. Therefore, they are not immediately added/updated/removed, but have a refresh rate handled by the APOC configuration apoc.trigger.refresh=<MILLISECONDS>, with default 60000 (milliseconds).

The examples in this section are based on the following graph, created in 'neo4j' db:

CREATE (:Counter {count:0})
CREATE (f:Foo);

To create a trigger that keeps a count of the number of nodes that have been deleted, run the following query:

CALL apoc.trigger.install(
  'neo4j',
  'count-removals',
  'MATCH (c:Counter)
   SET c.count = c.count + size([f IN $deletedNodes WHERE id(f) > 0])',
  {}
);
Table 2. Results
name query selector params installed paused

"count-removals"

MATCH (c:Counter) SET c.count = c.count + size([f IN $deletedNodes WHERE id(f)  0])

{}

{}

TRUE

FALSE

After a time defined by the configuration apoc.trigger.refresh, the Foo node will be deleted:

MATCH (f:Foo)
DELETE f;
Results
0 rows available after 20 ms, consumed after another 0 ms
Deleted 1 nodes

And finally, let’s check that the count property on our Counter node has been incremented:

MATCH (c:Counter)
RETURN c.count as count;
Table 3. Results
count

1

More documentation of apoc.trigger.install