diff --git a/src/BuildingBlocks/BuildingBlocks.csproj b/src/BuildingBlocks/BuildingBlocks.csproj
index 81ab599..8595b07 100644
--- a/src/BuildingBlocks/BuildingBlocks.csproj
+++ b/src/BuildingBlocks/BuildingBlocks.csproj
@@ -3,9 +3,9 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
-
   <ItemGroup>
     <PackageReference Include="Ardalis.GuardClauses" Version="4.0.1" />
     <PackageReference Include="Asp.Versioning.Abstractions" Version="7.0.0" />
@@ -144,7 +144,7 @@
     <PackageReference Include="Google.Protobuf" Version="3.21.12" />
     <PackageReference Include="Grpc.Net.Client" Version="2.51.0" />
     <PackageReference Include="Grpc.Net.ClientFactory" Version="2.51.0" />
-    
+
     <PackageReference Update="Meziantou.Analyzer" Version="1.0.758" />
     <PackageReference Update="AsyncFixer" Version="1.6.0" />
     <PackageReference Update="Roslynator.Analyzers" Version="4.2.0" />
diff --git a/src/BuildingBlocks/Core/Model/IVersion.cs b/src/BuildingBlocks/Core/Model/IVersion.cs
index 3dc2a27..e2b3138 100644
--- a/src/BuildingBlocks/Core/Model/IVersion.cs
+++ b/src/BuildingBlocks/Core/Model/IVersion.cs
@@ -1,5 +1,6 @@
 namespace BuildingBlocks.Core.Model;
 
+// For handling optimistic concurrency
 public interface IVersion
 {
     long Version { get; set; }
diff --git a/src/BuildingBlocks/EFCore/AppDbContextBase.cs b/src/BuildingBlocks/EFCore/AppDbContextBase.cs
index 4a2684c..9d33010 100644
--- a/src/BuildingBlocks/EFCore/AppDbContextBase.cs
+++ b/src/BuildingBlocks/EFCore/AppDbContextBase.cs
@@ -4,22 +4,24 @@ using System.Collections.Immutable;
 using Core.Event;
 using Core.Model;
 using Microsoft.EntityFrameworkCore;
-using Microsoft.EntityFrameworkCore.Storage;
+using Microsoft.Extensions.Logging;
 using Web;
 using Exception = System.Exception;
 using IsolationLevel = System.Data.IsolationLevel;
 
 public abstract class AppDbContextBase : DbContext, IDbContext
 {
-    private readonly ICurrentUserProvider _currentUserProvider;
-    private IDbContextTransaction? _currentTransaction;
+    private readonly ICurrentUserProvider? _currentUserProvider;
+    private readonly ILogger<AppDbContextBase>? _logger;
 
-    protected AppDbContextBase(DbContextOptions options, ICurrentUserProvider currentUserProvider) :
+    protected AppDbContextBase(DbContextOptions options, ICurrentUserProvider? currentUserProvider = null, ILogger<AppDbContextBase>? logger = null) :
         base(options)
     {
         _currentUserProvider = currentUserProvider;
+        _logger = logger;
     }
 
+
     protected override void OnModelCreating(ModelBuilder builder)
     {
     }
@@ -48,7 +50,29 @@ public abstract class AppDbContextBase : DbContext, IDbContext
     public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
     {
         OnBeforeSaving();
-        return await base.SaveChangesAsync(cancellationToken);
+        try
+        {
+            return await base.SaveChangesAsync(cancellationToken);
+        }
+        //ref: https://learn.microsoft.com/en-us/ef/core/saving/concurrency?tabs=data-annotations#resolving-concurrency-conflicts
+        catch (DbUpdateConcurrencyException ex)
+        {
+            foreach (var entry in ex.Entries)
+            {
+                var databaseValues = await entry.GetDatabaseValuesAsync(cancellationToken);
+
+                if (databaseValues == null)
+                {
+                    _logger.LogError("The record no longer exists in the database, The record has been deleted by another user.");
+                    throw;
+                }
+
+                // Refresh the original values to bypass next concurrency check
+                entry.OriginalValues.SetValues(databaseValues);
+            }
+
+            return await base.SaveChangesAsync(cancellationToken);
+        }
     }
 
