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Archive for the ‘Messaging’ Category
Sunday, May 6th, 2012
When people pull back the covers on something like MSMQ, particularly its private queues (the way NServiceBus uses it), and they see that MSMQ is storing its messages in C:WindowsSystem32, well, they’re not particularly happy.
One of the reasons they worry about these types of distributed or federated queue-based solutions has to do with physical failures. The concern is that messages would be lost if there was a hard drive failure.
The preference for centralized message broker type solutions is that we can set it up on a nice RAID infrastructure that will take care of any physical reliability concerns. (Just so that we’re clear here – I’m talking about an single datacenter, possibly connected to a disaster recovery site.)
So, here’s the thing:
Virtualization
You see, in a virtualized production environment, the C drive of a virtual machine is physically in the image file of that VM, which is sitting on a SAN (storage area network).
What that means is that when a message is sent from one processing node to another, the data of that message ends up being written to the SAN, with all of its redundant disks. Even if one of the machines has a critical failure and cannot start up again, all the VMs that were running on it can be started up on a different machine without any message loss.
In fact, most virtualized environments have monitoring and management capabilities built-in so that the VMs will be brought up automatically on another machine. Even if you aren’t using messaging, there are so many other benefits that virtualization brings that you probably should be planning on putting it in, if you haven’t already.
Databases too
In fact, many people do the same thing with databases.
The file partitions on which the database server actually stores its data are on the SAN.
Think about that for a second.
All the data in messages flowing through your queues, and the data in the database, on a SAN. This gives you the ability to do a fully consistent backup of the entire system with SAN snapshots, not to mention ship those to your disaster recovery site.
In closing
Distributed solutions are often misunderstood.
Bad experiences in the past with MSMQ can color perceptions in the present.
The thing is that today’s infrastructure is set up to handle distributed solutions much better.
Developers no longer have to turn to centralized broker or database technologies to get the centralized backup and restore capabilities administrators look for.
If you’ve been avoiding NServiceBus for these reasons, give it a try. Not only will it make your life as a developer easier, combined with this virtualization thing, it will make your administrators life easier too.
Posted in Architecture, EDA, ESB, Messaging, MSMQ, NServiceBus, Pub/Sub, Reliability, Scalability | 8 Comments »
Friday, April 13th, 2012
This Wednesday I’m going to be giving a talk at Skills Matter in London called Modeling Distributed Systems with NServiceBus Studio.
UPDATE: Recording is now online here.
You might remember that I announced NServiceBus Modeling Tools about 6 months ago, well this is going to be the next generation of those tools.
Here’s the abstract:
As developers build larger and more distributed systems than ever before, many questions come up around modeling like, how do I represent services, how do I compose these services into physical endpoints, and how should cross-cutting concerns like authentication and auditing be handled.
Join Udi for a first look at NServiceBus Studio – a modeling environment based on the decades-old principles behind NServiceBus yet fully integrated with code-generation into Visual Studio.
One of the important changes we made from the first version was separating the concept of an Endpoint from that of a Service. A Service is a logical concept which (in the perspective of code) is made up of multiple Components. Components can process messages and/or emit messages and are deployed to one or more Endpoints.
Also, in version 3.0 of NServiceBus we made the scaling out of your endpoints almost transparent – all you need to do is provide the name of the load-balancing machine in configuration and you’re good to go. In that sense, Endpoints are a bit more logical than physical.
While we haven’t gotten to it yet, all sorts of cross-cutting concerns like configuration, authentication, and logging will be able to be defined in one place and then applied to specific (or all) Endpoints.
We’re also planning to integrate tools that will hook into the central auditing already built into NServiceBus and visualize the message flow as well as debug and replay certain messages.
In short – this isn’t going to be UML centric.
This is a huge (and expensive) undertaking but I believe that it can push the level of abstraction developers work at significantly higher. Capabilities like round-tripping (sync-ing the visual model with changes to the code) are very hard to do well, yet are absolutely necessary for large-scale development. I am aware of how poorly the various code-generating tools have fared when dealing with The Real World, so I want to do this out in the open from early on to get very broad feedback on the vision while continuously providing value in the short term.
