Re: [jgroups-users] Ability to set the coordinator programaticaly and the ability to maintain a shared state

On 05/05/18 6:40 AM, Questions/problems related to using JGroups wrote:
> Hi all,
>
> Thank a lot for the answer. The reference 
> http://www.jgroups.org/manual4/index.html#MembershipChangePolicy 
> <http://www.jgroups.org/manual4/index.html#MembershipChangePolicy> seems 
> to be promising. It has most of the requirements I wanted to 
> implement. In fact, my solution requires some of the nodes are 
> prevented from becoming the coordinator. In other words, out of all 
> the nodes, there are set of nodes which are designated clients (they 
> shouldn't become the master at any point because they don't have the 
> data layer). Rest of them can become masters and there is a priority 
> order for leadership. So as I understood the MembershipChangePolicy 
> allows me to specify the priority of designated masters. But I am 
> still wondering whether I can prevent some of the nodes becoming the 
> coordinator while they get all the view changes and messages.

Well, you can *prioritize* certain members to become coordinators. 
However, if there are only members left in the cluster who are not 
supposed to become coordinators, then one of them will inevitably become 
coordinator. You cannot have a cluster _without_ a coord; one of the 
members has to be coord.

> Any help on this would be highly appreciated.
>
>
> Thanks & Regards!
> Tharindu
>
>
> Tharindu Munasinghe.
> Undergraduate, Department of Computer Science and Engineering
> University of Moratuwa.
> Contact no. +94770460887
>
>
>
> On Thu, May 3, 2018 at 12:56 PM, Questions/problems related to using 
> JGroups <[email protected] 
> <mailto:[email protected]>> wrote:
>
>
>
>     On 29/04/18 10:45, Questions/problems related to using JGroups wrote:
>
>         Hi all,
>
>         I am new to jGroups but I found this is an elegant library for
>         cluster management. I have 2 main requirements for one of my
>         projects.
>
>         First, is it possible to programmatically change the
>         coordinator of the jGroups cluster?. I am using the jgroups
>         coordinator as the application level master (for some
>         domain-specific application) of the server's cluster. But for
>         a given server to become the application level master there
>         are some application level criteria to be fulfilled. So in
>         case of the jGroups coordinator cannot fulfill those
>         requirements I want to change the coordinator of the
>         jGroups programmatically. I found [1] for this purpose, but I
>         wanted to understand whether this is a reliable method or is
>         there any side effects of doing this. If there is a
>         recommanded approach, please share that with me.
>
>
>     [1] certainly works, but is a kludge. I recommend implementing a
>     membership plugin [2], which determines the order of members in a
>     view. You can thus control who the coordinator is/becomes. [1] can
>     be used to change the coordinator when no view is getting
>     installed, e.g. triggered by the application.
>
>
>         The second question is about the ability to maintain a shared
>         state. Is it possible to maintain a state which is visible
>         across the cluster?. So the changes are synchronized among the
>         available members and visible to all members without
>         explicit message passing about the changes.
>
>
>     Yes, use state transfer, check the manual for details. Note that
>     the state is the same across *all* cluster members, which may lead
>     to heap exhaustion if you have a large state and/or many members.
>
>
>     [2]
>     http://www.jgroups.org/manual4/index.html#MembershipChangePolicy
>     <http://www.jgroups.org/manual4/index.html#MembershipChangePolicy>
>
>
>         [1]
>         https://github.com/belaban/JGroups/wiki/Changing-the-coordinator-of-a-cluster
>         <https://github.com/belaban/JGroups/wiki/Changing-the-coordinator-of-a-cluster>
>
>         Thanks & Regards!
>         Tharindu,
>
>         Tharindu Munasinghe.
>         Undergraduate, Department of Computer Science and Engineering
>         University of Moratuwa.
>         Contact no. +94770460887
>
>
>
>
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-- 
Bela Ban, JGroups lead (http://www.jgroups.org)


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