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Date: Tue, 29 Nov 2011 10:59:55 +0100
From: Emmanuel Lecharny <elecharny@gmail.com>
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To: Apache Directory Developers List <dev@directory.apache.org>
Subject: Re: Questions about the design of the "component-hub"
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On 11/28/11 10:56 AM, Alex Karasulu 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.
+1. This is definitively the way to go.
> 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.
The Config is read *before* the DS is started. This is actually the very 
first thing we do. You must assume the configuration partition is 
available, and it's browsable. You don't need the LDAP server or the DS 
to access the information stored there, as we have it managed by an 
AvlPartition (with its own searchengine and all what is needed).

>
> Makes sense?
>
>
>> 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.
We already have it. This is the way the config in dit is handled.
>   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.

Keep in mind that since 2.0.0-M1, we use the configuration in DIT, which 
is read before we initialize anything, as we need to read the 
configuration to be able to "cofigure" the server. This solve the 
chicken/egg issue.


-- 
Regards,
Cordialement,
Emmanuel Lécharny
www.iktek.com