This innocuous question comes up a lot. Usually I get this question after a short problem domain description. One of these came up on the nServiceBus discussion groups. Ayende took it and ran with it turning it into a nice blog post, An exercise in designing SOA systems. I’ve been meaning to write something myself. Bill put up a response already in his Service Granularity Example. So, I’m late to the party, again, but here we go.
It’s almost impossible to know, right away, which services are appropriate.
So, I’m going to focus more on the process of getting there, rather than describing the solution itself.
The domain deals with a placement agency placing physicians in positions at hospitals.
1. So, what does it actually do?
In Ayende’s post, he describes several services, but I’d rather look at them as use cases: registering an open position, registering a candidate, verifying their credentials, etc. It’s worth going through this requirements process. It doesn’t necessarily translate immediately to services, but there’s value in it.
2. What does it do it to?
We should also be looking at the data model, an entity relationship diagram (ERD) , where we see that we may have placed a certain physician at a number of positions. It’s also important for us to know about under which circumstances a physician finished their employment at a previous position before, say, trying to place them at a position in the same hospital or chain of hospitals. Don’t go thinking that this what the database schema will look like, it’s all about understanding connections between various bits of data.
3. When does that happen?
The next step is to map the uses cases above to the entities in the ERD, which entity is used in which use case. It’s also important to differentiate between entities (or even more importantly, specific fields of entities) that are used in a read-only fashion within a given use case. For instance, when registering a new position, we’ll want to check that against other open positions in the same hospital so we don’t end up registering the same position twice. Also, we might want to suggest verified physicians whose credentials match the position’s requirements. Data we wouldn’t be interested in might be which other physicians we placed at that hospital.
4. What just happened?
Another valuable perspective on the problem domain is the business process view – what are the interesting business events in the system and how they unfold over time. For instance, physician registered, position opened, physician’s credentials verified, and physician placed in position (or position filled by physician) are events that describe a different business perspective than use cases.
5. How do I decide?
Once we know what events there are, we can start looking at what kind of decisions we might want to make when those events occur and what data we’d need to make those decisions. These decisions may be as simple as updating a database or sending an email to a user. They also may include more advanced logic like when the profitability of an agreement with a specific hospital chain changes, prefer placing physicians in positions in that chain over others.
6. How do I deal with all this information?
After we have all of this information, we can start looking for cohesive bunching across all of these axes using these rules:
Data that is modified by a use case gets published as an event.
Data that is required by a use case for read-only purposes, arrives as the result of subscribing to some event.
Look for rules that differentiate behaviour based on the properties of data. Look for a correlation to some business concept. For instance, physicians probably won’t be changing their specialization, and open positions often deal with a certain specialization. Therefore, specific data instances tied to two different specializations can be said to be loosely coupled.
7. Which property slices across the domain?
Even though the ERD may not have made it clear, and the use cases didn’t show any particular break-down, nor did the events call out this point, the key to finding the way a business domain decomposes into services lies in decoupling specific data instances.
Actually, at this point we can clump autonomous components (mere technical bits) that handle a single message, into more granular business components.
If you think about it, it makes a lot of sense. The kind of credential checking you’d do for physicians specializing in brain surgery would likely be different than for general practitioners. The kind of information you’d store would, therefore, also be different.
But, which services do I need?
Quite frankly, I don’t have enough information to know.
But if we had continued this conversation, going through issues like transactional consistency, availability requirements, and other non-functional issues we could have gotten there.
If there’s one thing that I hope you got out of this, it’s that the questions are what’s important. The iterative process of looking at the problem domain from various perspectives, incorporating the new-found knowledge, and asking more questions is what leads us to a solution. But we don’t stop there. We keep looking for characteristics which split services apart into business components, and for consistency requirements that brings autonomous components together into services.
It’s not easy, but by focusing on these simple questions, you can get to a coherent service oriented architecture.
In this video, Greg Young, Martin Fowler, Evan Hoff, Dru Sellers, myself and some others discussed various aspects of event-based systems, how Domain-Driven Design works with them, what role messaging has, and how all these connect to architectural properties like scalability and fault tolerance.
One of the questions that Martin started answering was how teams can start getting into the messaging state-of-mind. Unfortunately, the conversation veered off into what kind of messaging interactions are appropriate leaving the original question unanswered.
I’m hoping to address this topic with some of the information I’m putting up on the nServiceBus site. There’s always Gregor and Bobby’s excellent EIP book that I think is a must for anybody writing distributed systems.
There’s a fairly intense discussion going on these days amongst the SOA illuminati. In the hopes that people will see me standing beside them and conclude that I too know something, I’ve decided to chip in.
Jim brought the concept of cohesion to the regular SOA discussions around loose coupling in his post Anemic Service Model, which I think, all in all, is a very good idea.
