Cumulative sum of AWS Cloudwatch Metric

Question

AWS Cloudwatch receives a count of 1 every time I start an image download. I am downloading 1,000s of images (on a cluster of EC2 instances) and would like to track the total progress.

I can't find any documentation on how to plot the cumulative sum of a metric. The AWS Cloudwatch Math Expressions looked promising, but they do not have an integrate function.

Currently, I can plot the sum of the started image downloads but only for periods, as seen below. Ideally, I'd like to plot the integral of this plot:

Answer 1

You can get a cumulative sum over the current range by using the SUM() function that is operated over the original range containing only the number One (1). Remember, you're looking for a single number in the end, so it's not much of a graph, but you need to turn the single value sum back into a time-series.

• Define m1 as your metric. This is the metric you will want to use SUM() on.
• Define an expression e1 as m1/m1. This results in a time-series with every value equal to 1. This is what will allow you convert that SUM back to a time-series.
• Define an expression e2 as SUM(m1) / e1. This is, effectively, the cumulative sum of m1 divided by one for every data-point in the original time-series. It will be a horizontal line on the graph, which will have every point on that horizontal line being the cumulative sum of metric m1. This is required because Cloudwatch can only plot a time-series on the chart, not a single value.
• Make m1 and e1 invisible. You need them, but you don't need to see them.
• Finally, change the chart type from Line to Number, since you only wanted the cumulative sum anyway.

The reason you can't use SUM() directly is because it is a single value. By dividing by a time-series containing all 1's, the entire graph is the result of the SUM(). Then, changing the chart to a Number effectively hides all the math and presents only the "final result".

Answer 2

You are correct. All Amazon CloudWatch metrics are for a defined period.

The maximum period for a metric is one day, so this is not suitable for a cumulative counter that you wish to continue beyond one day.

You would need to find an alternate method of storing the count, such as an Amazon DynamoDB table. Use an atomic counter via UpdateItem to increment the count.