     public IReadOnlyList<IDomainEvent> GetDomainEvents()
diff --git a/src/BuildingBlocks/PersistMessageProcessor/Data/DesignTimeDbContextFactory.cs b/src/BuildingBlocks/PersistMessageProcessor/Data/DesignTimeDbContextFactory.cs
index ce710a6..5609afe 100644
--- a/src/BuildingBlocks/PersistMessageProcessor/Data/DesignTimeDbContextFactory.cs
+++ b/src/BuildingBlocks/PersistMessageProcessor/Data/DesignTimeDbContextFactory.cs
@@ -11,6 +11,6 @@ public class DesignTimeDbContextFactory : IDesignTimeDbContextFactory<PersistMes
 
         builder.UseNpgsql("Server=localhost;Port=5432;Database=persist_message;User Id=postgres;Password=postgres;Include Error Detail=true")
             .UseSnakeCaseNamingConvention();
-        return new PersistMessageDbContext(builder.Options, null);
+        return new PersistMessageDbContext(builder.Options);
     }
 }
diff --git a/src/BuildingBlocks/PersistMessageProcessor/Data/PersistMessageDbContext.cs b/src/BuildingBlocks/PersistMessageProcessor/Data/PersistMessageDbContext.cs
index 56fd600..94c798a 100644
--- a/src/BuildingBlocks/PersistMessageProcessor/Data/PersistMessageDbContext.cs
+++ b/src/BuildingBlocks/PersistMessageProcessor/Data/PersistMessageDbContext.cs
@@ -3,19 +3,17 @@ using Microsoft.EntityFrameworkCore;
 
 namespace BuildingBlocks.PersistMessageProcessor.Data;
 
-using System.Net;
 using Configurations;
 using Core.Model;
-using global::Polly;
 using Microsoft.Extensions.Logging;
 using Exception = System.Exception;
 
 public class PersistMessageDbContext : DbContext, IPersistMessageDbContext
 {
-    private readonly ILogger<PersistMessageDbContext> _logger;
+    private readonly ILogger<PersistMessageDbContext>? _logger;
 
     public PersistMessageDbContext(DbContextOptions<PersistMessageDbContext> options,
-        ILogger<PersistMessageDbContext> logger)
+        ILogger<PersistMessageDbContext>? logger = null)
         : base(options)
     {
         _logger = logger;
@@ -34,23 +32,9 @@ public class PersistMessageDbContext : DbContext, IPersistMessageDbContext
     {
         OnBeforeSaving();
 
-        var policy = Policy.Handle<DbUpdateConcurrencyException>()
-            .WaitAndRetryAsync(retryCount: 3,
-                sleepDurationProvider: retryAttempt => TimeSpan.FromSeconds(1),
-                onRetry: (exception, timeSpan, retryCount, context) =>
-                {
-                    if (exception != null)
-                    {
-                        _logger.LogError(exception,
-                            "Request failed with {StatusCode}. Waiting {TimeSpan} before next retry. Retry attempt {RetryCount}.",
-                            HttpStatusCode.Conflict,
-                            timeSpan,
-                            retryCount);
-                    }
-                });
         try
         {
-            return await policy.ExecuteAsync(async () => await base.SaveChangesAsync(cancellationToken));
+            return await base.SaveChangesAsync(cancellationToken);
         }
         //ref: https://learn.microsoft.com/en-us/ef/core/saving/concurrency?tabs=data-annotations#resolving-concurrency-conflicts
         catch (DbUpdateConcurrencyException ex)
diff --git a/src/BuildingBlocks/PersistMessageProcessor/PersistMessageProcessor.cs b/src/BuildingBlocks/PersistMessageProcessor/PersistMessageProcessor.cs
index ff18f6e..d34d587 100644
--- a/src/BuildingBlocks/PersistMessageProcessor/PersistMessageProcessor.cs
+++ b/src/BuildingBlocks/PersistMessageProcessor/PersistMessageProcessor.cs
@@ -17,8 +17,6 @@ public class PersistMessageProcessor : IPersistMessageProcessor
     private readonly IMediator _mediator;
     private readonly IPersistMessageDbContext _persistMessageDbContext;
     private readonly IPublishEndpoint _publishEndpoint;
-    private SemaphoreSlim Semaphore => new SemaphoreSlim(1);
-
     public PersistMessageProcessor(
         ILogger<PersistMessageProcessor> logger,
         IMediator mediator,
@@ -108,7 +106,6 @@ public class PersistMessageProcessor : IPersistMessageProcessor
         }
     }
 
