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Archive for the ‘ESB’ Category



NServiceBus Videos Online

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.



Integration: How and Where

Friday, April 8th, 2011

integrationOne 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:

  1. 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.
  2. 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
  3. 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:

Distributed Integration

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.



Bus and Broker Pub/Sub Differences

Thursday, March 24th, 2011

differencesOne 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.



Careful with Content-Based Routing

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.



Polymorphism and Messaging

Thursday, January 13th, 2011

polymorphismOne 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.



NServiceBus 2.5 Released

Friday, December 31st, 2010

Just before we usher in the new year, I’m happy to announce the release of NServiceBus version 2.5.

Go to the NServiceBus website

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…

RedPillBluePill

http://www.NServiceBus.com

And have a happy New Year.



The Known Unknowns of SOA

Monday, November 15th, 2010

rumsfeldOne of the better known analysts in the enterprise software area, JP Morgenthal, wrote this post about the relationship between SOA, BPM, and EA. In it he defines SOA as follows:

“SOA is a practice that focuses on modeling the entities, and relationships between entities, that comprise the business as a set of services. This can be done on a small or large scale. Typically, the relationships in this model represent consumer/provider relationships.”

I have some serious concerns about the ramifications of this definition/description.

First of all, when reading “entities”, many people will interpret that to mean the entities found in Entity Relationship Diagrams [ERD] or in Object Oriented Analysis & Design [OOAD]. In both, these entities are identified as the “nouns” of the domain. Examples of these ERD/OOAD-type entities include things like Customer, Order, and Product.

These are almost always the wrong place to start for identifying services in SOA.

Second, on the consumer/provider relationship: on the one had, this fits very well with how web services can consume (or call) other web services. However, the downsides of using web services as services in SOA is becoming well enough known that even in the same post we see this warning:

“Web Services is not SOA, it is merely a standardized approach to accessing functionality on remote systems.”

But the question remains, if a producer/consumer relationship is OK for SOA-type services, why doesn’t that hold for web services? And the answer is… it depends on the type of producer/consumer relationship. The typical relationship is one of synchronous calls from consumer to producer, this is not OK for SOA-type services either.

You see, this synchronous producer/consumer implies a model where services are not able to fulfill their objectives without calling other services. In order for us to achieve the IT/Business alignment promised by SOA, we need services which are autonomous, ie. able to fulfill their objectives without that kind of external help.

Instead, we need to look for a more loosely coupled producer/consumer relationship – like publish/subscribe, where the producer emits events, and the consumer subscribes and handles those events. The reason that this kind of relationship doesn’t hurt autonomy is that it disconnects services on the dimension of time. In order for a service to be able to make a decision autonomously without synchronously calling any other service, using only information provided by events it received in the past, it must be strongly aligned with the business.

Most projects which bandy about the SOA acronym aren’t actually made up of services – they’re made up of XML over HTTP functions calling other XML over HTTP functions, eventually calling XML over HTTP databases. You can layer as much XML and HTTP as you want on top of it, but at the end of the day, most projects are just functions calling functions calling databases – in other words, procedural programming in the large, and no amount of SOAP will wash away the stink.

Here’s a different definition of services for SOA that may communicate a bit better what it’s all about:

A service is the technical authority for a specific business capability.
Any piece of data or rule must be owned by only one service.

What this means is that even when services are publishing and subscribing to each other’s events, we always know what the authoritative source of truth is for every piece of data and rule.

Also, when looking at services from the lense of business capabilities, what we see is that many user interfaces present information belonging to different capabilities – a product’s price alongside whether or not it’s in stock. In order for us to comply with the above definition of services, this leads us to an understanding that such user interfaces are actually a mashup – with each service having the fragment of the UI dealing with its particular data.

Ultimately, process boundaries like web apps, back-end, batch-processing are very poor indicators of service boundaries. We’d expect to see multiple business capabilities manifested in each of those processes.

