How to: CloudWatch Gotchas and Limitations
* 🔸Metrics in CloudWatch originate [on the hypervisor](https://forums.aws.amazon.com/message.jspa?messageID=403578). The hypervisor doesn't have access to OS information, so certain metrics (most notably memory utilization) are not available unless pushed to CloudWatch from inside the instance.
* 🔸You can not use [more than one metric for an alarm](https://forums.aws.amazon.com/thread.jspa?threadID=94984).
* 🔸Notifications you receive from alarms will not have any contextual detail; they have only the specifics of the threshold, alarm state, and timing.
* 🔸By default, CloudWatch metric resolution is 1 minute. If you send multiple values of a metric within the same minute, they will be aggregated into minimum, maximum, average and total (sum) per minute.
* 🐥In July 2017, a new [high-resolution option](http://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/publishingMetrics.html#high-resolution-metrics) was added for CloudWatch metrics and alarms. This feature allows you to record metrics with 1-second resolution, and to evaluate CloudWatch alarms every 10 seconds.
    - The [blog post introducing this feature](https://aws.amazon.com/blogs/aws/new-high-resolution-custom-metrics-and-alarms-for-amazon-cloudwatch/) describes how to publish a high-resolution metric to CloudWatch. Note that when calling the `PutMetricData` API, `StorageResolution` is an attribute of each item you send in the `MetricData` array, not a direct parameter of the `PutMetricData` API call.
* 🔸Data about metrics is kept in CloudWatch [for 15 months](https://aws.amazon.com/blogs/aws/amazon-cloudwatch-update-extended-metrics-retention-user-interface-update/), starting November 2016 (used to be 14 days). Minimum granularity increases after 15 days.

AMIs
----