d60 developing smart software solutions
Ride the bus! April 2012
Agenda
• What is messaging • Doing it with NServiceBus • Enter the saga
What is messaging?
void CreatePresentation(string name); -‐-‐ someObject.CreatePresentation(“Ride The Bus!”);
What is messaging?
• Networks are unreliable • Inter-‐process communicaKon is dead slow • The other process might not be there at the moment
• etc • etc
What is messaging?
void CreatePresentation(CreatePresentationArgs args); -‐-‐ var args = new CreatePresentationArgs {
Title = “Ride The Bus!” }; someObject.CreatePresentation(args);
What is messaging?
void Process(Args args); -‐-‐ someObject.Process(new CreatePresentation {
Title = “Ride The Bus!” }); someObject.Process(new Something()); someObject.Process(new SomethingElse());
What is messaging?
void
InteresKng properKes
• Can be persisted • Can be forwarded • etc
Persisted?
MSMQ can do that!
• Ships with all modern versions of Windows • Your MSMQ endpoint is always local – remoKng is taken care of via outgoing queues
• .NET client in System.Messaging (GAC).
Bus?
• Each service has its own input queue.
• When a service needs to send a message, the bus looks up the desKnaKon somehow.
• How desKnaKons are looked up depends on whether the service is sending or publishing.
Messaging Pa8erns
“Request/reply”
+ “Return address” + maybe “CorrelaKon idenKfier”
“Publish/subscribe”
“Publish/subscribe”
Request/reply vs. Publish/subscribe
“Process manager”
+ “CorrelaKon idenKfier”
How to do these things IRL
• Commercial – NServiceBus
• Free alternaKves – MassTransit – RhinoESB – Rebus(*)
NServiceBus
• Currently in version 3.0
• Licensing – <= 2.0 are free – >= 2.5 cost money
Show me the code
• Now let’s see how “Request/reply” looks with NServiceBus – We have a pre8y unreliable web service, umm “Whatchamacallit”, that we want to call.
– We build an integraKon service with NServiceBus that works as a messaging façade towards the web service.
– Our system requests stuff from the façade, the façade replies.
Show me the code
• Now, let’s see an example with some Pub/sub acKon – We have a service that processes something, “Somekindofprocessor”, when it receives a ProcessSomething command.
– In order to process stuff, it needs to get the important string from our unreliable web service from before.
– When processing is done, it should publish the result, allowing any other interested services to react to this.
What just happened?
What just happened?
The real world is a place...
• ...where we can’t always process messages in a stateless manner like this...
• ...where mulKple things need coordinaKon and will not always succeed/fail as a whole...
Enter the saga!
• A model for long-‐lived transacKons. • “A long-‐lived transacKon is a saga if it can be wri8en as a sequence of transacKons that can be interleaved with other transacKons.”
• For some reason, Mike Amundsen keeps a copy of the original whitepaper PDF here: h8p://www.amundsen.com/downloads/sagas.pdf
Sagas
• Implemented with an implementaKon of ISagaEntity, TEntity and deriving a message handler from Saga<TEntity>.
• All handled messages should somehow be correlated with the sata data.
Show me the code
• Just do it
Sagas
• Use them to – implement short conversaKons – implement long conversaKons – handle Kmeouts – implement idempotency
Image credits “It’s the same mouth, just flipped”: h8p://humor-‐image.com/its-‐the-‐same-‐mouth-‐just-‐flipped/ Thanks for lepng me borrow your awesome images – if you ever meet me, I’ll buy you guys and girls a beer. Seriously, I will.
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