The talk won’t be my usual “let me tell you how it is” kind of presentations but rather more of a “here’s what we’ve done, where we’re thinking about going, what do you think about that?” kind of discussion.
Anyway, I’d love for you to come out if you’re in and around the area of London on April 18 (register here) or if you could watch the video of the talk once it comes online and send me your feedback.
Posted in Community, General, Messaging, Modeling, MSMQ, NServiceBus, Presentations, Pub/Sub | No Comments »
Saturday, March 17th, 2012
My presentation at Oredev on “Who needs a service bus anyway?” is now online for your viewing pleasure. Enjoy.
Posted in Community, ESB, Messaging, MSMQ, NServiceBus, Presentations, Pub/Sub, Reliability | 3 Comments »
Sunday, June 26th, 2011
When I was at NDC a couple of weeks ago, I got together with Carl Franklin and we recorded a DNR-TV episode on NServiceBus. If you’re looking for a zero-to-sixty, code-centric explanation of NServiceBus – this is it.
NServiceBus on DNR-TV
For some more advanced stuff, I suggest looking at the Hidden NServiceBus Gems talk that I gave at Skills Matter the week after. Here we get into all sorts of things that you won’t tend to find by yourself through regular usage of NServiceBus.
Hidden NServiceBus Gems
Enjoy.
Posted in Community, ESB, Messaging, MSMQ, NServiceBus, Presentations, WCF | No Comments »
Friday, April 22nd, 2011
It looks like that CQRS has finally “made it” as a full blown “best practice”.
Please accept my apologies for my part in the overly-complex software being created because of it.
I’ve tried to do what I could to provide a balanced view on the topic with posts like Clarified CQRS and Race Conditions Don’t Exist.
It looks like that wasn’t enough, so I’ll go right out and say it:
Most people using CQRS (and Event Sourcing too) shouldn’t have done so.
Should we really go back to N-Tier?
When not using CQRS (which is the majority of the time), you don’t need N-Tier either.
You see, if you’re not in a collaborative domain then you don’t have multiple writers to the same logical set of data as an inherent property of your domain. As such, having a single database where all data lives isn’t really necessary.
Data is inherently partitioned by who owns it.
Let’s take the online shopping cart as an example. There aren’t any use cases where users operate on each others’ carts – ergo, not collaborative, therefore not a good candidate for CQRS. Same goes for user profiles, and tons of other cases.
So why is it that we need a separate tier to run our business logic?
Originally, the application server tier was introduced for improved scalability, but specifically around managing the connection pool to the database. Increasing numbers of clients (when each had its own user/account for connecting to the database) caused problems. Luckily, most web applications side-step this problem – that is, until someone got the idea that the web server was only supposed to run the UI layer, and the Business Logic layer would be on a separate application server tier.
Rubbish – see Fowler’s First Law of Distribution: Don’t.
Keep it all on one tier. Same goes for smart clients.
No, Silverlight, you don’t count – architecturally speaking, you’re a glorified browser.
But what about scalability?
In a non-collaborative domain, where you can horizontally add more database servers to support more users/requests/data at the same time you’re adding web servers – there is no real scalability problem (caveat, until you’re Amazon/Google/Facebook scale).
Database servers can be cheap – if using MySQL/SQL Express/others.
But what about the built-in event-log CQRS/ES gives us?
Architectural gold-plating / stealing from the business.
Who put you in a position to decide that development time and resources should be diverted from short-term business-value-adding features to support a non-functional requirement that the business didn’t ask for?
If you sat down with them, explaining the long-term value of having an archive of all actions in the system, and they said OK, build this into the system from the beginning, that would be fine. Most people who ask me about CQRS and/or Event Sourcing skip this step.
Finally, you can usually implement this specific requirement with some simple interception and logging. Don’t over-engineer the solution. If using messaging, you can get this by turning on journaling, or if you want to centralize this archive, NServiceBus can forward all messages to a specific queue.
Don’t forget that this storage has a cost – including administration. Nothing is free.
What about the “proof of correctness” in Event Sourcing
I’ve heard statements made that when you use the events that flowed into/through your system AS your system’s data, rather than transforming those events to some other schema (relational or otherwise) and storing the result – you can prove that your system behaves correctly.
