[GitHub] [jena] afs commented on issue #534: [WIP] Proof of concept for prometheus endpoint

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afs commented 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 inshared artifacts so the acknowledge clause applies.   No problem, just a task to do.
   
   #### 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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