Chapter 05: Project "GoTracker"

"Theory is nice. Practice is better."

We are going to build GoTracker. It is a microservice that listens for orders.created and simulates shipping.

1. The Specification

Tech Stack

• Language: Go
• Storage: Redis (For package status)
• Broker: Kafka (or Redpanda for dev)
• Protocol: gRPC (for admin queries) + HTTP (for public tracking)

Features

1. Ingest: Listen to Kafka topic orders.created.
2. Process:
   • Parse JSON.
   • Generate a "Tracking Number" (UUID).
   • Save to Redis: SET tracking:123 "Preparing"
3. Simulate:
   • Wait 10 seconds -> Update to "Shipped".
   • Wait 10 seconds -> Update to "Delivered".
4. Query:
   • GET /track/{id} returns the status.

2. Directory Structure

We will create a separate repository (or folder) for this.

go-tracker/
├── cmd/
│   └── tracker/
│       └── main.go
├── internal/
│   ├── consumer/ (Kafka Logic)
│   ├── store/    (Redis Logic)
│   └── service/  (Business Logic)
└── docker-compose.yml

3. Visual Signal (The Delivery Fleet) 🚚

Concept: Background Workers. Signal: A fleet of delivery trucks waiting for orders.

• Shop: The Storefront.
• Kafka: The Loading Dock.
• GoTracker: The Trucks that take boxes from the Dock and drive them to the customer.

4. The Challenge

This is an optional design exercise about physical deliveries, outside the digital-book project. No completed service or step-by-step implementation is included here.

Start with an in-memory event processor and a test: processing the same event twice creates one parcel. Then introduce persistence and test a process restart. Only after those work add the broker adapter and demonstrate a crash between saving a parcel and committing consumer progress. Explain why replay does not create a second parcel. Redis configuration and eviction rules must be explicit if it stores authoritative state.

If you cannot explain the first test, return to Middle chapters 4 and 11. If restart loses the parcel, revisit persistence before adding Kafka. Completion requires recorded failure experiments, not only a directory tree.