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138 changes: 138 additions & 0 deletions docs/system/architecture/AcknowledgmentEventPattern.md
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# 🧭 Dhanman Messaging Design: Acknowledgment Event Pattern

## **Objective**

Enable asynchronous, reliable communication between services using **event-based acknowledgments** instead of blocking request–response.
Each service that initiates a command will later receive a **targeted acknowledgment event** confirming completion and returning necessary identifiers.

---

## 🧩 **1. Design Philosophy**

- **Fire-and-Forget Commands:** Sales, Purchase, Payroll, and Community send commands to Common or other shared modules.
- **Targeted Acknowledgment Events:** The receiver (e.g., Common) publishes back an acknowledgment event containing IDs or status for the initiating entity.
- **Point-to-Point Routing:** Each acknowledgment event is delivered only to its respective source service, avoiding fanout noise.
- **Guaranteed Delivery:** All acknowledgment events use durable queues bound to a **direct exchange** with explicit routing keys.

---

## 🔁 **2. High-Level Flow Example**

```mermaid
sequenceDiagram
participant Sales as Dhanman.Sales
participant Common as Dhanman.Common

Sales->>Common: 🔶 CreateTransactionCommand (InvoiceId, Amount)
Common->>CommonDB: 💾 Create Transaction
Common-->>Sales: 🔵 TransactionPostedForInvoiceEvent (InvoiceId, TransactionId)
Sales->>SalesDB: 🧾 Update Invoice (TransactionId, Posted = true)
```

---

## 🧱 **3. Exchange and Queue Strategy**

| **Exchange** | **Type** | **Publisher** | **Routing Key Pattern** | **Consumer Service** | **Queue Example** |
| ------------------- | ---------- | ------------- | ----------------------- | -------------------- | ------------------------------------------------------------------ |
| `common.ack.events` | **direct** | Common | `invoice.posted` | Sales | `sales.invoice_posted.queue` |
| `common.ack.events` | **direct** | Common | `bill.posted` | Purchase | `purchase.bill_posted.queue` |
| `common.ack.events` | **direct** | Common | `salary.posted` | Payroll | `payroll.salary_posted.queue` |
| `common.ack.events` | **direct** | Common | `customer.created` | Sales / Community | `sales.customer_created.queue`, `community.customer_created.queue` |
| `common.ack.events` | **direct** | Common | `vendor.created` | Purchase | `purchase.vendor_created.queue` |
| `common.ack.events` | **direct** | Common | `employee.created` | Payroll | `payroll.employee_created.queue` |

---

## 🧩 **4. Complete List of Acknowledgment Events**

| **Origin Service (Sender)** | **Target Publisher (Responder)** | **Command Sent** | **Acknowledgment Event** | **Payload Fields (Suggested)** | **Consumer Queue** |
| --------------------------- | -------------------------------- | ---------------------------------------- | ----------------------------------------- | --------------------------------------------------------------------------------- | ------------------------------------- |
| **Sales** | Common | `CreateBasicCustomerCommand` | `CustomerCreatedForSalesEvent` | `CustomerId`, `CompanyId`, `OrganizationId`, `CorrelationId`, `CreatedOnUtc` | `sales.customer_created.queue` |
| | Common | `CreateTransactionCommand` | `TransactionPostedForInvoiceEvent` | `InvoiceId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `sales.invoice_posted.queue` |
| | Common | `CreateInvoicePaymentCommand` | `TransactionPostedForInvoicePaymentEvent` | `InvoicePaymentId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `sales.invoice_payment_posted.queue` |
| **Purchase** | Common | `CreateBasicVendorCommand` | `VendorCreatedForPurchaseEvent` | `VendorId`, `CompanyId`, `OrganizationId`, `CreatedOnUtc` | `purchase.vendor_created.queue` |
| | Common | `CreateTransactionCommand` | `TransactionPostedForBillEvent` | `BillId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `purchase.bill_posted.queue` |
| | Common | `CreateBillPaymentCommand` | `TransactionPostedForBillPaymentEvent` | `BillPaymentId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `purchase.bill_payment_posted.queue` |
| **Payroll** | Common | `CreateBasicEmployeeCommand` | `EmployeeCreatedForPayrollEvent` | `EmployeeId`, `CompanyId`, `OrganizationId`, `CreatedOnUtc` | `payroll.employee_created.queue` |
| | Common | `CreateTransactionCommand` | `TransactionPostedForSalaryEvent` | `SalaryId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `payroll.salary_posted.queue` |
| | Common | `CreateSalaryPaymentCommand` | `TransactionPostedForSalaryPaymentEvent` | `SalaryPaymentId`, `TransactionId`, `CompanyId`, `OrganizationId`, `PostedOnUtc` | `payroll.salary_payment_posted.queue` |
| **Community** | Common | `CreateBasicCustomerCommand` | `CustomerCreatedForCommunityEvent` | `CustomerId`, `UnitId`, `CompanyId`, `OrganizationId`, `CreatedOnUtc` | `community.customer_created.queue` |
| **Common (reverse ack)** | Community | (Triggered when Unit creates a Customer) | `CustomerLinkedToUnitEvent` | `UnitId`, `CustomerId`, `CompanyId`, `OrganizationId`, `LinkedOnUtc` | `community.customer_linked.queue` |

---

## ⚙️ **5. MassTransit Configuration Example**

### **Common.Api Publisher**

```csharp
await _eventPublisher.PublishAsync(new TransactionPostedForInvoiceEvent
{
InvoiceId = command.InvoiceId,
TransactionId = transaction.Id,
CompanyId = command.CompanyId,
OrganizationId = command.OrganizationId,
PostedOnUtc = DateTime.UtcNow
}, context =>
{
context.SetRoutingKey("invoice.posted");
});
```

### **Sales.Api Consumer**

```csharp
cfg.ReceiveEndpoint("sales.invoice_posted.queue", e =>
{
e.ConfigureConsumeTopology = false;
e.Bind("common.ack.events", x =>
{
x.RoutingKey = "invoice.posted";
});
e.ConfigureConsumer<TransactionPostedForInvoiceConsumer>(context);
});
```

---

## 🧠 **6. Why Use Direct Exchange**

✔ Only specific consumers receive relevant messages
✔ Reduces unnecessary message traffic
✔ Keeps event routing explicit and predictable
✔ Easy to extend — just add a new routing key and queue binding for another event type

---

## 🪶 **7. Schema Example (Event DTO)**

```csharp
public record TransactionPostedForInvoiceEvent(
Guid InvoiceId,
long TransactionId,
Guid CompanyId,
Guid OrganizationId,
DateTime PostedOnUtc,
Guid CorrelationId);
```

All acknowledgment events follow a consistent convention:

- Include **CorrelationId** from originating command
- Include **EntityId** (InvoiceId, BillId, etc.)
- Include **TransactionId**
- Include **OrganizationId**, **CompanyId**, and **Timestamp**

---

## 🧩 **8. Summary Checklist**

| ✅ Goal | 🧩 Implementation |
| -------------------------------- | ------------------------------------------------------------------ |
| Maintain async non-blocking flow | Fire commands, listen for acknowledgment events |
| Event naming consistency | Use `TransactionPostedForXEvent`, `CustomerCreatedForXEvent`, etc. |
| Routing isolation | Use direct exchange with service-specific routing keys |
| Observability | Include CorrelationId in every message |
| Reliability | Use Outbox pattern for event publishing |
| Maintainability | Group all acknowledgments under `common.ack.events` exchange |
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