feat: cost-saving measures

knowledge base/cloud computing/aws/cloudwatch.md

@@ -5,6 +5,7 @@ Observability service. with functions for logging, monitoring and alerting.
 1. [TL;DR](#tldr)
 1. [Queries of interest](#queries-of-interest)
 1. [Stream logs](#stream-logs)
+1. [Cost-saving measures](#cost-saving-measures)
 1. [Further readings](#further-readings)
    1. [Sources](#sources)
 
@@ -31,6 +32,19 @@ The [CloudWatch console] offers some default good queries.
 
 Logs in Log Groups can be [streamed][stream logs] elsewhere.
 
+CloudWatch retains metrics' data as follows:
+
+- Data points with a period of less than 60 seconds are available for 3 hours.<br/>
+  These are high-resolution custom metrics.
+- Data points with a period of 60 seconds (1 minute) are available for 15 days.
+- Data points with a period of 300 seconds (5 minutes) are available for 63 days.
+- Data points with a period of 3600 seconds (1 hour) are available for 455 days (15 months).
+
+Data points are aggregated together for long-term storage after the initial period.<br/>
+E.g., data using a period of 1 minute remains available for 15 days with 1-minute resolution, then it is aggregated and
+made available with a resolution of 5 minutes; after 63 days, it is further aggregated and made available with a
+resolution of 1 hour for 15 months.
+
 <details>
   <summary>CLI commands</summary>
 
@@ -101,6 +115,12 @@ Also refer [Streaming CloudWatch Logs data to Amazon OpenSearch Service] to stre
 
 Logs in CloudWatch Log Groups can be streamed [Kinesis], [Firehose] or [Lambda] by leveraging Logs subscriptions.
 
+## Cost-saving measures
+
+- Configure an _appropriate_ log retention period for any log groups.<br/>
+  Log groups containing development logs should not usually need more than 1w worth.
+- When in doubt, still configure a default, long log retention period for all log groups (10y?).
+
 ## Further readings
 
 - [Website]
@@ -113,6 +133,7 @@ Logs in CloudWatch Log Groups can be streamed [Kinesis], [Firehose] or [Lambda]
 - [Real-time processing of log data with subscriptions]
 - [Streaming CloudWatch Logs data to Amazon OpenSearch Service]
 - [Which log group is causing a sudden increase in my CloudWatch Logs bill?]
+- [Metrics concepts]
 
 <!--
   Reference
@@ -130,6 +151,7 @@ Logs in CloudWatch Log Groups can be streamed [Kinesis], [Firehose] or [Lambda]
 [firehose]: https://docs.aws.amazon.com/firehose/latest/dev/what-is-this-service.html
 [kinesis]: https://docs.aws.amazon.com/kinesis/
 [lambda]: https://docs.aws.amazon.com/lambda/
+[Metrics concepts]: https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/cloudwatch_concepts.html
 [real-time processing of log data with subscriptions]: https://docs.aws.amazon.com/cloudwatch/latest/logs/Subscriptions.html
 [services that publish cloudwatch metrics]: https://docs.aws.amazon.com/cloudwatch/latest/monitoring/aws-services-cloudwatch-metrics.html
 [streaming cloudwatch logs data to amazon opensearch service]: https://docs.aws.amazon.com/cloudwatch/latest/logs/CWL_OpenSearch_Stream.html