What is the difference between a CloudWatch Alarm and a CloudWatch Event?

Question

What is the difference between a CloudWatch Alarm and a CloudWatch Event? For me both seem to respond on a certain metric and trigger something on their target? What are important differences?

Answer 1

Some key differences:

• Events can self-trigger based on a schedule; alarms don't do this
• Alarms invoke actions only for sustained changes
• Alarms watch a single metric and respond to changes in that metric; events can respond to actions (such as a lambda being created or some other change in your AWS environment)
• Alarms can be added to CloudWatch dashboards, but events cannot
• Events are processed by targets, with many more options than the actions an alarm can trigger

Event:

Amazon CloudWatch Events delivers a near real-time stream of system events that describe changes in Amazon Web Services (AWS) resources. Using simple rules that you can quickly set up, you can match events and route them to one or more target functions or streams. CloudWatch Events becomes aware of operational changes as they occur. CloudWatch Events responds to these operational changes and takes corrective action as necessary, by sending messages to respond to the environment, activating functions, making changes, and capturing state information.

You can also use CloudWatch Events to schedule automated actions that self-trigger at certain times using cron or rate expressions.

Alarm:

You can create a CloudWatch alarm that watches a single CloudWatch metric or the result of a math expression based on CloudWatch metrics. The alarm performs one or more actions based on the value of the metric or expression relative to a threshold over a number of time periods. The action can be an Amazon EC2 action, an Amazon EC2 Auto Scaling action, or a notification sent to an Amazon SNS topic.

Alarms invoke actions for sustained state changes only. CloudWatch alarms don't invoke actions simply because they are in a particular state, the state must have changed and been maintained for a specified number of periods.

References:

• https://docs.aws.amazon.com/AmazonCloudWatch/latest/events/WhatIsCloudWatchEvents.html
• https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/AlarmThatSendsEmail.html
• https://www.edureka.co/blog/amazon-cloudwatch-monitoring-tool/