S3 request cost monitoring: which CloudWatch metrics actually matter and which ones will let a spike sneak past you Went through the exercise of setting up proper S3 cost monitoring recently and honestly the gap between "metrics that look useful" and "metrics that will actually catch the thing that destroys your bill" is bigger than I expected. Figured it would be worth writing up because the AWS docs don't really surface this in a practical way. The obvious starting point is NumberOfObjects and BucketSizeBytes. Both are in CloudWatch under the Daily Storage Metrics namespace. Useful for baseline awareness, completely useless for catching a request spike in anything close to real time because they only update once a day. If someone is hammering your bucket with GET requests you will not see it there until long after the damage is done. The metrics that actually matter for catching runaway costs are AllRequests, GetRequests, and BytesDownloaded under the Request Metrics namespace.…