Let me put it this way:
No programming technique used by humans will prevent those same humans from creating bugs.
No testing technique used by humans will prevent those same humans from not catching those bugs.
* Automated tests – see programming technique.
While having a full archive of all events can allow us to roll the system back to some state, fix a bug, and roll forwards, that assumes that we’re in a closed system. We have users which are outside the system. If a user made a decision based on data influenced by the bug, there’s no automated way for us to know that, or correct for it as we roll forwards.
In short, we’re interested in the business’ behavior – as composed of user and system behavior. No proof can exist.
Umm, so where should we use it
If you’ve uncovered a scenario where you’re wondering “first-one-wins, or last-one-wins”, that’s often a good candidate for a place where CQRS could make sense. Then re-read my Race Conditions Don’t Exist post.
Also, CQRS should not be your top-level architectural pattern – that would be SOA.
CQRS, if used at all, would be used inside a service boundary only.
Given that SOA guides us away from having a given 3rd normal form entity exist in any one service, it is unlikely that the building blocks of your CQRS design will be those kinds of entities. Most 3rd normal form one-to-many and many-to-many relationships simply do not exist when doing SOA and CQRS properly.
Therefore, I’m sorry to say that most sample application you’ll see online that show CQRS are architecturally wrong. I’d also be extremely wary of frameworks that guide you towards an entity-style aggregate root CQRS model.
In Summary
So, when should you avoid CQRS?
The answer is most of the time.
Here’s the strongest indication I can give you to know that you’re doing CQRS correctly: Your aggregate roots are sagas.
And the biggest caveat – the above are generalizations, and can’t necessarily be true for every specific scenario. If you’re Greg Young, then you probably can (and will) decide on your own on these matters. For everybody else, please take these warnings to heart. There have been far too many clients that have come to me all mixed up with their use CQRS in areas where it wasn’t warranted.
If you want to know everything you need to know to apply CQRS appropriately, please come to my course – there is so much unlearning to do first that just can’t happen via a series of blog posts.
Posted in Architecture, CQRS, Databases, DDD, Messaging, Scalability, Simplicity, SOA, Testing | 42 Comments »
Friday, April 8th, 2011
One of the topics that comes up a lot in the context of an Enterprise Service Bus (ESB) is that of integration. Unfortunately, many people take their ideas of reuse and design their integration as being done from a single place – both logical and physical. That unfortunately creates a bottleneck for all integration activities, where some are likely to be higher priority than others.
The ugly truth
You don’t need an ESB for integration.
There. I’ve said it.
Most of the ESB products on the market, in focusing on integration, are addressing the wrong problem.
What integration is about
There are three primary components to integration – data format translation, protocol bridging, and logic. Let’s take the Agile “simplest solution that could possibly work” motto and apply it to these elements:
- Data format translation
- Use something like MapForce (from Altova, the guys behind XMLSpy).
- At under $1200 for a dev license and handling mapping to/from XML, EDI, flat files, and relational databases, you can host the resulting mapping in any endpoint, as XSLT or even Java/C# – no need to have “the bus” do this for you.
- Protocol bridging
- Use something like /n software
- At about $1500 for a dev license, you get solutions for FTP, HTTP, SMTP, POP, IMAP, LDAP, DNS, RSS, SMS, Jabber, SOAP, WebDav, etc and, again – you can host the DLLs in any endpoint you want. Don’t need expensive buses for this
- Logic
This is all you – no technology can do this for you. If anything, the most important thing is to get all the ugly mapping and protocol bridging stuff out of the way and let you focus on your logic. This is Single Responsibility Principle (un)common sense.
Digging deeper into the logic
The interesting thing about most of the protocol stuff mentioned above is that they’re inherently unreliable and also their performance at runtime is unknowable due to the total load on the target server.
We wouldn’t want any of the above situations to cause our integration to “get stuck”. As such, it is best we think of our integration logic as a long-running process that manages other endpoints which do the actual protocol bridging and data transformations.
This is one of the areas where NServiceBus, with its sagas can actually help a lot. Just like the other pieces of integration mentioned above, these sagas can run on any endpoint.
