Herman Ciechanowiec created SLING-12367:
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Summary: Add MBeans Registration Upon OakSlingRepository
Construction
Key: SLING-12367
URL: https://issues.apache.org/jira/browse/SLING-12367
Project: Sling
Issue Type: Improvement
Affects Versions: JCR Oak Server 1.4.0
Reporter: Herman Ciechanowiec
Fix For: JCR Oak Server 1.4.2
*1. Summary*
By default, Apache Sling uses `OakSlingRepository` for repository
construction. `OakSlingRepository` is, in turn, backed by `Oak`, which delivers
some important MBeans (e.g., query statistics). However, due to the current
specifics of the `OakSlingRepository` implementation, those MBeans aren’t
registered in the MBean service registry (MBean Server). Therefore, those
MBeans aren’t recognized by Apache Sling as MBeans, and their MBean features
remain unavailable to the user. The proposed fix is to introduce the missing
MBeans registration.
The problem is related to Apache Sling JCR Oak Repository Server
(`org.apache.sling.jcr.oak.server`).
*2. Default Apache Sling Repository Construction*
Apache Sling uses Java Content Repository (JCR) for persistence. Connections
to that persistence layer are performed via `javax.jcr.Repository`
implementations.
There are several ways `javax.jcr.Repository` can be implemented and
constructed. By default, in Apache Sling 12, an instance of
`javax.jcr.Repository` is implemented and constructed in
[`org.apache.sling.jcr.oak.server.internal.OakSlingRepositoryManager#acquireRepository()`
method](https://github.com/apache/sling-org-apache-sling-jcr-oak-server/blob/0dcb90452e0cc27db3501a21a99d7de1f72c2425/src/main/java/org/apache/sling/jcr/oak/server/internal/OakSlingRepositoryManager.java#L131-L168)
from the `org.apache.sling.jcr.oak.server` bundle. To construct the
`javax.jcr.Repository`, the mentioned method utilizes
`org.apache.jackrabbit.oak.Oak` from `org.apache.jackrabbit:oak-core`.
*3. Whiteboard in Oak Repository*
During `javax.jcr.Repository` construction,
`org.apache.jackrabbit.oak.Oak#with(Whiteboard)` [can be provided with a
`org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard`](https://github.com/apache/jackrabbit-oak/blob/541815b62fd955d76f7069e87b9412bd7eba4daf/oak-core/src/main/java/org/apache/jackrabbit/oak/Oak.java#L566-L591)
from `org.apache.jackrabbit:oak-core-spi`.
The `org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard` provided to the
`org.apache.jackrabbit.oak.Oak` during `javax.jcr.Repository` construction is
later used inside the `org.apache.jackrabbit.oak.Oak`, among others, to perform
registration of services relevant for the repository (repository-specific
services). Importantly, `org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard`
is just an interface and doesn’t enforce any platform-specific logic for
service registration. For instance, the
`org.apache.jackrabbit.oak.spi.whiteboard.DefaultWhiteboard` implementation
just keeps services in the application memory, while the implementation
provided via `org.apache.jackrabbit.oak.osgi.OsgiWhiteboard` performs
registration of repository-specific services inside the OSGi service registry.
*4. MBeans in Oak Repository*
Some repository-specific services registered inside the
`org.apache.jackrabbit.oak.Oak` via the
`org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard` provided during
`javax.jcr.Repository` construction are MBeans. In general, those MBeans
provide crucial monitoring information on the state of the repository.
Specifically, at least the following MBeans are supposed to be registered:
org.apache.jackrabbit.oak:name=Oak Query Statistics
(Extended),type=QueryStats
org.apache.jackrabbit.oak:name=Oak Query Statistics,type=QueryStat
org.apache.jackrabbit.oak:name=Oak Repository
Statistics,type=RepositoryStats
org.apache.jackrabbit.oak:name=async,type=IndexStats
org.apache.jackrabbit.oak:name=async,type=PropertyIndexAsyncReindex
org.apache.jackrabbit.oak:name=nodeCounter,type=NodeCounter
org.apache.jackrabbit.oak:name=repository
manager,type=RepositoryManagement
org.apache.jackrabbit.oak:name=settings,type=QueryEngineSettings
In order to be recognized by the application as MBeans, repository-specific
services that are MBeans [must be registered in the MBean service registry
(MBean
Server)](https://www.oracle.com/technical-resources/articles/javase/jmx.html).
Otherwise, they will not expose their MBean features and will remain idle in
this regard. Importantly, there are no limitations that forbid the registration
of the same repository-specific service in separate and isolated service
registries simultaneously. On the contrary, if a given repository-specific
service is polymorphic and is supposed to be registered in separate and
isolated service registries simultaneously, such registration might even be
strongly advised. For instance, a given repository-specific service might be
constructed as both an MBean service and an OSGi service, which aren’t the same
and which provide different functionalities; hence it must be registered in
both the MBean service registry (MBean Server) and the OSGi service registry.