-
     public async Task ProcessAllAsync(CancellationToken cancellationToken = default)
     {
         var messages = await _persistMessageDbContext.PersistMessages
@@ -117,16 +114,7 @@ public class PersistMessageProcessor : IPersistMessageProcessor
 
         foreach (var message in messages)
         {
-            await Semaphore.WaitAsync(cancellationToken);
-
-            try
-            {
-                await ProcessAsync(message.Id, message.DeliveryType, cancellationToken);
-            }
-            finally
-            {
-                Semaphore.Release();
-            }
+            await ProcessAsync(message.Id, message.DeliveryType, cancellationToken);
         }
     }
 
diff --git a/src/Services/Booking/src/Booking/Booking.csproj b/src/Services/Booking/src/Booking/Booking.csproj
index 6f0239f..22fd0ed 100644
--- a/src/Services/Booking/src/Booking/Booking.csproj
+++ b/src/Services/Booking/src/Booking/Booking.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
@@ -29,11 +30,11 @@
   <ItemGroup>
     <Folder Include="GrpcClient\Protos" />
   </ItemGroup>
-  
+
   <ItemGroup>
     <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
       <_Parameter1>Integration.Test</_Parameter1>
     </AssemblyAttribute>
   </ItemGroup>
-  
+
 </Project>
diff --git a/src/Services/Booking/tests/IntegrationTest/Integration.Test.csproj b/src/Services/Booking/tests/IntegrationTest/Integration.Test.csproj
index c21a265..6b0d8b3 100644
--- a/src/Services/Booking/tests/IntegrationTest/Integration.Test.csproj
+++ b/src/Services/Booking/tests/IntegrationTest/Integration.Test.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Flight/src/Flight/Data/DesignTimeDbContextFactory.cs b/src/Services/Flight/src/Flight/Data/DesignTimeDbContextFactory.cs
index 6047782..b9a7a6c 100644
--- a/src/Services/Flight/src/Flight/Data/DesignTimeDbContextFactory.cs
+++ b/src/Services/Flight/src/Flight/Data/DesignTimeDbContextFactory.cs
@@ -11,7 +11,7 @@ namespace Flight.Data
 
             builder.UseNpgsql("Server=localhost;Port=5432;Database=flight;User Id=postgres;Password=postgres;Include Error Detail=true")
                 .UseSnakeCaseNamingConvention();
-            return new FlightDbContext(builder.Options, null);
+            return new FlightDbContext(builder.Options);
         }
     }
 }
diff --git a/src/Services/Flight/src/Flight/Data/FlightDbContext.cs b/src/Services/Flight/src/Flight/Data/FlightDbContext.cs
index b1945e3..8f9f77c 100644
--- a/src/Services/Flight/src/Flight/Data/FlightDbContext.cs
+++ b/src/Services/Flight/src/Flight/Data/FlightDbContext.cs
@@ -7,11 +7,13 @@ using Microsoft.EntityFrameworkCore;
 namespace Flight.Data;
 
 using BuildingBlocks.Web;
+using Microsoft.Extensions.Logging;
 
 public sealed class FlightDbContext : AppDbContextBase
 {
-    public FlightDbContext(DbContextOptions<FlightDbContext> options, ICurrentUserProvider currentUserProvider) : base(
-        options, currentUserProvider)
+    public FlightDbContext(DbContextOptions<FlightDbContext> options, ICurrentUserProvider? currentUserProvider = null,
+        ILogger<FlightDbContext>? logger = null) : base(
+        options, currentUserProvider, logger)
     {
     }
 