Naïve Service Composition
Jim first calls out a common anti-pattern that seems to have become quite rampant – I’d call it naïve service composition if only the things being composed could even be called services. And I think the tone being set is correct – a service needs to meet a stronger set of criteria than just being able to be composed. Multiple services sharing the same logical data store (in that the same actual rows/data elements are managed by multiple services) probably means there’s an encapsulation problem here. I agree with Jim sentiment here:
“On the one hand we’re inclined, and indeed encouraged by the SOA brigade, to think of this architecture as a good fit for purpose because it is very loosely coupled. Since every component or service is decoupled from every other component or service it should be possible to arrange and re-arrange them in a Lego-style in a myriad of useful ways. Building out “business services” from some more fundamental set of services is how the books tell us to do it. In fact we could even do that quite easily with point-and-[click] BPM tools, ruling out such overheads as developers and change management along the way. Right?”
MVC? There are, like, 6 of them!
However, I disagree with some of the conclusions that Jim draws from that point. Jim states “build your services to implement business processes”, and that services are “just an instance of MVC”. I’m going to leave alone the MVC statement since there are like 6 documented kinds of MVC not including the Front Controller stuff that the web guys are now calling MVC. I’m going to focus on the business process advice. JJ also doesn’t seem to agree with this advice. As Savas has already taken issue with the tone of JJ’s response, I’ll keep my focus on the content.
Visual Cobol
First of all, in my previous conversations with Jim he had already denounced the procedural nature of composing higher-level business processes out of smaller services which implement small bits of common activities. Visual Cobol was how he described it. In JJ’s follow-up post, he called out the necessary aspect of autonomy that jives with Jim’s cohesion principle.
I’m a bit concerned about the way JJ tends to version what SOA means over time. It might make it impossible to have intelligent design discussions without tagging each sentence with “as SOA meant in 2006”. I acknowledge that the accepted meaning of SOA by various vendors has changed over the years. However, I’ve found that meanings rooted in decades of computer science tend to last and provide value that outlasts much of the industry-buzzword-bingo (SOA 2.0 anyone?).
Cohesion, Business Domains, and Business Processes
My view of the original cohesion principles Steve discusses in his 2005 article Old Measures for New Services takes a business spin to Functional Cohesion:
A service should be responsible for one business domain.
If we jump off from this point, we’ll see that certain business processes which occur entirely in one business domain are fully encapsulated, whereas those macro-processes which cross many domains (like Order to Cash) cross multiple services – they do not become a service since that would break the “one business domain” rule. Given that services are loosely coupled, avoiding temporal coupling leads to services raising events. Thus, macro-processes are really just a series of events of various services where each service does its own internal business processes.
Enterprise Processes >> Business Processes
I think that maybe some of the difficulty in discussing concrete SOA guidance has to do with granularity. I’ve started calling those macro-processes something different from business processes, and that may just bring me full circle to Jim’s guidance.
An Enterprise Process is any process which involves multiple business domains.
Under that definition, a service may be responsible for multiple business processes in the same business domain. But still, one business process is usually not a service by itself.
Business Components & Autonomous Components to the Rescue
Finally, by introducing the additional levels of decomposition of business components and autonomous components I’ve found that we can focus the discourse on one concern at a time. My presentation on the topic can be found here. The 30 second pitch is this:
Business domains are inherently partitionable – data and rules. A business component represents one partition. An example of this is the domain of Sales being partitioned by strategic and non-strategic customers. Although the data structure might be similar or the same, the actual rows/data elements are not shared. Rules around discounts are different.
Within a business component, different activities should not interfere with each other. An autonomous component represents one activity. In our example, reporting on orders from strategic customers should not interfere with accepting their orders. As such, those activities should have different messages coming in on different endpoints. Each endpoint could have different characteristics, like durability. Losing a request for a report when a server restarts isn’t a big deal, however not a good idea for orders.
For more information you could check out these episodes from my podcast:
I often run into companies working on an SOA initiative where certain information aspects are given more importance than is warranted and, as a result, the overall service coupling is increased. Sometimes this takes the form of a Canonical Data Model or as a Master Data Management service. In both cases, information is divorced from its business context. Steve Jones (one of the SOA guiding lights out there) states “data only counts where it works” and I strongly agree. Due to the somewhat “wishy-washy” definition of services, I’ve found that the term Business Component captures that essence – data encapsulated in a business context.
In his post, Steve provides concrete guidance on how to look at data:
“The point about these bits of data is that they are about recording what has happened. Where this approach falls down is when you try and apply that approach to what is happening.”
One of the core ways that I suggest you avoid falling down the Data Services rabbit hole is to keep the context of time in mind as you analyse the data your services use and are responsible for. Ask yourself questions like:
Who creates this specific data element – not just this kind of data?
Can we partition these creators based on some property of the data?
When do they create it?
At what point can others access this data?
Do others need to use this data in their own business processes?
If so, how up-to-date does the data need to be? Up to the minute? Up to the millisecond?
Can we avoid transactions between those who create data and those who use it while maintaining business correctness?
In a follow up post, I’ll be analysing how we can identify services and business components in a domain by using these questions. More importantly, we’ll see how the message contracts of our services can be driven out by answering them.
There’s a new article up on MSDN describing how to write Smart Clients using WCF. The author is none other than WCF-Master Lowy and he goes over the multitude of ways you can deadlock yourself.
