diff --git a/src/Orchard.Web/Modules/Orchard.Workflows/Models/WorkflowDefinitionRecord.cs b/src/Orchard.Web/Modules/Orchard.Workflows/Models/WorkflowDefinitionRecord.cs
index ab585ca4d..ebb7ec26d 100644
--- a/src/Orchard.Web/Modules/Orchard.Workflows/Models/WorkflowDefinitionRecord.cs
+++ b/src/Orchard.Web/Modules/Orchard.Workflows/Models/WorkflowDefinitionRecord.cs
@@ -43,7 +43,7 @@ namespace Orchard.Workflows.Models {
         /// <summary>
         /// List of <see cref="WorkflowRecord"/> associated with this workflow definition.
         /// </summary>
-        [CascadeAllDeleteOrphan, Aggregate]
+        [CascadeAllDeleteOrphan]
         public virtual IList<WorkflowRecord> WorkflowRecords { get; set; }
     }
 }
\ No newline at end of file
diff --git a/src/Orchard/Environment/DefaultOrchardHost.cs b/src/Orchard/Environment/DefaultOrchardHost.cs
index ceafd25af..f7587a841 100644
--- a/src/Orchard/Environment/DefaultOrchardHost.cs
+++ b/src/Orchard/Environment/DefaultOrchardHost.cs
@@ -11,7 +11,6 @@ using Orchard.Environment.Descriptor;
 using Orchard.Environment.Descriptor.Models;
 using Orchard.Localization;
 using Orchard.Logging;
-using Orchard.Utility.Extensions;
 
 namespace Orchard.Environment {
     public class DefaultOrchardHost : IOrchardHost, IShellSettingsManagerEventHandler, IShellDescriptorManagerEventHandler {
@@ -23,7 +22,7 @@ namespace Orchard.Environment {
         private readonly IExtensionLoaderCoordinator _extensionLoaderCoordinator;
         private readonly IExtensionMonitoringCoordinator _extensionMonitoringCoordinator;
         private readonly ICacheManager _cacheManager;
-        private readonly object _syncLock = new object();
+        private readonly static object _syncLock = new object();
 
         private IEnumerable<ShellContext> _shellContexts;
         private IEnumerable<ShellSettings> _tenantsToRestart; 
@@ -54,13 +53,8 @@ namespace Orchard.Environment {
         public Localizer T { get; set; }
         public ILogger Logger { get; set; }
 
-        public IList<ShellContext> Current {
-            get { return BuildCurrent().ToReadOnlyCollection(); }
-        }
-
         public ShellContext GetShellContext(ShellSettings shellSettings) {
-            return Current
-                .Single(shellContext => shellContext.Settings.Name.Equals(shellSettings.Name));
+            return BuildCurrent().SingleOrDefault(shellContext => shellContext.Settings.Name.Equals(shellSettings.Name));
         }
 
         void IOrchardHost.Initialize() {
@@ -112,7 +106,7 @@ namespace Orchard.Environment {
         void StartUpdatedShells() {
             lock (_syncLock) {
                 if (_tenantsToRestart.Any()) {
-                    foreach (var settings in _tenantsToRestart.Distinct().ToList()) {
+                    foreach (var settings in _tenantsToRestart.ToList()) {
                         ActivateShell(settings);
                     }
 
@@ -155,7 +149,11 @@ namespace Orchard.Environment {
             Logger.Debug("Activating context for tenant {0}", context.Settings.Name); 
             context.Shell.Activate();
 
-            _shellContexts = (_shellContexts ?? Enumerable.Empty<ShellContext>()).Union(new [] {context});
+            _shellContexts = (_shellContexts ?? Enumerable.Empty<ShellContext>())
+                            .Where(c => c.Settings.Name != context.Settings.Name)
+                            .Concat(new[] { context })
+                            .ToArray(); 
+            
             _runningShellTable.Add(context.Settings);
         }
 
@@ -199,9 +197,13 @@ namespace Orchard.Environment {
             Logger.Information("Disposing active shell contexts");
 
             if (_shellContexts != null) {
-                foreach (var shellContext in _shellContexts) {
-                    shellContext.Shell.Terminate();
-                    shellContext.LifetimeScope.Dispose();
+                lock (_syncLock) {
+                    if (_shellContexts != null) {
+                        foreach (var shellContext in _shellContexts) {
+                            shellContext.Shell.Terminate();
+                            shellContext.LifetimeScope.Dispose();
+                        }
+                    }
                 }
                 _shellContexts = null;
             }
@@ -232,7 +234,9 @@ namespace Orchard.Environment {
                 
                 // if a tenant has been altered, and is not invalid, reload it
                 if (settings.State != TenantState.Invalid) {
-                    _tenantsToRestart = _tenantsToRestart.Where(x => x.Name != settings.Name).Union(new[] { settings });
+                    _tenantsToRestart = _tenantsToRestart
+                        .Where(x => x.Name != settings.Name)
+                        .Concat(new[] { settings });
                 }
             }
         }
@@ -284,12 +288,14 @@ namespace Orchard.Environment {
                     context = _shellContexts.First(x => x.Settings.Name == tenant);
                 }
 
