[
https://issues.apache.org/jira/browse/MESOS-1384?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14137280#comment-14137280
]
Bernd Mathiske commented on MESOS-1384:
---------------------------------------
[~karya], [~nnielsen], and I have yet another design that I will describe here.
We also have a prototype that seems to work for at least some simple example
modules. We stripped down the API for the module writer to as little as
possible and we also reduced versioning to as little as we could think of that
still makes sense and can be expanded later.
Definitions
--------------
Mesos version := Mesos releases.
Module API version := Bumped when the module management system changes.
Role := The purpose that a module fulfills. In a given Mesos implementation
this is tied to a specific object type, e.g. “Allocator”, “Isolator”,
“Authenticator”.
The API from the point of view of a module writer
To write a Mesos module, include the module API header file and a module role
type declaration, then place one macro call inside your library source code to
capture version information and add another macro call to declare a module.
Then add a function body that returns a module instance. That’s all. Here is a
minimal example that declares a module with the role “Calculator” and the name
“example”.
#include <mesos/module.hpp> // module system API
#include <calculator.hpp> // module role type declaration is in here
MESOS_MODULE_LIBRARY() // declares the module library (and its versions)
class ExampleCalculator: public Calculator
{
public:
// An example function that the module implements.
virtual int compute(char a, long b)
{
return a + b;
}
};
MESOS_MODULE(Calculator, example) // declares the module
{
return new ExampleCalculator(); // creating and returning the module instance
}
Additional compatibility checks
By default the above only works when the module client Mesos version and the
Mesos version against which the library has been compiled are exactly the same.
However, with this extra declaration, you can enable backwards compatibility
controlled by a table inside Mesos that allows earlier library versions.
MESOS_IS_MODULE_COMPATIBLE(example)
{
return true;
}
Here, instead of simply returning true, the module could also perform its own
checks, and return false under certain conditions. In the latter case the
module will not be admitted, no matter what the results of any other checks by
Mesos indicate. The module’s own checks are open-ended. In particular, they may
include queries about other loaded libraries and modules and their respective
versions. We will later provide an API for that.
The API from the point of view of a Mesos implementor
Only modules with pre-declared roles and binding sites can be loaded by Mesos.
There is no means of dynamic discovery or binding of extra roles. To declare a
loadable module, a Mesos developer needs to specify an abstract class with at
least one virtual method. Here is an example that matches the module code above.
class Calculator {
public:
Calculator() {}
virtual ~Calculator() {}
virtual int foo(char a, long b) = 0;
};
That’s all. To employ a specific instantiation of this module role/type, you
can write something like this.
#include <module/module_manager.hpp>
Try<Calculator*> module =
mesos::ModuleManager::loadModule<Calculator>("example");
if (module.isError()) {
… // error handling
}
Calculator calculator = loadedModule.get();
And then you can already use the module.
int n = calculator.compute(‘A’, 10);
Module Libraries and Module Versioning
Before loading the above module, a dynamic library that contains the module
needs to be loaded into Mesos. This happens early in Mesos startup code. The
Mesos developer does not need to touch that code when introducing new module
roles.
However, the developer is responsible for registering what versions of any
given module are expected to remain compatible with Mesos as it progresses.
This information is maintained in a table in src/module/module_manager.cpp. It
contains an entry for every possible module role that Mesos recognizes, each
with a corresponding Mesos release version number. This number needs to be
adjusted by the Mesos developer to reflect the current Mesos version whenever
compatibility between Mesos and modules that get compiled against it gets
broken. Given that module implementation for older Mesos versions can still be
written in the future, this may be impossible to tell and so in doubt it is
best to just bump the required module version to the current Mesos version. But
if one can be reasonably sure, assuming cooperative module developers, that a
certain kind of module will continue to function across several Mesos versions,
the table provides an easy way to specify this.
> Add support for loadable MesosModule
> ------------------------------------
>
> Key: MESOS-1384
> URL: https://issues.apache.org/jira/browse/MESOS-1384
> Project: Mesos
> Issue Type: Improvement
> Affects Versions: 0.19.0
> Reporter: Timothy St. Clair
> Assignee: Niklas Quarfot Nielsen
>
> I think we should break this into multiple phases.
> -(1) Let's get the dynamic library loading via a "stout-ified" version of
> https://github.com/timothysc/tests/blob/master/plugin_modules/DynamicLibrary.h.
> -
> *DONE*
> (2) Use (1) to instantiate some classes in Mesos (like an Authenticator
> and/or isolator) from a dynamic library. This will give us some more
> experience with how we want to name the underlying library symbol, how we
> want to specify flags for finding the library, what types of validation we
> want when loading a library.
> *TARGET*
> (3) After doing (2) for one or two classes in Mesos I think we can formalize
> the approach in a "mesos-ified" version of
> https://github.com/timothysc/tests/blob/master/plugin_modules/MesosModule.h.
> *NEXT*
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
