Can SQL Server R Services be enabled on a readable secondary?

Question

To avoid excess load on the primary SQL Server, I'd like to install and enable R Services on a secondary replica of the primary server. Is this possible?

Also, can I push computations from a remote R client onto the secondary server with R Services enabled?

Does this have any impact on the primary at all?

Answer 1

Readable Secondary is a feature of Always On Availability Groups and you can easily create one of these in Azure using the template from the gallery.

You have to do a little bit of extra config, to configure the read-only routing, something like this:

-- Configure the READ_ONLY_ROUTING_URL
ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-1'
WITH ( SECONDARY_ROLE( READ_ONLY_ROUTING_URL = N'TCP://sqlserver-1.contoso.com:1433') )

ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-0'
WITH ( SECONDARY_ROLE( READ_ONLY_ROUTING_URL = N'TCP://sqlserver-0.contoso.com:1433') )

-- Configure the READ_ONLY_ROUTING_LIST
ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-1' 
WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'sqlserver-0', N'sqlserver-1')))

ALTER AVAILABILITY GROUP [Contoso-ag] MODIFY REPLICA ON N'sqlserver-0' 
WITH (PRIMARY_ROLE(READ_ONLY_ROUTING_LIST=(N'sqlserver-1', N'sqlserver-0')))

Connecting to the readable secondary is then a matter of using the ApplicationIntent=ReadOnly facet in your connection string. I have not tried doing this from R but imagine you could. There's an easy way to verify whether or not you are connected to the secondary using sqlcmd which I used recently here.

As far as the push compute goes, this is not yet known. Looking on the Always On Availability Groups: Interoperability R Services is not listed.

I would personally be surprised if these features worked together in that way as R is so new, but it's a good thought. If it turns out it doesn't work, you could make it a feature request. I would also say though, when you connect to an AG listener and request read-only routing, it is only that, a request. As AOAG is a high-availability feature, if the secondary is down, you would connect to the primary. So let's say you only enabled R Services on the secondary, you create a situation where these scripts failed when the secondary was not available.

Unfortunately I can't test this out myself as I've burned all my Azure credits this month answering some question for @BrentO : ) Maybe later in the month.

Answer 2

Nick Burns wrote this Configuring R Services in SQL Server 2016 for SQLServerSentry.com.

Note his comments in the FINAL THOUGHTS:

Personally, I think that the integration of R with SQL Server 2016 is an incredibly exciting move from Microsoft. R provides enormous potential to analyze, explore and visualize the data in SQL Server. . . . But on a data warehouse, or a readable secondary perhaps the performance drain won't be too critical.

The work being done on the readable secondary should NOT affect the primary server. It is a readable but not writeable secondary, so the query ('R', 'TSQL', etc.) cannot leak back up to the primary server.

Do you have any reason to expect otherwise?