-                // don't update the settings themselves here
+                // don't flag the tenant if already listed
                 if(_tenantsToRestart.Any(x => x.Name == tenant)) {
                     return;
                 }
 
-                _tenantsToRestart = _tenantsToRestart.Union(new[] { context.Settings });
+                _tenantsToRestart = _tenantsToRestart
+                    .Concat(new[] { context.Settings })
+                    .ToArray();
             }
         }
     }
diff --git a/src/Orchard/Environment/RunningShellTable.cs b/src/Orchard/Environment/RunningShellTable.cs
index c0700a392..7713543a4 100644
--- a/src/Orchard/Environment/RunningShellTable.cs
+++ b/src/Orchard/Environment/RunningShellTable.cs
@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Web;
 using Orchard.Environment.Configuration;
 
@@ -17,34 +18,53 @@ namespace Orchard.Environment {
         private IEnumerable<ShellSettings> _shells = Enumerable.Empty<ShellSettings>();
         private IEnumerable<IGrouping<string, ShellSettings>> _shellsByHost = Enumerable.Empty<ShellSettings>().GroupBy(x => default(string));
         private ShellSettings _fallback;
+        private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim();
 
         public void Add(ShellSettings settings) {
-            _shells = _shells
-                .Where(s => s.Name != settings.Name)
-                .Concat(new[] { settings })
-                .ToArray();
+            _lock.EnterWriteLock();
+            try {
+                _shells = _shells
+                    .Where(s => s.Name != settings.Name)
+                    .Concat(new[] {settings})
+                    .ToArray();
 
-            Organize();
+                Organize();
+            }
+            finally {
+                _lock.ExitWriteLock();
+            }
         }
 
         public void Remove(ShellSettings settings) {
-            _shells = _shells
-                .Where(s => s.Name != settings.Name)
-                .ToArray();
+            _lock.EnterWriteLock();
+            try {
+                _shells = _shells
+                    .Where(s => s.Name != settings.Name)
+                    .ToArray();
 
-            Organize();
+                Organize();
+            }
+            finally {
+                _lock.ExitWriteLock();
+            }
         }
 
         public void Update(ShellSettings settings) {
-            _shells = _shells
-                .Where(s => s.Name != settings.Name)
-                .ToArray();
+            _lock.EnterWriteLock();
+            try {
+                _shells = _shells
+                    .Where(s => s.Name != settings.Name)
+                    .ToArray();
 
-            _shells = _shells
-                .Concat(new[] { settings })
-                .ToArray();
+                _shells = _shells
+                    .Concat(new[] {settings})
+                    .ToArray();
 
-            Organize();
+                Organize();
+            }
+            finally {
+                _lock.ExitWriteLock();
+            }
         }
 
         private void Organize() {
@@ -86,19 +106,21 @@ namespace Orchard.Environment {
         }
 
         public ShellSettings Match(string host, string appRelativePath) {
-            // optimized path when only one tenant (Default)
-            if (!_shellsByHost.Any()) {
-                return _fallback;
-            }
+            _lock.EnterReadLock();
+            try {
+                // optimized path when only one tenant (Default), configured with no custom host
+                if (!_shellsByHost.Any() && _fallback != null) {
+                    return _fallback;
+                }
 
-            var hostLength = host.IndexOf(':');
-            if (hostLength != -1)
-                host = host.Substring(0, hostLength);
+                var hostLength = host.IndexOf(':');
+                if (hostLength != -1)
+                    host = host.Substring(0, hostLength);
 
-            var mostQualifiedMatch = _shellsByHost
-                .Where(group => host.EndsWith(group.Key, StringComparison.OrdinalIgnoreCase))
-                .SelectMany(group => group
-                    .OrderByDescending(settings => (settings.RequestUrlPrefix ?? string.Empty).Length))
+                var mostQualifiedMatch = _shellsByHost
+                    .Where(group => host.EndsWith(group.Key, StringComparison.OrdinalIgnoreCase))
+                    .SelectMany(group => group
+                                             .OrderByDescending(settings => (settings.RequestUrlPrefix ?? string.Empty).Length))
                     .FirstOrDefault(settings => {
                         if (settings.State == TenantState.Disabled) {
                             return false;
@@ -112,7 +134,11 @@ namespace Orchard.Environment {
                                || appRelativePath.Equals("~/" + settings.RequestUrlPrefix, StringComparison.OrdinalIgnoreCase);
                     });
 
-            return mostQualifiedMatch ?? _fallback;
+                return mostQualifiedMatch ?? _fallback;
+            }
+            finally {
+                _lock.ExitReadLock();
+            }
         }
     }
 }