`org.apache.jackrabbit.oak.Oak` recognizes this polymorphic nature of some
repository-specific services and by default uses a custom
`org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard` implementation that
[registers repository-specific services both in the in-memory service registry
(`org.apache.jackrabbit.oak.spi.whiteboard.DefaultWhiteboard`) and additionally
- in the case of services that are also MBeans - in the MBean service registry
(MBean
Server)](https://github.com/apache/jackrabbit-oak/blob/541815b62fd955d76f7069e87b9412bd7eba4daf/oak-core/src/main/java/org/apache/jackrabbit/oak/Oak.java#L281-L359).
*5. Whiteboard in Default Apache Sling Repository Construction*
During `javax.jcr.Repository` construction performed by
`org.apache.sling.jcr.oak.server.internal.OakSlingRepositoryManager#acquireRepository()`,
the `org.apache.jackrabbit.oak.Oak` [is constructed with an instance of
`org.apache.jackrabbit.oak.osgi.OsgiWhiteboard`](https://github.com/apache/sling-org-apache-sling-jcr-oak-server/blob/0dcb90452e0cc27db3501a21a99d7de1f72c2425/src/main/java/org/apache/sling/jcr/oak/server/internal/OakSlingRepositoryManager.java#L133).
That instance is an implementation of
`org.apache.jackrabbit.oak.spi.whiteboard.Whiteboard` that registers
repository-specific services exclusively in the OSGi service registry. It means
that repository-specific services registered inside
`org.apache.jackrabbit.oak.Oak` that are both OSGi services and MBeans are
registered exclusively in the OSGi service registry and are not registered in
the MBean service registry (MBean Server). Therefore, those services do not
expose their MBean features and remain idle in this regard. Because of this,
the Apache Sling user does not receive crucial monitoring information on the
state of the repository delivered by those MBean features. For instance, they
do not have access to Oak Query Statistics, which complicates query
improvements.
*6. Difference in Proprietary Apache Sling Variations*
The problem of lacking MBean features delivered by repository-specific
services isn’t present in at least some proprietary variations of Apache Sling.
The reason for this is that those proprietary variations don’t rely on
`org.apache.sling.jcr.oak.server.internal.OakSlingRepositoryManager` used by
vanilla Apache Sling for `javax.jcr.Repository` construction but use custom
implementations. At least some of those implementations are performed correctly
so that repository-specific services that are both MBeans and OSGi services are
registered in both the MBean service registry (MBean Server) and the OSGi
service registry respectively. Thanks to this, users of such Apache Sling
variations do not lack MBean features of repository-specific services.
*7. Proposed Fix*
It is proposed to fix the problem of lacking MBean features delivered by
repository-specific services by registering the services that are MBeans not
only in the OSGi service registry, as is already implemented, but additionally
in the MBean service registry (MBean Server). To achieve this, the fix wraps
`org.apache.jackrabbit.oak.osgi.OsgiWhiteboard` currently used in the
`org.apache.sling.jcr.oak.server.internal.OakSlingRepositoryManager#acquireRepository()`
in the decorating class
`org.apache.sling.jcr.oak.server.internal.WithMBeanRegistration`. The decorator
passes all method calls to the wrapped object and additionally, when
registration and unregistration methods are called, registers and unregisters
respectively repository-specific services that are MBeans in the MBean service
registry (MBean Server). The logic of the implementation is a remake of the
logic [provided by
`org.apache.jackrabbit.oak.Oak`](https://github.com/apache/jackrabbit-oak/blob/541815b62fd955d76f7069e87b9412bd7eba4daf/oak-core/src/main/java/org/apache/jackrabbit/oak/Oak.java#L281-L359)
and discussed above.
*8. Verification of the Fix*
The proposed fix is verified in the integration and unit tests provided
along with the fix. Additionally, it was verified on a vanilla Apache Sling 12
instance. However, note that by default, vanilla Apache Sling 12 doesn’t
provide a user interface to directly interact with or see the state of the
MBean service registry nor to see the statistics provided by the MBean features
from repository-specific services. For such interaction, additional tooling
should be installed, like [Adobe Granite JMX
Support](https://mvnrepository.com/artifact/com.adobe.granite/com.adobe.granite.jmx)
(`com.adobe.granite:com.adobe.granite.jmx`,
http://localhost:8080/system/console/jmx) or
[Jolokia](https://mvnrepository.com/artifact/org.jolokia/jolokia-osgi)
(`org.jolokia:jolokia-osgi`, http://localhost:8080/jolokia/list).
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