I know that this may be more confusing than the traditional web services approach but, to paraphrase Donald Rumsfeld, it is better to know that you don’t know, than to not know that you don’t know :-)



Logical and Physical Architecture

Monday, November 8th, 2010

orthogonalOne architectural misunderstanding I see repeatedly in my work with clients is in the relationship between logical and physical architecture. The most common building-block of these misunderstandings is the web service (or it’s “upgraded” .net counterpart – the WCF service).

Don’t get me wrong, sometimes there is a place for a web service, just not everywhere.

So, what’s the problem?

Well, when developers and architects use web services as the building blocks of their designs, they are creating the same architecture for both the logical and physical elements of their system. Back in 1995, Philippe Kruchten documented his 4 + 1 Architectural View Model in which he outlined 4 + 1 different views that should be used to describe an architecture.

Even though since 1995 the number and types of recommended views of software architecture has evolved (with things like the Zachman Framework for enterprise architecture numbering some 30 views), there is broad agreement that (at the very least) the logical and physical artifacts should likely be designed differently.

Just because two distinct logical components have been identified in the architecture, that doesn’t necessarily mean they should be hosted separately (for example by making each one a web/wcf service). In fact, there are significant disadvantages to doing so (as described in the Fallacies of Distributed Computing).

In some cases, this mistake is exacerbated by a mistaking these components with SOA-type services, resulting in an attempt by developers to have each component have its own contract, which can then be independently versioned. This often results in the need for transformation between the structure of these so-called contracts, but not within the components themselves (oh-no, they’re “autonomous”), but rather in between them using some kind of “ESB” technology.

This architectural style is known as the Broker, Hub and Spoke, Mediator, and most importantly – not SOA. If you find a technology that fits this style perfectly (like BizTalk), that technology is not a Bus, not a Service Bus, and definitely not an Enterprise Service Bus.

One of the problems of this approach is that when any “service” contract changes, you have to change all the transformations in your broker that involve it. Unfortunately, most brokers have no unit-testing facility so it’s very much trial and error, and error, and error. The matter is even more serious since most brokers don’t enable you to have your transformations or orchestrations in source control, so you can’t diff to see what changed from the previous version.

It’s really amazing how much pain can be traced back to that one original misunderstanding.



NServiceBus Presentation Now Online

Wednesday, June 9th, 2010

Last April I was in Bergen Norway for some consulting and training and I also gave my first NServiceBus presentation to a user group. I don’t particularly like giving NServiceBus-specific presentations, preferring to talk about the patterns and concepts of service-based architectures and service buses – NServiceBus is just an implementation. Ultimately, that’s what happened in the presentation – in the first half (or so) I talked about the theory, and in the second I demonstrated that theory with NServiceBus.

Currently, the video is being graciously hosted by Jon Torresdal on his blog, so let’s hope that the bandwidth holds up.

Get it here.



ESB Differences Between Java and .NET

Monday, March 29th, 2010

At QCon London a couple of weeks ago I had a chat with Ross Mason, the founder of Mule – the open source Java ESB. After a while, I realized that NServiceBus is a bit different from Mule ESB in terms of scope.

While Mule has many features in terms of connectivity and integration, NServiceBus provides platform interop only. One could say that this is a product of the different backgrounds I and Ross come from.

On the other hand, the saga capabilities in NServiceBus for handling long-running processes are considered to be BPM functionality on the Java side of the industry, and as such, Mule does not have them.

In terms of other enterprise features like management and monitoring, Mule is more mature, but NServiceBus holds its own in terms of high availability and actually surpasses Mule with the grid and scale-out capabilities of its distributor.

Anyway, I think it’s about time each of these parts was explicitly described so that companies already invested in Java ESB tools will know what they’re getting with NServiceBus.

Until then, I hope this podcast describing the full spectrum of NServiceBus, from top level SOA services to in-the-weeds transaction management, will provide more information about what it is and why you might want to use it:

Deep Fried Bytes, episode 49 – Getting the right message about NServiceBus with Udi Dahan.

Comments most welcome.



   


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“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.

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