Complete implementation - Email dispatcher

.gitignore

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+################################################################################
+# This .gitignore file was automatically created by Microsoft(R) Visual Studio.
+################################################################################
+
+/EmailDispatcherAPI/bin
+/EmailDispatcherAPI/.vs/EmailDispatcherAPI
+/EmailDispatcherAPI/obj
+/EmailWorker/bin
+/EmailWorker/obj
+/EmailDispatcherAPI/appsettings.json
+/EmailDispatcherAPI/appsettings.Development.json
+/EmailWorker/appsettings.json
+/EmailWorker/appsettings.Development.json
+/EmailWorker/appsettings.json
+/EmailWorker/appsettings.Development.json
+/EmailRetryScheduler/appsettings.json
+/EmailRetryScheduler/appsettings.Development.json
+/EmailRetryScheduler/bin
+/EmailRetryScheduler/obj
+/EmailRetryScheduler/obj
+/EmailRetryScheduler/obj/Debug
+/EmailRetryScheduler/obj/Debug/net9.0
+/EmailRetryScheduler/appsettings.json
+/EmailRetryScheduler/appsettings.Development.json

Dev_Notes.md

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+## Stack
+- RabbitMQ
+- .NET Worker Service
+- Database (SQL Server)
+- SMTP / Email Provider
+
+## .Net Technical terms used :
+- WEB API
+- Entity Framework
+- Minimal Api
+- Global exception handling and developer exception page
+- DB First approach
+- Repository pattern and DI
+- Asynchronous Initialization via Hosted Service
+
+## RabbitMQ docker setup command
+# latest RabbitMQ 4.x
+- docker run -it --rm --name rabbitmq -p 5672:5672 -p 15672:15672 rabbitmq:4-management
+
+## RabbitMQ client
+- Should have single connection for whole application(singleton)
+- Can create multiple channel as required
+
+## Connection: Physical link to broker
+
+- Channel: Lightweight AMQP session
+
+- Exchange: Routes messages
+
+- Routing key: Address label
+
+- Queue: Stores messages
+
+- Binding: Routing rule
+
+- Consumer: Processes messages
+
+
+## 📨 RabbitMQ Publish Flow
+    ## 1️⃣ Connection
+
+      CreateConnectionAsync() opens a TCP connection
+
+      Heavy operation
+
+      Should be reused, not created per message
+
+      Thread-safe
+
+    ## 2️⃣ Channel
+
+      CreateChannelAsync() creates an AMQP channel
+
+      Lightweight
+
+      Used for publish/consume operations
+
+      Multiple channels can share one connection
+
+    ## 3️⃣ Queue Declaration
+
+      QueueDeclareAsync() ensures queue exists
+
+      Idempotent (safe to call multiple times)
+
+      Fails if queue exists with different settings
+
+      Flags
+
+      durable → survives broker restart
+
+      exclusive → only one connection can use it
+
+      autoDelete → deleted when last consumer disconnects
+
+    ## 4️⃣ Message Serialization
+
+      Messages are sent as byte[]
+
+      Common format: JSON → UTF-8 bytes
+
+    ## 5️⃣ Publishing Message
+
+      BasicPublishAsync() sends message to exchange
+
+      Default exchange ("") routes by queue name
+
+      Publish ≠ delivered
+
+      Publish ≠ persisted
+
+## Dependecy Issue for windows services
+
+While it seems intuitive to make everything a Singleton because a Windows Service runs continuously, you are running into a common dependency injection issue called a Scoped Leak.
+
+The Problem: Capturing a Scoped Service
+In Entity Framework Core, a DbContext is registered as Scoped by default. This means it is designed to be created and destroyed within a short lifetime (like a single HTTP request or a single loop of a worker).
+
+If your EmailRepository is a Singleton, it will be created once when the service starts. Because it depends on AppDBContext, it will "capture" that context and hold onto it forever.
+
+This leads to several issues:
+
+Memory Leaks: The DbContext keeps track of every entity it ever loads. Over days or weeks, your memory usage will climb indefinitely.
+
+Concurrency Crashes: A DbContext is not thread-safe. If your worker tries to do two things at once using the same singleton context, it will throw an exception.
+
+Stale Data: You won't see updates made to the database by other processes because the singleton context keeps its own internal cache.
+
+The Solution: Use a Service Scope
+The correct pattern for a continuous Worker (Windows Service) is to create a Scope manually inside your background loop. This ensures the DbContext is fresh for every "pulse" of work.
+
+
+## C# Rules to remeber 
+- All interface members are implicitly public This is a hard C# rule.
+- A public Classes cannot expose a less-accessible type like internal classes. So when you inject internal into public services means it show error.
+
+
+## RabbitMq
+
+When Send message Quename and Routing key should be same

