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Posted on Apr 4

🧩 What Is Spring Boot Actuator?

Actuator is like adding a health monitoring system to your Spring Boot app. It gives you real-time metrics, health checks, and other operational info — all via HTTP endpoints like:

http://localhost:8080/actuator/metrics

🎯 Why Is It Useful for Microservices?

Imagine you have 10 microservices running. You need to answer:

Is my app healthy?
Are the APIs responding fast?
Is the memory usage too high?
Are the DB connections overloaded?

👉 Actuator gives you these answers out-of-the-box.

🔍 What Metrics Does Actuator Provide?

Let’s explore the main types of metrics you’ll find useful for monitoring:

1. 🧠 JVM Metrics (Java Virtual Machine)

These tell you how your app is using memory, threads, and classes.

Metric	What It Tells You	Example
`jvm.memory.used`	How much memory is used by your app	128 MB
`jvm.memory.max`	The max memory your app can use	512 MB
`jvm.threads.live`	How many threads are running	100
`jvm.classes.loaded`	Number of classes loaded into memory	10,000

💡 Why care? If memory or thread usage spikes too much, your app could crash.

2. 🖥️ CPU & System Metrics

Metric	What It Tells You	Example
`system.cpu.usage`	Total CPU usage (system-wide)	0.60 (60%)
`process.cpu.usage`	CPU used by your app only	0.20 (20%)
`system.cpu.count`	Number of processor cores	8
`process.uptime`	How long the app has been running	4 hours

💡 Why care? High CPU usage may slow down your APIs.

3. 🌐 HTTP Request Metrics

Metric	What It Tells You	Example
`http.server.requests`	Number of API calls + response time	500 calls, avg 120ms
→ Tags: `uri`, `method`, `status`	Helps slice metrics per API route	`/login`, `POST`, `200 OK`

💡 Why care? You can find slow endpoints or too many errors (e.g. 500s).

4. 💽 Database (HikariCP) Metrics

If using a database connection pool (like HikariCP):

Metric	What It Tells You	Example
`hikaricp.connections.active`	Number of DB connections in use	5
`hikaricp.connections.idle`	Available connections in pool	10
`hikaricp.connections.max`	Maximum allowed connections	20

💡 Why care? If all DB connections are used, your app can freeze or slow down.

5. 📦 Cache Metrics

If you use caching (like Caffeine, Redis, etc.):

Metric	What It Tells You	Example
`cache.gets`	How many times you read from the cache	1000 hits
`cache.misses`	How many times data wasn't in cache	200 misses
`cache.puts`	How many times you added to cache	300 puts

💡 Why care? Too many cache misses = your DB is doing extra work.

6. 💓 Health Endpoint

Shows status of your app and its dependencies (DB, disk, etc.).

📍 Endpoint:

http://localhost:8080/actuator/health

Example output:

{
  "status": "UP",
  "components": {
    "db": { "status": "UP" },
    "diskSpace": { "status": "UP" }
  }
}

💡 Helps monitoring tools like Prometheus, Grafana, or Kubernetes to know if your service is healthy.

7. 📈 Custom Metrics (Optional but Powerful)

You can define your own metrics like:

meterRegistry.counter("orders.placed").increment();
meterRegistry.gauge("temperature", () -> getRoomTemp());

💡 Used to track business logic (e.g., number of signups, order volume).

⚙️ How to Enable Actuator Metrics

Add the dependency:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

In application.properties:

management.endpoints.web.exposure.include=*

For Prometheus/Grafana:

<dependency>
  <groupId>io.micrometer</groupId>
  <artifactId>micrometer-registry-prometheus</artifactId>
</dependency>

Then hit:

http://localhost:8080/actuator/prometheus

🧪 Use Case: Example for Microservice Monitoring

Let’s say you have a payment microservice:

/actuator/health: Used by Kubernetes for health checks.
/actuator/metrics: View API performance, DB usage.
http.server.requests: See if /pay API is slow.
hikaricp.connections.active: Monitor DB pool overload.
orders.placed: Track how many payments succeeded.

Would you like a dashboard setup in Grafana, or a custom Prometheus config example to visualize these metrics?

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