Problem

Building reactive applications on the Hiero network currently requires developers to implement significant boilerplate code. When a developer needs to monitor an account for transactions or a topic for messages, they are forced to manually handle polling, state tracking, and error recovery.

This friction prevents developers from easily fulfilling common enterprise requirements such as:

• Treasury Monitoring: "As a treasury manager, I want to be notified instantly when my account receives funds so I can trigger automated internal accounting."
• HCS Coordination: "As a DeFi developer, I want to monitor HCS messages for real-time coordination signals without writing custom polling loops."
• ERP Integration: "As an enterprise integrator, I want to react to successful settlements on the network to update our external ERP system in real-time."

I'm always frustrated when I have to rewrite this "plumbing" code for every new project. The current SDK provides great low-level primitives, but it lacks a high-level, declarative way to "just listen" to network events.

Solution

I propose adding a high-level Event Observer Framework that abstracts away the polling and state management logic, enabling a "plug-and-play" experience for the user stories mentioned above.

Key components of the solution:

• AbstractPollingObserver: A core background engine in hiero-enterprise-base that handles interval-based polling against the Mirror Node REST API.
• Deduplication Logic: Built-in tracking of consensus_timestamp to ensure events are delivered exactly once and in chronological order.
• Declarative Annotations: Introduction of @HieroTransactionListener and @HieroTopicListener to allow developers to subscribe to events by simply annotating a method.
• Spring Integration: A HieroListenerProcessor that automatically discovers annotated beans and manages the observer lifecycles.
• Repository Enhancements: Added timestamp-based filtering to TransactionRepository and TopicRepository to support efficient delta-polling.

Example Usage:

@HieroTransactionListener(account = "0.0.1234", interval = 5000)
public void onTreasuryUpdate(TransactionInfo tx) {
    log.info("Treasury received funds. Transaction ID: {}", tx.transactionId());
}
\```


### Alternatives

1.  **Raw gRPC Streaming**: While the SDK supports `TopicMessageQuery` via gRPC, it can be fragile for long-running observers in enterprise environments due to connection resets and the need for complex retry logic. The REST-based polling approach is often more stable for "always-on" background monitoring.
2.  **Manual Polling**: Developers can continue to write their own `Scheduled` tasks, but this leads to inconsistent implementations and high maintenance overhead across different teams.
3.  **Reactive Streams (Project Reactor)**: We considered a full `Flux/Mono` implementation, but decided that a simple annotation-driven model provides a much lower barrier to entry for the majority of enterprise developers while still being compatible with reactive stacks.