Here’s a taste:
UI Thread and Concurrency Management
Whenever you use hosting on the UI thread, deadlocks are possible. For example, the following setup is guaranteed to result with a deadlock: A Windows Forms application is hosting a service with UseSynchronizationContext set to true, and UI thread affinity is established. The Windows Forms application then calls the service over one of its endpoints. The call to the service blocks the UI thread, while WCF posts a message to the UI thread to invoke the service. That message is never processed, because of the blocking UI thread—hence, the deadlock.
Another possible case for a deadlock occurs when a Windows Forms application is hosting a service with UseSynchronizationContext set to true and UI thread affinity is established. The service receives a call from a remote client. That call is marshaled to the UI thread and is eventually executed on that thread. If the service is allowed to call out to another service, that can result in a deadlock if the callout causality tries somehow to update the UI or call back to the service’s endpoint, because all of the service instances that are associated with any endpoint (regardless of the service-instancing mode) share the same UI thread.
Similarly, you risk a deadlock if the service is configured for reentrancy and it calls back to its client. You risk a deadlock if the callback causality tries to update the UI or enter the service, because that reentrance must be marshaled to the blocked UI thread.
Actually, I have difficulty believing that Juval would go so far as to suggest that even the forms should be services, but he does:
Form as a Service
The main motivation for hosting a WCF service on the UI thread is if the service must update the UI or the form. The problem is always: How does the service reach out and obtain a reference to the form? While the techniques and ideas that appear thus far in the listings certainly work, it would be simpler yet if the form were the service and hosted itself. For this to work, the form (or any window) must be a singleton service. The reason is that singleton is the only instancing mode that enables you to provide WCF with a live instance to host. In addition, you would not want a per-call form that exists only during a client call (which is usually very brief), nor would you want a per-session form that only a single client can establish a session with and update.
When a form is also a service, having that form as a singleton service is the best instancing mode all around.
I think that this article serves as a great treatise leading to only one conclusion – you’d have to be crazy to try to do this without some higher level framework, preferably with a different low-level framework too 🙂 . Sucks Microsoft didn’t put one out – nor is there a pending beta, CTP, or even word about some project with a codename handling this. From what I know about Prism, it doesn’t intend to handle this issue either.
One thing that isn’t covered in the article is that if you do choose not to tie the client-side service to the UI thread, you open yourself up to race conditions. Reasons you’d want to handle messages on a different thread center around UI responsiveness. I’ve written about these things before:
The more I read things like this, the more I feel that I have to get going with my nServiceBus based solution. I’m fairly swamped as it is, so if anyone is interested in helping get this project off the ground, I’d be most grateful – as I think anyone else that had to build a smart client would.
In this article, I attempt to debunk some of the myths around stateless-ness as the key to scalability.
Here’s how it starts:
It was a sunny day in June 2005 and our spirits were high as we watched the new ordering system we’d worked on for the past 2 years go live in our production environment. Our partners began sending us orders and our monitoring system showed us that everything looked good. After an hour or so, our COO sent out an email to our strategic partners letting them know that they should send their orders to the new system. 5 minutes later, one server went down. A minute after that, 2 more went down. Partners started calling in. We knew that we wouldn’t be seeing any of that sun for a while.
The system that was supposed to increase the profitability of orders from strategic partners crumbled. The then seething COO emailed the strategic partners again, this time to ask them to return to the old system. The weird thing was that although we had servers to spare, just a few orders from a strategic customer could bring a server to its knees. The system could scale to large numbers of regular partners, but couldn’t handle even a few strategic partners.
This is the story of what we did wrong, what we did to fix it, and how it all worked out.
Ayende’s been going over nServiceBus, seeing how it’s built, and raising various questions and concerns. I’ll begin by taking them from the outside, in – that is, first API questions, and then internal structure issues.
SendLocal
First of all, the effect of calling SendLocal on IBus takes all the logical messages passed in (params IMessage[] messages), wraps them in a single TransportMessage, and puts that physical message at the end of the local queue. This call is equivalent to calling “Send(TransportMessage m, string destination);” on ITransport when passing in transport.Address as the parameter of destination.
There are numerous advantages to having this method, but one is the most important.
When client send a service a set of messages using “void Send(params IMessage[] messages);”, the client is requesting that the server treat this batch of messages as a unit of work. Under certain conditions, the service may choose to ignore the clients wishes – not least of which because the client has sent a ton of messages and the service doesn’t want ACID transactions to last a long time as they hurt throughput. In this case the server would use an intercepting message handler to go over those messages and call SendLocal for each. In other words, the server can set up units of work as it sees fit – taking into account client preference as well.
Other advantages include the ability to break apart complex or long-running logic into an “internal pipeline”. The Timeout Manager also makes use of this facility for “holding onto” messages until some condition occurs.
Return(errorCode)
The reason that integers are used as error codes is just so that you can push enums through them. This is the simplest way to get errors back to the client. More importantly, we take into account who on the client would be interested in this data.
Clients are often built using MVC with an additional Service Agent layer. Service Agents deal with translating the intent of Controllers into messages. Controllers don’t know about messaging, nor should they. However, they need to know when something fails with calls they initiated. As such, they are the final consumer of these error-code-enums, and integers are used to express them; that way Controllers don’t need to take a dependency on nServiceBus.
