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Saga Persistence and Event-Driven Architectures

Monday, April 20th, 2009.

imageWhen working with clients, I run into more than a couple of people that have difficulty with event-driven architecture (EDA). Even more people have difficulty understanding what sagas really are, let alone why they need to use them. I’d go so far to say that many people don’t realize the importance of how sagas are persisted in making it all work (including the Workflow Foundation team).

The common e-commerce example

We accept orders, bill the customer, and then ship them the product.

Fairly straight-forward.

Since each part of that process can be quite complex, let’s have each step be handled by a service:

Sales, Billing, and Shipping. Each of these services will publish an event when it’s done its part. Sales will publish OrderAccepted containing all the order information – order Id, customer Id, products, quantities, etc. Billing will publish CustomerBilledForOrder containing the customer Id, order Id, etc. And Shipping will publish OrderShippedToCustomer with its data.

So far, so good. EDA and SOA seem to be providing us some value.

Where’s the saga?

Well, let’s consider the behavior of the Shipping service. It shouldn’t ship the order to the customer until it has received the CustomerBilledForOrder event as well as the OrderAccepted event. In other words, Shipping needs to hold on to the state that came in the first event until the second event comes in. And this is exactly what sagas are for.

Let’s take a look at the saga code that implements this. In order to simplify the sample a bit, I’ll be omitting the product quantities.

   1:      public class ShippingSaga : Saga<ShippingSagaData>,
   2:          ISagaStartedBy<OrderAccepted>,
   3:          ISagaStartedBy<CustomerBilledForOrder>
   4:      {
   5:          public void Handle(OrderAccepted message)
   6:          {
   7:              this.Data.ProductIdsInOrder = message.ProductIdsInOrder;
   8:          }
   9:   
  10:          public void Handle(CustomerBilledForOrder message)
  11:          {
  12:               this.Bus.Send<ShipOrderToCustomer>(
  13:                  (m =>
  14:                  {
  15:                      m.CustomerId = message.CustomerId;
  16:                      m.OrderId = message.OrderId;
  17:                      m.ProductIdsInOrder = this.Data.ProductIdsInOrder;
  18:                  }
  19:                  ));
  20:   
  21:              this.MarkAsComplete();
  22:          }
  23:   
  24:          public override void Timeout(object state)
  25:          {
  26:              
  27:          }
  28:      }

First of all, this looks fairly simple and straightforward, which is good.

It’s also wrong, which is not so good.

One problem we have here is that events may arrive out of order – first CustomerBilledForOrder, and only then OrderAccepted. What would happen in the above saga in that case? Well, we wouldn’t end up shipping the products to the customer, and customers tend not to like that (for some reason).

There’s also another problem here. See if you can spot it as I go through the explanation of ISagaStartedBy<T>.

Saga start up and correlation

The “ISagaStartedBy<T>” that is implemented for both messages indicates to the infrastructure (NServiceBus) that when a message of that type arrives, if an existing saga instance cannot be found, that a new instance should be started up. Makes sense, doesn’t it? For a given order, when the OrderAccepted event arrives first, Shipping doesn’t currently have any sagas handling it, so it starts up a new one. After that, when the CustomerBilledForOrder event arrives for that same order, the event should be handled by the saga instance that handled the first event – not by a new one.

I’ll repeat the important part: “the event should be handled by the saga instance that handled the first event”.

Since the only information we stored in the saga was the list of products, how would we be able to look up that saga instance when the next event came in containing an order Id, but no saga Id?

OK, so we need to store the order Id from the first event so that when the second event comes along we’ll be able to find the saga based on that order Id. Not too complicated, but something to keep in mind.

Let’s look at the updated code:

   1:      public class ShippingSaga : Saga<ShippingSagaData>,
   2:          ISagaStartedBy<OrderAccepted>,
   3:          ISagaStartedBy<CustomerBilledForOrder>
   4:      {
   5:          public void Handle(CustomerBilledForOrder message)
   6:          {
   7:              this.Data.CustomerHasBeenBilled = true;
   8:   
   9:              this.Data.CustomerId = message.CustomerId;
  10:              this.Data.OrderId = message.OrderId;
  11:   
  12:              this.CompleteIfPossible();
  13:          }
  14:   
  15:          public void Handle(OrderAccepted message)
  16:          {
  17:              this.Data.ProductIdsInOrder = message.ProductIdsInOrder;
  18:   
  19:              this.Data.CustomerId = message.CustomerId;
  20:              this.Data.OrderId = message.OrderId;
  21:   
  22:              this.CompleteIfPossible();
  23:          }
  24:   
  25:          private void CompleteIfPossible()
  26:          {
  27:              if (this.Data.ProductIdsInOrder != null && this.Data.CustomerHasBeenBilled)
  28:              {
  29:                  this.Bus.Send<ShipOrderToCustomer>(
  30:                     (m =>
  31:                     {
  32:                         m.CustomerId = this.Data.CustomerId;
  33:                         m.OrderId = this.Data.OrderId;
  34:                         m.ProductIdsInOrder = this.Data.ProductIdsInOrder;
  35:                     }
  36:                     ));
  37:                  this.MarkAsComplete();
  38:              }
  39:          }
  40:      }

