Message Oriented Architecture using NServiceBus
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NServiceBus is a lightweight messaging framework for .NET that helps make distributed systems more reliable, scalable, and extensible using a message-oriented architecture. It provides transaction management, automatic retries on failure, and supports various queues including MSMQ. Messages are sent asynchronously in the form of commands or events. Commands are processed by a single endpoint while events can be published to multiple subscribers. This improves reliability over a traditional request/response model. The document discusses how NServiceBus can be used to address issues like transaction management and data consistency when processing orders across multiple services in a distributed system.
Message Oriented Architecture using NServiceBus
- 1. Message Oriented Architecture using NServiceBus Lars-Erik Kindblad Senior Consultant Twitter: @kindblad E-mail: [email protected]
- 2. What is NServiceBus? Lightweight messaging framework for designing distributed systems in .NET An Enterprise Service Bus Helps to make distributed systems more Reliable Scalable Extensible Founded in 2006 by UDI Dahan, an international renowned expert on software architecture and design Services and support provided by Particular Software Open Source but not free Source code available at https://github.com/Particular/NServiceBus Binaries available as NuGet packages
- 3. Remote Procedure Call (Request/Response) Request: Order Website Response: Order ID Order Service PlaceOrder(order) Synchronous – The website is blocked while calling the Order Service Messaged Oriented Architecture (One-way messaging) Message: PlaceOrder Website Message Queue Connect Receive PlaceOrder message Windows Service Process the PlaceOrder message Asynchronous – The website is NOT blocked while the place order is processed
- 4. NServiceBus Terminology & Components Sender /Send-Only Endpoint Message, Command or Event Message: PlaceOrder Website Message Queue Connect Receive PlaceOrder message Windows Service Process the PlaceOrder message Message Handler Worker Endpoint, Host or Receiver
- 5. Messages in NServiceBus Just simple .NET classes with properties 2 types of messages Command • Do something: PlaceOrder • Sent from one or more senders, processed by one endpoint • Implement ICommand Event • Something has happened: OrderWasPlaced • Published from one sender, processed by several endpoints/subscribers • Implement IEvent
- 6. Command Strongly coupled to the receiving endpoint Command: PlaceOrder Website Command: PlaceOrder Message Queue Endpoint Event (Publish/Subscribe) Subscribing Endpoint Loosely coupled to the receiving endpoints Event: OrderWasPlaced Worker NServiceBus will add 3 messages to the queue, one for each subscriber Event: OrderWasPlaced Message Queue Subscribing Endpoint Subscribing Endpoint
- 7. Queues First in-first out datastructure for storing and retrieving messages PlaceOrder message 4 First-In Queue PlaceOrder message 3 PlaceOrder message 2 PlaceOrder message 1 First-Out Default queue in NServiceBus is Microsoft Message Queuing (MSMQ) Available on all Windows editions since NT 4 and Windows 95 Also support for ActiveMQ RabbitMQ SQL Server Windows Azure Queues Windows Azure ServiceBus One endpoint has one queue, Send-Only endspoints do not have queues
- 8. How to Send & Handle Messages Command: PlaceOrder Website Send Command: PlaceOrder Message Queue Endpoint Handle (Command or Event)
- 9. Summary Sender endpoint Sends a message Can be a website, desktop application or even a worker endpoint Command Message where we want to do something concrete ICommand interface Event Message where we notify about something that has happened IEvent interface Message handler Processes a given message IHandleMessages<Message> interface Worker endpoint Finds and executes the handler for the message Can be a Windows Service, console application, WCF, website
- 10. NServiceBus Requirements MSMQ (default) or any other supported queue ActiveMQ RabbitMQ SQL Server Windows Azure Queues Windows Azure ServiceBus Distributed Transaction Cordinator (DTC) RavenDB (default) or a relational database Download the installer from NServiceBus from http://particular.net/ to setup the required components
- 11. NuGet packages NServiceBus Interfaces Interfaces that we need to define messages NServiceBus Most of the code that drives NServiceBus except hosting Use for send-only endpoints NServiceBus Host Console application that can be installed as a Windows Service Used to process/handle messages NServiceBus Testing Framework for testing NServiceBus endpoints
