Added details of database availability checks.

Author: AndyButland

16/umbraco-cms/SUMMARY.md

@@ -419,6 +419,7 @@
 * [Custom Swagger API](reference/custom-swagger-api.md)
 * [Umbraco Flavored Markdown](reference/umbraco-flavored-markdown.md)
 * [Content Type Filters](reference/content-type-filters.md)
+* [Database Availability Checks](reference/database-availability.md)
 
 ## Tutorials
 

16/umbraco-cms/reference/database-availability.md

@@ -0,0 +1,45 @@
+---
+description: Describes the checks Umbraco will do on startup to determine the availability of the database, and how this behavior can be customized.
+---
+
+# Database Availability Checks
+
+When Umbraco boots it will check for a configured database and, if found, verify that a connection can be made.
+
+The default behavior is to check five times with a one second delay between attempts. If, after that, a connection cannot be established, Umbraco will fail with a `BootFailedException`.
+
+## Implementing Custom Behavior
+
+For projects in development with the potential for misconfigured database settings, this is likely a reasonable approach to take.
+
+In production, when you have stable configuration, you may prefer to override the behavior to better handle cases where your hosting infrastructure might restart.
+
+We support this by abstracting the default behavior behind the `IDatabaseAvailabilityCheck` interface found in the `Umbraco.Cms.Infrastructure.Persistence` namespace.
+
+You can implement your own version of this interface and register it via a composer. This is shown in the following, stub example:
+
+```csharp
+using Umbraco.Cms.Core.Composing;
+using Umbraco.Cms.Infrastructure.Persistence;
+
+public class MyDatabaseAvailabilityCheckComposer : IComposer
+{
+    public void Compose(IUmbracoBuilder builder)
+    {
+        builder.Services.AddUnique<IDatabaseAvailabilityCheck, MyDatabaseAvailabilityCheck>();
+    }
+}
+
+internal class MyDatabaseAvailabilityCheck : IDatabaseAvailabilityCheck
+{
+    public bool IsDatabaseAvailable(IUmbracoDatabaseFactory databaseFactory)
+    {
+        // Provide your custom logic to check database availability, wait as required, and return true once a connection is established.
+        return true;
+    }
+}
+
+```
+
+For reference and inspiration, the default implementation can be found [here](https://github.com/umbraco/Umbraco-CMS/blob/main/src/Umbraco.Infrastructure/Persistence/DefaultDatabaseAvailabilityCheck.cs).
+