As you can imagine, the amount of metrics that CloudWatch collects from all clients in AWS is absolutely mind-boggling. Anyone who has ever designed or maintained any motoring system and the database the system relies on will appreciate the skill of the AWS CloudWatch team. The fact that CloudWatch is able to collect and store millions (or perhaps billions?) of data points concurrently is quite a feat. All I can say is kudos to the CloudWatch team!

The sheer volume of data being collected means that it is literally impossible to store the data indefinitely, which is why AWS has defined a retention period for each of the previously mentioned metrics as follows: 

• Any metrics with a collection interval of below 1 minute is available for 3 hours
• 1-minute metrics are available for 15 days
• 5-minute metrics are available for 63 days 
• 1-hour metrics are available for 15 months

Once the retention period expires, the data is aggregated to the next level and retained accordingly. For example, 1-second metrics will be aggregated to 1-minute metrics after 3 hours. The 1-minute metrics will in turn be aggregated to 5-minute metrics after 15 days. All 5-minute metrics are aggregated to 1-hour metrics after 63 days, and all metrics are discarded once they are more than 15 months old.