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From Emmanuel Lecharny <elecha...@gmail.com>
Subject Re: JNDI vs protocol vs Core API
Date Sat, 27 Sep 2008 00:36:28 GMT
Hi Alex,

I will add comments inline.

Alex Karasulu wrote:
> Emmanuel,
>
> On Fri, Sep 26, 2008 at 2:01 PM, Emmanuel Lecharny <elecharny@gmail.com>wrote:
>
>   
>> Hi guys,
>>
>> as i'm struggling with referrals for days now, I just discovered that there
>> are many places in the server which are not handling them correctly.
>>     
>
>
> I had removed referral handling from everywhere and placed it only at the
> protocol provider layer where it does work properly.
>   
yep, in the protocol layer, it works.
>
>   
>> Atm, the only part which is sane is the protocol layer. For embedded
>> servers, not passing through the protocol layer, I have to say that the
>> referrals are simply ignored.
>>
>>     
>
> This statement is not completely accurate.  When you embed ApacheDS as a
> server with the protocol layer then the protocol layer will handle
> referrals.  
but this is not the way people will embed the server. There is no 
advantage to pass through the protocol layer when embedding the server, IMO.
> You probably mean to say the DirectoryService and the JNDI
> Provider around the DirectoryService does not support referrals.
>   
exactly.
> Forgive me if I sound like I'm trying to correct you but just trying to make
> sure we use the right terminology to note exactly what we're talking about.
>   
Np at all. It's still a bit fuzzy so any clarification is perfect. i'm 
just trying to get as much information as possible in order to avoid to 
FU the server.
> <snip/>
>
>
>   
>> Now, the problem is that for referrals, as we have also removed the
>> ReferralInterceptor from the core server (a very good move, when you take
>> the time to think about it - and it took me two hours of discussion with
>> Alex to get it -), you have a new problem : where do we have to handle
>> referrals ?
>>
>>     
>
> Question is for what purpose do you want to handle referrals.  There are two
> ways to handle referrals:
>
> (1) Through the protocol
> (2) Via the JNDI Provided based on how JNDI is expected to behave with
> different values for the Context.REFERRAL property in the environment:
> throw, ignore, follow.
>   
well, I don't think we should see the referral handling this way. IIf 
you see it from another perspective, ie, from the user POV, a referral 
is just an indirection. Now, the way the server manage it is pretty much 
a black box for the client, as soon as the client is informed that the 
entry is not present in the server. If you consider that you want to 
embed the server, but without using JNDI, you are in a dead end. (and in 
fact, this is what we would like to do when implementing the protocol layer)
>
>   
>> The first idea was to move it to the protocol layer. So be it. But the
>> problem is that the JNDI referral handling was not anymore done (and as a
>> consequence, the associated tests were disabled, until the JNDI provider was
>> fixed to handle those referrals).
>>
>> I'm currently re-implementing the Referral handling in this JNDI layer, but
>> I suddenly discovered that this was not a good idea *at all*. And so was the
>> initial idea to implement the referrals handling into the protocol layer. We
>> now have 3 places where we must implement this process :
>> - in the protocol layer
>> - in the JNDI provider
>> - and in the CoreSession layer.
>>
>> Bad...
>>
>>     
>
> I don't think it should be done in the CoreSession.  The CoreSession should
> just provide an interface to perform operations against entries.  I think
> all entries should be treated the same with respect to this interface.
>   
That's fine, but then we have to add a thin layer on top of the 
CoreSession (be it the Operation manager or anything else, it's just 
something which will handle the referrals, when the plain entry is 
returned back, if it contains the Referral ObjectClass)). Otherwie, we 
will have to handle it in two places instead of one (ie, in the protocol 
and in JNDI).
> If code needs to do something different because the entry is a referral it's
> up to that code to do so.
>   
so the need of another layer on top of the CoreSession
> Referral handling is different depending on whether the protocol is handling
> it or whether the JNDI provider is handling it.  
In fact, if you abstract yoruself from the e=semantic, there is not that 
much a difference : either the referral is ignored, and we return the 
plain entry (=> ManageDsaIt is present _or_ the property is set to 
Ignore), or we want to be redirected, and we want the underlying server 
to give us the URL to go to (ManageDsaIt is not present _or_ the 
property is set to 'throw'). The last case (the property is set to 
follow) is pretty much the same, except that the server will 
automatically send a request to the other server (and we don't handle 
this case anyway)
> The reason why I separated
> these is because the concepts were getting mixed together and JNDI was
> contaminating the server.
>   
Removing those aspects from the server was a perfect move. But I think 
we can do better than adding handlers for referrals in both JNDI and 
protocol code. This is the reason why I posted this mail.
>
>   
>> So here is what I suggest : we should handle referrals into the CoreSession
>> layer, and deal with the thrown exceptions (LdapReferralExceptio,
>> LdapPartialResultException, etc) into the JNDI and protocol layer,
>> otherwise, we will do the job three times (and it's not a funny job ...)
>>
>> thoughts ?
>>
>>     
>
> We could do this but these exceptions are JNDI specific.  They've been
> defined to deal with a specific JNDI referral handling mode (throw
> specifically).  
IMO, 'follow' is the JNDI exception, not throw. And we don't handle the 
'follow' anyway :)
> Then the protocol handler code would need to change to
> understand these JNDI specifics.
>
> Perhaps we should just not deal with referrals at all in the CoreSession.
> Just add it to the JNDI provider for it's specific modes.
>   
That's also an option, but I'm afraid that may limit the embedded 
version of the server.
> My mindset my be wrong (and at times I do doubt it) but I feel the core API
> is purely a local access API to initialize and manipulate the DIT and its
> entries. 
I agree fully. The core should only return pure entries, not dealing 
with referrals as if they were special. But the layer on top of it must 
deal with referrals. The JNDI provider must do it, and my idea is to 
delegate this part to an intermediate layer, like :

client --> JNDI --> server API --> referral layer --> core --> ...
or
client --> server API --> referral layer --> core --> ...

>  It does not deal with following referrals since this would go over
> the network.  It does not deal with referrals as special entries since this
> is the responsibility of higher layers which deal with referrals in
> different ways.
>
>
>   
>> PS: don't be too picky with the names I'm using (like CoreSession is not
>> really the API we have built), but I'm sure that Alex can correct me. I
>> think however that the general picture is ok.
>>
>>
>>     
> Yeah just great - I understood easily what you meant.  I think reuse is good
> and I can certainly see how putting some referral handling functionality
> into the CoreSession is very tempting.  As coders we always want to reuse
> and that's good.  However sometimes reuse causes more problems than it is
> worth. 
hence my email ;)
>  I think this is a good example for that kind of situation.  Also
> sometimes we want to add additional functionality because it seems to make
> sense to have a richer interface that handles more cases.  However more
> complex interfaces are harder to work with as bricks to build other
> interfaces when their rich features start to get in the way.
>   
I think that the core session interface should not be modified. But we 
may need another thin layer on top of it. Maybe.
> <snip/>Then the referral handling code,
> whether in the protocol provider or in the JNDI provider, has a much easier
> task.  The respective code is clean and shows clearly the policy of how
> referrals are handled in their respective layers.
>   
this is the key : do we have to handle referrals in those two layers 
(protocol an JNDI) when we could manage it in an intermediate layer ? I 
think this is possoble. More than that, I think this can also be the 
seed for a new API which will replace the current JNDI API for those who 
would like to embed the server without being stuck into JNDI or to go 
through the protocol layer.


--
cordialement, regards,
Emmanuel L├ęcharny
www.iktek.com
directory.apache.org



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