Document IExecutionStrategy lifetime breaking change

Fixes #3662

entity-framework/core/what-is-new/ef-core-6.0/breaking-changes.md

@@ -2,7 +2,7 @@
 title: Breaking changes in EF Core 6.0 - EF Core
 description: Complete list of breaking changes introduced in Entity Framework Core 6.0
 author: ajcvickers
-ms.date: 10/25/2021
+ms.date: 01/10/2022
 uid: core/what-is-new/ef-core-6.0/breaking-changes
 ---
 
@@ -26,6 +26,7 @@ The following API and behavior changes have the potential to break existing appl
 | [TVF return entity type is also mapped to a table by default](#tvf-table)                                                             | Low        |
 | [Check constraint name uniqueness is now validated](#unique-check-constraints)                                                        | Low        |
 | [Added IReadOnly Metadata interfaces and removed extension methods](#ireadonly-metadata)                                              | Low        |
+| [IExecutionStrategy is now a singleton service](#iexecutionstrategy)                                                                  | Low        |
 | [SQL Server: More errors are considered transient](#transient-errors)                                                                 | Low        |
 | [Cosmos: More characters are escaped in 'id' values](#cosmos-id)                                                                      | Low        |
 | [Some Singleton services are now Scoped](#query-services)                                                                             | Low*       |
@@ -469,6 +470,26 @@ Default interface methods allow the implementation to be overridden, this is lev
 
 These changes shouldn't affect most code. However, if you were using the extension methods via the static invocation syntax, it would need to be converted to instance invocation syntax.
 
+<a name="iexecutionstrategy"></a>
+
+### IExecutionStrategy is now a singleton service
+
+[Tracking Issue #21350](https://github.com/dotnet/efcore/issues/21350)
+
+#### New behavior
+
+<xref:Microsoft.EntityFrameworkCore.Storage.IExecutionStrategy> is now a singleton service. This means that any added state in custom implementations will remain between executions and the delegate passed to <xref:Microsoft.EntityFrameworkCore.Infrastructure.RelationalDbContextOptionsBuilder%602.ExecutionStrategy%2A> will only be executed once.
+
+#### Why
+
+This reduced allocations on two hot paths in EF.
+
+#### Mitigations
+
+Implementations deriving from <xref:Microsoft.EntityFrameworkCore.Storage.ExecutionStrategy> should clear any state in <xref:Microsoft.EntityFrameworkCore.Storage.ExecutionStrategy.OnFirstExecution>.
+
+Conditional logic in the delegate passed to <xref:Microsoft.EntityFrameworkCore.Infrastructure.RelationalDbContextOptionsBuilder%602.ExecutionStrategy%2A> should be moved to a custom implementation of <xref:Microsoft.EntityFrameworkCore.Storage.IExecutionStrategy>.
+
 <a name="transient-errors"></a>
 
 ### SQL Server: More errors are considered transient