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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-3815) Add refresh callback support to Tracker
Date Tue, 05 Jan 2016 11:28:39 GMT

    [ https://issues.apache.org/jira/browse/OAK-3815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15082878#comment-15082878

Michael Dürig commented on OAK-3815:

With the callback based solution I'm a bit concerned re. the subtle issues which come along
with it (aka callback hell). You addressed the locking issue. However, what is expected from
the receiver of a callback? Can one register / unregister a service there? What length of
time may a callback take? May it block? May it throw an exception? Your implementation calls
it from within the register / unregister methods. Technically registration / unregistration
is not complete at this point, so what would you expect from a call to {{getServices}} at
this point?

> Add refresh callback support to Tracker
> ---------------------------------------
>                 Key: OAK-3815
>                 URL: https://issues.apache.org/jira/browse/OAK-3815
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: core
>            Reporter: Chetan Mehrotra
>            Assignee: Chetan Mehrotra
>            Priority: Minor
>             Fix For: 1.3.14
>         Attachments: OAK-3815-v1.patch
> Currently the {{Tracker}} only has access to list of registered services but there is
no way to know if the services have changed and one has to re obtain the set of services.
For certain usecases its desirable to know (get notified) if the set of services registered
against a specific type have changed. 
> For e.g. in OAK-3576 the services are to be registered against set of NodeTypes and each
service provides set of nodeTypes it support. Creating that structure (nodeType -> service)
is to be avoided on the critical path.
> So far Tracker serves the purpose well. One can just open the tracker and get the list
of services and create that nodeType -> service mapping. However in case of OSGi (or in
other modes also) if a new service is registered there is no way currently to get notified.
As of now one needs to have reference property added to some OSGi component with bind and
unbind callbacks. Given we already make use of {{ServicetTracker}} and do get notified when
services get updated we should easily be able to make use of that.
> For such cases I would like to propose registering a callback which should get notified
if there is some change in set of services registered against a specific type. Then one can
register the callback while creating the Tracker instance 

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