Understanding AWS CloudWatch Alarms: Listen to Your Cloud Signals

Jay TilluJay Tillu
5 min read

Arjun once asked himself a question that every AWS beginner stumbles upon:

“Okay, CloudWatch collects metrics… but how do I do something when something goes wrong?”

That’s where CloudWatch Alarms enter the chat — the silent watchdogs of your AWS infrastructure.


🎯 What Are CloudWatch Alarms?

CloudWatch Alarms monitor metrics and take action when thresholds are crossed.

Think of it like this:

“If CPU usage is over 80% for 5 minutes, alert me!”
“If disk space drops below 10%, reboot the instance!”

Simple rules, powerful outcomes.


🚦 Alarms Have 3 States

StateMeaning
OKAll is well.
INSUFFICIENT_DATANot enough data to decide.
🔴 ALARMThreshold crossed. Take action!

⏱️ The Heartbeat: Period

Period = how often you check the metric.

  • Can be as short as 10 seconds (high-resolution metric)

  • Or as long as 5 minutes, depending on what you need

Arjun set his alarm to check CPU every 60 seconds. That was enough for his use case.


🛠️ What Can an Alarm Do?

An alarm isn’t just about shouting.

It can actually do things:

  1. Take action on EC2 instances

    • Stop, terminate, reboot, recover
  2. Trigger Auto Scaling

    • Add/remove EC2s based on load
  3. Send notifications

    • 🔔 Push to SNS topic

    • 📩 Notify email/SMS

    • 🤖 Call Lambda for custom logic

When Arjun’s app went down once due to a full disk, he set up an alarm that automatically notified him AND triggered a Lambda to clean up logs. Crisis avoided.


🤹‍♂️ Composite Alarms — The Smart Combo

Arjun noticed a problem: too many alarms were firing. He needed to get smart.

So he used Composite Alarms.

Here’s how they work:

  • Combine multiple alarms using AND / OR logic.

  • Get notified only when multiple conditions are true.

Example:
✅ CPU > 80% AND Disk I/O high → Trigger alert
❌ CPU > 80% but Disk I/O normal → Ignore it

This helped Arjun reduce alarm noise and focus only on real issues.


🛡️ EC2 Instance Recovery with Alarms

Arjun wanted peace of mind in case EC2 failed. So he used CloudWatch Alarms to trigger EC2 recovery based on status checks:

Check TypeWhat It Monitors
Instance StatusEC2 OS health (like kernel panic)
System StatusHost hardware issues
EBS StatusAttached disk health

If the host failed, the alarm automatically moved his instance to a healthy host.
Same IP. Same data. No stress.


🧠 Bonus: Log-Based Alarms

Arjun was monitoring an app log file for the word ERROR. He created a CloudWatch Log Metric Filter, and tied that to an alarm.

Now if ERROR appears more than 10 times in 5 minutes, he gets an alert.

Simple setup. High confidence.


🧪 Test Before You Trust

Before going live, Arjun tested everything using this CLI command:

aws cloudwatch set-alarm-state \
  --alarm-name "CPUHigh" \
  --state-value ALARM \
  --state-reason "Testing alarm action"

This simulated a breach and confirmed SNS and Lambda triggers were working.


📌 SAA-Certified Knowledge

If you're prepping for the AWS SAA exam, remember:

✅ CloudWatch Alarms monitor metrics
✅ Can trigger EC2 actions, SNS, Auto Scaling
Composite Alarms = Combine logic across alarms
✅ Can be tied to Log Metric Filters
✅ Support EC2 recovery actions
✅ Use IAM roles properly when needed
✅ CLI command: set-alarm-state for testing


🧘 Arjun’s Takeaway

“CloudWatch collects data.
Alarms give it a voice.”

He now sleeps peacefully, knowing CloudWatch is watching over his infrastructure — and will shout (or act) if anything goes wrong.


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Written by

Jay Tillu
Jay Tillu

Hello! I'm Jay Tillu, an Information Security Engineer at Simple2Call. I have expertise in security frameworks and compliance, including NIST, ISO 27001, and ISO 27701. My specialities include Vulnerability Management, Threat Analysis, and Incident Response. I have also earned certifications in Google Cybersecurity and Microsoft Azure. I’m always eager to connect and discuss cybersecurity—let's get in touch!