CloudWatch 不会为自定义资源跨维度聚合数据——这究竟意味着什么？

Question

Let's lay out some definitions before getting to the question itself. Based on the CloudWatch concepts page in the docs.

So we have metric , which is a set of time-oredered set of datapoints. A metric is uniquely identitfied by it's namespace, name, and set of dimensions .

A dimension is a key=value pair which is part of the identity of a metric. So for example a metric called ServerStats with the dimensions Domain=Frankfurt,Server=Prod is not the same metric as the metric called ServerStats with the dimensions Domain=Rio,Server=Beta .

Now let's move to an example, and from there to the question:

Let's ride on the example given in the docs. Say I have 2 servers (Prod and Beta) in Frankfurt, and 2 servers (Prod and Beta) in Rio, which are publishing data points (which represent some count) regularly to CloudWatch, as such:

Dimensions: Server=Prod, Domain=Frankfurt, Unit: Count, Timestamp: 2016-10-31T12:30:00Z, Value: 105
Dimensions: Server=Beta, Domain=Frankfurt, Unit: Count, Timestamp: 2016-10-31T12:31:00Z, Value: 115
Dimensions: Server=Prod, Domain=Rio,       Unit: Count, Timestamp: 2016-10-31T12:32:00Z, Value: 95
Dimensions: Server=Beta, Domain=Rio,       Unit: Count, Timestamp: 2016-10-31T12:33:00Z, Value: 97

I've created a script to simulate this situation. If I go to my CloudWatch console, I can see my metrics:

So now that we have our example set up, I want to understand the statement in the docs:

For metrics produced by certain AWS services, such as Amazon EC2, CloudWatch can aggregate data across dimensions. For example, if you search for metrics in the AWS/EC2 namespace but do not specify any dimensions, CloudWatch aggregates all data for the specified metric to create the statistic that you requested. CloudWatch does not aggregate across dimensions for your custom metrics.

If I understand correctly, aggregating dimensions mean to specify a namespace of metrics, or a metric name, without any dimensions, and get an aggregate of the metrics of that name. So for example, in the AWS/EC2 namespace there are metrics called CPUUtilization . Some of them have the dimension InstanceId , and some have the dimension ImageId , and CloudWatch can aggregate those metrics to give us an overall CPUUtilization of all those metrics.

Now, in our example, It is possible to get an aggregate of all the Server=Prod metrics. If I run the following query:

SELECT SUM(ServerStats) FROM DataCenter WHERE Server = 'Prod'

I get an aggregate of 2 metrics: Server=Prod,Location=Frankfurt and Server=Prod,Location=Rio :

You can see that the Query1 metric's value is the addition of both other values (which are Prod Frankfurt and Prod Rio)

So I don't quite understand what is meant by this paragraph I've cited above, about that CloudWatch cannot aggregate data across dimensions for custom metrics.

Can someone clarify this?

Answer 1

It is poorly worded and out of date documentation on CloudWatch side. What they mean is that they won't aggregate and store your data as a new metric. There will be no one particular metric that will contain the aggregation you need.

You can however use metric math and metric insight feature to calculate the aggregation on the fly, like you did above. These methods have limits though. Metric math is limited to 500 metrics. Metric Insights can aggregate up to 10000 metrics, but it's limited to latest 3 hours of data.