EmailDispatcherAPI/Constant/AppConstant.cs

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+namespace EmailDispatcherAPI.Constant
+{
+    internal class AppConstant
+    {
+        public const string QueueName = "email.dispatcher.send";
+    }
+}

EmailDispatcherAPI/Constant/Enum/EmailStatus.cs

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+namespace EmailDispatcherAPI.Constant.Enum
+{
+    public enum EmailStatus
+    {
+        Pending = 1,
+        Scheduled = 2,
+        Sent = 3,
+        Failed = 4,
+        RetryQueued = 5,
+        Dead = 6
+    }
+}

EmailDispatcherAPI/Contract/IEmailRepository.cs

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+using EmailDispatcherAPI.Modal;
+
+namespace EmailDispatcherAPI.Contract
+{
+    public interface IEmailRepository
+    {
+        Task<EmailIdempotency?> GetEmailIdempotencyAsync(string idempotencyKey);
+        Task<EmailLog> CreateEmailLog(EmailLog emailLog);
+        Task<EmailIdempotency> CreateEmailIdempotency(EmailIdempotency emailIdempotency);
+        Task<bool> MarkEmailIdempotencyAsPublishedAsync(int id);
+    }
+}

EmailDispatcherAPI/Contract/IEmailService.cs

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+namespace EmailDispatcherAPI.Contract
+{
+    public interface IEmailService
+    {
+        Task<bool> IsValidEmail(string email);
+        Task SendEmail(string mailTo,int entityId);
+    }
+}

EmailDispatcherAPI/Contract/IRabbitMqConnection.cs

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+using RabbitMQ.Client;
+
+namespace EmailDispatcherAPI.Contract
+{
+    public interface IRabbitMqConnection
+    {
+        IConnection Connection { get; }
+    }
+
+}

EmailDispatcherAPI/Data/AppDBContext.cs

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+using EmailDispatcherAPI.Modal;
+using Microsoft.EntityFrameworkCore;
+
+namespace EmailDispatcherAPI.Data
+{
+    public class AppDBContext : DbContext
+    {
+        protected readonly IConfiguration Configuration;
+        public AppDBContext(IConfiguration configuration)
+        {
+            Configuration = configuration;
+        }
+        protected override void OnConfiguring(DbContextOptionsBuilder options)
+        {
+            options.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"));
+        }
+        public DbSet<EmailLog> EmailLog { get; set; }
+        public DbSet<EmailIdempotency> EmailIdempotency { get; set; }
+        public DbSet<EmailStatus> EmailStatus { get; set; }
+        public DbSet<EmailActionLog> EmailActionLog { get; set; }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<EmailIdempotency>()
+                .HasIndex(e => e.MessageKey)
+                .IsUnique();
+
+            modelBuilder.Entity<EmailLog>()
+                .HasIndex(e => new { e.EmailStatusId, e.NextAttemptAt });
+        }
+
+    }
+}

