Udi Dahan   Udi Dahan – The Software Simplist
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Archive for the ‘Autonomous Services’ Category

Service-Oriented Composition (with video)

Wednesday, July 30th, 2014

When telling people about my approach to SOA, in which a given service would have client/browser-side components running side-by-side in the same process and even in the same page as components from other services, I often get asked this question:

“Doesn’t all of this loosely-coupled composition come with a high cost, in terms of client to server chit-chat?”

So, I’ve finally buckled down and put together a slide to illustrate how the technocratic IT/Ops service I’ve talked about in the past can provide components to resolve these sorts of problems.

After putting the slide together, and realizing some animation would do it good, I went and made a short (5 min) video including some verbal explanation as to how it all works – just for clarity. Check it out or watch it here:

And here’s the image showing everything in one picture:

Service-oriented composition

People, Politics, and the Single Responsibility Principle

Monday, May 26th, 2014

PeopleIn one of Uncle Bob’s recent blog posts on the Single Responsibility Principle he uses the example of using people and organization boundaries as an indication of possible good software boundaries:

When you write a software module, you want to make sure that when changes are requested, those changes can only originate from a single person, or rather, a single tightly coupled group of people representing a single narrowly defined business function. You want to isolate your modules from the complexities of the organization as a whole, and design your systems such that each module is responsible (responds to) the needs of just that one business function.

This is something that often comes up when I teach people about service boundaries when it comes to SOA – organization boundaries are the most intuitive choice.

And, once up on a time, that intuition might have indeed held up.

Stepping back in time

In the age before computers, organizations had a very specific way of structuring themselves.

People who had to work closely together sat in close physical proximity to each other. Data that was required on an ongoing basis would be in file cabinets also physically co-located with the people using that data, and it would be structured in a way that was optimal for their specific purposes. All of this was due to the high cost of communicating with people farther away.

If you needed data from a different department, you had requisition it by filling out a special form, put it in your outbox, and then some guy from the mail room would pick it up, and physically schlep it to the right department, putting it in their inbox, and then someone there would get your data for you – putting it together with your original request, and then the mail guy would schlep it back. This inbox/outbox style of communication should ring a bell from the messaging patterns I talk about with NServiceBus.

As a result, different departments had to have very clearly delineated responsibilities with minimal overlap with each other. The organization just couldn’t function any other way.

And then a bunch of us geeks came along.

Enter the age of computers and networks

By introducing this technology, the cost of communication across large distances started falling – slowly at first, and then quite dramatically.

When anyone in an organization was able access data from anywhere in the blink of an eye, an interesting dynamic started to unfold. All of a sudden, the division of responsibility between departments wasn’t as critical as it was before. When an employee needed to do something, there wasn’t this “that isn’t our job, you need to go to so-and-so” reaction. Because things could be done instantly, that’s exactly what happened.

And then came the politics

By removing the cost of communication, it became possible for more power-hungry people in the organization to start making (or trying to make) decisions that they couldn’t have made before. The introduction of computers into an organization was heralded as a new way of doing business – that the old organizational boundaries were a relic that we should leave behind us.

And thus can the re-org (the first of many).

Responsibilities and people were shuffled around, managers vied for more power, and politics took its’ place as one of the driving forces in the company structure.

Nowadays, if you want a decision made in a company, there isn’t just one person who has the authority to sign off on it anymore. No, you need to have meetings – and more meetings, with people you never knew existed in the company, or why on earth they should have a say on how something is supposed to get done. But that is now our reality: endlessly partially overlapping responsibilities across the organization.

So, what of the Single Responsibility Principle

This just makes it that much harder to decide how to structure our software – there is no map with nice clean borders. We need to be able to see past the organizational dysfunction around us, possibly looking for how the company might have worked 100 years ago if everything was done by paper. While this might be possible in domains that have been around that long (like banking, shipping, etc) but even there, given the networked world we now live in, things that used to be done entirely within a single company are now spread across many different entities taking part in transnational value networks.

In short – it’s freakin’ hard.

But it’s still important.

Just don’t buy too deeply into the idea that by getting the responsibilities of your software right, that you will somehow reduce the impact that all of that business dysfunction has on you as a software developer. Part of the maturation process for a company is cleaning up its’ business processes in parallel to cleaning up its’ software processes.

