Changes the ApplicationEventHandler to check if the db is configured instead of being connected to which can cause issues if SQLCE file is locked by another app domain on startup. Then we verify in the CoreBootManager that the db can be connected to with a retry policy. Changes the db context CanConnect method to not store a static value since it might later be able to be connected to.

src/Umbraco.Core/CoreBootManager.cs

@@ -2,10 +2,12 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Threading;
 using System.Web;
 using AutoMapper;
 using Umbraco.Core.Cache;
 using Umbraco.Core.Configuration;
+using Umbraco.Core.Exceptions;
 using Umbraco.Core.IO;
 using Umbraco.Core.Logging;
 using Umbraco.Core.Models.Mapping;
@@ -261,6 +263,9 @@ namespace Umbraco.Core
 
 		    FreezeResolution();
 
+            //Here we need to make sure the db can be connected to
+		    EnsureDatabaseConnection();
+
             using (DisposableTimer.DebugDuration<CoreBootManager>(
                 () => string.Format("Executing {0} IApplicationEventHandler.OnApplicationStarted", ApplicationEventsResolver.Current.ApplicationEventHandlers.Count()),
                 () => "Finished executing IApplicationEventHandler.OnApplicationStarted"))
@@ -299,6 +304,32 @@ namespace Umbraco.Core
 			return this;
 		}
 
+        /// <summary>
+        /// We cannot continue if the db cannot be connected to
+        /// </summary>
+        private void EnsureDatabaseConnection()
+        {
+            if (ApplicationContext.IsConfigured == false) return;
+            if (ApplicationContext.DatabaseContext.IsDatabaseConfigured == false) return;
+
+            var currentTry = 0;
+            while (currentTry < 5)
+            {
+                if (ApplicationContext.DatabaseContext.CanConnect)
+                    break;
+
+                //wait and retry
+                Thread.Sleep(1000);
+                currentTry++;
+            }
+
+            if (currentTry == 5)
+            {
+                throw new UmbracoStartupFailedException("Umbraco cannot start. A connection string is configured but the Umbraco cannot connect to the database.");
+            }
+
+        }
+
         /// <summary>
         /// Freeze resolution to not allow Resolvers to be modified
         /// </summary>