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From "Chetan Mehrotra (JIRA)" <j...@apache.org>
Subject [jira] [Comment Edited] (OAK-3815) Add refresh callback support to Tracker
Date Wed, 06 Jan 2016 07:30:39 GMT

    [ https://issues.apache.org/jira/browse/OAK-3815?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15085125#comment-15085125
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Chetan Mehrotra edited comment on OAK-3815 at 1/6/16 7:30 AM:
--------------------------------------------------------------

bq. Ok then... but wouldn't it be simpler to check whether a returned list of services differ
from a previous one and only rebuild the data structure in that case

Can be done but then this would need to done in all places where derived data structure is
being used. Another way would be to avoid use of Tracker (as dynamics only apply for OSGi
case) and handle it in a different way. Can be done but would involve bit of boiler plate
code as we have 2 such services and with SCR need to add all those bind/unbind methods and
handle the update case

bq. With the callback based solution I'm a bit concerned re. the subtle issues which come
along with it (aka callback hell).

Yes that is possible but then this is similar to ServiceTracker itself which is callback when
services are registered. 

bq. 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?

That we can ensure in implementation and say that once callback is invoked the {{getServices}}
call would reflect the updated state. Or we can pass the updated list as part of callback
call itself. This would avoid any reverse invocation to Tracker from within the callback implementation




was (Author: chetanm):
bq. Ok then... but wouldn't it be simpler to check whether a returned list of services differ
from a previous one and only rebuild the data structure in that case

Can be done but then this would need to done in all places where derived data structure is
being used. Another way would be to avoid use of Tracker (as dynamics only apply for OSGi
case) and handle it in a different way. Can be done but would involve bit of boiler plate
code as we have 2 such services and with SCR need to add all those bind/unbind methods and
handle the update case

bq. With the callback based solution I'm a bit concerned re. the subtle issues which come
along with it (aka callback hell).

Yes that is possible but then this is similar to ServiceTracker itself which is callback when
services are registered. 

bq. 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?

That we can ensure in implementation and say that once callback is invoked the {{getServices}}
call would reflect the updated state. 



> 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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