Till Toenshoff created MESOS-1905:
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Summary: 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: Blocker
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, 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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