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Always on HA
• SQL Server Always ON feature is the new
comprehensive high availability and disaster
recovery solution which increases application
availability. Always ON provides availability at
either the application database or instance
level.
• Features
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Multiple database failover
Multiple secondaries (conceptwise similar to a Mirror in Database Mirroring), at
the max 4 secondaries are allowed
Data movement can be synchronous and asynchronous
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For additional data protection there is support for two synchronous
secondaries.
Manual, Automatic and Forced Failover
Flexible policy for failover
Automated Page Repair
Active Secondary
 Readable secondary
 Secondary backup
Automatic redirection of application using virtual name
SSMS for SQL 2012
Structure of Always on HA
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An availability group is a collection of databases which behave as a single unit.
During a Failover, even if one of the databases fails then it affects all the databases
and hence the whole Availability Group will fail.
Structure of Always on HA
Structure of Always on HA
AG Listener:
Availability Group Listener provides client connectivity to the database of given
Available Group. Listener is a virtual network name to which client connects to the
database in the Availability Groups. A Listener enables a client to connect to an
availability replica without knowing the name of the physical instance of SQL
Server to which the client is connecting.
Availability Replica Roles:
The Availability Replicas are the database replicas which can reside in the
Availability groups. For a single database there can exist a maximum of 4 replicas.
When one replica is made Primary the other replicas are considered as Secondary
and can be used as database back up. One availability group can own one replica at
a time.
Data Synchronization mode:
The Data Synchronization mode is the mode of data replication between the Primary &
Secondary Replicas. The data replication can be in Synchronous and Asynchronous Mode.
Synchronous commit:
In Synchronous Commit mode, when a transaction/data hits the primary replica
then it is reflected in the Secondary replica as well in same time interval. The Secondary
replica then sends a Check point and only then the Transaction is committed in both the
Primary & Secondary Replicas. Output is in synchronous with input which means it occurs
at the same time. The Synchronous commit data replication is applicable for maximum of
two replicas.
Asynchronous commit:
In Asynchronous Commit mode, when a transaction/data hits the primary replica
then it is reflected in the Secondary replica as well in same time interval. The transaction is
committed as soon as the Primary replica sends a check point. The Synchronous commit
data replication is applicable for maximum of four replicas.
Connection Mode in Secondary:
The connection mode is set for the Secondary replicas present in the Availability
Groups. Allow all connections, Allow only Read-Only connections, Disallow all
connections are the different types of connections. The Client connectivity towards
the Secondary replicas is determined by the type of Connection Mode.
Failover Modes:
Always ON feature in SQL Server 2012 gives a choice of two types of Failovers. This feature
operates on Automatic and Manual Failover.
Automatic Failover:
An Automatic Failover causes a Synchronized available Secondary Replica to
transit to a Primary Role without Data Loss after a Failover in the Primary Replica and the
Primary Replica transits to a Secondary Role. The Primary & Secondary Replicas should
mandatorily be in Synchronized commit mode.
Manual Failover:
Manual Failover can also be called as Planned Manual Failover. If the Failover is
done on the Secondary replica forcefully, then it becomes a Manual Failover. In this
Scenario the Secondary replica on which the Failover has been performed performs
Primary role and the Primary replica performs a Secondary Role. The Primary & Secondary
Replicas should mandatorily be in Synchronized commit mode.
Always ON Architecture
Availability Group Failover:
 The Client connects to the Primary replica SQL – AG1 through Availability Group
Listener.
 The SQL – AG1 is Primary and SQL – AG2 & SQL – AG3 Servers are Secondary.
 When a failover is initiated the SQL – AG1 notifies the database administrator and SQL
– AG1 goes offline. And then the client disconnects from the AG1.
 The Availability Group Listener moves to the next available replica SQL – AG2 [refer
above fig].
 The Client is automatically connected to the SQL- AG2 availability group through
Availability Group Listener.
 At this point the SQL – AG1 becomes Secondary & SQL – AG2 becomes the Primary
replica. The SQL – AG3 remains secondary and the Synchronization among all the
Replicas resumes.
SQL Server 2012 Always ON feature offers High Availability & Disaster Recovery
 The Availability Groups enhances the capability of database mirroring.
 Multiple database coordination during Failovers and Simplified application
connectivity.
 Synchronous and Asynchronous data movement along primary and secondary
replicas.
 Automatic & manual failover modes with configurable failover trigger levels.
 Ability to read from Secondary replicas and can be used as Back up.
 Availability Dashboard for monitoring the health of Availability Groups.