Availability Group Listener: Benefits and Configurations
Table of Contents
What is Listener?
An Availability Group Listener is a Virtual Network Name (VNN). Clients can connect to this VNN to access the database in a primary replica. They can also connect to a secondary replica of an Always on Availability Group.
Listener allows application to connect to the replica without knowing physical name of instance of SQL server.
Only TCP protocol is supported by availability group listener
When an availability group failover occurs, the listener terminates its connection with the affected instance. It then establishes a connection with the new primary instance. Because of this we do not need to make any changes on application side after failover.
Read only routing can be set up for one or more readable secondary replicas. In this setup, read-intent client connections to the listener are redirected automatically. They are directed to a readable secondary replica.
Listener Parameters
A) A unique DNS Name
This is also known as Virtual Network Name(VNN).
Active Directory naming rules for DNS host name are applied here.
B) Virtual IP Addresses
Listener can have one or more virtual IP addresses.
These IP addresses are configured for one or more subnets to which the availability group can failover.
C) IP Address Configuration
We can use dynamic IP address but as per recommendations you should use static IP address.
If the Availability groups that extends to multiple subnets must use Static IP addresses.
D) Listener Port
If you use default port 1433 for listener you can directly connect to AG using listener name.
If you are using a port other than 1433, you must mention the port number. You need to include it with the listener name explicitly in the connection string.
You can use below views to perform checks on availability group listener.
1. sys.availability_group_listener_ip_addresses
2. sys.availability_group_listeners
3. sys.dm_tcp_listener_states
