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Archive for the ‘Web Services’ Category



Service-Oriented API implementations

Monday, December 10th, 2012

gearsIt’s quite common for our systems to need to expose an API for external parties to call that isn’t exactly aligned with our service boundaries – at least, when you follow the “vertical services” model rather than the “layered services” approach. I’ve blogged many times about the problems with layering, so I won’t go into that now beyond to say that you really, REALLY, should avoid it.

A short intro to SOA, done right

In the “vertical services” approach I espouse, you often see components from multiple services deployed to any given endpoint. While these services usually don’t need to communicate with each other at all, occasionally you’ll see them leaving “breadcrumbs” behind for each other – things like UserId or OrderId in the session. What’s especially important is that these IDs are accessible even before the entity is finalized – this enables each service to collect its own data without needing any other service to know about that data.

In an ecommerce environment, we would see one service owning money, another the product catalog (excluding prices, those would be owned by the previous service), another service owning the customers’ payment info (credit cards, etc), and yet another owning shipping addresses – all of these separate from the one that owns the shopping cart. Let’s call these Finance, Catalog, Payment, Shipping, and finally Shopping – just so that we have something to reference later.

The API

While we can do all sorts of cool browser composition with the UI in our own system, enabling each service to collect and display its own information, if we want to expose an API for clients to call, we wouldn’t want to force those clients to have to make a separate call to each service in order to make a purchase. Instead, we’d want something that looks like:

MakePurchase(Guid orderId, Dictionary cart, CreditCardInfo cc, Address shippingAddress)

In case you were wondering, the service which owns the definition of this API is different from all of the above services – it is a service that is primarily technical in nature and is responsible for things like integration and data transformations. I call this service IT/Ops.

Getting the data from the API to the services

So, we don’t want any of our business-centric services to know about anybody else’s data structures, so that leaves it to IT/Ops to pass the data to them. The thing is that we still want to do that in the most loosely-coupled way possible – with messaging being a good candidate for that.

So, what we’ll do is have IT/Ops send a message containing the data to the other services, but with a slight twist.

Here’s what the code would look like with NServiceBus:

Bus.SendLocal(new Order { Id = orderId, Cart = cart, 
                          CreditCard = cc, ShippingAddress = shippingAddress });

Why the SendLocal?

So that the components from the other services can all run together with IT/Ops in the same process giving a nice tight deployment model.

UPDATE:

If you’re using a transport like RabbitMQ that is set up as a remote broker, the overhead of going back through the broker might not be worth the improved reliability you’d get by going through messaging. In that case, you might want to consider the Domain Events approach. This would give you a similar level of decoupling but then IT/Ops would need to set up an surrounding transaction, well, that is if you want all the services processing to succeed/fail as one unit. If you don’t, you’d probably be better off just sending messages from IT/Ops to endpoints that host each of the components of the other services.

End Update

Before we get to the services, let me show you the Order class – specifically, the interfaces it implements:

public class Order : ShoppingOrder, PaymentsOrder, ShippingOrder
{
    public Guid OrderId { get; set; }
    public Dictionary<Guid, int> Cart { get; set; }
    public CreditCardInfo CreditCard { get; set; }
    public Address ShippingAddress { get; set; }
}

public interface ShoppingOrder
{
    Guid OrderId { get; set; }
    Dictionary<Guid, int> Cart { get; set; }
}

public interface PaymentsOrder
{
    Guid OrderId { get; set; }
    CreditCardInfo CreditCard { get; set; }
}

public interface ShippingOrder
{
    Guid OrderId { get; set; }
    Address ShippingAddress { get; set; }
}

Each of the above interfaces represents the data that each service cares about. Therefore, each service will provide an assembly that handles that message/data and persists it to its database, like this:


public class ShoppingAPIHandler : IHandleMessages<ShoppingOrder>
{
    public void Handle(ShoppingOrder message)
    {
        //persist to shopping service db
    }
}

public class PaymentsAPIHandler : IHandleMessages<PaymentsOrder>
{
    public void Handle(PaymentsOrder message)
    {
        //persist to payment service db
    }
}

public class ShippingAPIHandler : IHandleMessages<ShippingOrder>
{
    public void Handle(ShippingOrder message)
    {
        //persist to shipping service db
    }
}

Since the Order object being sent is a polymorphic match for all of these interfaces, NServiceBus knows to invoke all of these handlers. By the way, if you care about the order of invocation, then you can control that as well (but I won’t get into that here).

Also, since all of these handler assemblies are deployed to the same endpoint, and the Order object is sent just once, this means that all the handlers will be invoked in a single transaction on a single thread – either all of them succeeding, or all failing. Since they’re all connected on the same Order Id, referential integrity can be preserved as well.