- 12. The Case – The eBook Shop 1. Create the order in the database 2. Auto-pay the order using a previously stored credit card 3. Send an order confirmation by e-mail 4. Deliver the book to the Windows Phone device
- 13. The Usual Way - Request/Response Place order 1. Add order Browser OrderController Confirmation page with order ID Database 2. Charge credit card PaymentService 3. Send mail SMTP-Server 4. Deliver ebook DeviceService
- 14. Problem #1 – The order is lost during an error Place order 1. Add order Browser OrderController Database Error page Exception 2. Charge credit card Error PaymentService 3. Send mail SMTP-Server 4. Deliver ebook DeviceService Conseqence The database is rolled back User receives an error Order must be sent again
- 15. Problem #2 – Poor Transaction Management Place order 1. Add order Browser OrderController Database Error page 2. Charge credit card PaymentService 3. Send mail SMTP-Server Exception 4. Deliver ebook Error DeviceService Conseqence: The credit card is charged (no transaction support) A mail saying the order was successful was sent (no transaction support) The order do not exist in the database (supports transaction – rolled back) The user will receive an error
- 16. Let’s Fix This With NServiceBus Change to a message oriented architecture Add PlaceOrder message Place order Browser Queue OrderController Confirmation page Old New
- 17. The Message Must Also Be Processed MSMQ 1. Connect 2. Receive message PlaceOrder Worker (Windows Service) PlaceOrder 3. PlaceOrder Message Handler 4. Add order Database 5. Charge credit card PaymentService 6. Send mail SMTP-Server 7. Deliver ebook DeviceService
- 18. When an Error Occur 4. Rollback. Put the message back on the queue and retry MSMQ 1. Connect 2. Receive message PlaceOrder Worker (Windows Service) PlaceOrder 3. PlaceOrder Error Message Handler
- 19. More about MSMQ Max 4MB message size Persists messages to files. Default location C:WINDOWSSystem32msmqstorage Guranteed once-and-only-once delivery The message will only be handled once Supports transactional queues and distributed transactions (using DTC) If an error occur both changes to the database and to the queue can be rolled back Supports store-and-forward MSMQ will store messages locally before forwarding it to the queue on another server MSMQ will confirm when the message has been written to the disk in the outgoing queue, not when it has been delivered to the server Important feature for distributed systems with unrealiable networks Server A Sender MSMQ Outgoing queue Server B Message MSMQ Input queue Worker
- 20. How NServiceBus Handles Failure Transaction management and rollback are automatically handled by NServiceBus in message handlers When an error occur 1. The message is put back on the queue 2. Attempt to process the message 5 times, wait for 10 seconds 3. Attempt to process the message 5 times, wait for 20 seconds 4. Attempt to process the message 5 times, wait for 30 seconds 5. Attempt to process the message 5 times, send the message to an error queue The time interval is configurable Messages can be moved back to the original queue for replay using ReturnToSourceQueue.exe tool.
- 21. A challenge – The Order ID Old New The order ID has not been generated yet – it’s generated in the message handler at the worker endpoint
- 22. Possible Solutions Change the UI to not show the order ID Use a GUID (Guid.NewGuid()) or find another way to generate the ID in the controller instead of in the database • message.OrderId = Guid.NewGuid(); Bus.Send(message); ViewBag.OrderId = message.OrderId; • 5773DD0E-0AB0-446B-8649-B2D43D7DA4AA is not a very user friendly ID Move OrderRepository.Add(order) to the Controller: • var orderId = OrderRepository.Add(order); Bus.Send(message); • Less reliable and more error prone solution Use Send/Reply to simulate request/response • The controller will wait for a PlaceOrderCompleted message that the PlaceOrder handler will send • Too much overhead • An Anti-pattern
- 23. We Still Have Poor Transaction Management 1. Add order Handle PlaceOrder Database 2. Charge credit card PaymentService 3. Send mail SMTP-Server 4. Deliver ebook Exception Error DeviceService Conseqence: The credit card is charged (no transaction support) A mail saying the order was successful was sent (no transaction support) The order do not exist in the database (supports transaction – rolledback) The message is put back on the queue and will be retried = credit card will be charged twice
- 24. Solution Split into man small messages - One message per transactional boundary Place order OrderController Browser 1. Add PlaceOrder message PlaceOrder message 2. Handle PlaceOrder Add PayOrder message If an error occur only DeliverEbook is affected. The message will be put back on the queue and retried SendMail message 4. Handle SendMail DeliverEbook message 5. Handle DeliverEbook PayOrder message Queue Add SendMail & DeliverEbook message 3. Handle PayOrder