And that brings us to…

Saga persistence

We already saw why Shipping needs to be able to look up its internal sagas using data from the events, but what that means is that simple blob-type persistence of those sagas is out. NServiceBus comes with an NHibernate-based saga persister for exactly this reason, though any persistence mechanism which allows you to query on something other than saga Id would work just as well.

Let’s take a quick look at the saga data that we’ll be storing and see how simple it is:

   1:      public class ShippingSagaData : ISagaEntity
   2:      {
   3:          public virtual Guid Id { get; set; }
   4:          public virtual string Originator { get; set; }
   5:          public virtual Guid OrderId { get; set; }
   6:          public virtual Guid CustomerId { get; set; }
   7:          public virtual List<Guid> ProductIdsInOrder { get; set; }
   8:          public virtual bool CustomerHasBeenBilled { get; set; }
   9:      }

You might have noticed the “Originator” property in there and wondered what it is for. First of all, the ISagaEntity interface requires the two properties Id and Originator. Originator is used to store the return address of the message that started the saga. Id is for what you think it’s for. In this saga, we don’t need to send any messages back to whoever started the saga, but in many others we do. In those cases, we’ll often be handling a message from some other endpoint when we want to possibly report some status back to the client that started the process. By storing that client’s address the first time, we can then “ReplyToOriginator” at any point in the process.

The manufacturing sample that comes with NServiceBus shows how this works.

Saga Lookup

Earlier, we saw the need to search for sagas based on order Id. The way to hook into the infrastructure and perform these lookups is by implementing “IFindSagas<T>.Using<M>” where T is the type of the saga data and M is the type of message. In our example, doing this using NHibernate would look like this:

   1:      public class ShippingSagaFinder : 
   2:          IFindSagas<ShippingSagaData>.Using<OrderAccepted>,
   3:          IFindSagas<ShippingSagaData>.Using<CustomerBilledForOrder>
   4:      {
   5:          public ShippingSagaData FindBy(CustomerBilledForOrder message)
   6:          {
   7:              return FindBy(message.OrderId)
   8:          }
   9:   
  10:          public ShippingSagaData FindBy(OrderAccepted message)
  11:          {
  12:              return FindBy(message.OrderId)
  13:          }
  14:   
  15:          private ShippingSagaData FindBy(Guid orderId)
  16:          {
  17:              return sessionFactory.GetCurrentSession().CreateCriteria(typeof(ShippingSagaData))
  18:                  .Add(Expression.Eq("OrderId", orderId))
  19:                  .UniqueResult<ShippingSagaData>();
  20:          }
  21:   
  22:          private ISessionFactory sessionFactory;
  23:   
  24:          public virtual ISessionFactory SessionFactory
  25:          {
  26:              get { return sessionFactory; }
  27:              set { sessionFactory = value; }
  28:          }
  29:      }

For a performance boost, we’d probably index our saga data by order Id.

On concurrency

Another important note is that for this saga, if both messages were handled in parallel on different machines, the saga could get stuck. The persistence mechanism here needs to prevent this. When using NHibernate over a database with the appropriate isolation level (Repeatable Read – the default in NServiceBus), this “just works”. If/When implementing your own saga persistence mechanism, it is important to understand the kind of concurrency your business logic can live with.

Take a look at Ayende’s example for mobile phone billing to get a feeling for what that’s like.

Summary

In almost any event-driven architecture, you’ll have services correlating multiple events in order to make decisions. The saga pattern is a great fit there, and not at all difficult to implement. You do need to take into account that events may arrive out of order and implement the saga logic accordingly, but it’s really not that big a deal. Do take the time to think through what data will need to be stored in order for the saga to be fault-tolerant, as well as a persistence mechanism that will allow you to look up that data based on event data.

If you feel like giving this approach a try, but don’t have an environment handy for this, download NServiceBus and take a look at the samples. It’s really quick and easy to get set up.

  
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13 Comments

  1. BjartN Says:

    Well written and interesting. Thx.


