Event-driven Order Processing System

A serverless, event-driven backend system that processes orders asynchronously using Azure Functions, Queue Storage, and Cosmos DB. The system demonstrates real-world patterns such as decoupling, retry handling, and eventual consistency.

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Architecture

Client
│
▼
createOrder (HTTP Function)
│
▼
Azure Queue Storage ←─── (decoupling + async boundary)
│
▼
processOrder (Queue Trigger Function)
│
▼
Cosmos DB

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Flowchart

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Tech Stack

• Azure Functions (Python) — Serverless compute for API and worker
• Azure Queue Storage — Message queue for asynchronous processing
• Azure Cosmos DB (SQL API) — NoSQL database for order state
• Azurite — Local development for storage emulation
• Postman — API testing

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Workflow

1. Client sends request to createOrder API
2. System generates:
   • orderId
   • status = CREATED
3. Order is stored in Cosmos DB
4. Order message is pushed to Azure Queue
5. API returns response immediately (no blocking)

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Asynchronous Processing

• Queue triggers the processOrder worker function
• Worker updates status to PROCESSING
• Executes business logic (simulated processing)

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Fault Tolerance & Retry

• Failures are simulated during processing
• Azure Queue automatically retries failed messages
• Retry count is tracked using dequeue_count

Behavior:

• If retries remain → message is reprocessed
• If retries exhausted → order marked as FAILED

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State Transitions

CREATED → PROCESSING → COMPLETED/FAILED (after fixed number of retries)

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Data Persistence

All order states are stored in Cosmos DB:

• CREATED (initial state)
• PROCESSING (during execution)
• COMPLETED / FAILED (final state)

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API Endpoints

Create Order: POST /api/createOrder

Example: https://order-system-func-mk31.azurewebsites.net/api/createOrder

Request

{
  "item": "item_name"
}

Response

{
  "orderId": "...",
  "status": "CREATED"
}

Get Order Status: GET /api/getOrder/{orderID}

Example: https://order-system-func-mk31.azurewebsites.net/api/getOrder/2bb9c503-fe22-4fb5-96ff-4e5d0b67b627 Returns the latest order state from Cosmos DB.

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Deployment

Deployed on Azure Functions: Click here for deployed link.

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Key Concepts Demonstrated

• Event-driven architecture
• Asynchronous processing
• Decoupling via message queues
• Retry mechanisms & failure handling
• Eventual consistency
• Serverless deployment (Azure Functions)

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Version Evolution

V1 (Basic)

• main branch
• Azure Queue Storage
• Simple async processing

V2 (Current)

• redis-servicebus branch
• Service Bus integration
• Redis-based inventory reservation
• Compensation logic
• Application Insights logging

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Future Improvements

• Add authentication (Azure AD / API keys)
• Replace Queue with Azure Service Bus for advanced messaging
• Add monitoring via Application Insights
• Optimize latency by co-locating resources
• Implement dead-letter queue handling