DoNotContinueDispatchingCurrentMessageToHandlers
This method on bus is used by intercepting message handlers in order to instruct the bus not to pass the current message on to subsequent handlers in the pipeline. This is often used by authentication and authorization handlers when those checks fail. This is what makes the message handling pipeline possible.
BuildAndDispatch
This method is defined on IBuilder and is used by the bus when dispatching messages to handlers. The reason that this exists instead of just having the bus ask the builder to create the handler and dispatch the call itself has to do with client-side threading. You can find the full explanation here – Object Builder, the place to fix system-wide threading bugs.
Summary
NServiceBus has grown over the years in environments where I’ve had the luxury of deciding most, if not all of the design of the systems involved. As such, it has taken on just the responsibilities needed from infrastructure in order to develop robust, flexible, and scalable systems. Check out the nServiceBus site.
Note to those who didn’t attend the session: you didn’t hear me say it, so I’ll repeat it: I hate WSDL almost as much as I hate Las Vegas. Ask me why sometime, or if I get enough of a critical mass of questions, I’ll blog it. If you’ve seen me do talks on Web Services, though, you’ve probably heard the rant: WSDL creates tightly-coupled endpoints precisely where loose coupling is necessary, WSDL encourages schema definitions that are inflexible and unevolvable, and WSDL intrinsically assumes a synchronous client-server invocation model that doesn’t really meet the scalability or feature needs of the modern enterprise. And that’s just for starters.
I hate WSDL.
I still hate Vegas more, though.
Web Services, and WSDL by connection have taken hold of the industry like cancer – inhibiting the minds of otherwise intelligent developers and architects. Whenever I get the “Web Services Question” (Does X support Web Services – where X is some design pattern, tool, and sometimes nServiceBus), I have to suppress an urge to groan – I’ve got the question that many times. The other day I was at a client and Sam, their head architect asked me that question. I gave my stock response:
“When you say ‘Web Services’, are you referring to SOAP or WSDL, and is HTTP a necessary component too?”
See how good I got at the suppressing thing?
Sam conceded that Web Services over TCP is OK too, so I pressed on with:
“What about UDP? FTP? MSMQ? Is it still ‘Web Services’ then? Is the rule then that ‘Web Services’ == SOAP?”
At that point, Sam was beginning to get a little flustered.
“And what’s so great about SOAP? Is it the interoperability? Because that’s just because it’s based on XSD.”
He didn’t know how to reply. Instead, he walked away from the whiteboard and sat down. I didn’t let up:
“And what if we want to do something other than Request/Response? How about one request with many responses? How about many requests and one response? And why does this decision need to be rigid? Shouldn’t we just be able to decide programmatically how many responses we want to return? Wouldn’t that flexibility be better than creating huge response structures for web methods to return?”
Sam made his last stand:
“Look, we can’t go and do something different from the rest of the industry. Everybody else is doing Web Services. It’s not like the technology doesn’t work.”
I gave way, a little:
“If you want, we can offer two interfaces. One, the flexible, robust, scalable XSD over messaging based solution. The second, an icky, synchronous Web Services facade which calls into our first interface.
I’m not saying that the technology doesn’t work – but both of us know that every problem has multiple solutions, some are fragile and error prone like WS, others are more elegant and have decades of knowledge behind them like messaging.
But we can do both if you like. How’s that?”
And it was agreed. The entire system would be built on one-way messaging patterns using XSD in cases where interoperability was required. And WS would be layered on, like a tiny little pig on top of a gigantic lipstick … thing – hmm, that metaphor isn’t really working – well, you get the idea.
The brilliant guys at 2xSundbland have launched their architect academy and it looks quite promising. I haven’t yet taken the trial lesson, but its in the queue. I have taken a look at the articles they have on the site as well, and they’re quite good. I especially like the Software Architecture vs. Software Engineering one. There is one topic in that article where I beg to differ, and it’s around services. The article (on page 7) describes the following scenario:
Typically, in such an environment [SOA], services tend to be parts of multiple systems. For example, consider a Products service! It might start its life as part of a sales system. Later it might be involved in a purchasing system, a product development system, a marketing system, a warehousing system, and perhaps in several other systems too. This process may take years, and it really never ends. The service is the same, but its responsibilities and its external exposure are increased with each system it’s enrolled in.
One of the core tenets of SOA that all vendors and analysts agree upon is that there should be loose coupling between services. If you were to design such a product service, it’s clear that changing part of its interface could break almost every system in the enterprise. That doesn’t sound like loose coupling to me.
If there’s one place that is the source of loose coupling – it’s the business. Warehousing is viewed by the business as being fairly independent of Marketing. While Sales might make use of data created in Product Development, business wouldn’t want any problems in IT related to Product Development to inhibit Sales ability to accept orders. That is another kind of loose coupling – the ability of one service to make use of “not-accurate-up-to-the-millisecond” data created by another service. That’s known as loose “temporal” coupling, as in loose coupling in the dimension of time.