  2. DotNetShoutout Says:

    Saga Persistence and Event-Driven Architectures…

    Thank you for submitting this cool story – Trackback from DotNetShoutout…


  3. Habib Says:

    The clearest explanation of “Saga” i have ever seen.
    Thank you very much.


  4. Martin Says:

    Thanks for this great post explaining Sagas!


  5. Raghunath Says:

    Udi you post have been great. Recently I have been refreshing thro an earlier read of NService Bus and Saga.
    As I have understood long running process flow(consiciously avoiding the word workflow), effectively implemented in state machine scheme of things constitute a Saga. Saga represented a blueprint of process flow and on how it is supposed to behave in deterministic manner. We could have numerous instance of this blueprint aided by the messaging infrastructure, SagaMessage and handler.
    When we go ahead try to implement various such process flow in the form of numerous blueprints of saga library I think the code base could gets voluminous, lot of code and time consuming. Thinking in this direction would it be helpful to make some portion of saga declarative. By declarative part I do not mean config associated but could we think of declarative saga using XAML which self describe the saga in XAML if possible associating declarative saga with programmatic saga.
    If that would be task, I would like to understand, from you, thoughts on what it would take to do such a thing.


  6. udidahan Says:

    Raghunath,

    There is some interesting some going on in declarative saga declaration in the Mass Transit project – that is under consideration for inclusion in nServiceBus.

    “I think the code base could gets voluminous, lot of code and time consuming”

    What I’ve found is that these sagas don’t just pile up but belong in the context of higher-level business services and bounded contexts. This partitioning prevents things from getting out of hand.

    About the “time consuming” parts, well, it’s really not. The fact that the code is isolated from most everything else, as well as being highly unit-testable (with the NServiceBus.Testing library), removes most obstacles from getting it functional and stable – performance and scalability are handled “by default” by the messaging aspects around it.

    Does that answer your questions?


  7. Raghunath Says:

    Udi,
    I see the direction saga would move towards.
    I may be vague in what I say, I was just thinking if some thing like what is said this post http://blog.pixelingene.com/?p=32
    is possible with saga using concepts such as
    – Saga
    – Initial Trigger
    – Event Messages
    – Transitions
    – Handler
    and linking programmatic saga thro XAML Extensions which save you lot of hand-written code improve readility.
    I also see WF4 also being more declarative.
    Thanks,
    Raghunath


  8. udidahan Says:

    Raghunath,

    Not all sagas have very much state-management to them.
    Many are just simple integration pieces.

    I have no direct problem with things being declarative, it’s just that that’s not an end unto itself, but a means that does not make sense in all cases.

    Hope that makes sense.


  9. Andy Z Says:

    After sending my last post I realized that the object which is receiving the event notification will likely not have access to any context other than the domain object itself. So, if the domain object (in this case the Customer) does not have an associated list of Email domain objects there may not be a way to add an Email domain object into the context of the Customer or to hold on to an email request and have it sent as part of (or directly after) the context in which the Customer object exists is serialized.

    I hope that this apparent problem is just my lack of understanding rearing its head and not something which is difficult to overcome.

    Thanks again,
    Andy


  10. Living in the Tech Avalanche Generation » A Linq To SQL Saga Persister for NServiceBus Says:

    [...] features in the technology of our choice, and the subject of this post regards extending the Saga Persister. However at this point let me just say that it may well be worth your while to read Udi’s post on [...]


  11. Testing Message Handlers with NServiceBus » Kang The Koder Says:

    [...] is a dll called “NServiceBus.Testing” which provides us with the ability to easily test your sagas. This enables us to move forward with greater confidence in knowing that the code that we have [...]


  12. Getting Started With NServiceBus: Part 3 Sagas « Plan B Software Says:

    [...] gives a much better explanation of them here and there is some more information about them on the NServiceBus Wiki [...]


  13. Living in the Tech Avalanche Generation » NServiceBus - Linq To SQL Saga Persister Part 2.0 Says:

    [...] how to use the extensibility points provided by NServiceBus to create a persistence mechanism for Sagas. Notice I said two persisters, one for LINQ To SQL and another for the Entity Framework, this post [...]


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On the personal level - Udi is a great communicator and can persuade even the most difficult audiences (I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”

Adam Dymitruk Adam Dymitruk, Director of IT at Apara Systems
“I met Udi for the first time at DevTeach in Montreal back in early 2007. While Udi is usually involved in SOA subjects, his knowledge spans all of a software development company's concerns. I would not hesitate to recommend Udi for any company that needs excellent leadership, mentoring, problem solving, application of patterns, implementation of methodologies and straight out solution development.
There are very few people in the world that are as dedicated to their craft as Udi is to his. At ALT.NET Seattle, Udi explained many core ideas about SOA. The team that I brought with me found his workshop and other talks the highlight of the event and provided the most value to us and our organization. I am thrilled to have the opportunity to recommend him.”