You could alternatively look at other technologies like BizTalk and Business Process Execution Language (BPEL) engines, though many of those are designed to be physically centralized (just like many of the ESBs out there).
By the way, if you do want to use NServiceBus together with BizTalk, Michael Stephenson has published some great white papers on getting the two to work together. His latest is about integrating NServiceBus into BizTalk’s RFID processes – check it out.
Putting it all together
When we take all of these pieces and look at them in a cohesive architecture, here’s what it can look like:
As you can see, integration is physically distributed across multiple endpoints.
Not only that, but the integration logic is kept separate from the protocol bridging and data transformation, enabling independent versioning of each. Just as important, it makes it much easier to unit test that the integration logic is correct as we don’t have to simulate the target technologies.
Costs
As you can see, you can get integration capabilities just as powerful as if you went with something like BizTalk, but without creating a single point of failure in your architecture. In terms of costs, it’s also quite a bit cheaper.
For a high availability BizTalk deployment, you’ll be paying over $40,000 per CPU for the Enterprise Edition, not including the extra $7000 per CPU for SQL Server Standard Edition (clustered). For a clustered 4-CPU mid-size deployment, you’d be in the area of $200,000.
For the distributed integration solution above, you’d be paying around $2700 in dev licenses (for /n software and Altova MapForce), and $500 per core for NServiceBus Standard Edition. The reason there’s per-core licensing for NServiceBus is that in a virtualized environment, you’ll be provisioning virtual cores to your virtual machines. No reason to pay for a quad-core CPU when all you’re using is a single core. You can also use any number of cores running NServiceBus Express Edition at no cost, so a mid-size deployment with say 8 cores running Standard Edition, and another 24 cores running Express Edition would cost (with the volume discount) an additional $3800 – a total cost of $6,500.
BizTalk: $200,000 NServiceBus: $6,500
‘Nuff said.
In closing
We’ve been bitten by centralized architectures before.
By having our integration distributed, we can version, upgrade, and scale each piece independently.
You’ve seen how we can use simple, lightweight, and inexpensive technologies to create distributed integration solutions just as powerful and robust as the centralized ESB vendor-offerings out there, but at a tiny fraction of the cost.
Next time the topic of integration is brought up, you’ll know not to be suckered in by re-branded EAI brokers.
Learn more about NServiceBus.
Posted in Architecture, BizTalk, ESB, Messaging, NServiceBus, Pub/Sub, SOA | 8 Comments »
Thursday, March 24th, 2011
One of the things which often confuses people using NServiceBus for the first time is that it only allows an endpoint to subscribe to a given event from a single other publishing endpoint. The rule that there can only be a single publisher for a given event type is one of the things that differentiates buses from brokers, though both obviously allow you to have multiple subscribers.
Brokers
Message brokers, more broadly known and used on the Java platform, don’t come with this constraint. For example, when using ActiveMQ, you can have any number of endpoints come to the broker and publish a message under a given topic.
So where’s the problem?
It’s all about accountability.
Let’s say you’ve subscribed to a given topic, and have received two events – one telling you that the price of bananas next week will be $1/kg and another telling you that it’ll be $2/kg.
Which one is right?
Especially given that those events may have been published by any other endpoint via the broker.
Is it first one wins? Last one wins? How about first one sent vs. first one received? Ditto for last. As a subscriber, can you really be held accountable for having the logic to choose the right one? Shouldn’t this responsibility have fallen to the publishing side?
This is one of the big drawbacks of the broker, hub and spoke architecture. No responsibility. No single source of truth – unless everybody’s going to some central database, in which case – what’s the point of all this messaging anyway?
Buses
The Bus Architectural Style is all about accountability. If you are going to publish an event, you are accountable for the correctness of the data in that event – there is no central database that a subscriber can go to “just in case”. And the only way that you can be held accountable, is if you have full responsibility – ergo, you’re the only one who can publish that type of event.
If you say bananas are going to cost $1/kg next week, that’s that. Subscribers will not hear from anybody else on that topic.
Now, this is not to say that you can’t have more than one physical publishing endpoint.
You see, buses differentiate between the logical and the physical. Brokers tend to assume that the physical hub-and-spoke topology is also the logical.
