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From Alex Karasulu <akaras...@gmail.com>
Subject Re: Replication configuration : second thought
Date Mon, 16 Mar 2009 15:47:35 GMT
Hi Emmanuel,

On Mon, Mar 16, 2009 at 7:16 AM, Emmanuel Lecharny <elecharny@apache.org>wrote:

> Hi guys,
>
> after having thought about the configuration a bit more, I'm not sure that
> it should be associated with a replication interceptor. I'm not even sure
> that such an interceptor makes sense.
>
> If we consider both side, a server can be either a client, a server, or
> both. But we will consider each case as distinct.
>
> If the server is a client (a replica, or a consumer), then  it connects to
> a master server using a RefreshAndPersist control associated to a search
> request. This connection can be managed in a specific interceptor, but in
> any case, the searchResults will have to go through the whole chain.  As
> it's also a persistent connection, it would be better to deal with it at the
> DirectoryService level.
>

> If the server is a master (a producer), then we will consider that a client
> search request has been received, and persists. All the resulting entries
> are filtered by the chain, and AFAICT, we don't need to add an interceptor
> to filter the results. We just have to consider that the clinet is a
> standard client.
>

Regarding Repl Terminology
----------------------------------------

OK let's get religious about using replication specific terminology.
Instead of talking about client, server, master, slave, let's use the terms
supplier and consumer.  The reason why I like these terms besides their
common use is because it clearly denotes directionality and breaks down
elements of the replication agreements to their atomic components. So I'd
like to think in terms of a consumer configuration (with replication
agreement) and a supplier configuration (also with a repl-agmt).


Regarding Configuration
-----------------------------------

On a separate note, I've been torn between two ways of thinking about
configuration for some time.  Regardless of whether we're talking about
replication subsystem or not, we could apply this general discussion to any
feature/facet of the server which also contains an interceptor.  So this
discussion applies across the board as a configuration issue.

So where do we configure a subsystem? In the interceptor configuration? Or
as a separate component under the directory service.  Over the years I've
made many mistakes with this stuff.  I like the idea of having a high level
subsystem bean that contains all the configuration for the feature in one
place instead of being distributed all over the configuration tree.
Localization is always good because then the user and developer only needs
to goto one place to get this information or modify the code for it.  It's
more manageable.

I was always uncomfortable with bloating these interceptors with code.
Instead I just wanted to leave them as simple hooks that funneled events
(calls) into the subsystem.  The intercetor is then a simple listener that
belongs to the subsystem in question. So the listener detects events and
shuffles them into the subsystem to properly respond.  This allows us to
keep the configuration in the top level subsystem facade while properly
designing the various parts of the subsystem underneath with clarity instead
of jamming all the subsystem handling logic tightly into interceptors which
leads to code bloat in the interceptor modules leading to a jumbled up mess
that is hard to manage.

So I recommend the following:

  (1) Implement a subsystem for replication with a top level facade bean
that can be configured via XBean.
  (2) Setup a means to setup a set of replication agreements.  The same DSA
can be both a consumer and a supplier to many other DSAs.  So we'll have a
set of consumer agreements and a set of supplier agreements.
  (3) Build the interceptor out to be just a simple hook into other Classes
in the subsystem.  Make the subsystem return values for LDAP methods in the
chain that require return values.
  (4) Don't wory about adding the replication intercetor to the chain via
XBean, just have the subsystem inject the intercetor programatically.  The
user does not need to know this interceptor even exists right?


DirectoryService vs. LdapServer
----------------------------------------------

Now I don't know the answer to this but something itches me about putting
the replication subsystem into the DirectoryService.  For some time now the
DS corresponding to the top level facade representing what we always refered
to as the core.  The core was never supposed to be networked because then
that would pull dependencies like MINA.  It was the frontend that had
network capability.  I know mitosis broke from this but seems the moron who
wrote it broke all the rules.

Question is do we want replication to be a high level system in the frontend
since it leverages the protocol? Or do we want it in the DS which was
traditionally core services without networking?

As I write this I am leaning towards the LdapServer.  What we have to ask
ourselves is does the replication subsystem need access to protocol layer
structures and information? Another question is does anything other than the
interceptor need access to the ReplicationManager facade (presuming this is
what we named it).

This is a very important question.



>
> So I would suggest to move the replication configuration at a higher level
> (DirectoryService), and remove the ReplicationInterceptor.
>
> This is not definitive, I'm waiting for any comment before moving on this
> direction regarding to the interceptor removal (I'm not 100% sure that we
> don't need some replication specific filtering, for instance).
>
> wdyt ?
>
> --
> --
> cordialement, regards,
> Emmanuel L├ęcharny
> www.iktek.com
> directory.apache.org
>
>
>

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