The good news is that you’ll always have a job :-)

On that Microservices thing

Monday, March 31st, 2014

antsSeems that I’m a bit late to the Microservices party – original article here.

But since I’ve been getting repeated requests to weigh in on the topic, I guess I’ll have to risk fanning the flames up again.

Also, since quite a few reactions have already been written on the topic (and I don’t want to repeat them here), I’ll just point to this post by Arnon which sums them all up pretty well.

Now, I don’t entirely agree with all the commentary Arnon pointed to, or all of his thoughts on the topic, but I’ll try to take those up some other time.

And before jumping into it, let me say that there is a lot of good stuff in the article and that, regardless of naming, spreading the word more broadly on these approaches has value.

So, where do I stand on the topic

First of all, for those of you who have been following my blog for a while I’d say this:

Microservices almost equals Autonomous Components.

Why “almost”?

Because an Autonomous Component (AC) isn’t necessarily a physical unit of deployment – very often we’ll see multiple ACs deployed in the same physical process. One of the most common occurrences is in a web front end built as a composite UI. In the same web server process we’ll see components from multiple Services.

This is something that was hardly mentioned in the original article.

On Services and Systems

In my world, Services are a larger organizing principle that are meant to align solution domain boundaries with problem domain boundaries.

Now, that might sound very similar to this passage from the original article:

“The microservice approach to division is different, splitting up into services organized around business capability. Such services take a broad-stack implementation of software for that business area, including user-interface, persistant storage, and any external collaborations. Consequently the teams are cross-functional, including the full range of skills required for the development: user-experience, database, and project management.”

Now, this isn’t entirely surprising because I did have several conversations with both James and Martin on the topic over the past couple of years.

Still, there is something important missing here that I believe is very important to achieve loose-coupling, and that is that Services necessarily have to span system boundaries.

Let me repeat that: a Service will need to have components that are deployed to more than one system.

Here’s why:

Let’s say you have a piece of data like the price of a product. Not only will that data be visible in one system, often it will need to be shown (as well as updated) in other systems too. In order to have appropriate encapsulation of that concept, the owning service will need to be the one that owns the components that operate on that concept in the other systems.

This means that if we need to show the product price on an invoice in a back-end system, then that invoice would have to be a composite UI as well, and the service which owns the price will have a component deployed there which would be responsible for showing the price on the invoice.

In this manner, no code outside the service boundary would know about the concept of the product price and thus could not end up coupled to it.

Although the original article does get into this to some degree (when talking about Decentralized Data Management), I don’t really see how a microservice/AC could end up having this level of ownership of data.

Still, the point made about different persistence technologies is valid at the level of services (though not ACs).

How big is a service

Now, if the price is not shared outside the boundary of the service, then how would order totals be calculated?

The answer is that the totals must (MUST) be calculated in the same service.

This shouldn’t be surprising as it’s just good old OO – encapsulating data with the logic that operates on it. Or, if you’d like, call it the Single Responsibility Principle: there should only be a single service impacted by a change to the definition of this data.

As a result, you’ll tend to see services that aren’t all that small, and probably not so many of them. In my experience, I’ve seen between 7 and 15 services the majority of the time.

Cross-service collaboration

Although I am glad to see the recommendation for event-based interactions between microservices, the focus on cross-process communication ignores some extremely important collaboration scenarios – the most important of which is in the client tier.

In a web application, it is quite common to have components written in javascript from multiple services interacting among themselves in the browser – one publishing events, others subscribing to those JS events. It is also quite common to see those JS components request some data from back-end components in the same service in response to those JS events.

This type of synchronous RPC communication within a service boundary is perfectly acceptable, although it stands in contrast to the recommendations of the microservices approach.

Caveat on sharing data

I’ve been going on and on about the importance of not sharing data, but there is one exception to that rule.

There is a special service that I call IT/Ops which (among other things) is responsible for integration with 3rd party systems. As a part of this integration, it encapsulates data transformation logic and, as a result, needs to be able to receive data from the other more business-centric services.

As you can imagine, this puts IT/Ops in the risky position of coupling itself to a lot of things and thus needs to be done carefully. As a result, I recommend that many of the most skilled technical people work within the IT/Ops team, also serving in a consultative capacity to the other service teams.