- 25. Code - The Messages
- 26. Code – Message Handlers #1
- 27. Code – Message Handlers #2
- 28. Yet another transactional issue MSMQ Processed ok Handle PayOrder message Charge credit card Request Handle PayOrder Request: Error PaymentService Payment OK Network error External Payment Gateway The message will be put back on the queue and processed again Consequence: The credit card will be charged more than once
- 29. Solution Operations should be idempotent Idempotent operations can be run many times without changing the result MSMQ Handle PayOrder message Charge credit card Request Handle PayOrder Payment OK PaymentService Has the order been paid? Use OrderId Database Payment OK External Payment Gateway Has the message already been processed? Use order ID or a GUID Database
- 30. Another Challenge – Eventual Consistency Users clicks on View Order History The PlaceOrder message is processed async by another process If it has not gotten processed yet then the new order will not be in the list Solution Some UI trick? Move the OrderRepository.Add to the controller Make sure the messages are handled fast enough • Define a SLA on the message to help monitoring
- 31. Publish/Subscribe Place order OrderController Browser 1. Add PlaceOrder message PlaceOrder command 2. Handle PlaceOrder Add PayOrder command With Publish/Subscribe the Subscribers are loosely coupled to the publisher Subscribe to OrderWasPaid 4. Handle SendMail Subscribes to OrderWasPaid Queue 3. Handle PayOrder Add OrderWasPaid event OrderWasPaid Subscribes to OrderWasPaid 4. Handle DeliverEbook PayOrder command OrderWasPaid NServiceBus will add 2 messages to the queue, one for each subscriber
- 32. Summary Request/Response Command Event X X Automatically retries X X Full transactional support X X Reliable Consistency X Can return ID generated by the database to the sender X Loosely coupled X Reads is synchronous and should be request/response Using a queue for synchronous operations gives little value, only overhead Writes can be asynchronous one-way messaging or events There are no silver bullet – We are always trading one set of problems for another set of problems
- 33. Scaling Scale up Easy to scale up if business processes are split into multiple messages Increase the number of threads in the config – NServiceBus will concurrently process multiple messages Scale out Use the Distributor or the Master (Both a distributor and a worker) Worker #1 Message MSMQ Distributor Worker #2 Worker #3 The distributor will forward the message to a worker that is ready to process the message
- 34. How the Distributor Works 1. Worker: Send I’m ready message I’m ready message Distributor: Control Queue I’m ready message Distributor Any messages in the input queue? PlaceOrder message 1. Website: Send PlaceOrder message Send PlaceOrder message from input queue PlaceOrder message Distributor Input Queue Yes No PlaceOrder message Distributor Find worker through message in Storage queue Store I’m ready message Storage Queue
- 35. Other Features Sagas Long running workflow like processes State is shared between multiple message handlers Scheduling Send a message every x minute. A handler will handle the message Unobtrusive mode Use your own ICommand, IEvent etc. to reduce coupling to NServiceBus Supports Various dependency injection containers Logging frameworks Etc. ++
- 36. NServiceBus License & Pricing Open source Used to be free, but not anymore Indefinitely license with 1 year maintenance updates • $500 USD per processing core • $250 USD per sending core Monthly subscription • $35 USD per processing core • $17 USD per sending core Development and testing environments + disaster recovery and passive backups are free Developers need a license (free) that must be renewed every 3rd month through particular.net
- 37. Demo
- 38. About Capgemini With more than 120,000 people in 40 countries, Capgemini is one of the world's foremost providers of consulting, technology and outsourcing services. The Group reported 2011 global revenues of EUR 9.7 billion. Together with its clients, Capgemini creates and delivers business and technology solutions that fit their needs and drive the results they want. A deeply multicultural organization, Capgemini has developed its own way of working, the Collaborative Business ExperienceTM, and draws on Rightshore ®, its worldwide delivery model. Rightshore® is a trademark belonging to Capgemini www.capgemini.com The information contained in this presentation is proprietary. © 2012 Capgemini. All rights reserved.