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https://issues.apache.org/jira/browse/SLING-12367?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Munteanu reassigned SLING-12367:
---------------------------------------
Assignee: Robert Munteanu
> 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
> Assignee: Robert Munteanu
> Priority: Major
> Labels: Sling-13-Release-Notes
> Fix For: JCR Oak Server 1.4.4
>
>
> *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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