So lets think about a practical example. We want to expose a REST service that is visible to the inside via its direct url on the server that exposes the service and also via a proxy server where this service will have a different url.

So the server side topology manager would detect that the service is to be published. It would call the distribution provider to export the service.

The provider would then return two ExportRegistrations one for the direct access and one for the access over the proxy. They could each have a special property like "zone" to distinguish them (like "backend" and "frontend").

The TopologyManager on the front end client side then could be configured to only consider Endpoints that have (zone=frontend) for imports.

I think on the client I like this approach. It would also need only minimal additions to the TopologyManager code. We would either need a config setting for a import filter or a SPI where the user can supply custom filtering logic.

On the server side I am not so sure. The distribution provider would need to know about the proxy and it might even need to communicate with it to get the alias address for the service it exports. Another problem is that in such a scenario the server administrator would be able to poke new holes into the firewall. On the positive side I agree that the DistributionProvider might know more about the details of the protocol to be qualified to create a suitable address than the TopologyManager.

Generally I think I would prefer a more central approach where you have a system that is part of the firewall that manages which services to expose to the outside world. Maybe this can also be done with Remote Service Admin.

How about this:


We use a system with two instances of the zookeeper discovery. One that communicates with the backend zookeeper and one that communicates with the frontend zookeeper.

The backend discovery would report all internal services to the TopologyManager. The TopologyManager would select the backend services for import using a special proxy DistributionProvider.

The proxy DistributionProvider would create an OSGi service with the necessary prorperties to be exported as a proxy for the frontend. The TopologyManager would detect this service and export it using the same proxy DistributionProvider which would create an ExportRegistration with the url of the proxy Endpoint it would also either implement an HTTP Proxy for the service or configure an external proxy server.

This approach might even be able to cover both cases the case where we have one zookeeper with all addresses and the case where we have spearate zookeepers for frontend and backend.

If I understand correctly then the Endpoint information for the proxy service would always be sent to all discovery providers, or can the TopologyManager select where to export it? If it can not then I think we would need a filtering config for the zookeeper discovery so the frontend zookeeper will only contain the frontend services and the backend zookeeper would only contain the backend services.

What do you think?


On 21.09.2016 11:49, Timothy Ward wrote:
Hi Christian,

 From an RSA perspective this is a Topology Management issue, not a discovery 
issue. Services should be exposed by the Distribution Provider with multiple 
ExportRegistrations (and hence multiple EndpointDescriptions), one for the 
“internal” URL and one for the “proxied” URL The Topology Manager on the client 
side should then import the Endpoint Description that gives the behaviour that 
it desires.

Using multiple discovery services to do this sort of scoping is possible, but 
much more complicated than controlling it using the Topology Manager. Adding a 
simple filter to the client side’s Topology Manager’s EndpointEventListener 
would probably be sufficient, and could be done easily using Config Admin.



On 19 Sep 2016, at 13:52, Christian Schneider <> wrote:

I just had a discussion with Panu Hämäläinen about the DiscoveryPlugin 
See and .

What he needs is to have two zones of services, a backend zone and a frontend 

The (or some) services in the backend will be published with http. Inside the 
backend zone the services should be available using this http url.

In the frontend zone these services should also be visible but their url should 
point to a proxy server that offer a https connection and potentially some 
additional security mechanisms.

So we can not simply have one (zookeeper or other) discovery view. Instead we 
need a different discovery view for backend and frontend and some mechanism to 
make some services from one zone available in the other while also applying 
some changes like pointing to a proxy.

Do you think it makes sense to support this case in Aries RSA in some way?

I thought we might also be able to interact with one of the existing proxy 
servers to automatically register the proxy for each service. Such a mechanism 
is very typically for cloud enabled architectures. So that might also bring us 
nearer to good cloud support.

I would be happy about any ideas and feedback.


Christian Schneider

Open Source Architect

Christian Schneider

Open Source Architect

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