Barrier

Barriers listen for multiple events from different datasets and fires a downstream event when arrival conditions are met.

In order to join one or more datasets, data partitioned on equivalent keys must be available, the join keys.

A join barrier waits until data arrives for a given partition in each declared dataset, when all the sets have satisfied the join condition, an event is published describing the participants in the join.

It is up to the processing node to complete the join.

A join barrier is described by a join expression, every time the expression is satisfied by an arriving event, a new event is published.