Message Oriented Architectureusing NServiceBus

Lars-Erik KindbladSenior Consultant

Twitter: @kindbladE-mail: le@kindblad.com

What is NServiceBus?

Lightweight messaging framework for designing distributed systems in .NET

An Enterprise Service BusHelps 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 SoftwareOpen Source but not free

Source code available at https://github.com/Particular/NServiceBus

Binaries available as NuGet packages

Remote Procedure Call (Request/Response)

Order ServicePlaceOrder(order)

Request: Order

Website Response: Order ID

Synchronous – The website is blocked while calling the Order Service

Message: PlaceOrderWebsite

Asynchronous – The website is NOT blocked while the place order is processed

Message Queue

Windows ServiceProcess the PlaceOrder message

ConnectReceivePlaceOrder message

Messaged Oriented Architecture (One-way messaging)

NServiceBus Terminology & Components

Message: PlaceOrderWebsite Message Queue

Windows ServiceProcess the PlaceOrder message

ConnectReceivePlaceOrder message

Sender/Send-Only Endpoint

Message,Command or Event

Worker Endpoint,Host or

Receiver

Message Handler

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

Command

Event (Publish/Subscribe)

Command: PlaceOrderWebsite Message Queue Endpoint

Command: PlaceOrder

Event: OrderWasPlacedWorker Message Queue Subscribing

Endpoint

Event: OrderW

asPlaced

Subscribing Endpoint

Subscribing Endpoint

Event: OrderWasPlaced

Event: OrderWasPlaced

Loosely coupled to the receiving

endpoints

Strongly coupled to the receiving

endpoint

NServiceBus will add 3 messages to the queue, one for each subscriber

Queues

First in-first out datastructure for storing and retrieving messages

Queue

PlaceOrder message 4

PlaceOrder message 3

PlaceOrder message 2

PlaceOrder message 1

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

First-In

First-Out

How to Send & Handle Messages

Command: PlaceOrderWebsite Message Queue Endpoint

Command: PlaceOrder

Send Handle (Command or Event)

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

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

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

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

The Usual Way - Request/Response

OrderController

Place order

Browser Database

PaymentService

DeviceService

Confirmation pagewith order ID

1. Add order

2. Charge credit card

4. Deliver ebook

SMTP-Server

3. Send mail

Problem #1 – The order is lost during an error

Conseqence The database is rolled back User receives an error Order must be sent again

OrderController

Place order

Browser Database

PaymentService

DeviceService

Error page

1. Add order

2. Charge credit card

4. Deliver ebook

SMTP-Server

3. Send mail

ErrorException

Problem #2 – Poor Transaction Management

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

OrderController

Place order

Browser Database

PaymentService

DeviceService

Error page

1. Add order

2. Charge credit card

4. Deliver ebook

SMTP-Server

3. Send mail

ErrorException

New

Let’s Fix This With NServiceBus

Change to a message oriented architecture

OrderController

Place order

Browser

Confirmation page

Add PlaceOrdermessage

Queue

Old

The Message Must Also Be Processed

MSMQ

Worker(Windows Service)

1. Connect 2. Receive message PlaceOrder

3. PlaceOrderMessage Handler

PlaceOrder

Database

PaymentService

DeviceService

4. Add order

5. Charge credit card

7. Deliver ebook

SMTP-Server

6. Send mail

When an Error Occur

MSMQ

Worker(Windows Service)

1. Connect 2. Receive messagePlaceOrder

3. PlaceOrderMessage Handler

PlaceOrderError

4. Rollback. Put the message back on the queueand retry

Server A

More about MSMQ

Max 4MB message sizePersists messages to files. Default location C:\WINDOWS\System32\msmq\storage

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

MSMQOutgoing

queueSender

Server B

WorkerMSMQ

Input queue

Message

How NServiceBus Handles Failure

Transaction management and rollback are automatically handled by NServiceBus in message handlers

When an error occur1. 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.

A challenge – The Order ID

The order ID has not been generated yet – it’s generated in the message handler at the worker endpoint

NewOld

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

We Still Have Poor Transaction Management

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

Handle PlaceOrder Database

PaymentService

DeviceService

1. Add order

2. Charge credit card

4. Deliver ebook

SMTP-Server

3. Send mail

ErrorException

Solution

Split into man small messages - One message per transactional boundary

OrderController

Place order

Browser

1. Add PlaceOrder message

Queue

2. Handle PlaceOrder

4. Handle SendMail

5. Handle DeliverEbook

PlaceOrder message

Add PayOrder message

PayOrder message

Add SendMail & DeliverEbook message

SendMail message

DeliverEbook message

3. Handle PayOrder

If an error occur only DeliverEbook is affected. The

message will be put back on the queue and retried

Code - The Messages

Code – Message Handlers #1

Code – Message Handlers #2

Yet another transactional issue

The message will be put back on the queue and processed againConsequence: The credit card will be charged more than once

Handle PayOrder

Request

MSMQ

Handle PayOrder message

PaymentServiceRequest: Error

Charge credit cardExternal PaymentGateway

Network error

Processed ok

Payment OK

Solution

Operations should be idempotent Idempotent operations can be run many times without changing the result

Database

Has the order been paid?Use OrderId

Handle PayOrder

Request

MSMQ

Handle PayOrder message

PaymentService

Charge credit cardExternal PaymentGateway

Payment OK

Database

Has the message already been processed?Use order ID or a GUID

Payment OK

Another Challenge – Eventual Consistency

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 listSolution

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

Users clicks on View Order History

Publish/Subscribe

OrderController

Place order

Browser

1. Add PlaceOrder message

Queue

2. Handle PlaceOrder

4. Handle SendMailSubscribes to OrderWasPaid

4. Handle DeliverEbookSubscribes to OrderWasPaid

PlaceOrder command

Add PayOrder command

PayOrder command

Add OrderWasPaid event

OrderWasPaid

OrderWasPaid

3. Handle PayOrderSubscribe to OrderWasPaid

With Publish/Subscribe the Subscribers are loosely coupled

to the publisher

NServiceBus will add 2 messages to the queue, one for each subscriber

Summary

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 eventsThere are no silver bullet – We are always trading one set of problems for

another set of problems

Request/Response Command Event

Reliable X X

Consistency X

Can return ID generated by the database to the sender

X

Automatically retries X X

Full transactional support

X X

Loosely coupled X

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)

The distributor will forward the message to a worker that is ready to process the message

DistributorMSMQ

Worker #1

Worker #2

Worker #3

Message

Message

How the Distributor Works

Distributor:Control Queue

1. Worker:Send I’m ready

message

I’m ready messageDistributor

Store I’m ready message

Storage Queue

I’m ready message

PlaceOrder message Send PlaceOrder message from input

queue No

Any messages in the input queue?

Yes

DistributorInput Queue

1. Website:Send

PlaceOrder message

PlaceOrder message Distributor

PlaceOrder message

Find worker through

message in Storage queue

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.

++

NServiceBus License & Pricing

Open sourceUsed 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

Demo

The information contained in this presentation is proprietary.© 2012 Capgemini. All rights reserved.

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