[jira] [Issue Comment Deleted] (MESOS-1905) Enable module metadata to be accessed by the user

["Gavin (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MESOS-1905?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Gavin updated MESOS-1905:
-------------------------
    Comment: was deleted

(was: www.rtat.net)

> Enable module metadata to be accessed by the user
> -------------------------------------------------
>
>                 Key: MESOS-1905
>                 URL: https://issues.apache.org/jira/browse/MESOS-1905
>             Project: Mesos
>          Issue Type: Improvement
>          Components: modules
>            Reporter: Till Toenshoff
>            Priority: Minor
>
> h4. Motivation
> I would love to be able to get custom, meta-data information from a module without needing to create a kind instance. 
> h4. Use Case
> When slave authentication is activated on the master, the user has to supply credentials (as our current implementation demands them). Given that alternative authentications will not rely on such credentials, we need a way to make sure that only Authenticator's that need this information will demand them from the user. I would like to prevent instantiating a kind for that module as I will not make further use of that instance at that (early) point - let me call this the "capabilities instance". 
> Options are; (a) delete it right away or (b) hold on to it.
> a: definitely is possible but does not seem elegant to me right now.
> b: holding on onto that instance and reusing it later is not really a good fit as the master will instantiate new Authenticator's per connected slave. So for the first slave, I would have to use that capabilities instance and for all further slave connections I would have to create new Authenticators (possible but ugly as hell).
> So by extending the Module structure specialization with a {{bool needsCredentials()}}, I could solve this rather neatly:
> {noformat}
> template <>
> struct Module<Authenticator> : ModuleBase
> {
>   Module(
>       const char* _moduleApiVersion,
>       const char* _mesosVersion,
>       const char* _authorName,
>       const char* _authorEmail,
>       const char* _description,
>       bool (*_compatible)(),
>       bool (*_needsCredentials)(),
>       Authenticator* (*_create)())
>     : ModuleBase(
>         _moduleApiVersion,
>         _mesosVersion,
>         "Authenticator",
>         _authorName,
>         _authorEmail,
>         _description,
>         _compatible),
>       needsCredentials(_needsCredentials),
>       create(_create)
>   { }
>   bool (*needsCredentials)();
>   Authenticator* (*create)();
> };
> {noformat}
> Within the implementation I would simply use that function just like we are using {{compatible()}} already.
> h4. Status Quo
> ModuleManager does not support returning the {{ModuleBase*}} to the user. The only way for such information to be returned by a module is to instantiate it and ask its implementation for it - that is, the module interface needs to include a method returning such info. 
> h4. Idea
> For being able to get information on a module without an instance kind, a method within the ModuleManager that looks something like this would help:
> {noformat}
> static Try<ModuleBase*> peek(const std::string& moduleName) 
> {noformat}
> h4. Discussion
> Am I possibly attempting something too hacky here - are there better alternatives I missed?



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