Lync Server 2013 Pool Pairing

As we all know, in Lync Server 2010 and 2013, the High Availability feature we have is Pooling concept. If a server running a certain server role fails, the other servers in the pool running the same role take the load of that server. This applies to Front End Servers, Edge Servers, Mediation Servers, and Directors.

In Lync Server 2013 Microsoft introduced a new Disaster Recovery feature called "Pool Pairing". To have best DR option, we should deploy two FE pools across two physically dispersed location. Each site contains a Front End pool which is paired with a corresponding Front End pool in the other site. Both sites are active, and the Lync Server Backup Service provides real-time data replication to keep the pools synchronized. The Backup Service is a new feature in Lync Server 2013, designed to support the disaster recovery solution. It is installed on a Front End pool when you pair the pool with another Front End pool.

Remember that the relationship between these pools is ‘Active-Active’, so both pools can be serving their own associated users. With this is mind, always be sure that you have enough capacity on one pool to cater for it’s own users as well as those of its partner if a failover were to occur.

Each pool will act as a backup registrar for other pool. we can pair pools only with 1:1 ratio. meaning that a pool cannot be paired multiple times, and that each pool should consist of servers that are patched to the same level.

Supported pool pairing scenario

Standard edition pool --> Standard edition pool
Enterprise edition pool --> Enterprise edition pool
Physical pool --> Physical pool
Virtual pool --> Virtual pool

Pool Failover and Failback time

For pool failover and pool failback, the engineering target for recovery time objective (RTO) is 30 minutes.This is the time required for the failover to happen, after administrators have determined there was a disaster and initiated the failover procedures. It does not include the time for administrators to assess the situation and make a decision, nor does it include the time for users to sign in again after failover is complete.

For pool failover and pool failback, the engineering target for recovery point objective (RPO) is 30 minutes. This represents the time measure of data that could be lost due to the disaster, due to replication latency of the Backup Service. For example, if a pool goes down at 10:00 A.M., and the RPO is 30 minutes, data written to the pool between 9:30 A.M. and 10:00 A.M.might not have replicated to the backup pool, and would be lost.

For Enterprise Voice default failover and failback time is respectively 300 and 600 Seconds but we can customise these values.

Once we added the "Associated Backup Pool" from LCP, we should invoke backup service to start the initial sync.

Invoke-CsBackupServiceSync –PoolFqdn pool1
Invoke-CsBackupServiceSync –PoolFqdn pool2

To get the status of the backup service

Get-CSBackupServiceStatus

CMS Failover

When you establish a pool pairing that includes the pool hosting the Central Management store, a backup Central Management store database is set up in the backup pool, and Central Management store services are installed in both pools. At any point in time, one of the two Central Management store databases is the active master, and the other is a standby. The content is replicated by the Backup Service from the active master to the standby.

During a pool failover that involves the pools hosting the Central Management store, the administrator must fail over the Central Management store before failing over the Front End pool.

RTO & RPO is 5 mins for CMS failover.

Pool pairing replication security

There is no secured channel between primary and backup pool pairing replication using Lync backup service. Microsoft recommends to use IPSec for this and replication network should be internal.

User Experience for an Affected User in a Pool in Failback

User state or task	During failback	After failback completion
User state of user already logged in	User stays signed in and connected to backup pool. At some point user will be signed out and sign back in to the original home pool, in Resiliency mode.	User remains signed in and goes into regular mode.
New user logging in	User can sign in to the home pool in Resiliency mode.	User can sign in to the original home pool in regular mode.
Ongoing conferences organized by affected user	All modalities of conference are terminated. Rejoin button will appear, but no users can rejoin while the affected user is in Resiliency mode.	All modalities now work. Every participant needs to click to rejoin the conference.
Ongoing conferences organized by unaffected user	Conference continues and affected user can stay in the conference. Affected user is restricted to what he/she can do in Resiliency mode.	Conference continues, and affected user can stay in the conference and all modalities work after user exits Resiliency mode.
Scheduling or modifying scheduled meetings, creating ad-hoc conferences	Not possible while user is in Resiliency mode.	Available for all modalities.
Presence as seen by other users in the same pool	Presence unknown while user is signed into backup pool during Resiliency mode.	Shows the last presence state set by the user, and presence changes will now be reflected.
Contacts list and Address Book Service availability	Not available	Available
All peer-to-peer sessions and modalities	Available	Available

User Experience for a User Homed in an Unaffected Pool During Failback of Another Pool

User task	During failback	After failback completion
Viewing presence of affected user	Shows the last presence state set by the affected user.	Working. Unaffected users see updates made by affected users.
Ongoing conferences organized by affected user	All modalities of conference are terminated.	All modalities now work. Every participant needs to click to rejoin the conference.
Ongoing conferences organized by unaffected user	Conference continues, and affected user can stay in the conference and all modalities work.	Conference continues, and affected user can stay in the conference and all modalities work.
All peer-to-peer sessions and modalities	Available	Available

Edge Server DR

Pool pairing concept is not for Edge servers. We have to configure two separate edge pool at two sites and configure SRV records with different priorities.