Alarm actions (SNS, Auto Scaling) in AWS - Step-by-Step Execution

Process Flow - Alarm actions (SNS, Auto Scaling)

Metric monitored

↓

Alarm threshold reached?

No→Wait

Yes↓

Trigger Alarm

↓

Send SNS notification

→Subscribers notified

↓

Trigger Auto Scaling action

→Scale up/down instances

The system monitors a metric, triggers an alarm when a threshold is crossed, then sends notifications and/or scales resources automatically.

Execution Sample

AWS

Alarm:
  Metric: CPUUtilization
  Threshold: > 70%
  Actions:
    - SNS Topic
    - Auto Scaling Policy

This alarm watches CPU usage, and when it goes above 70%, it sends a message and adjusts server count.

Process Table

Step	Metric Value	Condition (>70%)	Alarm State	Action Taken
1	45%	False	OK	No action
2	65%	False	OK	No action
3	72%	True	ALARM	Send SNS notification, Trigger Auto Scaling
4	75%	True	ALARM	Send SNS notification, Trigger Auto Scaling
5	68%	False	OK	No action

💡 Metric drops below threshold at step 5, alarm state returns to OK, actions stop.

Status Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5
Metric Value	N/A	45%	65%	72%	75%	68%
Alarm State	OK	OK	OK	ALARM	ALARM	OK
Action Taken	None	None	None	SNS + Auto Scaling	SNS + Auto Scaling	None

Key Moments - 2 Insights

Why does the alarm only trigger actions when the metric is above 70%?

Because the condition (>70%) is only true at steps 3 and 4, as shown in the execution_table rows where Alarm State changes to ALARM and actions occur.

What happens when the metric falls back below the threshold?

At step 5, the metric is 68%, condition is false, so the alarm state returns to OK and no actions are taken, as seen in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Alarm State at step 2?

A OK

B ALARM

C INSUFFICIENT_DATA

D UNKNOWN

💡 Hint

Check the 'Alarm State' column at step 2 in the execution_table.

At which step does the alarm first trigger Auto Scaling actions?

A Step 1

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Action Taken' column in the execution_table to find when Auto Scaling starts.

If the threshold was changed to > 60%, how would the alarm state at step 2 change?

A It would become INSUFFICIENT_DATA

B It would remain OK

C It would change to ALARM

D It would be UNKNOWN

💡 Hint

Step 2 metric is 65%, which is above 60%, so condition would be true.

Concept Snapshot

Alarm actions watch a metric and trigger when thresholds are crossed.
Actions include sending SNS notifications and Auto Scaling adjustments.
Alarm state changes from OK to ALARM when triggered.
Actions stop when metric returns below threshold.
Use alarms to automate responses to system changes.

Full Transcript

This visual execution shows how an alarm monitors a metric like CPU usage. When the metric goes above 70%, the alarm state changes to ALARM. This triggers two actions: sending a notification via SNS and triggering an Auto Scaling policy to adjust server count. When the metric falls back below 70%, the alarm state returns to OK and actions stop. The execution table tracks metric values, alarm state, and actions step-by-step. This helps beginners see exactly when and why actions happen in response to metric changes.

Practice

1. What is the primary purpose of connecting an AWS CloudWatch alarm to an SNS topic?

easy

A. To create backups of alarm configurations

B. To send notifications when the alarm state changes

C. To store alarm history logs

D. To automatically scale EC2 instances

Solution

1. Step 1: Understand SNS topic role

   SNS topics are used to send messages or notifications to subscribers.

2. Step 2: Connect alarm to SNS for notifications

   When an alarm changes state, it can trigger SNS to notify users or systems.

3. Final Answer:

   To send notifications when the alarm state changes -> Option B

4. Quick Check:

   Alarm + SNS = Notifications [OK]

Hint: SNS topics send messages on alarm changes [OK]

Common Mistakes:

• Confusing SNS with Auto Scaling actions
• Thinking SNS stores logs
• Assuming SNS creates backups

2. Which of the following is the correct way to specify an Auto Scaling action in a CloudWatch alarm configuration?

easy

A. {
"AlarmActions": ["arn:aws:autoscaling:region:account-id:scalingPolicy:policy-id:autoScalingGroupName/group-name:policyName/policy-name"]
}

B. AlarmActions: ["arn:aws:sns:region:account-id:topic-name"]

C. AlarmActions: ["arn:aws:lambda:region:account-id:function:function-name"]

D. AlarmActions: ["arn:aws:s3:::bucket-name"]

Solution

1. Step 1: Identify correct ARN format for Auto Scaling policy

   Auto Scaling policy ARNs include 'autoscaling' and specify scalingPolicy and group name.