Loosely-Coupled Services
So, in the example described we’d see the following services:
Sales
Purchasing
Product Development
Marketing
Warehousing / Inventory
Product data would flow between the services but each would have a very different internal view of it.
Product Development would be more interested in managing the schedule and risk around a product’s development.
Marketing would probably be more focused on its relation to competing products and pricing.
Purchasing would be maintaining data as to which suppliers are being used to supply raw materials for the production of the product.
Sales would be looking at actually accepting orders and giving discounts.
Warehousing would be focused on the storage and transportation aspects of the product.
As you can see, there is very little overlap in the data between these services even on something similar like product data. The logic of each service around the management of its data would be even more different. This leads to services with a high level of autonomy.
There Be Dragons…
Without starting at this business-level loose coupling, I doubt that any technical effort will succeed. That is to say every time I’ve seen this style implemented it has failed, but that’s no proof. Conversely, every time that we did start our SOA efforts by identifying the clear business fracture lines, we were able to maintain loose coupling all the way down. That is not to say that it always will succeed, but the logic is sound.
I suppose that the difference between my view on SOA and Sundblad’s stems from the fact that they put systems at a higher level of abstraction than services, and I put services on top. Regardless, I do agree with their views about architecture and engineering and consider them quite valuable.
Most people getting started with DDD and the Domain Model pattern get stuck on this. For a while I tried answering this on the discussion groups, but here we have a nice example that I can point to next time.
The underlying problem I’ve noticed over the past few years is that developers are still thinking in terms of querying when they need more data. When moving to the Domain Model pattern, you have to “simply” represent the domain concepts in code – in other words, see things you aren’t used to seeing. I’ll highlight that part in the question below so that you can see where I’m going to go with this in my answer:
I have an instance where I believe I need access to a service or repository from my entity to evaluate a business rule but I’m using NHibernate for persistence so I don’t have a real good way to inject services into my entity. Can I get some viewpoints on just passing the services to my entity vs. using a facade?
Let me explain my problem to provide more context to the problem.
The core domain revolves around renting video games. I am working on a new feature to allow customers to trade in old video games. Customers can trade in multiple games at a time so we have a TradeInCart entity that works similar to most shopping carts that everybody is familiar with. However there are several rules that limit the items that can be placed into the TradeInCart. The core rules are:
1. Only 3 games of the same title can be added to the cart. 2. The total number of items in the cart cannot exceed 10. 3. No games can be added to the cart that the customer had previously reported lost with regards to their rental membership. a. If an attempt is made to add a previously reported lost game, then we need to log a BadQueueStatusAddAttempt to the persistence store.
So the first 2 rules are easily handled internally by the cart through an Add operation. Sample cart interface is below.
1: class TradeInCart{
2: Account Account{get;}
3: LineItem Add(Game game);
4: ValidationResult CanAdd(Game game);
5: IList<LineItems> LineItems{get;}
6: }
However the #3 rule is much more complicated and can’t be handled internally by the cart, so I have to depend on external services. Splitting up the validation logic for a cart add operation doesn’t seem very appealing to me at all. So I have the option of passing in a repository to get the previously reported lost games and a service to log bad attempts. This makes my cart interface ugly real quick.
1: class TradeInCart{
2: Account Account{get;}
3: LineItem Add(
4: Game game,
5: IRepository<QueueHistory> repository,
6: LoggingService service);
7:
8: ValidationResult CanAdd(
9: Game game,
10: IRepository<QueueHistory> repository,
11: LoggingService service);
12:
13: IList<LineItems> LineItems{get;}
14: }
The alternative option is to have a TradeInCartFacade that handles the validations and adding the items to the cart. The façade can have the repository and services injected though DI which is nice, but the big negative is that the cart ends up totally anemic.
Any thought on this would be greatly appreciated.
Thanks, Jesse
As I highlighted above, the thing that will help you with your business rules is to introduce the Customer object (that you probably already have) with the property GamesReportedLost (an IList<Game>). Your TradeInCart would have a reference to the Customer object and could then check the rule in the Add method.
Before I go into the code, it looks like your Account object might be used the same way, but your description of the domain doesn’t mention accounts, so I’m going to assume that that’s unrelated for now:
1: public class Customer{
2:
3: /* other properties and methods */
4:
5: private IList<Game> gamesReportedLost;
6: publicvirtual IList<Game> GamesReportedLost
7: {
8: get
9: {
10: return gamesReportedLost;
11: }
12: set
13: {
14: gamesReportedLost = value;
15: }
16: }
17: }
Keep in mind that the GamesReportedLost is a persistent property of Customer. Every time a customer reports a game lost, this list needs to be kept up to date. Here’s the TradeInCart now:
1: publicclass TradeInCart
2: {
3: /* other properties and methods */
4:
5: private Customer customer;
6: publicvirtual Customer Customer
7: {
8: get { return customer; }
9: set { customer = value; }
10: }
11:
12: private IList<LineItem> lineItems;
13: publicvirtual IList<LineItem> LineItems
14: {
15: get { return lineItems; }
16: set { lineItems = value; }
17: }
18:
19: publicvoid Add(Game game)
20: {
21: if (lineItems.Count >= CONSTANTS.MaxItemsPerCart)
Your service layer class that calls the Add method of TradeInCart would first subscribe to the relevant events in FailureEvents. If one of those events is raised, it would do the necessary logging, external system calls, etc.