Wrapping up

When you are building a system on SOA principles, you’ll often find that you need a service like IT/Ops to handle data transformation and other broker-centric tasks. While much of SOA is based on the Bus Architectural Style – meaning primarily publish/subscribe interaction between services – that doesn’t mean that your business-centric services cannot have their components deployed in the same process.

I’d go so far as to say that if you aren’t deploying components from multiple services in process with each other at least some of the time then it’s quite likely that your service boundaries are probably incorrect.

Anyway, I hope you found this post interesting. Shout out to Slawek who gave me the idea for this post.

By the way, if you’d like to learn more about these kinds of patterns, the next batch of courses is open for registration – but the early bird prices are almost over, so you’d better hurry.

Register for   Denver CO, USA,     Bad Ems, Germany,     Perth, Australia.



JPA with REST, OData, and SQL

Sunday, November 4th, 2012

Feeling a little bit rant-y today, as I just saw some more abuse of remote calls, this time on the Java side of things.

JPA is the Java Persistence API – a kind of ORM, as you’d expect. Luckily, a lot of the web services stuff was already on the way out by the time that EclipseLink DBWS came out. DBWS allowed you to expose database artifacts as web services.

I mean, it’s not like we have any other interoperable ways of accessing data, right?

Anyway, like I said, that didn’t take off, but now they’re reinventing it – this time with REST!

In case you had any doubts, REST is pure awesomeness and adding it to anything else makes it awesome too. Lest anybody take this out of context (it’s happened before), I’m being sarcastic.

Here it is.

God knows they couldn’t let Microsoft totally dominate this area with OData coming out quite some time ago. In case you were wondering, OData was designed to provide standard CRUD access of a data source over HTTP.

Of course, none of these support any transactions so if you actually wanted to do some meaningful business logic on top of this CRUD, you wouldn’t have any consistency. And, let’s face it, if you’re not doing any meaningful business logic, just basic persistence, you just do it. That problem’s been solved a long time ago.

Can we please stop reinventing SQL already?



Bandwidth, Priority, and Service Contracts

Monday, August 20th, 2012

contractHere’s a small quick tip that can help you improve the performance of important use cases in your systems. It doesn’t require very many changes to your code and can improve matters when your system is under load but won’t make much of a difference when you have capacity to spare.

This is something I talk about in the first day of my course when going through the fallacies of distributed computing – specifically fallacy #3 which talks about bandwidth.

What about bandwidth?

When it comes to network bandwidth in your datacenter, there’s a pretty good chance you’re still on gigabit ethernet. When most developers hear that prefix, “giga”, there is an instantaneous translation in their brain to “so much I don’t need to worry about it”.

The thing is that it’s GigaBIT, not GigaByte, so we’re talking about 128 MBps.

Also, keep in mind that hardly anybody programs at the level of ethernet, we’re several layers up the stack. You can expect roughly 40% the bandwidth of ethernet up in TCP land due to its collision detection, exponential backoff, etc. So that’s roughly 50MBps, not counting overhead for serialization (which can be very significant if it’s text-based like XML or JSON).

In practice, you might be getting something like 25MBps – definitely not so much that you don’t even need to think about it.

Everybody’s talking scalability in terms of number of servers and memory, storage, CPU per server – but what about the network? More importantly, what happens when (not if) you run out? Well, the latency of your calls increase – and that can be quite substantial.

Business Priority

And now we get to the crux of the matter.

Consider a “Customer” web service with a bunch of methods on it, including these two: GenerateTopCustomersByRegionReport and MarkCustomerAsFraud.

Now, your system is under significant load and there’s just enough bandwidth left for one call to make it across the network without timing out. Two users invoke the functionality above – one doing the report, the other doing the fraud. Should the fact that one user clicked the button a millisecond before the other mean that the MarkCustomerAsFraud should be delayed to the point of failure?

I’m fairly sure that if we asked a business stakeholder the answer would be a clear no.

While we could try asking the network engineers to give higher priority to that webmethod, let’s face it, that’s never going to happen. Since both methods are on the same web service, clients are bound to the address where that web service is hosted, regardless of which methods they call.

The problem with “the” network

If there’s one word I loathe in the English language, it’s the word “the”.

Such a small word, tacked on in front of so many other words that, without you even noticing it, traps your mind into thinking you can have only one of that thing.

The network.
The database.

But you CAN have more than one – it’s YOUR system. Design it however you like.

Most servers (if not all) can have more than one network card these days. And even if yours couldn’t – you can ask your network engineers to set up multiple virtual networks on top of the physical network and divide up the bandwidth between them.

Putting it together

The next step is to simply put the MarkAsFraud method on a different web service. This way, you can decide at deployment time which web service should be hosted over which network.

