In previous version of Lync 2010, user services was provided by the Backend database, and that’s why when the SQL backend goes down, the Lync clients goes into “Limited Functionalities” status. But this has changed with Lync 2013 server architecture, where users services were moved from the backend databases to the Lync Front end servers. MS is calling this architecture as "Loosely coupled backend store".
What is windows fabric
Windows Fabric is a kind of clustering technology, with Lync 2013 Microsoft recommended to use 3 nodes in an Enterprise pool, and in case you cannot use 3 nodes in a pool then deploy two standard edition pools and use pool pairing for high availability.
we can verify the settings at cluster manifest file from the below path.
C:\Program file\windows fabric\bin\clustermanifest.current
Routing Groups
In Lync 2010, a hash algorithm determines where a user exist in a front end pool and administrators can not control it. Because of this, we can easily identify on which FE server a user is homed.
Lync server 2013 doesn't use any hash algorithm to home a user. Instead, it will map a user to one routing group when we enable that user for Lync. Each routing group is assigned to one FE server until that server is rebooted or goes down. we can identify routing information from msRTCSIP-UserRoutingGroupId attribute from user AD properties also from powershell.
Each routing group has primary, active backup and idle backup copy on three different FE servers. Windows fabric won't allow Single FE server to hold multiple primary copies. That way, if a Front End server goes down, user disruption is limited. windows fabric will make sure a FE server contains only one copy of a single routing group.
we can get this information from the below cmdlet
Get-CSUserPoolInfo -Identity "SIP:testuser@mylab.com"
Note : When it comes in backend database routing group is nothing but a Lync user.
Server down scenario
In our example consider we have four FE servers in a Lync pool.
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| FE Servers and their Routing Groups |
suddenly FE3.mylab.com server is down. What will be the impact to the primary copy of the Routing Group3. In response to this failure, the primary and the backup copies of routing groups are rearranged on the servers that are still running. Below figure shows the new arrangement. Notice that the primary copies of the three routing groups are on separate Front End servers. Also notice that some of the secondary and tertiary copies were moved so that each Front End server contains only one copy of a routing group.
When the Front End server goes down, users will experience a momentary disconnect. The Lync client disconnects during the shuffling of routing groups on the Front End servers, then reconnects once the user's secondary copy of the routing group establishes a connection to another Lync Front End server.
Because of this reshuffling, the routing groups might be assigned to different Front End servers.
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| Routing Group location after server outage |
After the FE3 server is up, the Routing Groups will be reshuffeled once again. However the Routing Groups need not to be located on the same FE server where they resided before the server down.
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| FE Server and Routing Group location after FE3 is up |
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