CloudWatch Concepts

Metrics
Metrics are the fundamental concept in CloudWatch.

A metric represents a time-ordered set of data points that are published to CloudWatch.

Think of a metric as a variable to monitor, and the data points as representing the values of that variable over time.
For example, the CPU usage of a particular EC2 instance is one metric provided by EC2.

By default, many AWS services provide free metrics for resources (such as Amazon EC2 instances, Amazon EBS volumes, and Amazon RDS DB instances).

For a charge, you can also enable detailed monitoring for some resources.

For custom metrics, you can add the data points in any order, and at any rate you choose.
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Metrics exist only in the Region in which they are created.

Each data point in a metric has a time stamp, and (optionally) a unit of measure.
Dimensions
A dimension is a name/value pair that is part of the identity of a metric.

You can assign up to 10 dimensions to a metric.

Think of dimensions as categories for those metrics.
Namespaces
A namespace is a container for CloudWatch metrics.

Metrics in different namespaces are isolated from each other, so that metrics from different applications are not mistakenly aggregated into the same statistics.

There is no default namespace.
You must specify a namespace for each data point you publish to CloudWatch.
Resolution
Standard resolution = one-minute granularity.

High resolution = granularity of one second.

EC2 Basic monitoring = Metrics every 5 minutes.

EC2 Detailed monitoring = Metrics every minute; it is a paid option.
Custom Metrics
You can define custom metrics for your own use.

Once Amazon CloudWatch contains your custom metrics, you can view those in the CloudWatch console.

Uses API called PutMetricData.
CloudWatch Alarms
You can use an alarm to automatically initiate actions on your behalf.

An alarm watches a single metric over a specified time period, and performs one or more specified actions, based on the value of the metric relative to a threshold over time.

The action is a notification sent to an Amazon SNS topic or an Auto Scaling policy.

When creating an alarm, select an alarm monitoring period that is greater than or equal to the metric's resolution.
For example, basic monitoring for EC2 is metrics every 5 minutes.
So, when setting an alarm, select a period greater than 5 minutes.

An alarm can be in three possible states:
OK - The metric or expression is within the defined threshold.

ALARM - The metric or expression is outside of the defined 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.

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