Expense Tracker — Observability Demo

See how your Spring Boot app connects to Prometheus and Grafana. This is a bonus demo for the Expense Tracker workshop — showing DevOps engineers the bridge between the app they built and the monitoring they already know.

What's Inside

Component	Port	URL
Spring Boot App	8080	http://localhost:8080/swagger-ui.html
Actuator Metrics	8080	http://localhost:8080/actuator/prometheus
Prometheus	9090	http://localhost:9090
Grafana	3000	http://localhost:3000 (admin/admin)

Quick Start

1. Start Prometheus + Grafana:

docker compose up -d

2. Start the Spring Boot app:

cd app
./mvnw spring-boot:run

3. Open the dashboards:

• Grafana: http://localhost:3000 (login: admin / admin)
• The "Expense Tracker" dashboard is pre-configured

4. Generate some traffic:

Open Swagger UI at http://localhost:8080/swagger-ui.html and:

• Add a few expenses (POST /expenses)
• List them (GET /expenses)
• Delete some (DELETE /expenses/{id})

Watch the Grafana dashboard update in real time!

What You'll See

Actuator Endpoints

Endpoint	What it shows
/actuator/health	App health status (UP/DOWN)
/actuator/info	App info
/actuator/prometheus	All metrics in Prometheus format
/actuator/metrics	List of available metric names

Grafana Dashboard Panels

Panel	What it shows
HTTP Request Rate	Requests per second by endpoint
Response Time (p95)	95th percentile response time
Expenses Added	Counter of POST operations
Expenses Deleted	Counter of DELETE operations
JVM Heap Memory	Memory usage over time
JVM Threads	Number of live threads
GC Pause Time	Garbage collection impact
CPU Usage	Process CPU utilization
Uptime	How long the app has been running

Custom Business Metrics

The app exposes custom Micrometer counters:

expenses_added_total    — number of expenses created
expenses_deleted_total  — number of expenses deleted

These are defined in ExpenseController.kt using Counter.builder().

How It Works

Browser/Swagger UI
       │
       ▼
Spring Boot App (:8080)
  ├── /expenses          — REST API
  └── /actuator/prometheus — metrics endpoint
       │
       ▼ (scrapes every 5s)
Prometheus (:9090)
       │
       ▼ (queries)
Grafana (:3000)
  └── Dashboard: Expense Tracker

1. Spring Boot Actuator exposes metrics at /actuator/prometheus
2. Micrometer collects JVM, HTTP, and custom metrics
3. Prometheus scrapes the endpoint every 5 seconds
4. Grafana queries Prometheus and visualizes the data

Key Files

File	Purpose
app/pom.xml	spring-boot-starter-actuator + micrometer-registry-prometheus
app/src/.../application.yaml	Actuator endpoint exposure config
app/src/.../ExpenseController.kt	Custom Micrometer counters
docker-compose.yml	Prometheus + Grafana containers
prometheus/prometheus.yml	Scrape config targeting localhost:8080
grafana/dashboards/expense-tracker.json	Pre-built Grafana dashboard

Cleanup

docker compose down

Related

• expense-tracker-spring — Workshop Session 1
• expense-tracker-frontend — React frontend
• expense-tracker-workshop-02 — Workshop Session 2