In a bus, while there can only be one logical endpoint publishing a given type of event, that endpoint can be physically scaled out across multiple machines. It is the responsibility of the bus to provide infrastructure facilities to allow for that to happen in such a way that to subscribers, it still appears as if there is really only one publishing endpoint.
The same is true about the subscriber – one logical subscribing endpoint may be scaled out across multiple machines.
Product Mix-ups
Unfortunately, there are many broker-style technologies out there that are being marketed under the banner of the Enterprise Service Bus. While some products have the ability to be deployed in both a centralized and distributed fashion (sometimes called “federated” or “embedded” mode), many do not enforce the “single publishing logical-endpoint per event-type” rule.
Without this constraint, it is just too easy to make mistakes.
NServiceBus
By enforcing this constraint, we see the same kind of question appear on the discussion group time and time again:
“I have an Audit event that I’d like all of my machines to publish, and have one machine subscribe to them all, but NServiceBus won’t let me. How do I make NServiceBus support this scenario?”
And the answer is the same every time:
“You should have all the machines Send the Audit message (configured to go to the single machine handling that message), and not Publish. It is not an event until its been handled by the endpoint responsible for it.”
The semantics of the message matter a lot.
When looking at Service-Oriented Architecture, these messages are the contract and, as any lawyer will tell you, contracts need to be explicit and the intentions really need to be spelled out – otherwise the contract is practically worthless.
In closing
Friction is sometimes a good thing – it prevents us from making mistakes. It keeps cars on the road. And because that’s not enough friction, we introduce curbs as well.
If you’re looking for a service bus technology for your next project, check that it’ll give you the friction that you need to keep everybody safe. Really check what it is that the vendors are offering you – more often than not, it’s some ESB lipstick on a broker pig.
To learn more about how NServiceBus supports this kind of publish/subscribe, click here.
Posted in Architecture, BizTalk, EDA, ESB, Messaging, MSMQ, NServiceBus, Pub/Sub, SOA | 22 Comments »
Sunday, March 20th, 2011
Every once in a while I get clients who ask me why NServiceBus doesn’t support content-based routing. My answer sometimes surprises them, “because it is a dangerous pattern that should usually be avoided”.
Content-Based Routing and ESBs
Since content-based routing often appears on feature lists for various ESBs, many people consider them to be a necessary part of systems built on SOA principles. The pattern also appears in the book Enterprise Integration Patterns, which apparently is also a convincing reason to use it, even though the book specifically states:
“When implementing a Content-Based Router, special caution should be taken to make the routing function easy to maintain as the router can become a point of frequent maintenance.”
That’s right – maintenance nightmare.
For Example
Let’s take a real-world example – a trading system where users can put orders for stock and for forex (money from other countries).
We’d start by creating a message that can be sent by a client:
public class PlaceOrder : IMessage
{
public OrderTypeEnum OrderType { get; set; }
public string Code { get; set; }
public double Amount { get; set; }
}
public enum OrderTypeEnum
{
Stock,
Forex
}
Clients would send messages by using code like the following:
Bus.Send<PlaceOrder>(m => {
m.OrderType = OrderTypeEnum.Stock;
m.Code = "MSFT";
m.Amount = 300;
m.SetHeader("AccountId", myAccountId);
});
The header is the way that clients identify themselves in the system.
Let’s say that the logic for handling the different types of orders is different, but also that we’d like the logic to be deployed to different endpoints. One reason we might want to do this is so that we can independently scale each of these endpoints. This is where it would appear we’d need some content-based routing – having some code that looks at the OrderType property and decides where to route based on its value.
Before we get into solutions, let’s make this more involved.
Not only do we want to route based on OrderType, but we want to use the account ID in the header to check in our database (or via a web service) if the account belongs to one of our VIP customers, and if so, it should be given higher priority.
Content-Based Routing and Brokers
We can see that if we were to go with a content-based routing solution, this would drive our architecture to a hub-and-spoke model where the hub becomes quite large and complex, as well as likely becoming a bottleneck in terms of performance.
This logically centralized place through which all communication flows defines the Broker architectural style – not that of a Bus. In the Bus architectural style, there is no logical (or physical) hub. You can think of it as a kind of peer-to-peer setup. Just like you wouldn’t want ethernet getting involved in applicative routing decisions, neither should your bus get involved.
Business-Topology Mapping Solutions
Instead, when following the Bus architectural style – we look at mapping stable business characteristics to bus-level topology. At one level, that would mean defining two different message types for our different types of trades:
public abstract class PlaceOrder
{
public string Code { get; set; }
public double Amount { get; set; }
}
public class PlaceStockOrder : PlaceOrder { }
public class PlaceForexOrder : PlaceOrder { }
Once we have two different message types, then we can configure the client to have those statically sent to different endpoints like this:
<UnicastBusConfig>
<MessageEndpointMappings>
<add Messages="Messages.PlaceStockOrder, Messages" Endpoint="Stock" />
<add Messages="Messages.PlaceForexOrder, Messages" Endpoint="Forex" />
</MessageEndpointMappings>
</UnicastBusConfig>
And when it comes to handling our VIP customers, the recommendation would be to have those customers be served by a different set of web servers – we wouldn’t want a sudden flux in regular customers stealing all the HTTP connections from our VIP customers. Then we’d statically configure our VIP front-end to talk to our VIP back-end like this:
<UnicastBusConfig>
<MessageEndpointMappings>
<add Messages="Messages.PlaceStockOrder, Messages" Endpoint="VIP_Stock" />
<add Messages="Messages.PlaceForexOrder, Messages" Endpoint="VIP_Forex" />
</MessageEndpointMappings>
</UnicastBusConfig>
And the logic to identify VIP customers would be done in the login screen which, from there, would direct the user to the appropriate web server farm.
Static vs. Dynamic
It may appear that the above statically configured solution is less flexible then the afore-mentioned hub-and-spoke content-based routing solution. And that’s probably correct. But the question is, are the business requirements (better called “objectives” in this case) likely to change? If we make the technological solution 10x more flexible than the business needs, but at the cost of maintainability (read time to market), we probably haven’t used the right tool for the job.
Many times we can find stable business objectives and align the topology of our solution with them. Not all that is dynamic and flexible is necessarily better than that which is static.
I’d go so far to say that if a solution makes heavy use of content-based routing, it is likely a more fragile solution as implementations of stable business objectives and volatile requirements are mixed up together.
In Closing
NServiceBus intentionally does not support content-based routing so as not to make it easy for developers to make architectural blunders that could require full-system rewrites a couple of years down the road.
If you want to learn more about these kinds of architectural principles, I suggest coming to my course. I’m afraid that New York City is already sold out, but I’ll be coming back to the US again around October. Stockholm, London, Sydney, and Oslo are now all open for registration.
Hope to see you there.
Posted in Architecture, ESB, Messaging, NServiceBus, SOA | 25 Comments »
Thursday, January 13th, 2011
One of the questions that came up from my NServiceBus – .NET Service Bus Smackdown post was about the Polymorphic Message Dispatch and Polymorphic Message Routing features. People wanted to know what those are, why they’re important, and if other technologies (specifically WCF and BizTalk) support them.
Messaging Basics
First of all, when building a system using messaging, you don’t have methods that are invoked on some remote object (a.k.a “service”) to which you pass parameters. Instead, you use some generic piece of infrastructure (in the world of Java, this is most commonly a Message Broker) to send a message where a message can be thought of as a serializable class. Here’s an example of a message:
public class UserCreated : IMessage
{
public Guid UserId { get; set; }
public string Name { get; set; }
}
This message would be published using NServiceBus like this:
bus.Publish<UserCreated>( m =>
{
m.UserId = Guid.NewGuid();
m.Name = "John Smith";
});
This can be contrasted with RPC models like WCF where you need to define a “service” that has methods on it, where those methods accepts parameters. Sometimes developers try to make this more generic by having a single “service” with one method on it named something like “Process” where the parameter it accepts is of the type “object”, or they introduce generics like this: Process<T>(T message);
This is where Polymorphic Message Dispatch comes in:
Polymorphic Message Dispatch
While you can pull of the WCF generics thing, one thing that is more difficult (without writing your own dispatch model) is to have a pipeline of classes which can be invoked based on their relationship to the type passed in. Using NServiceBus, both of the following message handlers will be invoked when UserCreated arrives:
public class Persistence : IHandleMessages<UserCreated>
{
public void Handle(UserCreated message) { }
}
public class Audit : IHandleMessages<IMessage>
{
public void Handle(IMessage message) { }
}
Now some might say that WCF, BizTalk, and the .NET Service Bus allow you to do auditing in their own internal pipeline, and that’s true. The place where this becomes more powerful is when you need to build V2 of your system, and the publisher now publishes a slightly different event – that a user was created as a part of a campaign, requiring the subscriber to register statistics about the campaign. Of course, this event also means that a user was created. Here’s how you’d do that with NServiceBus:
public class UserCreatedFromCampaign : UserCreated
{
public Guid CampaignId { get; set; }
}
//publisher code
bus.Publish<UserCreatedFromCampaign>( m =>
{
m.UserId = Guid.NewGuid();
m.Name = "John Smith";
m.CampaignId = theCampaignId;
}
//subscriber code
public class Statistics : IHandleMessages<UserCreatedFromCampaign>
{
public void Handle(UserCreatedFromCampaign message) { }
}
The important part is what you don’t see – since UserCreatedFromCampaign inherits from UserCrated, the Persistence handler we had from V1 will also be invoked, and so will the Audit handler of course. You don’t have to make your new code call the old code like you would in a method based dispatch model. This makes sure that the coupling in your service layer code remains constant over time as you grow the functionality of your system.
This was one of the main benefits mentioned by Rackspace in their use of NServiceBus (here):
“The main benefit NServiceBus has brought us so far is developer scalability due to lower coupling and higher consistency in our code.”
But, when looking at the above scenario, we can obviously expect that all sorts of things can happen in relation to campaigns – it is a separate concern, and thus should be handled by a separate subscriber. And this bring us to…
Polymorphic Message Routing
The challenge that we have here is that we no longer have a hierarchy where something clearly belongs on top of something else. We have users created and activities happening related to campaigns – that may happen in any combination. By having separate subscribers, we could then introduce new handlers/subscribers to our environment without touching or taking down any of the other subscribers. Here’s what the subscribers would look like:
public class Persistence : IHandleMessages<UserCreated>
{
public void Handle(UserCreated message) { }
}
public class Statistics : IHandleMessages<CampaignActivityOccurred>
{
public void Handle(CampaignActivityOccurred message) { }
}
But if each of the above messages were a class, how could we define a message which inherited from both?
Before answering that, we need to understand why the publisher wouldn’t just publish both of the above messages. You see, the publisher can’t make any assumptions about its subscribers – it could be that one of them has logic that correlates across both of these messages that could end up counting the occurrence as happening twice rather than once, possibly charging the account associated with the campaign twice. Publishing two messages results in two transactions when there really should have been one.
So, here’s how to define messages so that we can have multiple inheritance:
public interface UserCreated : IMessage
{
Guid UserId { get; set; }
string Name { get; set; }
}
public interface CampaignActivityOccurred : IMessage
{
Guid CampaignId { get; set; }
Guid ActivityId { get; set; }
}
public interface UserCreatedFromCampaign
: UserCreated,
CampaignActivityOccurred
{
}
And when the publisher publishes UserCreatedFromCampaign, the event would be routed to both the UserCreated subscriber and the CampaignActivityOccurred subscriber. The power of this approach is felt as we handle new requirements around purchases made related to a campaign. Now we can have another event which inherits from CampaignActivityOccurred and not have to worry since the existing subscriber will be routed those messages automatically.
Since WCF doesn’t have publish/subscribe capabilities, we might as well move along.
Not to throw a burning match on an ocean of oil, but REST doesn’t really support this either.
Not Content-Based Routing
This may sound like the content-based router pattern from EIP (CBR), but it’s not. The important difference is that there isn’t some part of the routing that depends on the structure of the messages. The major drawback of CBR is that it creates a central place in your system that needs to be changed any time *syntactic* changes happen to message structure *in addition to* to changes in the subscribers.
Now, this is where the BizTalk guys would say that “that’s why we can do message transformations”, and then the subscribers wouldn’t need to be changed. However, can we really know when getting a requirement that the change is syntactic and not semantic? I mean, it’s quite common that changes to message structures happen together with changes to processing logic.
You may be beginning to get the feeling that more and more logic is being sucked out of the subscribers into some monolithic black hole that is likely going to be unmaintainable and quite slow.
This is one of the main differences between using a bus and a broker – a bus supports the correct distribution of logic keeping the system loosely coupled; brokers are useful integration engines when you absolutely can’t change the applications being integrated. Enterprise Application Integration (EAI) brokers don’t usually make good Enterprise Service Bus (ESB) technology.
In Closing
NServiceBus has all sorts of features you didn’t know you needed until you saw what life could be like when you had them. Most of these features don’t have snazzy drag-and-drop demos that make people ooh-and-aah and TechEds and PDCs, but they’re really necessary to avoid finding yourself in yet another big-ball-of-mud code base telling your manager/customer (again) that it would be faster to rebuild the system from scratch than to implement that new requirement in the old one.
Take NServiceBus for a spin and see for yourself.
Posted in Architecture, BizTalk, EDA, ESB, Messaging, NServiceBus, Pub/Sub, WCF, Web Services | 7 Comments »
Friday, December 31st, 2010
Just before we usher in the new year, I’m happy to announce the release of NServiceBus version 2.5.
Yes, there’s a new logo, and the website’s been redesigned.
It’s been a long time coming – the previous version (2.0) was released in March.
I’m really quite excited about this version as it rolls up all the bug fixes and enhancements that customers have asked for as they ran version 2.0 under the most severe types of production environments. The next thing that is a big deal that many have been asking for is a licensed version of NServiceBus – that is, the ability to purchase a commercial license and receive support.
We all know how managers like having a throat to choke.
And now they’ll have one – NServiceBus Ltd is the company that will be providing licensing, services, and support for all customers’ NServiceBus needs. After more than 33,000 downloads and over 1000 developers in the community, the demand has really grown. Who would’ve thought all this would happen when I started NServiceBus 4 years ago (before it even had a name).
Why NServiceBus is better than WCF for your distributed systems
This question comes up repeatedly for people hearing about NServiceBus for the first time.
The answer is simple – reliability.
A system built with NServiceBus is so much more reliable to all kinds of production conditions than WCF that it’s hardly a fair comparison at all. While WCF can be configured to provide something kind of close to the same level of reliability, you need to do a fair amount of spelunking through the various options of netMsmqBinding to get it right.
The second reason to use NServiceBus instead of WCF is publish/subscribe.
The ability to make use of events and the observer pattern not just to achieve loose coupling within a single process, but across many processes, machines, and sites. Can you imagine going back to programming without events? Shudder. But that’s exactly what it’s like to use WCF in your distributed system. NServiceBus brings you the best of object-oriented programming but in a distributed and reliable infrastructure.
Don’t wait any longer
Take NServiceBus for a spin.
But things may look a bit different after you do…
http://www.NServiceBus.com
And have a happy New Year.
Posted in EDA, ESB, Messaging, MSMQ, NServiceBus, Pub/Sub, Reliability, WCF, Web Services | 4 Comments »
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Recommendations
Bryan Wheeler, Director Platform Development at msnbc.com
“ Udi Dahan is the real deal.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
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“Udi, one of the great minds in this area. A man I respect immensely.”
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“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.”
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“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.”
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I highly recommend consulting with Udi when it comes to large scale mission critical systems in any domain.”
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I am honored to have Udi as a colleague and a friend (plus having his cell phone on my speed dial).”
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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.”
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“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.”
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It was a distinct pleasure and a unique opportunity to learn from someone who is among the best at what he does.”
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“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.”
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“ 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.”
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“Udi has provided us with guidance in system architecture and supports our implementation of NServiceBus in our core business application.
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.”
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“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.”
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“ Awesome. Just awesome.
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!”
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“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.”
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“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.”
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“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.”
Ray Houston, Director of Development at TOPAZ Technologies
“Udi's SOA class made me smart - it was awesome.
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.”
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“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.”
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“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.”
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