When your system is under load, you will then be guaranteed that even if there are a large number of low priority calls being invoked, they will not use up the network bandwidth reserved for your higher priority calls. You will likely still need to take care of processing and other IO concerns on your servers, but if worst comes to worst, you can partition your server farm as well.

While this may sound a bit CQRS-ish but it would be more accurate to say that CQRS is a more specific case of this pattern – that of partitioning the API according to business priority.

One of the interesting things about messaging is that we tend to forgo the traditional “service contracts” where many methods are put together on a single “service”. Instead, each message definition stands on its own and can be routed to any destination.

In summary

If you are still using WCF and web services, be aware that these apparently little things can have an impact on how your system behaves under load. Even if you do use MSMQ under WCF, the traditional service contract made of multiple methods will still govern your routing.

If you do go all the way with this pattern, you’ll see that each of your service contracts ends up with only one method on it. This might make you wonder what’s the point of the whole service contract thing in WCF – that would be a very good issue to resolve.

Remember the first rule of remote communication – Don’t.



Service Boundaries Aren’t Process Boundaries

Sunday, July 3rd, 2011

boundariesRichard Veryard blogged about the topic of service boundaries in SOA, specifically asking why aren’t more people talking about service boundaries – especially if they’re such a core principle in SOA.

I can only speak for myself on this one, but I guess it’s that there’s just so many times you can repeat yourself.

So, why this post?

Well, Richard was able to dig up an old (2004) presentation I gave about SOA in which I said:

“Services run in a separate process from their clients
A boundary must be crossed to get from the client to the service – network, security, …”

And 7 years later I can say, hand on heart, I was wrong.

Luckily, I’ve spent much of those past 7 years trying to correct that recommendation. One blog post in which I tried to do that (in mid-2007) was On Intermediation and SOA in which I described the relationship between systems (i.e process boundaries) and services:

“all of these “systems” might just end up within the same service, or having parts of them being used by multiple services

There can also be multiple services (or, more accurately, parts of multiple services) deployed together in the same system/process.

And this is nothing new – in the 4+1 Architectural View Model by Philippe Kruchten (1995) we can see very clearly the differentiation between the Logical View (our services) and the Physical View (a.k.a the Deployment View).

These views are orthogonal to each other – multiple elements from one view can map to a single element in another view (and vice versa).

This, if anything, makes it that much harder to identify service boundaries – if they have nothing to do with the existing applications and systems, then what are they? In my blog post on The Known Unknowns of SOA I point to the fact that Business Capabilities are much more appropriate constructs than, say, web services which (as it says in the referenced post) “[are] merely a standardized approach to accessing functionality on remote systems”.

As I bring this post to a close, I’m feeling more comfortable rehashing material I’ve published before:

Logical and Physical Architecture

and the rest of the SOA category on my blog here.

Happy boundary hunting.



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 – .NET Service Bus Smackdown

Wednesday, August 4th, 2010

I get this question quite often: “what is the difference between NServiceBus and the .NET Service Bus from Microsoft?” And I’m afraid the answer is that the two technologies were designed to handle a very different set of problems.

The .NET Service Bus was designed to bridge internet communications using the cloud to enable a variety of devices to communicate using a WCF-remote-procedure-call type of API. NServicebus was designed to simplify the design of on-premise distributed systems using reliable messaging.

Still, people seem to want a kind of comparison, so here’s a quick one off-the-top-of-my-head:

Feature .NET Service Bus NServiceBus
Cloud-based messaging Yes No
Internet-spanning communication Yes Yes – via additional gateway process
Lightweight client support Yes Yes – via exposed WCF endpoint
Full duplex communication Yes Yes – not including lightweight clients
Publish / Subscribe support Yes Yes
Interop with non-.Net platforms Yes Yes
Maximum message size 64 KB 4012 KB
Long-running stateful processes Using WF on top Yes
On-premise messaging No Yes
Client can send messages if server is offline No Yes
Poison message detection and dispatching No Yes
Poison messages re-processing No Yes
Subscriptions persist after restart No Yes
Polymorphic message dispatch No Yes
Polymorphic message routing No Yes
Message-driven unit testing No Yes

This is not meant to be an exhaustive list, or even an accurate representation of the relative strengths and weaknesses of the technologies because, as I said, they’re designed for different purposes.

One could even plug the .NET Service Bus into NServiceBus instead of its MSMQ transport to get broad reach where required, and then switching back to the on-premise strengths of MSMQ. The pluggability of NServiceBus makes it easy to swap out almost all implementation components like the subscription storage, transport, authorization mechanisms, containers, etc.

For more information on the .NET Service Bus see the Azure AppFabric page. Juval Lowey also has a nice article on it up on MSDN magazine here.

For more information on NServiceBus see the new Particular.net site. Also take a look at the pages giving you comparisons to WCF and BizTalk.



