Chapter 1. High available event-driven decisioning on Red Hat OpenShift Container Platform

You can use the decision engine to implement high available event-driven decisioning on Red Hat OpenShift Container Platform.

An event models a fact that happened in a specific point in time. The decision engine offers a rich set of temporal operators to compare, correlate, and accumulate events. In event-driven decisioning, the decision engine processes complex series of decisions based on events. Every event can alter the state of the engine, influencing decisions for subsequent events.

You cannot use a standard deployment of Red Hat Decision Manager on Red Hat OpenShift Container Platform, as described in Deploying a Red Hat Decision Manager environment on Red Hat OpenShift Container Platform using Operators, to run high available event-driven decisioning. The deployment includes Decision Server (KIE Server) pods, which remain independent of each other when scaled. The states of the pods are not synchronized. Therefore, only stateless calls can be processed reliably.

The Complex Event Processing (CEP) API is useful for event-driven decisioning with the decision engine. The decision engine uses CEP to detect and process multiple events within a collection of events, to uncover relationships that exist between events, and to infer new data from the events and their relationships. For more information about CEP in the decision engine, see Decision engine in Red Hat Decision Manager.

You can implement high available event-driven decisioning on Red Hat OpenShift Container Platform based on the reference implementation provided with Red Hat Decision Manager. This implementation provides an environment with safe failover.

In this reference implementation, you can scale the pod with the processing code. The replicas of the pod are not independent. One of the replicas is automatically designated leader. If the leader ceases to function, another replica is automatically made leader, and the processing continues without interruption or data loss.

The election of the leader is implemented with Kubernetes ConfigMaps. Coordination of the leader with other replicas is performed with exchanged messages through Kafka. The leader is always the first to process an event. When processing is complete, the leader notifies other replicas. A replica that is not the leader starts executing an event only after it has been completely processed on the leader.

When a new replica joins the cluster, this replica requests a snapshot of the current Drools session from the leader. The leader can use a recent existing snapshot if one is available in a Kafka topic. If a recent snapshot is not available, the leader produces a new snapshot on demand. After receiving the snapshot, the new replica deserializes it and eventually executes the last events not included in the snapshot before starting to process new events in coordination with the leader.