diff --git a/src/Services/Flight/src/Flight/Flight.csproj b/src/Services/Flight/src/Flight/Flight.csproj
index b8d25fe..5ec1a9a 100644
--- a/src/Services/Flight/src/Flight/Flight.csproj
+++ b/src/Services/Flight/src/Flight/Flight.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Flight/tests/EndToEndTest/EndToEnd.Test.csproj b/src/Services/Flight/tests/EndToEndTest/EndToEnd.Test.csproj
index 5d4348e..6acf245 100644
--- a/src/Services/Flight/tests/EndToEndTest/EndToEnd.Test.csproj
+++ b/src/Services/Flight/tests/EndToEndTest/EndToEnd.Test.csproj
@@ -4,7 +4,6 @@
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
-
     <IsPackable>false</IsPackable>
   </PropertyGroup>
 
diff --git a/src/Services/Flight/tests/IntegrationTest/Integration.Test.csproj b/src/Services/Flight/tests/IntegrationTest/Integration.Test.csproj
index 1807978..ed55732 100644
--- a/src/Services/Flight/tests/IntegrationTest/Integration.Test.csproj
+++ b/src/Services/Flight/tests/IntegrationTest/Integration.Test.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Flight/tests/UnitTest/Common/DbContextFactory.cs b/src/Services/Flight/tests/UnitTest/Common/DbContextFactory.cs
index 62c5bcb..a00335c 100644
--- a/src/Services/Flight/tests/UnitTest/Common/DbContextFactory.cs
+++ b/src/Services/Flight/tests/UnitTest/Common/DbContextFactory.cs
@@ -24,7 +24,7 @@ public static class DbContextFactory
         var options = new DbContextOptionsBuilder<FlightDbContext>()
             .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString()).Options;
 
-        var context = new FlightDbContext(options, currentUserProvider: null);
+        var context = new FlightDbContext(options, currentUserProvider: null, null);
 
         // Seed our data
         FlightDataSeeder(context);
diff --git a/src/Services/Flight/tests/UnitTest/Unit.Test.csproj b/src/Services/Flight/tests/UnitTest/Unit.Test.csproj
index 7ebb982..d69eeb2 100644
--- a/src/Services/Flight/tests/UnitTest/Unit.Test.csproj
+++ b/src/Services/Flight/tests/UnitTest/Unit.Test.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Identity/src/Identity/Data/IdentityContext.cs b/src/Services/Identity/src/Identity/Data/IdentityContext.cs
index ff67c9a..2195862 100644
--- a/src/Services/Identity/src/Identity/Data/IdentityContext.cs
+++ b/src/Services/Identity/src/Identity/Data/IdentityContext.cs
@@ -16,14 +16,16 @@ namespace Identity.Data;
 
 using System;
 using Microsoft.EntityFrameworkCore.Storage;
+using Microsoft.Extensions.Logging;
 
 public sealed class IdentityContext : IdentityDbContext<User, Role, Guid,
     UserClaim, UserRole, UserLogin, RoleClaim, UserToken>, IDbContext
 {
-    private IDbContextTransaction? _currentTransaction;
+    private readonly ILogger<IdentityContext>? _logger;
 
-    public IdentityContext(DbContextOptions<IdentityContext> options) : base(options)
+    public IdentityContext(DbContextOptions<IdentityContext> options, ILogger<IdentityContext>? logger = null) : base(options)
     {
+        _logger = logger;
     }
 
     protected override void OnModelCreating(ModelBuilder builder)
@@ -58,7 +60,29 @@ public sealed class IdentityContext : IdentityDbContext<User, Role, Guid,
     public override async Task<int> SaveChangesAsync(CancellationToken cancellationToken = default)
     {
         OnBeforeSaving();
-        return await base.SaveChangesAsync(cancellationToken);
+        try
+        {
+            return await base.SaveChangesAsync(cancellationToken);
+        }
+        //ref: https://learn.microsoft.com/en-us/ef/core/saving/concurrency?tabs=data-annotations#resolving-concurrency-conflicts
+        catch (DbUpdateConcurrencyException ex)
+        {
+            foreach (var entry in ex.Entries)
+            {
+                var databaseValues = await entry.GetDatabaseValuesAsync(cancellationToken);
+
+                if (databaseValues == null)
+                {
+                    _logger.LogError("The record no longer exists in the database, The record has been deleted by another user.");
+                    throw;
+                }
+
+                // Refresh the original values to bypass next concurrency check
+                entry.OriginalValues.SetValues(databaseValues);
+            }
+
+            return await base.SaveChangesAsync(cancellationToken);
+        }
     }
 
