Fix #3400, ignore server-provided failover partner by mdaigle · Pull Request #3625 · dotnet/SqlClient

mdaigle · 2025-09-22T22:47:58Z

Description

A Basic Availability Group (BAG), is a basic high availability configuration for standard edition, on-prem servers (not applicable to azure). It involves two servers, one a primary replica and the other a secondary replica (a.k.a. failover partner / mirroring partner). When the primary is unavailable, clients will connect to the secondary. When connecting to the primary in a BAG, the server specifies the domain name of the secondary to the client via a Real Time Log Shipping ENVCHANGE token. This allows the BAG's secondary configuration to be updated dynamically without a code change. We call this the server-provided failover partner. Clients can also specify a failover partner directly in the connection string. The client-provided failover partner is used only if the initial connection to the primary replica fails (no server-side failover partner available). Otherwise, the server-provided failover partner supersedes the client-provided failover partner.

When configuring a BAG, replicas are added to the group by specifying their NetBIOS or network name (CREATE AVAILABILITY GROUP (Transact-SQL) - SQL Server | Microsoft Learn), NOT their protocol, fully qualified domain name, and port. This network name is the value returned to the client as the server-provided failover partner. This causes an issue when using TCP for either replica because the server-provided failover partner will not contain the protocol and FQDN.

The goal of this PR is to provide support for TCP in BAGs while respecting the following constraints:

We must continue to prefer server-provided failover partners by default. This behavior is useful and changing it would not be backwards compatible.
Low cost. Avoid server-side or TSQL changes.

We introduce a client-side app context switch that, when enabled, ignores the server-provided failover partner and only uses the failover partner provided in the connection string.

Example usage

Connecting to a BAG via TCP:

AppContext.SetSwitch("Switch.Microsoft.Data.SqlClient.IgnoreServerProvidedFailoverPartner", true);

SqlConnection connection = new SqlConnection("Data Source=tcp:localhost,1234;Failover Partner=tcp:localhost,5678;...");
connection.Open();

Issues

#3400

Testing

Testing capabilities are currently limited in this area.
#3488 will add basic integration level test coverage for this area.

Copilot

Pull Request Overview

This PR introduces an app context switch to ignore server-provided failover partners in Basic Availability Groups (BAGs), addressing issues with custom ports when the server provides only the NetBIOS name without port information.

Adds IgnoreServerProvidedFailoverPartner app context switch to allow client-side control of failover behavior
Refactors failover partner property from private field to public property for better encapsulation
Updates failover logic to conditionally ignore server-provided partners based on the new switch

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
LocalAppContextSwitches.cs	Adds new app context switch configuration and property
SqlInternalConnectionTds.cs (netfx)	Implements conditional failover partner logic and refactors property access
SqlInternalConnectionTds.cs (netcore)	Mirror implementation for .NET Core with same conditional logic
SqlCommand.cs (netfx)	Updates property name reference for consistency
SqlCommand.cs (netcore)	Updates property name reference for consistency

src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs

src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs

…8489-retain-failover-port

cheenamalhotra · 2025-09-23T02:11:24Z

Can you add an example here on how should a user specify secondary failover replica info with this change and enable app context switch alongside?

edwardneal · 2025-09-23T05:10:48Z

This might also be the root cause of #2610.

Besides this, another issue (#2365) has suggested a similar idea, but controlled using a connection string option.

mdaigle · 2025-09-23T14:15:35Z

This might also be the root cause of #2610.

Besides this, another issue (#2365) has suggested a similar idea, but controlled using a connection string option.

Yes, looks like this is the same issue. The network name used to configure a BAG doesn't square with most people's setups these days.

codecov · 2025-09-23T15:42:02Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.82%. Comparing base (d244be2) to head (5e0b12e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3625       +/-   ##
===========================================
+ Coverage   66.43%   90.82%   +24.38%     
===========================================
  Files         271        6      -265     
  Lines       60277      316    -59961     
===========================================
- Hits        40048      287    -39761     
+ Misses      20229       29    -20200

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addons	`90.82% <ø> (?)`
netcore	`?`
netfx	`?`

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…8489-retain-failover-port

… simulated server test coverage.

src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionFailoverTests.cs

* Add IgnoreServerProvidedFailoverPartner app context switch. * Add behavior skip to netfx. * Consolidate to single property for failover partner value. * Rework checks to preserve server provided value, but ignore it. * Fix import. * Skip server failover partner override in �LoginNoFailover method. Add simulated server test coverage.

