[NIFI-10666] PrometheusReportingTask does not use UTF-8 encoding on /metrics/ endpoint

• Type: Bug
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 1.17.0, 1.16.3, 1.18.0, 1.23.2, 1.25.0, 2.0.0-M2
• Fix Version/s: 1.27.0, 2.0.0-M4
• Component/s: Extensions
• Labels: encoding prometheus utf-8
• Environment: JVM with non-UTF-8 default encoding (e.g. default Windows installation)
• Assignee: Jim Steinebrey
• Reporter: René Zeidler
• Created: 19/Oct/22 13:03
• Updated: 27/Jun/24 14:46
• Resolved: 20/Jun/24 18:24

Description

We have created a default PrometheusReportingTask for our NiFi instance and tried to consume the metrics with Prometheus. However, Prometheus threw the following error:

ts=2022-10-19T12:25:18.110Z caller=scrape.go:1332 level=debug component="scrape manager" scrape_pool=nifi-cluster target=http://***nifi***:9092/metrics msg="Append failed" err="invalid UTF-8 label value" 

Upon further inspection, we noticed that the /metrics/ endpoint exposed by the reporting task does not use UTF-8 encoding, which is required by Prometheus (as documented here: [Exposition formats | Prometheus|https://prometheus.io/docs/instrumenting/exposition_formats/]).

Our flow uses non-ASCII characters (in our case German umlauts like "ü"). As a workaround, removing those characters fixes the Prometheus error, but this is not practical for a large flow in German language.

Opening the /metrics/ endpoint in a browser confirms that the encoding used is not UTF-8:

> document.characterSet
'windows-1252' 

---

The responsible code might be here:

https://github.com/apache/nifi/blob/2be5c26f287469f4f19f0fa759d6c1b56dc0e348/nifi-nar-bundles/nifi-prometheus-bundle/nifi-prometheus-reporting-task/src/main/java/org/apache/nifi/reporting/prometheus/PrometheusServer.java#L67

The PrometheusServer used by the reporting task uses an OutputStreamWriter with the default encoding, instead of explicitly using UTF-8. The Content-Type header set in that function also does not get passed along (see screenshot).

Issue Links

• relates to: NIFI-12669 EvaluateXQuery processor incorrectly encodes result attributes
• relates to: NIFI-12670 JoltTransform processors incorrectly encode/decode text in the Jolt Specification