While we normally prefer the CLI (or CloudFormation) for creating AWS resources the wizard for creating alarms does a lot of work for you, so it is a good place to start. Once you know what kinds of alarms you are interested in, you can automate them.

The CloudWatch console is a great place to keep an eye on the performance of your resources. Often when looking at the metrics you might find a scenario that you would want to be notified of, and quickly create an alarm on it.

We start by selecting the metric we are interested in; in this case, it is errors from the example-lambda-function, but the process would work the same regardless of the metric you select.

You must define a name for the alarm, and you can optionally create a description. One of the most important parts of the alarm is how you define the threshold that will trigger it. You can choose not only the value and comparison operator used (for example, greater than (>), less than (<), greater than or equal to (>=), and so on), but also the number of failing data points that must occur before the alarm is triggered. This can stop you being alerted unnecessarily for temporary spikes in metric values. In this scenario we want to know if there are any errors, so we set the value to 1.

On the right-hand side you can define the check period and the statistic used (for example, Average, Maximum, Minimum, and so on). You can also see the recent history of the selected metric in the top-right corner. The red line on the graph is where the currently defined threshold will sit, so you can quickly see if the alarm would have been triggered.

In the Actions section of the alarm, you define what action will be taken when triggered. While you can select an existing SNS topic, we will define a new one by clicking on New list. You are then prompted for the details of the new topic; you must give both a name and an e-mail address to subscribe to the topic.

When you click Create Alarm, you will see the status of the subscription. After receiving the e-mail and clicking on the confirmation link, the status will automatically update. It doesn't matter if you navigate away from the window before you confirm the subscription. Just remember that your target e-mail address won't receive any notifications if you do not confirm the subscription.

Viewing the newly created alarm shows its current state, and its recent history. An alarm has three possible states:

• ALARM: The metric is over the defined threshold
• INSUFFICIENT_DATA: There were not enough data points to determine if the metric is under or over the threshold
• OK: The metric is under the defined threshold

You can filter alarms by their state by the links on the side menu, which also show an updated view of how many alarms are in each state.

Behind the scenes, the wizard has created an SNS topic for you. The topic is what handles converting the alarm message to an e-mail, and sending it. Without the SNS topic the alarm would still alert (that is change state), but there would be no way to tell without looking at the metric in the CloudWatch dashboard.