Asynchronous Messaging

Sergey Shishkin

http://shishkin.org

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AgendaAgenda

Why Messaging?– And what’s wrong with good old RPC?

Sample scenario– News module of the Prism’s StockTrader RI

Under the hood of Message Bus– Building a Message Bus in 37 lines of code– And what those 37 lines did not cover?

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Communication Styles

Remote Procedure Calls–Synchronous–High coupling

One-Way Messages–Asynchronous–Loose coupling

Fallacies of Distributed Computing*Fallacies of Distributed Computing*

* * http://en.wikipedia.org/wiki/Fallacies_of_Distributed_Computing

1.The network is reliable2.Latency is zero3.Bandwidth is infinite4.The network is secure5.Topology doesn't change6.There is one administrator7.Transport cost is zero8.The network is homogeneous

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Composite ApplicationsComposite Applications

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Modularized applications are similar to distributed ones

GUI composition is solved by:•Common region infrastructure•Event-based GUI synchronization

Same pattern in broader context•Common Message Bus infrastructure•Message-based components integration

Tenets of Service-Orientation*

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Boundaries are explicit

Services are autonomous

Services share schema and contract, not class

Service compatibility is based upon policy

* http://msdn.microsoft.com/en-us/magazine/cc164026.aspx

Messaging Patterns

* http://www.enterpriseintegrationpatterns.com/toc.html

Message Bus is a central place for implementing integration patterns*

Advantages of Messaging (I)

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Loose coupling

Separation of concerns

Orthogonal architecture

Advantages of Messaging (II)

Testing

Scalability

Disadvantages of MessagingDependency tracing•Who is publishing that message?•Is publishing of that message required?

Debugging

•Same as debugging of multithreaded apps

Learning curve•It’s not a new technology, it’s a paradigm shift•Requires different mindset and discipline

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Guidelines

Consider messaging to loose coupling

Avoid RPC across modules/business services/bounded contexts*

* In terms of DDD

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Look aheadLook ahead

Messaging is the default communication style for cloud-based services

Silverlight does not support synchronous communication out of the box

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DEMO

References• Composite WPF guidance aka “Prism”, StockTrader RI

– http://compositewpf.codeplex.com/• StockTrader with messages (revision 12, as shown in demo)

– http://code.google.com/p/messagingwpf/source/browse/trunk/src/StockTraderRI/• Some OSS messaging frameworks

– http://code.google.com/p/masstransit/– http://www.nservicebus.com– https://rhino-tools.svn.sourceforge.net/svnroot/rhino-tools/trunk/esb/

• Fallacies of distributed computing– http://en.wikipedia.org/wiki/Fallacies_of_Distributed_Computing

• Four tenets of service orientation– http://msdn.microsoft.com/en-us/magazine/cc164026.aspx

• Enterprise integration patterns– http://www.enterpriseintegrationpatterns.com/toc.html