     public IReadOnlyList<IDomainEvent> GetDomainEvents()
diff --git a/src/Services/Identity/src/Identity/Identity.csproj b/src/Services/Identity/src/Identity/Identity.csproj
index 26628df..cf433e1 100644
--- a/src/Services/Identity/src/Identity/Identity.csproj
+++ b/src/Services/Identity/src/Identity/Identity.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Identity/tests/IntegrationTest/Integration.Test.csproj b/src/Services/Identity/tests/IntegrationTest/Integration.Test.csproj
index 5cefc2e..7153e37 100644
--- a/src/Services/Identity/tests/IntegrationTest/Integration.Test.csproj
+++ b/src/Services/Identity/tests/IntegrationTest/Integration.Test.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Passenger/src/Passenger.Api/Passenger.Api.csproj b/src/Services/Passenger/src/Passenger.Api/Passenger.Api.csproj
index 91eafd6..f18f014 100644
--- a/src/Services/Passenger/src/Passenger.Api/Passenger.Api.csproj
+++ b/src/Services/Passenger/src/Passenger.Api/Passenger.Api.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Passenger/src/Passenger/Data/DesignTimeDbContextFactory.cs b/src/Services/Passenger/src/Passenger/Data/DesignTimeDbContextFactory.cs
index 87860fb..d56404d 100644
--- a/src/Services/Passenger/src/Passenger/Data/DesignTimeDbContextFactory.cs
+++ b/src/Services/Passenger/src/Passenger/Data/DesignTimeDbContextFactory.cs
@@ -11,6 +11,6 @@ public class DesignTimeDbContextFactory: IDesignTimeDbContextFactory<PassengerDb
 
         builder.UseNpgsql("Server=localhost;Port=5432;Database=passenger;User Id=postgres;Password=postgres;Include Error Detail=true")
             .UseSnakeCaseNamingConvention();
-        return new PassengerDbContext(builder.Options, null);
+        return new PassengerDbContext(builder.Options);
     }
 }
diff --git a/src/Services/Passenger/src/Passenger/Data/PassengerDbContext.cs b/src/Services/Passenger/src/Passenger/Data/PassengerDbContext.cs
index 3f855eb..2aa1227 100644
--- a/src/Services/Passenger/src/Passenger/Data/PassengerDbContext.cs
+++ b/src/Services/Passenger/src/Passenger/Data/PassengerDbContext.cs
@@ -5,10 +5,13 @@ using Microsoft.EntityFrameworkCore;
 
 namespace Passenger.Data;
 
+using Microsoft.Extensions.Logging;
+
 public sealed class PassengerDbContext : AppDbContextBase
 {
-    public PassengerDbContext(DbContextOptions<PassengerDbContext> options, ICurrentUserProvider currentUserProvider) :
-        base(options, currentUserProvider)
+    public PassengerDbContext(DbContextOptions<PassengerDbContext> options,
+        ICurrentUserProvider? currentUserProvider = null, ILogger<PassengerDbContext>? logger = null) :
+        base(options, currentUserProvider, logger)
     {
     }
 
diff --git a/src/Services/Passenger/src/Passenger/Passenger.csproj b/src/Services/Passenger/src/Passenger/Passenger.csproj
index afe0565..40cf04a 100644
--- a/src/Services/Passenger/src/Passenger/Passenger.csproj
+++ b/src/Services/Passenger/src/Passenger/Passenger.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/Services/Passenger/tests/IntegrationTest/Integration.Test.csproj b/src/Services/Passenger/tests/IntegrationTest/Integration.Test.csproj
index f96ccb9..686c844 100644
--- a/src/Services/Passenger/tests/IntegrationTest/Integration.Test.csproj
+++ b/src/Services/Passenger/tests/IntegrationTest/Integration.Test.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFramework>net7.0</TargetFramework>
     <IsPackable>false</IsPackable>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>