As you can see, the API of TradeInCart doesn’t need to make use of any external repositories, nor do you need to inject any other external dependencies in.
One thing I didn’t do in the above code to keep it “short” is to define the relevant custom EventArgs for bubbling up the information as to which game was reported lost or already have 3 of those in the cart. That is something that definitely should be done so that the service layer can pass this information back to the client.
Here’s a look at Service Layer code:
1: publicclass AddGameToCartMessageHandler :
2: BaseMessageHandler<AddGameToCartMessage>
3: {
4: publicoverridevoid Handle(AddGameToCartMessage m)
5: {
6: using (ISession session = SessionFactory.OpenSession())
7: using (ITransaction tx = session.BeginTransaction())
It’s important to remember to clean up your event subscriptions so that your Service Layer objects get garbage collected. This is one of the primary causes of memory leaks when using static events in your Domain Model. I’m hoping to find ways to use lambdas to decrease this repetitive coding pattern. You might be thinking to yourself that non-static events on your Domain Model objects would be easier, since those objects would get collected, freeing up the service layer objects for collection as well. There’s just on small problem:
The problem is that if an event is raised by a child (or grandchild object), the service layer object may not even know that that grandchild was involved and, as such, would not have subscribed to that event. The only way the service layer could work was by knowing how the Domain Model worked internally – in essence, breaking encapsulation.
If you’re thinking that using exceptions would be better, you’d be right in thinking that that won’t break encapsulation, and that you wouldn’t need all that subscribe/unsubscribe code in the service layer. The only problem is that the Domain Model needs to know that the service layer had a default catch clause so that it wouldn’t blow up. Otherwise, the service layer (or WCF, or nServiceBus) may end up flagging that message as a poison message (Read more about poison messages). You’d also have to be extremely careful about in which environments you used your Domain Model – in other words, your reuse is shot.
Conclusion
I never said it would be easy 🙂
However, the solution is simple (not complex). The same patterns occur over and over. The design is consistent. By focusing on the dependencies we now have a domain model that is reusable across many environments (server, client, sql clr, silverlight). The domain model is also testable without resorting to any fancy mock objects.
One closing comment – while I do my best to write code that is consistent with production quality environments, this code is more about demonstrating design principles. As such, I focus more on the self-documenting aspects of the code and have elided many production concerns.
Do you have a better solution?
Something that I haven’t considered?
Do me a favour – leave me a comment. Tell me what you think.
We brought him on site to give our development staff the 5-day “Advanced Distributed System Design” training. The course profoundly changed our understanding and approach to SOA and distributed systems.
Consider some of the evidence: 1. Months later, developers still make allusions to concepts learned in the course nearly every day 2. One of our developers went home and made her husband (a developer at another company) sign up for the course at a subsequent date/venue 3. Based on what we learned, we’ve made constant improvements to our architecture that have helped us to adapt to our ever changing business domain at scale and speed If you have the opportunity to receive the training, you will make a substantial paradigm shift.
If I were to do the whole thing over again, I’d start the week by playing the clip from the Matrix where Morpheus offers Neo the choice between the red and blue pills. Once you make the intellectual leap, you’ll never look at distributed systems the same way.
Beyond the training, we were able to spend some time with Udi discussing issues unique to our business domain. Because Udi is a rare combination of a big picture thinker and a low level doer, he can quickly hone in on various issues and quickly make good (if not startling) recommendations to help solve tough technical issues.” November 11, 2010
Ian Robinson, Principal Consultant at ThoughtWorks
"Your blog and articles have been enormously useful in shaping, testing and refining my own approach to delivering on SOA initiatives over the last few years. Over and against a certain 3-layer-application-architecture-blown-out-to- distributed-proportions school of SOA, your writing, steers a far more valuable course."
Shy Cohen, Senior Program Manager at Microsoft
“Udi is a world renowned software architect and speaker. I met Udi at a conference that we were both speaking at, and immediately recognized his keen insight and razor-sharp intellect. Our shared passion for SOA and the advancement of its practice launched a discussion that lasted into the small hours of the night. It was evident through that discussion that Udi is one of the most knowledgeable people in the SOA space. It was also clear why – Udi does not settle for mediocrity, and seeks to fully understand (or define) the logic and principles behind things. Humble yet uncompromising, Udi is a pleasure to interact with.”
Glenn Block, Senior Program Manager - WCF at Microsoft
“I have known Udi for many years having attended his workshops and having several personal interactions including working with him when we were building our Composite Application Guidance in patterns & practices. What impresses me about Udi is his deep insight into how to address business problems through sound architecture. Backed by many years of building mission critical real world distributed systems it is no wonder that Udi is the best at what he does. When customers have deep issues with their system design, I point them Udi's way.”
Karl Wannenmacher, Senior Lead Expert at Frequentis AG
“I have been following Udi’s blog and podcasts since 2007. I’m convinced that he is one of the most knowledgeable and experienced people in the field of SOA, EDA and large scale systems.
