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From Alex Karasulu <>
Subject Re: Implementing Interceptor Extendibility
Date Mon, 31 Oct 2011 09:17:28 GMT
On Sun, Oct 30, 2011 at 7:02 PM, Göktürk Gezer <>wrote:

> Hi Devs,
> I'd like to do some brainstorming about Interceptor extension mechanism
> which we're about to implement. Ideas those will come out from that thread
> will enlighten our way on other extension points of the ApacheDS.
> The first main issue is whether we're going to preserve old standalone
> ApacheDS or not?

Let's be really careful about the common terminology we will be using for
this discussion. When you say "standalone ApacheDS", I automatically think
about the standalone ApacheDS maven module that holds the ApacheDS main()
application launcher.

In a previous thread Pierre recommended keeping such a module but modifing
it so that it starts up an OSGi container which launches the ApacheDS
bundles. However the user experience does not change much. This merely
allows users to start up ApacheDS from the command line as before but
behind the scenes an OSGi container is launched as the main() application.
This is one option and I think I like this approach because this way we
don't have to maintain a monolithic application startup and a OSGi startup.

> Preserving API in nonOSGI environments had purpose. And with the very
> simple method we've used in API, we can also preserve ApacheDS's nonOSGI
> version too.

Sure we can do anything but will it add additional complexity and overhead
for maintaining code and what's the gain that we get? Will it make
debugging easier to maintain this non-OSGi configuration?

> Doing it will have pros and cons.

Ahh there you go with the pros and cons :-).

> The main advantage will be of course letting existing user semantics
> unchanged.

It appears to me that you're talking about more than just the application
launcher verses the OSGi based launch sequence. I guess you're referring to
all the configuration mechanism changes that will impact the different

To me these usage semantic changes are small compared to the overall gains
we will get.

> But implementing extendibility without breaking ApaheDS's nonOSGI version
> will put some limits on our extension capability on individual components.
> Besides those limitations, we'll also end up having two different runtime
> behaviour of ApacheDS. Documentation will be obviously painful. And some
> aspects won't be implementable if we choose to preserve nonOSGI version.
> For example, OSGI can give us the opportunity to don't restart server after
> some changes on configuration, which will be perfect improvement of course.
> But to make this we must go fully OSGI. List is long.

Yeah I prefer to forget about maintaining the non-OSGi version. The
"standalone ApacheDS" maven module can launch the OSGi container or Karaf
then have the OSGi environment fire up ApacheDS as a full OSGi application.

> We've talked about it before but i wanted to ask that question again,
> because its an alive topic and subject to change. Maybe some of yours mind
> could have been changed, and i want to know if it did. If we proceed like
> what we've been talked earlier, then I'm going to make ApacheDS (not
> shared) an OSGI application, and use all of its loveliness.

> To talk about Interceptors specifically, here is the design that i'm
> thinking. To manage OSGI heavy dynamic nature, we must implement our
> Interceptor related code in the closest module that is using Interceptors,
> which is InterceptorChain in our case. First step will be implement a
> component called InterceptorHub which is responsible for keeping list of
> all Interceptors installed in the container(means installed in OSGI as
> bundle). This hub will create an instance of every Interceptor per every
> ApacheDS instance and keep it.
> When an InterceptorChain want to iterate through interceptors, it will get
> them from InterceptorHub rather then DirectoryService.getInterceptors(). So
> it will get the most recent list of interceptors every time.

Just a question: why not just make the InterceptorChain act as or perform
the duties of the hub? I guess the hub is intended as an internal component
of the chain that hides some details relating to OSGi from the chain?

> Currently in our config.ldif file, there is an interceptors entries. We
> can leave those entries there for specifying mandatory interceptors. So if
> any of them is not exists at the time server is starting we can throw an
> exception saying necessary interceptor is not exist. But we won't be have
> to list additional interceptors in that list. They will be automatically
> attached to the active ApacheDS instances.
> We'll also change the Interceptor implementations to be an IPojo
> component. Those components will publish two main properties. First is
> their ordering and the second is their bypass list.
> Ordering is something we've talked about but i couldn't get a clear
> explanation about it. What i suggest is like this: we publish three kind of
> ordering from interceptor: strict, level and relax. "strict" means strict
> ordering, this will be published from core interceptors and that strict
> order will create a level. If we publish it as level we must also attach an
> index with it, such as level-3. If we publish it as relax it means it can
> be called anywhere in the chain after the strict ones. So lets suppose we
> documented the core interceptor ordering like:
> 1- InterceptorA
> 2- InterceptorB
> 3- InterceptorC
> So there is 3 level in interceptor chain. If somebody implement an
> additional interceptor named InterceptorD and publish its ordering
> as level-2, it is saying it must be called after InterceptorA(level-1). So
> the new list will be:
> 1- InterceptorA
> 2- InterceptorD
>     InterceptorB
> 3- InterceptorC
> I didnt't numbered B as 3 but rather i grouped it with the InterceptorD,
> putting InterceptorD above it. Why? What happens if vendor wants to
> implement two custom interceptor named InterceptorD and InterceptorE and
> want 2 of them to be called right after InterceptorA(after 1th level) but
> want InterceptorD to be called first. So we must suborder those 2
> interceptors. That's why i treated the first core list as levels rather
> than orders ! So the ordering which is published from custom interceptor is
> not sufficient to describe good ordering. We must also publish subordering
> which specifies the ordering between that specific vendor's interceptors.
This is rather interesting. This approach never occurred to me but it's
appealing. I'll give this some thought and also would like to hear from
others on the list about it.

So lets suppose InterceptorD(ordering="strict-2",subordering="foo1"),
> InterceptorE(ordering="strict-2", subordering="foo2"). Both interceptors
> want to be executed after 1th level(means after InterceptorA). But they
> also publish subordering with the same token, "foo". So they will be called
> after 1.level which is InterceptorA, but also they will be called in order
> InterceptorD - interceptorE with the help of their subordering string.
> We can also depend on string ordering of Interceptor class names here, but
> i think its not good. We can also merge ordering and subordering in one,
> like (ordering="level2-foo1"). This way, we can define infinite subordering
> by separating them with dashes.
> So this is my way to give interceptor writers a way to describe their
> interceptor's order in the chain. If i'm missing something here please say
> it and i'll put it in consideration too.

Very interesting ideas. I think we're in total agreement regarding going
fully OSGi and making the standalone launcher just start the container then

I think I really like this ordering scheme you've proposed but I have to
give it some time and thought. Also interested in other commentary.  This
looks great and thanks for your effort.

Best Regards,
-- Alex

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