EmailDispatcherAPI/Dto/RabbitMQConfig.cs

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+namespace EmailDispatcherAPI.Dto
+{
+    public class RabbitMQConfig
+    {
+        public string HostName { get; set; } = "localhost";
+        public int Port { get; set; } = 5672;
+        public string UserName { get; set; } = "guest";
+        public string Password { get; set; } = "guest";
+    }
+}
+

EmailDispatcherAPI/EmailDispatcherAPI.csproj

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+<Project Sdk="Microsoft.NET.Sdk.Web">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="9.0.0" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="9.0.1" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="9.0.1" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.Tools" Version="9.0.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
+    <PackageReference Include="Newtonsoft.Json" Version="13.0.4" />
+    <PackageReference Include="RabbitMQ.Client" Version="7.2.0" />
+    <PackageReference Include="Swashbuckle.AspNetCore" Version="9.0.1" />
+  </ItemGroup>
+
+</Project>

EmailDispatcherAPI/EmailDispatcherAPI.csproj.user

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+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <ActiveDebugProfile>https</ActiveDebugProfile>
+  </PropertyGroup>
+</Project>

EmailDispatcherAPI/EmailDispatcherAPI.http

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+@EmailDispatcherAPI_HostAddress = http://localhost:5213
+
+GET {{EmailDispatcherAPI_HostAddress}}/weatherforecast/
+Accept: application/json
+
+###

EmailDispatcherAPI/EmailDispatcherAPI.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.6.33801.468
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "EmailDispatcherAPI", "EmailDispatcherAPI.csproj", "{46D2A6BB-80B6-41F1-AD62-599EFB8799EE}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "EmailWorker", "..\EmailWorker\EmailWorker.csproj", "{F70C08CC-2CDD-4EC4-BDDA-C99C994D2D40}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "EmailRetryScheduler", "..\EmailRetryScheduler\EmailRetryScheduler.csproj", "{C666BCBF-D1C1-470A-BE3F-EA19BAEC5370}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{46D2A6BB-80B6-41F1-AD62-599EFB8799EE}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{46D2A6BB-80B6-41F1-AD62-599EFB8799EE}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{46D2A6BB-80B6-41F1-AD62-599EFB8799EE}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{46D2A6BB-80B6-41F1-AD62-599EFB8799EE}.Release|Any CPU.Build.0 = Release|Any CPU
+		{F70C08CC-2CDD-4EC4-BDDA-C99C994D2D40}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{F70C08CC-2CDD-4EC4-BDDA-C99C994D2D40}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{F70C08CC-2CDD-4EC4-BDDA-C99C994D2D40}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{F70C08CC-2CDD-4EC4-BDDA-C99C994D2D40}.Release|Any CPU.Build.0 = Release|Any CPU
+		{C666BCBF-D1C1-470A-BE3F-EA19BAEC5370}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{C666BCBF-D1C1-470A-BE3F-EA19BAEC5370}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{C666BCBF-D1C1-470A-BE3F-EA19BAEC5370}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{C666BCBF-D1C1-470A-BE3F-EA19BAEC5370}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {6CD7770A-E955-4903-AB51-0582BCF4B728}
+	EndGlobalSection
+EndGlobal

EmailDispatcherAPI/EmailDispatcherAPI.slnLaunch.user

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+[
+  {
+    "Name": "New Profile",
+    "Projects": [
+      {
+        "Path": "EmailDispatcherAPI.csproj",
+        "Action": "Start"
+      },
+      {
+        "Path": "..\\EmailWorker\\EmailWorker.csproj",
+        "Action": "Start"
+      },
+      {
+        "Path": "..\\EmailRetryScheduler\\EmailRetryScheduler.csproj",
+        "Action": "Start"
+      }
+    ]
+  }
+]

EmailDispatcherAPI/Exception/CustomException.cs

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+namespace EmailDispatcherAPI.Exception
+{
+    public sealed class ResourceAlreadyExistsException : System.Exception
+    {
+        public ResourceAlreadyExistsException(string message) : base(message) {}
+    }
+}
+
+