Eytan Michaeli Eytan Michaeli, CTO Korentec
“Udi was responsible for a major project in the company, and as a chief architect designed a complex multi server C4I system with many innovations and excellent performance.”


Carl Kenne Carl Kenne, .Net Consultant at Dotway AB
“Udi's session "DDD in Enterprise apps" was truly an eye opener. Udi has a great ability to explain complex enterprise designs in a very comprehensive and inspiring way. I've seen several sessions on both DDD and SOA in the past, but Udi puts it in a completly new perspective and makes us understand what it's all really about. If you ever have a chance to see any of Udi's sessions in the future, take it!”

Avi Nehama, R&D Project Manager at Retalix
“Not only that Udi is a briliant software architecture consultant, he also has remarkable abilities to present complex ideas in a simple and concise manner, and...
always with a smile. Udi is indeed a top-league professional!”

Ben Scheirman Ben Scheirman, Lead Developer at CenterPoint Energy
“Udi is one of those rare people who not only deeply understands SOA and domain driven design, but also eloquently conveys that in an easy to grasp way. He is patient, polite, and easy to talk to. I'm extremely glad I came to his workshop on SOA.”

Scott C. Reynolds Scott C. Reynolds, Director of Software Engineering at CBLPath
“Udi is consistently advancing the state of thought in software architecture, service orientation, and domain modeling.
His mastery of the technologies and techniques is second to none, but he pairs that with a singular ability to listen and communicate effectively with all parties, technical and non, to help people arrive at context-appropriate solutions. Every time I have worked with Udi, or attended a talk of his, or just had a conversation with him I have come away from it enriched with new understanding about the ideas discussed.”

Evgeny-Hen Osipow, Head of R&D at PCLine
“Udi has helped PCLine on projects by implementing architectural blueprints demonstrating the value of simple design and code.”

Rhys Campbell Rhys Campbell, Owner at Artemis West
“For many years I have been following the works of Udi. His explanation of often complex design and architectural concepts are so cleanly broken down that even the most junior of architects can begin to understand these concepts. These concepts however tend to typify the "real world" problems we face daily so even the most experienced software expert will find himself in an "Aha!" moment when following Udi teachings.
It was a pleasure to finally meet Udi in Seattle Alt.Net OpenSpaces 2008, where I was pleasantly surprised at how down-to-earth and approachable he was. His depth and breadth of software knowledge also became apparent when discussion with his peers quickly dove deep in to the problems we current face. If given the opportunity to work with or recommend Udi I would quickly take that chance. When I think .Net Architecture, I think Udi.”

Sverre Hundeide Sverre Hundeide, Senior Consultant at Objectware
“Udi had been hired to present the third LEAP master class in Oslo. He is an well known international expert on enterprise software architecture and design, and is the author of the open source messaging framework nServiceBus. The entire class was based on discussion and interaction with the audience, and the only Power Point slide used was the one showing the agenda.
He started out with sketching a naive traditional n-tier application (big ball of mud), and based on suggestions from the audience we explored different solutions which might improve the solution. Whatever suggestions we threw at him, he always had a thoroughly considered answer describing pros and cons with the suggested solution. He obviously has a lot of experience with real world enterprise SOA applications.”

Raphaël Wouters Raphaël Wouters, Owner/Managing Partner at Medinternals
“I attended Udi's excellent course 'Advanced Distributed System Design with SOA and DDD' at Skillsmatter. Few people can truly claim such a high skill and expertise level, present it using a pragmatic, concrete no-nonsense approach and still stay reachable.”

Nimrod Peleg Nimrod Peleg, Lab Engineer at Technion IIT
“One of the best programmers and software engineer I've ever met, creative, knows how to design and implemet, very collaborative and finally - the applications he designed implemeted work for many years without any problems!

Jose Manuel Beas
“When I attended Udi's SOA Workshop, then it suddenly changed my view of what Service Oriented Architectures were all about. Udi explained complex concepts very clearly and created a very productive discussion environment where all the attendees could learn a lot. I strongly recommend hiring Udi.”

Daniel Jin Daniel Jin, Senior Lead Developer at PJM Interconnection
“Udi is one of the top SOA guru in the .NET space. He is always eager to help others by sharing his knowledge and experiences. His blog articles often offer deep insights and is a invaluable resource. I highly recommend him.”

Pasi Taive Pasi Taive, Chief Architect at Tieto
“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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