skreebydba

I finally am getting to work with SQL Server 2016 and spent the past two days building out an Availability Group to test the new read-only replica functionality.  This allows connections with an ApplicationIntent of ReadOnly to round-robin between all instances in the READ_ONLY_ROUTING_LIST, load balancing the read-only activity.

For my test, I built out three replicas and set all three to be readable secondaries.  Using Microsoft's documentation on configuring read-only routing, I coded the following:

ALTER AVAILABILITY GROUP SKREEBYHA
MODIFY REPLICA ON 'AGNODE1\SKREEBYAG'
WITH
(
PRIMARY_ROLE
(
READ_ONLY_ROUTING_LIST=('AGNODE2\SKREEBYAG','AGNODE3\SKREEBYAG')
)
);

To test the new setup, I used the SQLCMD examples from Eric Russo's blog post on Load Balancing 101.  Running the following command:

SQLCMD -S SKREEBYLIST -K ReadOnly -d testdb -Q "SELECT @@SERVERNAME"

returned AGNODE2\SKREEBYAG in each case. I could not figure out what I was doing wrong. After much frustration, I noticed the routing check query at the bottom of Eric's post:

SELECT ar_Primary.replica_server_name 'Primary',
ar_secondary.replica_server_name 'Route_to',
ror.routing_priority 'Priority'
FROM sys.availability_read_only_routing_lists ror
INNER JOIN sys.availability_replicas ar_Primary
ON ror.replica_id = ar_Primary.replica_id
INNER JOIN sys.availability_replicas ar_secondary
ON ror.read_only_replica_id = ar_secondary.replica_id
INNER JOIN sys.availability_groups ag
ON ag.group_id = ar_Primary.group_id
WHERE ag.name = 'SKREEBYHA'
ORDER BY 1,3

Running it returned the following results:

So, in technical terms, something was screwy.  Looking at Eric's post again, I noticed his third example for setting a routing list:

ALTER AVAILABILITY GROUP SQL2016AG
MODIFY REPLICA ON 'MyNode3'
WITH
(
PRIMARY_ROLE
(
READ_ONLY_ROUTING_LIST= (('MyNode4','MyNode1','MyNode2'))
)
)

Notice the second set of parentheses around the READ_ONLY_ROUTING_LIST value. Based on this, I changed my code to:

ALTER AVAILABILITY GROUP SKREEBYHA
MODIFY REPLICA ON 'AGNODE1\SKREEBYAG'
WITH
(
PRIMARY_ROLE
(
READ_ONLY_ROUTING_LIST=(('AGNODE2\SKREEBYAG','AGNODE3\SKREEBYAG'))
)
);

and, voila, the routing list check query returned the following:

Rerunning the SQLCMD test, the read-only connections round-robined between AGNODE2 and AGNODE3.

Many thanks to Eric Russo (b|t) for a great blog post.  I hope this saves you some frustration when working with read-only routing.