Graph databases are emerging as the leading data management technology for
storing large knowledge graphs; significant efforts are ongoing to produce new
standards (such as the Graph Query Language, GQL), as well as enrich them with
properties, types, schemas, and keys. In this article, we propose PG-Triggers,
a complete proposal for adding triggers to Property Graphs, along the direction
marked by the SQL3 Standard. We define the syntax and semantics of PG-Triggers
and then illustrate how they can be implemented on top of Neo4j, one of the
most popular graph databases. In particular, we introduce a syntax-directed
translation from PG-Triggers into Neo4j, which makes use of the so-called APOC
triggers; APOC is a community-contributed library for augmenting the Cypher
query language supported by Neo4j. We also illustrate the use of PG-Triggers
through a life science application inspired by the COVID-19 pandemic. The main
result of this article is proposing reactive aspects within graph databases as
first-class citizens, so as to turn them into an ideal infrastructure for
supporting reactive knowledge management.Comment: 12 pages, 4 figures, 3 table