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Policy engine should support state persist and restore. Siddhi is one major policy engine which Eagle currently supports, and Siddhi supports state persist and store.

public interface Snapshotable {
    public Object[] currentState();
    public void restoreState(Object[] state);
    public String getElementId();
}

In Eagle AlertExecutor is the processing element which handles policy evaluation, policy lifecycle management, so AlertExecutor is the place where policy state can be persisted and restored.

Current policy state = snapshot of policy state + series of events after snapshot

When to do snapshot

Option 1: each AlertExecutor creates a daemon thread which is dedicated for taking snapshot for all polices being processed by this AlertExecutor

Option 2: use storm tick tuple to notify AlertExecutor do a snapshot

While doing snapshot, the whole AlertExecutor is halted to keep state consistent.

How to do delta events persistence

Option1: write-ahead log and move to centralized storage in the backend

Option2: persist each event into Kafka

Option3: persist each event into eagle

Policy state recovery

Recovery only happens when AlertExecutor is first started or restarted by framework.

Recovery includes restoring snapshot and applying delta events