Dear AWS User,

I understand that you are observing high CPU usage on your DocumentDB cluster the moment your Application begins to read from the ChangeStreams even when the cluster is relatively idle. From the graphs, I do see that we are maxing out in terms of vCPUs on the primary instance and hence we will need to identify the queries reported at Performance Insights (PI) to have more visibility on the issue.

While the usage of ChangeStreams does consume some resources it shouldn't necessarily lead to maxing out of CPUs and we will need to look over the associated queries.

When you attempt to get the full query from PI dashboard, there is a probability that it will get truncated. This is because by default, each row in the Top queries table shows 500 bytes of query text for each query statement. When a digest statement exceeds 500 bytes, you can view more text by opening the statement in the Performance Insights dashboard. In this case, the maximum length for the displayed query is 1 KB.

If you want to view a full query statement, you can also choose Download [ 1 ] [ 2 ].

Kindly note that performance insights does not an option to map a specific query to its correlating opid. In light of this, you can find it useful using the following step sequence in helping identify and mitigating queries that cause high CPU issues here [ 3 ].

These will help you to get more insights into what queries are leading to the excessive CPU usage. Then accordingly, I will advice you to followup with your DBAs/ AWS Support Team via a support ticket to troubleshoot more on the issue.

Have a great and wonderful day ahead!

Reference links:

[ 1 ]: Overview of the Top queries tab - Accessing statement query text - https://docs.aws.amazon.com/documentdb/latest/developerguide/performance-insights-top-queries.html#performance-insights-top-queries-accessing-text

[ 2 ]: Overview of the Top queries tab - Viewing and downloading statement query text - https://docs.aws.amazon.com/documentdb/latest/developerguide/performance-insights-top-queries.html#performance-insights-top-queries-viewing-downloading

[ 3 ]: Performance and Resource Utilization - How Do I Determine the Cause of High CPU Utilization on One or More Cluster Instances? - https://docs.aws.amazon.com/documentdb/latest/developerguide/user_diagnostics.html#user_diagnostics-cpu_utilization