Udi helped Frequentis to design a major subsystem of a large mission critical system with a nationwide deployment based on NServiceBus. It was impressive to see how he took the initial architecture and turned it upside down leading to a very flexible and scalable yet simple system without knowing the details of the business domain.
I highly recommend consulting with Udi when it comes to large scale mission critical systems in any domain.”
Simon Segal, Independent Consultant
“Udi is one of the outstanding software development minds in the world today, his vast insights into Service Oriented Architectures and Smart Clients in particular are indeed a rare commodity. Udi is also an exceptional teacher and can help lead teams to fall into the pit of success. I would recommend Udi to anyone considering some Architecural guidance and support in their next project.”
Ohad Israeli, Chief Architect at Hewlett-Packard, Indigo Division
“When you need a man to do the job Udi is your man! No matter if you are facing near deadline deadlock or at the early stages of your development, if you have a problem Udi is the one who will probably be able to solve it, with his large experience at the industry and his widely horizons of thinking , he is always full of just in place great architectural ideas.
I am honored to have Udi as a colleague and a friend (plus having his cell phone on my speed dial).”
Ward Bell, VP Product Development at IdeaBlade
“Everyone will tell you how smart and knowledgable Udi is ... and they are oh-so-right. Let me add that Udi is a smart LISTENER. He's always calibrating what he has to offer with your needs and your experience ... looking for the fit. He has strongly held views ... and the ability to temper them with the nuances of the situation. I trust Udi to tell me what I need to hear, even if I don't want to hear it, ... in a way that I can hear it. That's a rare skill to go along with his command and intelligence.”
Eli Brin, Program Manager at RISCO Group
“We hired Udi as a SOA specialist for a large scale project. The development is outsourced to India. SOA is a buzzword used almost for anything today. We wanted to understand what SOA really is, and what is the meaning and practice to develop a SOA based system.
We identified Udi as the one that can put some sense and order in our minds. We started with a private customized SOA training for the entire team in Israel. After that I had several focused sessions regarding our architecture and design.
I will summarize it simply (as he is the software simplist): We are very happy to have Udi in our project. It has a great benefit. We feel good and assured with the knowledge and practice he brings. He doesn’t talk over our heads. We assimilated nServicebus as the ESB of the project. I highly recommend you to bring Udi into your project.”
Catherine Hole, Senior Project Manager at the Norwegian Health Network
“My colleagues and I have spent five interesting days with Udi - diving into the many aspects of SOA. Udi has shown impressive abilities of understanding organizational challenges, and has brought the business perspective into our way of looking at services. He has an excellent understanding of the many layers from business at the top to the technical infrstructure at the bottom. He is a great listener, and manages to simplify challenges in a way that is understandable both for developers and CEOs, and all the specialists in between.”
Yoel Arnon, MSMQ Expert
“Udi has a unique, in depth understanding of service oriented architecture and how it should be used in the real world, combined with excellent presentation skills. I think Udi should be a premier choice for a consultant or architect of distributed systems.”
Vadim Mesonzhnik, Development Project Lead at Polycom
“When we were faced with a task of creating a high performance server for a video-tele conferencing domain we decided to opt for a stateless cluster with SQL server approach. In order to confirm our decision we invited Udi.
After carefully listening for 2 hours he said: "With your kind of high availability and performance requirements you don’t want to go with stateless architecture."
One simple sentence saved us from implementing a wrong product and finding that out after years of development. No matter whether our former decisions were confirmed or altered, it gave us great confidence to move forward relying on the experience, industry best-practices and time-proven techniques that Udi shared with us.
It was a distinct pleasure and a unique opportunity to learn from someone who is among the best at what he does.”
Jack Van Hoof, Enterprise Integration Architect at Dutch Railways
“Udi is a respected visionary on SOA and EDA, whose opinion I most of the time (if not always) highly agree with. The nice thing about Udi is that he is able to explain architectural concepts in terms of practical code-level examples.”
Neil Robbins, Applications Architect at Brit Insurance
“Having followed Udi's blog and other writings for a number of years I attended Udi's two day course on 'Loosely Coupled Messaging with NServiceBus' at SkillsMatter, London.
I would strongly recommend this course to anyone with an interest in how to develop IT systems which provide immediate and future fitness for purpose. An influential and innovative thought leader and practitioner in his field, Udi demonstrates and shares a phenomenally in depth knowledge that proves his position as one of the premier experts in his field globally.
The course has enhanced my knowledge and skills in ways that I am able to immediately apply to provide benefits to my employer. Additionally though I will be able to build upon what I learned in my 2 days with Udi and have no doubt that it will only enhance my future career.
I cannot recommend Udi, and his courses, highly enough.”
Nick Malik, Enterprise Architect at Microsoft Corporation
“You are an excellent speaker and trainer, Udi, and I've had the fortunate experience of having attended one of your presentations. I believe that you are a knowledgable and intelligent man.”
He accompanied us in all stages of our development cycle and helped us put vision into real life distributed scalable software. He brought fresh thinking, great in depth of understanding software, and ongoing support that proved as valuable and cost effective.