The Fallacy Of ReUse

Sunday, June 7th, 2009

This industry is pre-occupied with reuse.

There’s this belief that if we just reused more code, everything would be better.

Some even go so far as saying that the whole point of object-orientation was reuse – it wasn’t, encapsulation was the big thing. After that component-orientation was the thing that was supposed to make reuse happen. Apparently that didn’t pan out so well either because here we are now pinning our reuseful hopes on service-orientation.

Entire books of patterns have been written on how to achieve reuse with the orientation of the day.
Services have been classified every which way in trying to achieve this, from entity services and activity services, through process services and orchestration services. Composing services has been touted as the key to reusing, and creating reusable services.

I might as well let you in on the dirty-little secret:

Reuse is a fallacy

Before running too far ahead, let’s go back to what the actual goal of reuse was: getting done faster.

That’s it.

It’s a fine goal to have.

And here’s how reuse fits in to the picture:

If we were to write all the code of a system, we’d write a certain amount of code.
If we could reuse some code from somewhere else that was written before, we could write less code.
The more code we can reuse, the less code we write.
The less code we write, the sooner we’ll be done!

However, the above logical progression is based on another couple of fallacies:

Fallacy: All code takes the same amount of time to write

Fallacy: Writing code is the primary activity in getting a system done

Anyone who’s actually written some code that’s gone into production knows this.

There’s the time it takes us to understand what the system should do.
Multiply that by the time it takes the users to understand what the system should do :-)
Then there’s the integrating that code with all the other code, databases, configuration, web services, etc.
Debugging. Deploying. Debugging. Rebugging. Meetings. Etc.

Writing code is actually the least of our worries.
We actually spend less time writing code than…

Rebugging code

Also known as bug regressions.

This is where we fix one piece of code, and in the process break another piece of code.
It’s not like we do it on purpose. It’s all those dependencies between the various bits of code.
The more dependencies there are, the more likely something’s gonna break.
Especially when we have all sorts of hidden dependencies,
like when other code uses stuff we put in the database without asking us what it means,
or, heaven forbid, changing it without telling us.

These debugging/rebugging cycles can make stabilizing a system take a long time.

So, how does reuse help/hinder with that?

Here’s how:

Dependencies multiply by reuse

It’s to be expected. If you wrote the code all in one place, there are no dependencies. By reusing code, you’ve created a dependency. The more you reuse, the more dependencies you have. The more dependencies, the more rebugging.

Of course, we need to keep in mind the difference between…

Reuse & Use

Your code uses the runtime API (JDK, .NET BCL, etc).
Likewise other frameworks like (N)Hibernate, Spring, WCF, etc.

Reuse happens when you extend and override existing behaviors within other code.
This is most often done by inheritance in OO languages.

Interestingly enough, by the above generally accepted definition, most web services “reuse” is actually really use.

Let’s take a look at the characteristics of the code we’re using and reusing to see where we get the greatest value:

The value of (re)use

If we were to (re)use a piece of code in only one part of our system, it would be safe to say that we would get less value than if we could (re)use it in more places. For example, we could say that for many web applications, the web framework we use provides more value than a given encryption algorithm that we may use in only a few places.

So, what characterizes the code we use in many places?

Well, it’s very generic.

Actually, the more generic a piece of code, the less likely it is that we’ll be changing something in it when fixing a bug in the system.

That’s important.

However, when looking at the kind of code we reuse, and the reasons around it, we tend to see very non-generic code – something that deals with the domain-specific behaviors of the system. Thus, the likelihood of a bug fix needing to touch that code is higher than in the generic/use-not-reuse case, often much higher.

How it all fits together

Goal: Getting done faster
Via: Spending less time debugging/rebugging/stabilizing
Via: Having less dependencies reasonably requiring a bug fix to touch the dependent side
Via: Not reusing non-generic code

This doesn’t mean you shouldn’t use generic code / frameworks where applicable – absolutely, you should.
Just watch the number of kind of dependencies you introduce.

Back to services

So, if we follow the above advice with services, we wouldn’t want domain specific services reusing each other.
If we could get away with it, we probably wouldn’t even want them using each other either.

As use and reuse go down, we can see that service autonomy goes up. And vice-versa.
Luckily, we have service interaction mechanisms from Event-Driven Architecture that enable use without breaking autonomy.
Autonomy is actually very similar to the principle of encapsulation that drove object-orientation in the first place.
Interesting, isn’t it?

In summary

We all want to get done faster.

Way back when, someone told us reuse was the way to do that.

They were wrong.

Reuse may make sense in the most tightly coupled pieces of code you have, but not very much anywhere else.

When designing services in your SOA, stay away from reuse, and minimize use (with EDA patterns).

The next time someone pulls the “reuse excuse”, you’ll be ready.


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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.

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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