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

[Sanne Grinovero]

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