Udi has the unique ability to analyze the business problem and come up with a simple and elegant solution for the code and the business alike. With Udi's attention to details, and knowledge we avoided pit falls that would cost us dearly.”
Børge Hansen, Architect Advisor at Microsoft
“Udi delivered a 5 hour long workshop on SOA for aspiring architects in Norway. While keeping everyone awake and excited Udi gave us some great insights and really delivered on making complex software challenges simple. Truly the software simplist.”
Motty Cohen, SW Manager at KorenTec Technologies
“I know Udi very well from our mutual work at KorenTec. During the analysis and design of a complex, distributed C4I system - where the basic concepts of NServiceBus start to emerge - I gained a lot of "Udi's hours" so I can surely say that he is a professional, skilled architect with fresh ideas and unique perspective for solving complex architecture challenges. His ideas, concepts and parts of the artifacts are the basis of several state-of-the-art C4I systems that I was involved in their architecture design.”
We’d been meaning to delve into messaging at Eleutian after multiple discussions with and blog posts from Greg Young and Udi Dahan in the past. We weren’t entirely sure where to start, how to start, what tools to use, how to use them, etc. Being able to sit in a room with Udi for an entire week while he described exactly how, why and what he does to tackle a massive enterprise system was invaluable to say the least.
We now have a much better direction and, more importantly, have the confidence we need to start introducing these powerful concepts into production at Eleutian.”
Gad Rosenthal, Department Manager at Retalix
“A thinking person. Brought fresh and valuable ideas that helped us in architecting our product. When recommending a solution he supports it with evidence and detail so you can successfully act based on it. Udi's support "comes on all levels" - As the solution architect through to the detailed class design. Trustworthy!”
Chris Bilson, Developer at Russell Investment Group
“I had the pleasure of attending a workshop Udi led at the Seattle ALT.NET conference in February 2009. I have been reading Udi's articles and listening to his podcasts for a long time and have always looked to him as a source of advice on software architecture. When I actually met him and talked to him I was even more impressed. Not only is Udi an extremely likable person, he's got that rare gift of being able to explain complex concepts and ideas in a way that is easy to understand. All the attendees of the workshop greatly appreciate the time he spent with us and the amazing insights into service oriented architecture he shared with us.”
Alexey Shestialtynov, Senior .Net Developer at Candidate Manager
“I met Udi at Candidate Manager where he was brought in part-time as a consultant to help the company make its flagship product more scalable. For me, even after 30 years in software development, working with Udi was a great learning experience. I simply love his fresh ideas and architecture insights. As we all know it is not enough to be armed with best tools and technologies to be successful in software - there is still human factor involved. When, as it happens, the project got in trouble, management asked Udi to step into a leadership role and bring it back on track. This he did in the span of a month. I can only wish that things had been done this way from the very beginning. I look forward to working with Udi again in the future.”
Christopher Bennage, President at Blue Spire Consulting, Inc.
“My company was hired to be the primary development team for a large scale and highly distributed application. Since these are not necessarily everyday requirements, we wanted to bring in some additional expertise. We chose Udi because of his blogging, podcasting, and speaking. We asked him to to review our architectural strategy as well as the overall viability of project.
I was very impressed, as Udi demonstrated a broad understanding of the sorts of problems we would face. His advice was honest and unbiased and very pragmatic. Whenever I questioned him on particular points, he was able to backup his opinion with real life examples.
I was also impressed with his clarity and precision. He was very careful to untangle the meaning of words that might be overloaded or otherwise confusing. While Udi's hourly rate may not be the cheapest, the ROI is undoubtedly a deal.
I would highly recommend consulting with Udi.”
Robert Lewkovich, Product / Development Manager at Eggs Overnight
“Udi's advice and consulting were a huge time saver for the project I'm responsible for. The $ spent were well worth it and provided me with a more complete understanding of nServiceBus and most importantly in helping make the correct architectural decisions earlier thereby reducing later, and more expensive, rework.”
The class was very well put together. The materials were clear and concise and Udi did a fantastic job presenting it. It was a good mixture of lecture, coding, and question and answer. I fully expected that I would be taking notes like crazy, but it was so well laid out that the only thing I wrote down the entire course was what I wanted for lunch. Udi provided us with all the lecture materials and everyone has access to all of the samples which are in the nServiceBus trunk.
Now I know why Udi is the "Software Simplist." I was amazed to find that all the code and solutions were indeed very simple. The patterns that Udi presented keep things simple by isolating complexity so that it doesn't creep into your day to day code. The domain code looks the same if it's running in a single process or if it's running in 100 processes.”
Ian Cooper, Team Lead at Beazley
“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do.”
Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading. Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project. 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, 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, 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, .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, 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, 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, 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, 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, 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, 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, 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, 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.”
It’s important to remember to clean up your event subscriptions so that your Service Layer objects get garbage collected. This is one of the primary causes of memory leaks when using static events in your Domain Model. I’m hoping to find ways to use lambdas to decrease this repetitive coding pattern. You might be thinking to yourself that non-static events on your Domain Model objects would be easier, since those objects would get collected, freeing up the service layer objects for collection as well. There’s just on small problem:
