directory-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Göktürk Gezer <gokt...@apache.org>
Subject Re: Questions about the design of the "component-hub"
Date Mon, 28 Nov 2011 16:24:15 GMT
On Mon, Nov 28, 2011 at 11:56 AM, Alex Karasulu <akarasulu@apache.org>wrote:

> On Mon, Nov 28, 2011 at 2:25 AM, Göktürk Gezer <gokturk@apache.org> wrote:
> > Hi Devs,
> > As i go through implementing component-hub, i couldn't think clearly
> about
> > some things, so i need some pointers on them. I've made some decisions on
> > code based on my experiments but i need to hear more from you on these
> > specific issues.
> > component-hub is responsible for providing main extendibility facilities,
> > which contains the DIT configuration hooks. component-hub is providing
> the
> > DIT configuration hooks as a way for users to configure the component
> > instances.( the instance of a EventInterceptor for example). Then some
> > DirectoryListener implementation will listen for specified DIT entries
> for
> > changes and then reconfigure the instances. First question is whether
> should
> > i use PersistentSearch or implement DirectoryListener interface myself.
> > DirectoryListeners are called by EventInterceptor so that will make the
> > listening code have to go through LdapCoreSession. How PersistentSearch
> is
> > different? Is there any mechanism other than these two??
>
> OBJECTIVE: Detect changes to LDAP backed component configurations and
> react to them.
>
> PersistentSearch is a protocol mechanism (over the wire) used to
> detect changes to entries (in remote LDAP servers) implemented using
> controls. You issue a search request with the persistent search
> control and specifying an area of the DIT to watch. The search never
> completes unless you abort it and it just sends you back entries if
> they change with a control indicting the nature of the change. It's a
> hack if you ask me. So this mechanism can work for remote and local
> servers but you'll go through the wire, the protocol, and will pay a
> big price for it from the number of threads used to encoding/decoding
> etc etc.
>
> Another way to detect changes but only to a local (in the same process
> space) DIT, is by using the event listener mechanism provided by the
> Event subsystem of the DirectoryService. This does essentially the
> same thing that persistent search does for the same local directory
> service but better and with less overhead. "Better", in that it allows
> you to see exactly what attributes changed and how which is a tiny bit
> more resolution than from the response control you get back with the
> persistent search mechanism.
>
> SO YOU SHOULD MOST LIKELY USE THE EVENT MECHANISM. This because:
>
> 1). You're local to the data anyway, configuring local components
> 2). Less overhead, more efficient faster ... yada yada
> 3). It's a better programatic interface.
> 4). Does everything you need it to do.
>
> This of course is for the solid state of configuration in DIT
> operation after the server has already bootstrapped and is online. To
> actually bootstrap the server you'll probably need to read data kept
> in LDIF files in the configuration partition folder on disk as if it's
> stored in an LDAP server or a live partition but there is no LDAP
> server yet, nor is there a live partition yet.
>
> Makes sense?
>

Okey then, i just wondered what PersistentSearch is meant to. compenent-hub
uses event mechanism already. I'm leaving it like that.

>
>
> > Second thing is like chicken-and-egg problem, specifically about
> > Interceptors. To add the custom generated schemas and the DIT
> configuration
> > hooks for instances, the current code access the server through session
> > (LdapCoreSessionConnection). But for LdapCoreSession to work as expected,
> > the interceptors are needed(at least, to work in the way it meant to).
> How i
> > solved this issue is by deferring all the write operations until
> > DirectoryService is fully initialized. component-hub will be able to
> access,
> > instantiate and use the interceptors( and other types of components).
> > However, until its being notified of full initialization of
> > DirectoryService, it defers the custom schema and DIT configuration hook
> > entries to be written to DIT. Is there any problem with that approach
> that i
> > couldn't see? This approach is clear but it may introduce some rendezvous
> > like concurrency management into code, increasing server startup about 1
> > second. The other way to do is initializing component-hub with the
> > DirectoryService reference having its Schema and Config Partition set.
> And
> > then we access the partitions directly without going through nexus and
> > interceptors. But to handle it with the second way, we must be sure
> nothing
> > is dependent on Interceptors until server is fully initialized.(*1).
> Which
> > way would you choose?
>
> I'd implement an LDIF file reading LdapConnection that is just
> READ-ONLY with just simple operations enabled to read what we need
> from the configuration at startup. Then you can use this object at
> initialization/bootstrap time to get at the configuration data you
> need in the LDIF files stored on disk. Then there is no chicken and
> egg problem. Then after bootstrapping you just throw away this
> LdifFileLdapConnection or whatever you want to call it and can use the
> session connections instead for steady solid state operation after
> startup.
>

I couldn't get my answer on that actually. First of all, the configuration
information for component-hub and individual components are not all in ldif
format. All the configurations end up being an entry in DIT but i thought
saving the configuration like that is not feasible for many reasons. We
have to do many housekeeping to leave DIT clear, after every OSGI operation
if we go that way. That's why configuration is kept per component:
* When we uninstall an individual component from the OSGI framework, all of
its schema and configuration entries in DIT are cached, leaving DIT as this
component has never been installed.
* When we install that component later time, its cache is stored in schema
and config partitions.

Schema caches are kept as ldif files, but configuration caches are kept as
java property files. This is gives component-hub an ability to fetch the
configuration but configuration alone, and then hook it somewhere in the
DIT dynamically. This caching system is designed for server runtime in
focus, but its making server startups-shutdowns bulky.

But in the time i'm writing this, i realized this system is not resilliant
for disaster scenarios. I must change it to load configurations from
server's config partition itself and do some housekeeping to make DIT
reflect the current status of the OSGI container as much as it can. I like
it when ideas come to your mind in the middle of writing an email :)


> > (*1) init() method of SubEntryInterceptor is using nexus to do some
> search.
> > But at that point in the code no partition is added to the nexus, not
> even
> > schema partition is set on DirectoryService. What exactly the code there
> is
> > doing, is it broken or am i missing something?
>
> Sorry don't have the code up in your branch. Let me set that up and I
> can be more useful to you.
>

Its on the trunk also.


>
> --
> Best Regards,
> -- Alex
>

Regards,
Gokturk

Mime
View raw message