SUB Command Overview
Purpose
The SUB command is a cornerstone of EnSync's event distribution system, enabling services to register interest in specific event types and receive real-time updates. This command implements the subscriber side of the publish-subscribe pattern, allowing for dynamic, scalable event-driven architectures.
Key Capabilities
Dynamic Subscription
Services can subscribe to specific event types at runtime, allowing for flexible system configurations without code changes. This dynamic nature enables systems to adapt to changing requirements and traffic patterns.
Competing Consumer Pattern
Multiple instances of the same service can subscribe using the same access key, implementing the competing consumer pattern. This pattern ensures that events are processed exactly once by one of the available service instances, enabling horizontal scaling.
Hierarchical Event Filtering
The SUB command supports hierarchical event filtering using path-like event names (e.g., domain/resource/action). Subscribers can use wildcards to receive all events within a specific domain or of a specific action type.
Position Tracking
The system maintains precise tracking of each client's position within each event stream, enabling reliable resume capabilities after disconnections or service restarts.
Technical Implementation
The SUB command works by:
Authenticating the subscriber using their client ID
Verifying that the subscriber has permission to receive the specified event type
Registering the subscription in the client registry
Loading initial events based on the client's last position
Delivering events in real-time as they become available
This implementation ensures that subscribers receive all events they are authorized to access, without gaps or duplicates, even across disconnections.
Integration Points
The SUB command integrates with:
Authentication System: Verifies subscriber identity and permissions
Client Registry: Tracks active subscriptions and client state
Position Tracking System: Maintains client position within event streams
Event Storage: Retrieves historical and new events for delivery
Competing Consumer System: Ensures events are delivered to exactly one instance
Use Cases
Building reactive microservices that respond to system events
Creating real-time dashboards and monitoring applications
Implementing event-driven workflows across distributed systems
Developing collaborative applications with live updates
Building notification systems that respond to specific triggers
Implementing CQRS (Command Query Responsibility Segregation) patterns