In modern fraud detection systems, a critical challenge emerges: how do you achieve both lightning-fast response times and unwavering reliability? Most architectures force you to choose between speed and consistency, but there's a sophisticated solution that delivers both.

Traditional event-driven systems excel at immediate processing but struggle with sparse activity patterns and external query requirements. When events don't arrive, these systems can leave aggregations incomplete and state stale - a significant liability in financial services where every millisecond and every calculation matters.

This post explores hybrid event-based aggregation - an architectural pattern that combines the immediate responsiveness of event-driven systems with the reliability of timer-based completion. We'll examine real-world implementation challenges and proven solutions that have processed billions of financial events in production.

The Core Challenge: When Event-Driven Systems Fall Short

Event-driven architectures have transformed real-time processing, but they reveal critical limitations in fraud detection scenarios. Understanding these constraints is essential for building robust financial systems.

Problem 1: The Inactivity Gap

Consider a fraud detection system that processes user behavior patterns. When legitimate users have sparse transaction activity, purely event-driven systems encounter a fundamental issue.

Figure 1: Pure event-driven systems struggle with sparse user activity, leading to incomplete aggregations

Without subsequent events to trigger completion, aggregation state persists indefinitely, creating several critical issues:

• Stale State Accumulation: Outdated calculations consume memory and processing resources
• Logical Incorrectness: Temporary spikes trigger persistent alerts that never reset automatically
• Resource Leaks: Unclosed aggregation windows create gradual system degradation

Problem 2: The External Query Challenge

Real-world fraud systems must respond to external queries regardless of recent event activity. This requirement exposes another fundamental limitation of pure event-driven architectures.

Figure 2: External systems requesting current state may receive stale data when no recent events have occurred

When external systems query for current risk scores, they may receive stale data from hours-old events. In fraud detection, where threat landscapes evolve rapidly, this staleness represents a significant security vulnerability and operational risk.

The Hybrid Solution: Dual-Trigger Architecture

The solution lies in combining event-driven responsiveness with timer-based reliability through a dual-trigger approach. This architecture ensures both immediate processing and guaranteed completion.

Core Design Principles

The hybrid approach operates on four fundamental principles:

1. Event-Triggered Processing: Immediate reaction to incoming data streams
2. Timer-Triggered Completion: Guaranteed finalization of aggregations after inactivity periods
3. State Lifecycle Management: Automatic cleanup and resource reclamation
4. Query-Time Consistency: Fresh state available for external system requests

Production Architecture: Building the Hybrid System

Let's examine the technical implementation of a production-ready hybrid aggregation system. Each component plays a crucial role in achieving both speed and reliability.

Event Ingestion Layer

Figure 3: Event ingestion layer with multiple sources flowing through partitioned message queues to ensure ordered processing

Key Design Decisions:

• Partitioning Strategy: Events partitioned by User ID ensure ordered processing per user
• Event Time vs Processing Time: Use event timestamps for accurate temporal reasoning
• Watermark Handling: Manage late-arriving events gracefully

2. Stream Processing Engine (Apache Beam Implementation)

# Simplified Beam pipeline structure
def create_fraud_detection_pipeline():
    return (
        p 
        | 'Read Events' >> beam.io.ReadFromPubSub(subscription)
        | 'Parse Events' >> beam.Map(parse_event)
        | 'Key by User' >> beam.Map(lambda event: (event.user_id, event))
        | 'Windowing' >> beam.WindowInto(
            window.Sessions(gap_size=300),  # 5-minute session windows
            trigger=trigger.AfterWatermark(
                early=trigger.AfterProcessingTime(60),  # Early firing every minute
                late=trigger.AfterCount(1)  # Late data triggers
            ),
            accumulation_mode=trigger.AccumulationMode.ACCUMULATING
        )
        | 'Aggregate Features' >> beam.ParDo(HybridAggregationDoFn())
        | 'Write Results' >> beam.io.WriteToBigQuery(table_spec)
    )

3. Hybrid Aggregation Logic

The core of our system lies in the HybridAggregationDoFn that handles both event and timer triggers:

Figure 4: State machine showing the dual-trigger approach - events trigger immediate processing while timers ensure guaranteed completion

Implementation Pattern:

class HybridAggregationDoFn(beam.DoFn):
    USER_STATE_SPEC = beam.transforms.userstate.BagStateSpec('user_events', beam.coders.JsonCoder())
    TIMER_SPEC = beam.transforms.userstate.TimerSpec('cleanup_timer', beam.transforms.userstate.TimeDomain.PROCESSING_TIME)
    
    def process(self, element, user_state=beam.DoFn.StateParam(USER_STATE_SPEC), 
                cleanup_timer=beam.DoFn.TimerParam(TIMER_SPEC)):
        user_id, event = element
        
        # Cancel any existing timer
        cleanup_timer.clear()
        
        # Process the event and update aggregation
        current_events = list(user_state.read())
        current_events.append(event)
        user_state.clear()
        user_state.add(current_events)
        
        # Calculate aggregated features
        aggregation = self.calculate_features(current_events)
        
        # Set new timer for cleanup (e.g., 5 minutes of inactivity)
        cleanup_timer.set(timestamp.now() + duration.Duration(seconds=300))
        
        yield (user_id, aggregation)
    
    @beam.transforms.userstate.on_timer(TIMER_SPEC)
    def cleanup_expired_state(self, user_state=beam.DoFn.StateParam(USER_STATE_SPEC)):
        # Finalize any pending aggregations
        current_events = list(user_state.read())
        if current_events:
            final_aggregation = self.finalize_features(current_events)
            user_state.clear()
            yield final_aggregation

4. State Management and Query Interface

Figure 5: Multi-tier state management with consistent query interface for external systems

State Consistency Guarantees:

• Read-Your-Writes: Queries immediately see the effects of recent events
• Monotonic Reads: Subsequent queries never return older state
• Timer-Driven Freshness: Timers ensure state is never more than X minutes stale

5. Complete System Flow

Figure 6: End-to-end system architecture showing data flow from event sources through hybrid aggregation to fraud detection and external systems

Advanced Implementation Considerations

Watermark Management for Late Events

Figure 7: Timeline showing event time vs processing time with watermark advancement for handling late-arriving events

Late Event Handling Strategy:

• Grace Period: Accept events up to 5 minutes late
• Trigger Configuration: Process immediately but allow late updates
• State Versioning: Maintain multiple versions for consistency

Conclusion

Hybrid event-based aggregation represents a significant advancement in building production-grade fraud detection systems. By combining the immediate responsiveness of event-driven processing with the reliability of timer-based completion, organizations can build systems that are both fast and reliable.

The architecture pattern described here addresses the core limitations of pure event-driven systems while maintaining their performance benefits. This approach has been proven in high-scale financial environments, providing a robust foundation for modern real-time fraud prevention systems.

Key benefits include:

• Sub-10ms response times for critical fraud decisions
• Guaranteed state consistency and completion
• Scalable processing of millions of events daily
• Automated resource management and cleanup

As fraud techniques become more sophisticated, detection systems must evolve to match both their speed and complexity. Hybrid event-based aggregation provides exactly this capability.

This architecture has been successfully deployed in production environments processing billions of financial events annually. The techniques described here are based on real-world implementations using Apache Beam, Google Cloud Dataflow, and modern stream processing best practices.