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Re: [ecf-dev] handling pub-sub providers with RFC 119

Hi Phillip,


Konradi, Philipp wrote:
Hi Scott,

<stuff deleted>
[Konradi, Philipp] I'd prefer option two, since option one doesn't
support the transparency which RFC 119 targeted for OSGi application
(services&clients) bundles.

Agreed, option 1 doesn't support the intended usage of RFC119 (transparent remoting)...but in fact as I understand it RFC 119 in it's current form doesn't actually support this model (pub/sub) of remoting/remote services (because there is only the notion of 'endpoint'...and no notion of pubsub group).


So we (ECF) will/are introducing the API described below to make it possible to use RFC 119 publication and proxy creation within pubsub groups (e.g. JMS). It is, of course, also possible for people to use the ECF remote services API to explicitly access proxies and the other forms of remote communication on IRemoteService as well.


BTW: in [0]:
Consider the following architecture:

A  <-->  B <--> C

In this architecture, we have two clients 'A' and 'C' and a server
'B'...all participating in the same publish and subscribe group.
To verify I've understood it correctly: 'server' is in case of JMS the
JMS broker?

Sort of :). In ECF's JMS provider implementation (with TCP anyway) the situation looks like this:


A <--> (JMS Broker) <--> C
| B


The JMS Broker can be (and usually will be) run on the same process/host as B (the ECF 'group manager'). But this isn't required, actually...as the JMS Broker and B can/could be run on a process separate from B. You might ask...why have B at all? Well, the answer is that B manages the group membership for the entire group, so that reliable group membership changes can be communicated to the others (A and C). It also can/does manage any authentication that occurs for access to the group.


...By 'these cases'
I
mean publish and subscribe groups that are used for remote services
between members of those groups (e.g. JMS groups, etc).

Currently, there's no direct support in RFC119 for remote services
within pubsub groups... because although RFC119 has the notion of an
'endpoint ID', this endpoint ID gives the location of the remote
service's host...which assumes that the host is a server (i.e. can be
reached *directly* from a client), rather than indirectly (i.e. via a
pub/sub group).
[Konradi, Philipp] That's correct, RFC 119 really does not say a word
about pub-sub... though my thinking was always that pub-sub is easily
possible with RFC 119 as well. But let's see, this discussion sounds
interesting... I assume by saying 'endpoint ID' you mean RFC 119
ServicePublication.PROP_KEY_ENDPOINT_LOCATION property.
ServicePublication defines also a property called PROP_KEY_ENDPOINT_ID
but this is just some unique service endpoint ID (e.g. an UUID) allowing
to find out whether service metadata discovered via different discovery
providers belongs to the same service instance or not.
My interpretation of RFC 119 PROP_KEY_ENDPOINT_LOCATION property is that
it's the URL under which the advertised service endpoint can be reached.

Right this is my understanding.

So in case of an Web service endpoint the url would probably contain
local host and port, in case of JMS (indirect communication) the url
would contain the host name of the JMS broker, port and the queue or
topic name.

Perhaps, but this isn't necessary the endpoint for all clients participating in the group. That is, what if A (above) publishes a service? What would its endpoint be in the service publication?


BTW: Would it make sense to add some (configuration) mechanisms to
select whether JMS queues or topics (request-response or pub-sub) should
be used for exposure of a service? I could think of using either intents for this or/and additional
configuration data with the service.

Effectively, ECF already now already has this with the IHostContainerFinder. It would allow the usage of intents (or any other service properties) to determine how/what container (and what it's configuration might be) is made available for usage as the distribution provider ...and for JMS can/could determine what JMS queues/topics are used to expose the service (e.g. by creating JMS IContainer instances of appropriate type).


A custom IHostContainerFinder can replaced/substituted by creating an implementation of IHostContainerFinder and registering it as a service (whiteboard pattern) via the service registry with a service.ranking that is higher than the default one (which is Integer.MIN_VALUE).

Thanks,

Scott