…ner. (#3704) * Fix #3400, ignore server-provided failover partner (#3625) * Add IgnoreServerProvidedFailoverPartner app context switch. * Add behavior skip to netfx. * Consolidate to single property for failover partner value. * Rework checks to preserve server provided value, but ignore it. * Fix import. * Skip server failover partner override in �LoginNoFailover method. Add simulated server test coverage. * Fix compilation issues. * Clean up duplicate line. Adjust app context switch logic to match existing. * Review changes. * Add test case. Doesn't compile. * Adjust usage of test servers and app context. * Add failover and prelogin count capabilities to test server. Update test case. * Set backing field using reflection. * Try clearing pool to make test more reliable. * Use string.Empty

* Add IgnoreServerProvidedFailoverPartner app context switch. * Add behavior skip to netfx. * Consolidate to single property for failover partner value. * Rework checks to preserve server provided value, but ignore it. * Fix import. * Skip server failover partner override in �LoginNoFailover method. Add simulated server test coverage.

…ner. (#3702) * Fix #3400, ignore server-provided failover partner (#3625) * Add IgnoreServerProvidedFailoverPartner app context switch. * Add behavior skip to netfx. * Consolidate to single property for failover partner value. * Rework checks to preserve server provided value, but ignore it. * Fix import. * Skip server failover partner override in �LoginNoFailover method. Add simulated server test coverage. * Adjust category level so test runs in ci. * Review changes. * Clear pool to make test more reliable. * Serialize unit tests that touch app context switches * Are connections getting carried over in the pool somehow? * Skip test in pipeline.

mdaigle added 5 commits September 22, 2025 11:11

Add IgnoreServerProvidedFailoverPartner app context switch.

1b9b8eb

Add behavior skip to netfx.

35eda78

Consolidate to single property for failover partner value.

de0fafe

Rework checks to preserve server provided value, but ignore it.

6a36110

Fix import.

99fe120

mdaigle requested a review from a team as a code owner September 22, 2025 22:47

Copilot AI review requested due to automatic review settings September 22, 2025 22:47

Copilot AI reviewed Sep 22, 2025

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src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/LocalAppContextSwitches.cs Show resolved Hide resolved

src/Microsoft.Data.SqlClient/netfx/src/Microsoft/Data/SqlClient/SqlInternalConnectionTds.cs Show resolved Hide resolved

Merge branch 'main' of github.com:dotnet/SqlClient into dev/mdaigle/3…

92741c4

…8489-retain-failover-port

mdaigle requested a review from samsharma2700 September 22, 2025 22:51

This was linked to issues Sep 23, 2025

Add option to ignore server provided failover partner #2365

Closed

Fail to connect after database failover #2610

Closed

Connection fails and ignores FailoverPartner port after Linux SQL Server BAG failover, defaults to port 1433 #3400

Closed

paulmedynski previously approved these changes Sep 23, 2025

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paulmedynski self-assigned this Sep 23, 2025

cheenamalhotra previously approved these changes Sep 23, 2025

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cheenamalhotra added this to the 7.0-preview2 milestone Sep 23, 2025

mdaigle added 2 commits September 25, 2025 11:49

Merge branch 'main' of github.com:dotnet/SqlClient into dev/mdaigle/3…

a5c9a18

…8489-retain-failover-port

Skip server failover partner override in �LoginNoFailover method. Add…

5e0b12e

… simulated server test coverage.

mdaigle dismissed stale reviews from cheenamalhotra and paulmedynski via 5e0b12e September 26, 2025 16:37

paulmedynski approved these changes Sep 26, 2025

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cheenamalhotra approved these changes Sep 26, 2025

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samsharma2700 approved these changes Sep 26, 2025

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src/Microsoft.Data.SqlClient/tests/UnitTests/SimulatedServerTests/ConnectionFailoverTests.cs Show resolved Hide resolved

mdaigle merged commit fa7132e into main Sep 29, 2025
252 checks passed

mdaigle deleted the dev/mdaigle/38489-retain-failover-port branch September 29, 2025 16:11

This was referenced Oct 20, 2025

[6.1] Backport BAG failover fix. Ignore server provided failover partner. #3702

Merged

[6.0] Backport BAG failover fix. Ignore server provided failover partner. #3703

Merged

mdaigle mentioned this pull request Oct 20, 2025

[5.1] Backport BAG failover fix. Ignore server provided failover partner. #3704

Merged

dependabot bot mentioned this pull request Nov 12, 2025

Bump Microsoft.Data.SqlClient from 7.0.0-preview1.25257.1 to 7.0.0-preview2.25289.6 stack-falconmsr47/UUID#20

Closed

dependabot bot mentioned this pull request Dec 9, 2025

Bump Microsoft.Data.SqlClient from 7.0.0-preview1.25257.1 to 7.0.0-preview3.25342.7 stack-falconmsr47/UUID#29

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Fix #3400, ignore server-provided failover partner#3625

Fix #3400, ignore server-provided failover partner#3625
mdaigle merged 8 commits intomainfrom
dev/mdaigle/38489-retain-failover-port

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