Why Metrics collection (Prometheus) in Microservices? - Purpose & Use Cases

The Big Idea

What if you could spot problems in your apps before anyone else does, without endless manual checks?

The Scenario

Imagine running many small apps (microservices) that talk to each other. You want to know if they are healthy and fast. Without tools, you check each app one by one, looking at logs and guessing what went wrong.

The Problem

Checking each app manually is slow and tiring. You might miss problems or get wrong info. It's like trying to find a broken light bulb in a huge building by walking every room instead of using a smart system.

The Solution

Prometheus automatically collects important numbers (metrics) from all your apps. It shows you clear pictures and alerts if something is wrong. You get fast, reliable info without searching everywhere.

Before vs After

✗ Before

curl http://service1/health
curl http://service2/health
# Repeat for many services

✓ After

# Prometheus scrapes all services' metrics automatically
# One place to see all data

What It Enables

With Prometheus, you can watch all your microservices' health and performance in one place, catching problems before users notice.

Real Life Example

A company runs dozens of microservices for an online store. Prometheus helps them see if checkout is slow or if a payment service fails, so they fix issues quickly and keep customers happy.

Key Takeaways

Manual checks are slow and error-prone for many microservices.

Prometheus collects and shows metrics automatically and clearly.

This helps catch problems early and keep apps running smoothly.

Practice

1. What is the main purpose of Prometheus in a microservices environment?

easy

A. To collect and store metrics from services for monitoring

B. To deploy microservices automatically

C. To manage user authentication

D. To serve web pages to users

Solution

1. Step 1: Understand Prometheus role

   Prometheus is designed to collect numerical data called metrics from running services.

2. Step 2: Identify monitoring purpose

   These metrics help monitor service health and performance in microservices.

3. Final Answer:

   To collect and store metrics from services for monitoring -> Option A

4. Quick Check:

   Prometheus = Metrics collection [OK]

Hint: Prometheus is for metrics, not deployment or auth [OK]

Common Mistakes:

• Confusing Prometheus with deployment tools
• Thinking Prometheus manages users
• Assuming Prometheus serves web content

2. Which YAML configuration snippet correctly defines a Prometheus scrape job for a service at http://localhost:8080/metrics?

easy

A. jobs:
- job: 'myservice'
endpoints: ['localhost:8080']

B. scrape_configs:
- job_name: 'myservice'
static_configs:
- targets: ['http://localhost:8080/metrics']

C. scrape_configs:
- job_name: 'myservice'
static_configs:
- targets: ['localhost:8080']

D. scrape_jobs:
- name: 'myservice'
targets: ['localhost:8080/metrics']

Solution

1. Step 1: Check Prometheus YAML syntax

   Prometheus uses scrape_configs with job_name and static_configs listing targets as host:port without URL path.

2. Step 2: Validate target format

   Targets must be host:port only, no http:// or path like /metrics.

3. Final Answer:

   scrape_configs: - job_name: 'myservice' static_configs: - targets: ['localhost:8080'] -> Option C

4. Quick Check:

   Targets = host:port only [OK]

Hint: Targets list host:port only, no URL scheme or path [OK]

Common Mistakes:

• Including http:// or /metrics in targets
• Using wrong YAML keys like scrape_jobs or jobs
• Misnaming job_name or static_configs

3. Given this Prometheus query: rate(http_requests_total[5m]), what does it calculate?

medium

A. The average rate of HTTP requests per second over the last 5 minutes

B. The current number of active HTTP requests

C. The total number of HTTP requests since service start

D. The maximum number of HTTP requests in the last 5 minutes

Solution

1. Step 1: Understand rate() function

   The rate() function calculates the per-second average increase of a counter over a time window.

2. Step 2: Apply to http_requests_total[5m]

   This means it measures how fast the total HTTP requests counter increased in the last 5 minutes, giving requests per second.

3. Final Answer:

   The average rate of HTTP requests per second over the last 5 minutes -> Option A

4. Quick Check:

   rate() = per-second average increase [OK]

Hint: rate() gives per-second average over time window [OK]

Common Mistakes:

• Thinking rate() returns total count
• Confusing rate() with current active requests
• Assuming rate() returns max value

4. You configured Prometheus to scrape localhost:9090 but no metrics appear. Which fix is correct?

medium

A. Change target to localhost:9090/metrics in YAML

B. Remove job_name from config

C. Restart Prometheus to reload config

D. Add metrics_path: '/metrics' under the scrape job

Solution

1. Step 1: Understand default metrics path

   Prometheus scrapes /metrics path by default, but if the service uses a different path, you must specify it.

2. Step 2: Fix missing metrics path

   Adding metrics_path: '/metrics' explicitly tells Prometheus where to get metrics if not default or to confirm path.

3. Final Answer:

   Add metrics_path: '/metrics' under the scrape job -> Option D

4. Quick Check:

   metrics_path fixes scrape URL [OK]

Hint: Use metrics_path to set correct scrape URL path [OK]

Common Mistakes:

• Adding path in targets instead of metrics_path
• Restarting without config fix
• Removing job_name breaks config

5. You want to monitor error rates in a microservice using Prometheus. The service exposes http_requests_total with labels status and method. Which query shows the error rate (status codes 500-599) over the last 10 minutes as a percentage of all requests?

hard

A. rate(http_requests_total{status=~"5.."}[10m]) / rate(http_requests_total[10m]) * 100

B. sum(rate(http_requests_total{status=~"5.."}[10m])) / sum(rate(http_requests_total[10m])) * 100

C. sum(rate(http_requests_total{status=~"5.."}[10m])) * 100

D. sum(rate(http_requests_total{status!~"5.."}[10m])) / sum(rate(http_requests_total[10m])) * 100

Solution

1. Step 1: Filter error status codes 500-599

   Use regex status=~"5.." to select error codes in the 500 range.

2. Step 2: Calculate error rate as percentage

   Sum the rate of error requests and divide by sum of all requests rate, then multiply by 100 for percentage.

3. Final Answer:

   sum(rate(http_requests_total{status=~"5.."}[10m])) / sum(rate(http_requests_total[10m])) * 100 -> Option B

4. Quick Check:

   Error rate % = error requests / total requests * 100 [OK]

Hint: Sum rates before division for correct percentage [OK]

Common Mistakes:

• Dividing single rates instead of sums
• Using wrong label regex
• Multiplying before division