Defining CloudWatch Alarms for Common Compute Symptoms
Explore how to create and validate CloudWatch alarms that detect common compute issues on EC2 instances. Understand alarm state transitions, metric selection, threshold settings, and missing data handling to build actionable monitoring and ensure timely incident response in AWS environments.
A CloudWatch alarm only becomes useful after it behaves like a tested state machine. An alarm reads a metric stream, applies a threshold and evaluation rule, then transitions between OK, ALARM, and INSUFFICIENT_DATA. The proof lives in the alarm history and the underlying metric timestamps.
The EC2 reachability signal provides a clear operational meaning. StatusCheckFailed implies that the instance is failing an AWS status check, directly impacting the customer through dropped connections. It also affects automation decisions, such as replacing an instance in an Auto Scaling group.
The diagram below shows how an EC2 status check turns into a CloudWatch alarm state change.
When the alarm transitions, evidence from two places needs to agree in time. The alarm history shows the exact moment CloudWatch evaluated the condition and changed state, and the EC2 instance status shows when the system and instance checks started failing. That timestamp alignment is what makes an alarm actionable in an incident ticket, not the fact that a notification was sent.
Status checks as the smallest meaningful alarm
An EC2 status check alarm is anchored on a metric that is already emitted without an agent, so missing data usually indicates a collection or ...