Project: Resilient IoT Bridge (MQTT to REST)

📌 The Problem

In many IoT deployments, devices push data faster than backend REST APIs can ingest them. Traditional "fire-and-forget" MQTT subscribers often suffer from:

• Backpressure: Slow API responses cause the MQTT subscriber to lag, eventually dropping connections.

• Data Loss: Messages are overwritten or lost during network partitions.

• Lack of Observability: No way to track which specific messages failed or to re-run them.

🚀 The Solution

This project implements a Resilient Subscriber Pattern using .NET Background Workers. It acts as a high-speed buffer between an ESP32 sensor array and a legacy REST API.

Key Features

• Non-Blocking Ingestion: Uses System.Threading.Channels to decouple MQTT reception from processing.

• Local Persistence: Every message is "Staged" in a local LiteDB instance before transmission.

• Circuit Breaker Pattern: Automatically halts API traffic during downstream outages to prevent system exhaustion.

• Self-Healing: An "Outbox" worker identifies unprocessed records and retries them with exponential backoff.

System Design

[!TIP] This architecture ensures that even if the REST API is down for hours, no data is lost as long as the local disk has space.

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Project Architecture Overview

The core of your solution relies on decoupling. Instead of a direct pipe where the MQTT subscriber is blocked by the REST API's response time, you are creating a "Buffer and Forward" system.

1. The Resilience Flow

1. Ingestion: The .NET Background Worker subscribes to Mosquitto.

2. Internal Handoff: It pushes the message into a System.Threading.Channels (Bounded Channel). This allows the MQTT thread to return immediately, keeping the broker connection healthy.

3. Persistence (The "Safety Net"): A consumer reads from the channel and writes the raw JSON to the LiteDB/SQLite database immediately.

4. The Circuit Breaker: Using Polly, if the REST API starts failing or slowing down (latency > X ms), the Circuit Breaker opens.

5. Offline Mode: While the circuit is open, the system continues to save to the DB but stops attempting the API calls, preventing a backlog of "zombie" requests.

6. Reprocessing: A separate worker polls the DB for isProcessed = false records and attempts to sync them once the API is healthy.

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Suggested Tech Stack Refinement

Component	Recommendation	Why?
Edge Device	ESP32 (C++ / Arduino)	More performant for high-frequency MQTT than nanoFramework in most "real-time" scenarios.
Local Database	LiteDB	Excellent for .NET; it’s a NoSQL document store that is "serverless" (file-based) like SQLite, but handles JSON natively.
Resilience	Polly v8	Use the new "Resilience Pipelines" for combining Retries and Circuit Breakers.
Communication	Refit	Keeps your REST client code clean and interface-based.

Integration Notes

1. Security nuance: TLS vs. Payload Signature

• TLS (Transport Layer Security): Encrypts the data while it is moving between the ESP32 and Mosquitto. It prevents "Eavesdropping."

• Digital Signature (Application Level): Protects the data while it is "at rest" in your LiteDB and during the REST API hop. It prevents "Tampering."

2. .NET Worker & ESP32 Configuration

• The ESP32 will need the ca.crt to verify it's talking to the right broker.

• The .NET Worker will use the client.crt and client.key if you decide to enforce mutual TLS (mTLS), or just the ca.crt for a standard encrypted connection.

Infrastructure Notes

• The broker is configured to support secure MQTT communication over TLS with mutual TLS (mTLS), leveraging a private Certificate Authority and X.509 certificates.