Here we are, at Service Number Two on our High Availability Top 10. And it is Amazon CloudWatch! Now, CloudWatch is seriously useful. The de facto monitoring servers for AWS resources, it's like the pulse of your solution, providing you the metrics to quickly diagnose and dynamically adjust any availability or scalability issue. CloudWatch logs aspects such as CPU, disk, and network activity. Amazon RDS database instances, Amazon DynamoDB, Elastic Load Balancer, or Amazon Elastic Block Store volumes, are all examples of where CloudWatch can monitor your services for you. CloudWatch Basic Monitoring for Amazon EC2 instances provides seven pre-selected metrics at five minute frequencies, and three status-check metrics at one minute frequencies. Basic Monitoring is enabled by default in an account. Detailed Monitoring for Amazon EC2 instances, all the same metrics available to Basic Monitoring, but at a one minute frequency. That comes at an additional charge. Instances with Detailed Monitoring enabled allows data aggregation by Amazon EC2, AMI IDs and instance types. Now, monitoring data is retained for two weeks. Even if your AWS resources have been terminated, CloudWatch provides the ability to set alarms on these and other metrics. It is also possible to include custom metrics and create CloudWatch alarms. You can create a CloudWatch alarm, that sends an Amazon Simple Notification Service message, when the alarm changes state. An alarm watches a single metric over a time period you specify, and performs one or more actions, based on the value of the metric, relative to a given threshold, over a number of time periods. The action is a notification, sent to the Amazon Simple Notification Service topic or Auto Scale policy. Alarms invoke actions for sustained state changes only. CloudWatch alarms will not invoke actions, simply because they are in a particular state. The state must have changed and been maintained for a specified number of periods. After an alarm invokes an action due to a change in state, its subsequent behavior depends on the type of action that you have associated with the alarm. For Auto Scaling policy notifications, the alarm continues to invoke the action for every period the alarm remains in the new state. For Amazon Simple Notifications Service notifications, no additional actions are invoked. An alarm has three possible states. Try to remember these: OK, the metric is within the defined threshold Alarm, the metric is outside of the design threshold Insufficient Data, the alarm has just started, the metric is not available or not enough data is available for the metric to determine the alarm state.