2. Step 2: Match with given options

   { "AlarmActions": ["arn:aws:autoscaling:region:account-id:scalingPolicy:policy-id:autoScalingGroupName/group-name:policyName/policy-name"] } matches the correct ARN format for Auto Scaling alarm action.

3. Final Answer:

   Correct Auto Scaling policy ARN format -> Option A

4. Quick Check:

   Auto Scaling action uses autoscaling ARN [OK]

Hint: Auto Scaling ARNs contain 'autoscaling' keyword [OK]

Common Mistakes:

• Using SNS ARN instead of Auto Scaling ARN
• Confusing Lambda or S3 ARNs as alarm actions
• Incorrect ARN structure

3. Given this CloudWatch alarm configuration snippet:

{
  "AlarmName": "HighCPU",
  "MetricName": "CPUUtilization",
  "Namespace": "AWS/EC2",
  "Threshold": 80,
  "ComparisonOperator": "GreaterThanThreshold",
  "EvaluationPeriods": 2,
  "AlarmActions": ["arn:aws:sns:us-east-1:123456789012:NotifyMe"]
}

What happens when the CPU utilization exceeds 80% for 2 evaluation periods?

medium

A. An SNS notification is sent to subscribers of the NotifyMe topic

B. The EC2 instance automatically scales up

C. The alarm resets without any action

D. A Lambda function is triggered

Solution

1. Step 1: Analyze AlarmActions ARN

   The AlarmActions list contains an SNS topic ARN, meaning notifications are sent on alarm.

2. Step 2: Understand alarm trigger

   When CPUUtilization > 80% for 2 periods, alarm state changes and triggers SNS notification.

3. Final Answer:

   An SNS notification is sent to subscribers of the NotifyMe topic -> Option A

4. Quick Check:

   Alarm with SNS ARN sends notifications [OK]

Hint: SNS ARN in AlarmActions means notification sent [OK]

Common Mistakes:

• Assuming Auto Scaling happens without scaling policy
• Thinking alarm resets silently
• Confusing SNS with Lambda triggers

4. You configured a CloudWatch alarm to trigger an Auto Scaling policy, but the instances are not scaling when the alarm state changes. What is the most likely cause?

medium

A. The EC2 instances are stopped

B. The SNS topic subscription is missing

C. The alarm threshold is set too low

D. The alarm's AlarmActions does not include the correct Auto Scaling policy ARN

Solution

1. Step 1: Check AlarmActions configuration

   If the Auto Scaling policy ARN is missing or incorrect, the alarm cannot trigger scaling.

2. Step 2: Rule out other options

   SNS subscription missing affects notifications, not scaling. Threshold too low triggers alarm but no scaling if policy missing. Stopped instances do not scale but alarm triggers still need correct action.

3. Final Answer:

   The alarm's AlarmActions does not include the correct Auto Scaling policy ARN -> Option D

4. Quick Check:

   Correct Auto Scaling ARN needed in AlarmActions [OK]

Hint: Auto Scaling needs correct policy ARN in AlarmActions [OK]

Common Mistakes:

• Confusing SNS subscription with scaling trigger
• Ignoring ARN correctness
• Assuming stopped instances auto scale

5. You want to automatically increase the number of EC2 instances when CPU usage is high and send a notification to your team. Which combination of alarm actions should you configure?

hard

A. Add only the Auto Scaling policy ARN to AlarmActions

B. Add only the SNS topic ARN to AlarmActions

C. Add both the Auto Scaling policy ARN and the SNS topic ARN to the alarm's AlarmActions

D. Create two separate alarms, one for scaling and one for notifications

Solution

1. Step 1: Understand alarm actions for scaling and notifications

   To scale automatically, the alarm must trigger the Auto Scaling policy. To notify, it must trigger the SNS topic.

2. Step 2: Combine both actions in one alarm

   Adding both ARNs in AlarmActions ensures the alarm triggers scaling and sends notifications simultaneously.

3. Final Answer:

   Add both the Auto Scaling policy ARN and the SNS topic ARN to the alarm's AlarmActions -> Option C

4. Quick Check:

   Multiple AlarmActions allow scaling + notifications [OK]

Hint: Use both Auto Scaling and SNS ARNs in AlarmActions [OK]

Common Mistakes:

• Thinking one ARN can do both scaling and notifications
• Creating multiple alarms unnecessarily
• Missing one of the required ARNs