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There are a few ways to configure the Presence Services in a Sametime cluster:
1 instance of thePresence Services handling all n users in the community.
m instances of the Presence Services, each handling all n users in the community.
m instances of the Presence Services, each handling n/m users from the community.
m instances of the Presence Services, each handling overlapping subsets of users from the community, such that each user is handled by at least two instances.
Each option has different performance and availability characteristics so please plan according to your system requirements and needs.
Option 1 doesn’t create any duplicate telephony presence-related traffic in the cluster, but the instance of the Presence Services is a single point of failure.
Option 2 generates m times as much telephony presence-related traffic than option 1, but it has the virtue that presence will continue to be provided as long as one of the m instances is still running.
Option 3 doesn’t create any duplicate telephony presence traffic, but telephony presence for n/m users will cease to function when one of the instances becomes unavailable.
Option 4 generates duplicated telephony presence-related traffic for each user monitored by more than one instance. Yet, telephony presence for a given user will continue functioning if one of the instances becomes unavailable as long as another instance monitoring that user is still running. This option combines options 2 and 3 to grant greater flexibility for administrators in fine-tuning multiple instances of the Presence Services, while ensuring that telephony presence will continue uninterrupted.
Options 1 and 3 result in n monitors being added via the AE Services Server. Option 2 is more CPU intensive on that account: it adds n*m monitors.
Keep in mind that each instance of the Presence Services must be running on a separate machine to achieve m instances. For a Community Server cluster this can easily be achieved since IBM only supports horizontal clustering in that type of environment. The Media Manager can be vertically clustered so if this is your deployment scenario beware that you will only be able to install 1 instance of the Presence Services since there is only one physical machine.
With version 1.3 of the AE Services Sametime Plug-in, the Telephony Presence Services can be installed as a standalone service on a separate non-Sametime server. The standalone Telephony Presence Services should not be installed on a server hosting Sametime server software. This feature provides a high-availability solution for the Presence Services. By installing and configuring the Presence Services as a standalone service on an additional machine, a Sametime community can have duplicated telephony presence for the Sametime users. If one of the servers providing telephony service goes offline or becomes otherwise unavailable, the other server guarantees uninterrupted telephony presence. Note that the above considerations for resource use, multiple AE Services Server monitors, and network traffic limitations will still apply.