afs edited a comment on issue #534: [WIP] Proof of concept for prometheus endpoint URL: https://github.com/apache/jena/pull/534#issuecomment-475252662 This is looking good. I have it working in Fuseki Full (webapp form) and in Fuseki Main (the more configurable servlet form) - details for running in Fuseki Main below. #### Configuration Something we have to think about carefully is using `ServiceLoader`. Jena already has an [init mechanism](https://jena.apache.org/documentation/notes/system-initialization.html) and it allows controlling the order of initialization. It is quite simple - there is an `init()` method which might be a limitation. `ActionPrometheus` could use this to wire in a function lambda to call it's `perform()` into a registry. I don't know at the moment which of a "different ServiceLoader" or "single ServiceLoader" is better. #### Naming Rather than hide under `/$/extras/` I think I prefer `/$/metrics/` or something else to cover just statistics reporting. This is a significant functionality, not to be hidden under "extras". #### Dispatch The `ActionExtras.perform` could dispatch on the final component of the URI. No fixed string! It might even be better to look for the dispatch string in `init(ServletConfig)` to find the prefix for this servlet instance. #### HTTP MIME Type The HTTP return is set as `application/json;charset=utf-8` but it isn't JSON. If there isn't a specific MIME type, then `text/plain;charset=utf-8` would be better (?). #### jena-prometheus jar The shaded JAR has got slf4j classes in it. I don't think they are needed. Can they be excluded ? either in the POM or in the shader. Maybe it is an effect of the `<scope>provided</scope>`. [jena-fuseki2/jena-fuseki-server/pom.xml](https://github.com/apache/jena/blob/master/jena-fuseki2/jena-fuseki-server/pom.xml) has an example of shading Jena including license control. #### Licensing `org.hdrhistogram` and `org.latencyutils` are 2-clause BSD license and we a shipping them in shared artifacts so the acknowledge clause applies. No problem, just a task to do. The right LICENSE and NOTICE need to end up in the shaded jar. #### Fuseki Main example Illustration of running in `FusekiMain`: public static void mainServer(String ... a) { FusekiLogging.setLogging(); FusekiServer server = FusekiServer.create() .add("/ds", DatasetGraphFactory.createTxnMem()) .port(3030) .verbose(true) .addServlet("/$/extras/*", new ActionExtras()) .build(); try { server.start().join(); } finally { server.stop(); } }
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