[glue-wg] Endpoints and ComputingService, relationships between services. Some thoughs

[stephen.burke at stfc.ac.uk]

Florido Paganelli [mailto:florido.paganelli at hep.lu.se] said:
> Your observations are right, however the model of service you have in
> mind is monolithic and not distributed; I am in fact speaking about
> something that was not considered during the GLUE2 "making of".

As a general question Service-Service relations weren't considered much because in practice we had no relevant use cases, and as far as I know we still haven't. The schema is not supposed to be able to represent any conceivable situation, just to satisfy the things we know about. The schema does have a Service-Service relation, but as far as I know we have no uses of it at the moment and the semantics would need to be defined for a specific service if we did have a need. It may also be the case that you can infer the relationships from other information - for example if multiple VOMS servers are related to the same VO.

> 1) the service, by design, *has* distributed endpoints, that might run
> on different hosts.

As far as the schema is concerned it doesn't matter what hosts the endpoints are on - it will often be the case that a Service with many Endpoints will involve many hosts. It may of course be a question of the information system technology as to whether such a configuration can easily be published in practice. In particular if you have endpoints on different sites it may be difficult to co-ordinate the publishing. However within a site it should be fairly easy - for example CREAM is done like that (in the CLUSTER mode). Each head node publishes its own Endpoint(s) and they get merged in the site BDII.

> How to relate these records and infer, for example, that they're on the
> same machine and/or serve the same resource?

Why would you care if they run on the same machine? In general you can't get that even from the URL, because it may refer to a DNS alias and not the real host. If they serve the same Resource in the schema sense you can known because they will all have relations to the same Resource object, i.e. ExecutionEnvironment for computing services.

Stephen