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Document common "I see no data" question #660

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Mar 3, 2024
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Document common "I see no data" question #660

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13 changes: 13 additions & 0 deletions README.md
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Expand Up @@ -145,6 +145,19 @@ For certain metrics which update relatively rarely, such as from S3,
a timestamp. This is as the true timestamp from CloudWatch could be so old that
Prometheus would reject the sample.

#### FAQ: I can see the metrics in `/metrics` but not in the Prometheus web console

The metrics will be visible in Prometheus if you look more than `delay_seconds` in the past.
Try the graph view.

This is an unfortunate result of a fundamental mismatch between CloudWatch and Prometheus.
CloudWatch metrics converge over time, that is, the value at time `T` can change up to some later time `T+dT`.
Meanwhile, Prometheus assumes that once it has scraped a sample, that is the truth, and the past does not change.

To compensate for this, by default the exporter [delays fetching metrics](https://github.com/prometheus/cloudwatch_exporter/blob/master/README.md#timestamps), that is, it only asks for data 10 minutes later, when _almost_ all AWS services have converged.
It also reports to Prometheus that this sample is from the past.
Because Prometheus, for an instant request, only looks back 5 minutes, it never sees any data "now".

### Special handling for certain DynamoDB metrics

The DynamoDB metrics listed below break the usual CloudWatch data model.
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