SQL Server Mirroring keeps giving error when I want to add more tables

[sholy29]

I currently mirror SQL Server from our on-prem SQL Server to Fabric. I have couples of table currently mirrored to fabric. I want to add more tables to mirrored, but when i click on the configure replication to add more table, I keep geeting the following error

The error: The database cannot be mirrored to Fabric due to below error: Test connection failed. Request failed with status code 400 Activity ID: undefined

Mirroring still works fine for other tables that were previouslly added. I am really confused about. On a few occasion it just work but most of the time, i get the above error.

I will appreciate any help or pointers. Thanks

[v-karpurapud]

Hi @sholy29 

We have not yet received a response from you regarding your query if you find the response helpful, kindly mark it as the accepted solution and provide kudos, as this will aid other members with similar queries.

Thank You.

[v-karpurapud]

Hi @sholy29 

May I ask if you have resolved this issue? If so, please mark the helpful reply and accept it as the solution. This will be helpful for other community members who have similar problems to solve it faster.

Thank you.

[sholy29]

Thank you.

However, the error still occur. Previously, I activated CDC for all tables and entity in the SQL Server database. Based on your recommendation, I thought I must have actived CDC on an unsupported schema elements.

I went back and only activated CDC on the selected modules/table that I need to mirror. I though this would have resolved it and I was wrong. I still get the error. See below for screenshot.

I doubt if it is gateway misconfigurations because, when I use Dataflow Gen 2 or Copy Job to copy table from our sql server, I do not get this error.

What do you mean by "Ensure SQL authentication is used for connectivity". We currently use an Azure linked services as our username and password to connect to our on-prem sql server

What you do recomend. Thanks

[v-karpurapud]

Hi @sholy29 

Thank you for the detailed update and the screenshot.

The error message “Test connection failed. The Data Factory runtime is busy now. Please retry the operation later” shows that the issue is now due to a runtime resource contention in Microsoft Fabric’s backend, specifically the Data Factory runtime that manages mirroring and data movement. This usually happens when there are many concurrent operations, like active mirroring or data transformation jobs, which can temporarily use up available resources.

To address this, it’s recommended to wait 15–30 minutes and then try again, as these issues often resolve themselves. Also, try to avoid running overlapping workloads at the same time pause any active pipelines, Dataflows, or Power BI dataset refreshes if possible.

Additionally, If you have access, use a dedicated Fabric capacity workspace, since lower SKUs or trial capacities are more likely to encounter this issue. You might also try using a new browser session or incognito window to prevent session token problems. For authentication, using a linked service with a stored SQL username and password is valid for mirroring, so no changes are needed there.


Thank you for being part of Fabric Community Forum.

Regards,
Karpurapu D,
Microsoft Fabric Community Support Team.

[v-karpurapud]

Hi @sholy29 

We have not yet heard back from you about whether our response addressed your query. If it did not, please share more details so we can assist you more effectively.

Thank You.

[v-karpurapud]

Hi @sholy29 

I wanted to check if you had the opportunity to review the information provided. Please feel free to contact us if you have any further questions. If my response has addressed your query, please accept it as a solution and give a 'Kudos' so other members can easily find it.

Thank you.

[v-karpurapud]

Hi @sholy29 

Welcome to the Microsoft Fabric Community Forum. 

The error encountered when attempting to add more tables to an existing SQL Server mirroring setup in Microsoft Fabric, specifically “Test connection failed. Request failed with status code 400,” is typically caused by transient backend service issues, gateway misconfigurations, unsupported schema elements, or disabled tenant settings. While previously mirrored tables continue to function, new additions may trigger revalidation processes that expose these underlying issues. 

To resolve this, retry the operation after a short delay, as transient errors often self-resolve. Additionally, ensure the on-premises data gateway is properly configured and restarted, and that SQL authentication is used for connectivity. Review the schema of the new tables to confirm they do not contain deprecated data types such as TEXT, NTEXT, or IMAGE. 

In the Microsoft Fabric Admin Portal, verify that the tenant setting “Users can access data stored in OneLake with apps external to Fabric” is enabled. If previous mirroring attempts left residual metadata, use SQL commands such as sp_change_feed_drop_table_group to clean up stale entries. Finally, add tables incrementally and monitor replication status using diagnostic views like sys.dm_change_feed_errors to isolate and address specific issues. These steps have been validated across multiple incident reports and align with Microsoft’s recommended practices for ensuring reliable mirroring to Fabric.

If this response resolves your query, kindly mark it as Accepted Solution to help other community members. A Kudos is also appreciated if you found the response helpful.

Thank you for being part of Fabric Community Forum.

Regards,
Karpurapu D,
Microsoft Fabric Community Support Team.