Accept header mismatch for OpenMetrics

[jonatan-ivanov]

The latest(2.30.0) version of Prometheus sends out the following request when scraping:

GET /actuator/prometheus HTTP/1.1
Host: host.docker.internal:8080
User-Agent: Prometheus/2.30.0
Accept: application/openmetrics-text; version=0.0.1,text/plain;version=0.0.4;q=0.5,*/*;q=0.1
Accept-Encoding: gzip
X-Prometheus-Scrape-Timeout-Seconds: 10

In the Accept header, the version of openmetrics-text is 0.0.1 (see scrape.go).
In the Java client, the Content-Type constant does not match to this, its version is 1.0.0 (see TextFormat):

CONTENT_TYPE_OPENMETRICS_100 = "application/openmetrics-text; version=1.0.0; charset=utf-8";

The python client uses 0.0.1 too. Shouldn't all use the same version (1.0.0) or should the Java client use (0.0.1)?

It also seems that TextFormat::chooseContentType takes this into account:

client_java/simpleclient_common/src/main/java/io/prometheus/client/exporter/common/TextFormat.java

Lines 37 to 41 in 7a29af0

	for (String accepts : acceptHeader.split(",")) {
	if ("application/openmetrics-text".equals(accepts.split(";")[0].trim())) {
	return CONTENT_TYPE_OPENMETRICS_100;
	}
	}

[dhoard]

@jonatan-ivanov In my opinion The calling clients should be fixed to send application/openmetrics-text; version=1.0.0

Some analysis (my thought process forming my opinion) from looking at TextWriter.

--

TextWriter currently only supports two formats...

• text/plain;version=0.0.4

or...

• application/openmetrics-text; version=1.0.0

Because there are currently only two versions of the metrics format supported, any version in the accept header is ignored.

--

Per RFC2616 https://datatracker.ietf.org/doc/html/rfc2616/#section-10.4.7 the server has two options...

• look at the content-type version, and send back a 406 Not Acceptable if it can't return a response to satisfy the request.

or...

• send back a version that may not be acceptable to the client. The client is responsible for determining if it can handle the content-type returned by the server. (per the Note in RFC2616 https://datatracker.ietf.org/doc/html/rfc2616/#section-10.4.7)

The code has taken the second option, based on the content-type prefix mapping...

• application/openmetrics-text -> application/openmetrics-text; version=1.0.0; charset=utf-8
• text/plain -> text/plain; version=0.0.4; charset=utf-8

--

Once additional versions are supported, then I feel that TextWriter should be changed to parse/process the version correctly.

[jonatan-ivanov]

@dhoard By calling client you mean the Prometheus server right?
So scrape.go should send application/openmetrics-text; version=1.0.0, right?

[dhoard]

@dhoard By calling client you mean the Prometheus server right? So scrape.go should send application/openmetrics-text; version=1.0.0, right?

That's my opinion.

As it stands, the version doesn't really matter... at least based on the code in TextWriter.

I'm not a maintainer, so I probably missed something. Maybe this is for compatibility with older versions?

[jonatan-ivanov]

Yepp, got it, I agree with you.

[fstab]

I agree, too. According to the OpenMetrics Spec:

The content type MUST be: application/openmetrics-text; version=1.0.0; charset=utf-8