Cost of GetMetricData Api calls when using metric math? - amazon-web-services

Question

I'm working on ingesting metrics from Lambda into our centralized logging system. Our first idea is too costly so I'm trying to figure out if there is a way to lower the cost (instead of ingesting 3 metrics from 200 lambdas every 60s).
I've been messing around with MetricMath and have pretty much figured out what I want to do. I'd run this as a k8s cron job like thing and variabilize the start and end time.
How would this be charged? Is it the number of metrics used to perform the math or the number of values that I output?
i.e. m1 and m2 are pulling Errors and Invocations from 200 lambdas. To pull each of these individually would be 400 metrics.
In this method, would it only be 1, 3, or 401?
{
"MetricDataQueries": [
{
"Id": "m1",
"MetricStat": {
"Metric": {
"Namespace": "AWS/Lambda",
"MetricName": "Errors"
},
"Period": 300,
"Stat": "Sum",
"Unit": "Count"
},
"ReturnData": false
},
{
"Id": "m2",
"MetricStat": {
"Metric": {
"Namespace": "AWS/Lambda",
"MetricName": "Invocations"
},
"Period": 300,
"Stat": "Sum",
"Unit": "Count"
},
"ReturnData": false
},
{
"Id": "e1",
"Expression": "m1 / m2",
"Label": "ErrorRate"
}
],
"StartTime": "2020-02-25T02:00:0000",
"EndTime": "2020-02-26T02:05:0000"
}
Output:
{
"Messages": [],
"MetricDataResults": [
{
"Label": "ErrorRate",
"StatusCode": "Complete",
"Values": [
0.0045127626568890146
],
"Id": "e1",
"Timestamps": [
"2020-02-26T19:00:00Z"
]
}
]
}
Example 2:
Same principle. This is pulling the invocations by of each function by FunctionName. It then sorts them and outputs the most invoked. Any idea how many metrics this would be?
{
"MetricDataQueries": [
{
"Id": "e2",
"Expression": "SEARCH(' {AWS/Lambda,FunctionName} MetricName=`Invocations` ', 'Sum', 60)",
"ReturnData" : false
},
{
"Id": "e3",
"Expression": "SORT(e2, SUM, DESC, 1)"
}
],
"StartTime": "2020-02-26T12:00:0000",
"EndTime": "2020-02-26T12:01:0000"
}
Same question. 1 or 201 metrics?
Output:
{
"MetricDataResults": [
{
"Id": "e3",
"Timestamps": [
"2020-02-26T12:00:00Z"
],
"Label": "1 - FunctionName",
"Values": [
91.0
],
"StatusCode": "Complete"
}
],
"Messages": []
}

Answer 1

Billing is on metrics requested: https://aws.amazon.com/cloudwatch/pricing/
In the first example, you're requesting only 2 metrics. These metrics are aggregates of per lambda function metrics, but as far you're concerned, that's only 2 metrics and you will be billed for 2. You're not billed for the metric math, only for metrics you request.
In the second example, the number of metrics the search returns is the amount you will be billed for, 200 in your case.