Re: [hibernate-dev] "Service" in Hibernate Search: history, lessons learned and rewrite

The "Service" and "ServiceManager" concepts in Hibernate Search have a specific meaning which is often misunderstood and/or abused, causing trouble. They also changed over time: victim of two major refactorings which evolved the purpose and stretching its intent So I'll change the definition again :-) But hopefully clarifying it, so here is a draft of the rules which I plan to both implement and document carefully on the javadoc, with some comments to highlight what is changing: # A service type is identified by a Class: a Service interface Nothing new here. Yet: worth stressing that this implies that only one implementation will be around. # There can only be ONE IMPLEMENTATION of a service type used by the whole Hibernate Search instance In other words: it was not a good idea to have the LuceneWorkSerializer to be modelled as a Service, back when we supposedly could use a different serialization strategy for a different index. Yet it is a good idea nowadays to have LuceneWorkSerializer extend Service, as we dropped that level of flexibility. This implies that there's a single type of serializer (at most) and it's totally fine to expose this as: SearchIntegrator#getLuceneWorkSerializer() [this method doesn't exist yet, but I'm thinking of adding it for our convenience and the following other points] P.S. we're only maintaining - and bundling - a single Serializer implementation so it's no surprise that we can handle only one.. yet this implies people wanting to override it have to either hack our bootstrap or physically remove our implementation. # A Service implementation can be provided by having it injected at bootstrap (i.e. org.hibernate.search.cfg.spi.SearchConfiguration.getProvidedServices() ) Not a new rule either: repeating for clarity. We call these "provided" services. # If a service isn't "provided", then we attempt to create one using java.util.ServiceLoader Currently this expects a single implementation to be available: there's no way to pick which one if there are multiple implementations on the classpath. I think we'll need to be able to pass a "hint" or similar to the requestService to allow expression of preferences, handle shortnames, etc.. a proposal for that will follow when there will be need: at this point it's important to clarify the limitation, as this expresses what a Service is not able to model today. Currently implementations are looked up "on demand". I plan to allow "pre-initialize" services as it removes some trouble; these components could have convenience getters, not least to remove the concurrency overhead. Remember that since there's only one implementation for a given type around, there's no reason to not do this: the intent of the Service contract is to allow people to inject a customized implementation. # If a Service implementation also implements Startable, or Stoppable, we'll invoke the respective methods once at start and/or at stop of the Search instance - unless they are provided in which case they are ignored. The current javadoc suggests that it's illegal for a provided implementation to also implement Startable and/or Stoppable; I don't remember why that was, but today it seems unfitting: people might want to extend one of our implementations, or reuse some of the implementations normally auto-started but reuse them "by instance" by providing them to multiple Search instances to save memory (we actually have a need for this for Index Affinity in Infinispan). The important concept which will survive, is that we don't start or stop stuff which is provided as that's clearly responsibility of another component. # All non-provided Services will be stopped once, and only once as final step when the SearchIntegrator is stopped. This is a significant difference with today's code: we expect the Service consumers to "open / close", hopefully in a finally block, to the point that Gunnar enhanced it to at least allow AutoClose semantics. Yet, I don't want runtime code to open and close these frequently as it has been a bottleneck in the past. It also led to the creations of issues like HSEARCH-1589 : we might start/stop the same service frequently, and need to improve with reference counters. I suspect that historically the reasoning was to make sure that the order of teardown would follow the inverse order of bootstrap as components would cleanup after themselves, but having clarified that Service instances are unique globally, should also imply that their state doesn't depend on other Services. So the teardown order doesn't matter anymore.. we'll start one for each, but only close it at shutdown. # Hierarchy? We've talked about global components so far. It's clear that the IndexManager has a central role in the overall architecture, as we tend to allow per-index customisations. Or per-family customisations, as suggested in my previous email. An example which affects Service: The "JestClient client" [the Service we use to connect to Elasticsearch] could be considered a good fit for being a "Service" as this allows people to override the client implementation and/or inject a pre-configured instance.. yet it's not a good fit if we want to allow people to connect to different hosts for different indexes. I don't plan to implement the hierarchical ServiceManager right now, but proposing it already so that we can agree on the above cleanups in contract, with the perspective that there are cleaner solutions also for the scoped use case. Implementing these changes resolves or obsoletes at least 10 JIRA issues in one shot.. Thanks, Sanne _______________________________________________ hibernate-dev mailing list hibernate-dev(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/hibernate-dev