In closing

I am extremely thankful to Martin and James for writing the Microservices article.

I think that the conversations it has sparked are timely, and hopefully more people will ponder these questions of how to structure their code-bases in order to avoid them becoming monolithic.

And while I think that it’s great to consider aligning team boundaries with service boundaries, people need to understand that it will need to be an evolutionary process – it will take time to transition an existing code base and an existing team structure to this new model, especially since these teams will have to continue to deliver features and bug fixes through the transition period. Jumping to the new model directly may cause more harm than good.

This is actually one of the most salient topics of my course (next one in NYC in May) – how do you get there from here. In my experience there are 4 phases that companies go through, often taking at least a couple of years, with larger environments taking potentially longer.

In any case, let’s keep the conversation going.

What are your thoughts? Have you been applying the Microservices approach, or possibly the one I talk about (I really should give it a name). What’s been working well for you? What hasn’t?

Leave me a comment or write your own blog post.

Data Duplication and Replication

Tuesday, August 28th, 2012

agent_smith_replicationOccasionally I’ll get questions from people who have been going down the CQRS path about why I’m so against data duplication. Aren’t the performance benefits of a denormalized view model justified, they ask. This is even more pronounced in geographically distributed systems where the “round-trip” may involve going outside your datacenter over a relatively slow link to another site.


As his been said several times before by many others, it’s not the denormalized view model that defines CQRS.

One of the things that sometimes surprising people after going through my course is that in most cases you don’t need a denormalized view model, or at least, not the kind you think. Yes, that’s right: MOST cases.

But I don’t want to get too deep into the CQRS thing in this post – that can wait.


The big thing I’m against is raw business data being duplicated between services.

Data that can be expected to be accessible in multiple services includes things like identifiers, status information, and date-times. These date-times are used to anchor the status changes in time so that our system will behave correctly even if data/messages are processed out of order. Not all status information necessarily needs to be anchored in time explicitly – sometimes this can be implicit to the context of a given flow through the system.

For example, the Amazon.com checkout workflow.

In that flow, if you provide a shipping address that is in the US, you are presented with one set of options for shipping speed, whereas an international address will lead you to a different set of options.

Assuming that the address information of the customer and the shipping speed options are in different services, we need to propagate the status InternationalAddress(true/false) between these services in that same flow. In this case, there isn’t a need to explicitly anchor that status in time.

But what’s so bad about duplication of data between services?

The danger is that functionality ultimately follows raw business data.

You start with something small like having product prices in the catalog service, the order service, and the invoice service. Then, when you get requirements around supporting multiple currencies, you now need to implement that logic in multiple places, or create a shared library that all the services depend on.

These dependencies creep up on you slowly, tying your shoelaces together, gradually slowing down the pace of development, undermining the stability of your codebase where changes to one part of the system break other parts. It’s a slow death by a thousand cuts, and as a result nobody is exactly sure what big decision we made that caused everything to go so bad.

That’s the thing, it wasn’t viewed as a “big decision” but rather as just one “pragmatic choice” for that specific case. The first one excuses the second, which paves the way for third, and from that point on, it’s a “pattern” – how we do things around here; the proverbial slippery slope.

So what’s with the word “Replication” in the title of this post?

While data duplication between services is very dangerous, replication of business data WITHIN a service is perfectly alright.

Let’s get back into multi-site scenarios, like a retail chain that has a headquarters (HQ) and many stores. Prices are pushed out from the HQ and orders are pushed back from the stores according to some schedule.

We know that we can’t guarantee a perfect connection between all stores and the HQ at all times, therefore we copy the prices published from the HQ and store them locally in the store. Also, since we want to perform top-level analytics on the orders made at the various stores, that would be best done by having all of those orders copied locally at the HQ as well.

We should not view this movement of data from one physical location to another as duplication, but rather as replication done for performance reasons. If there were some magical always-on zero-latency network that existed, we wouldn’t need to do any of this replication.

And that’s just the thing – logical boundaries should not be impacted by these types of physical infrastructure choices (generally speaking). Since services are aligned with logical boundaries, we should expect to see them cross physical boundaries – this includes SYSTEM boundaries (since a system is really nothing more than a unit of deployment).

I know that you might be reading that and thinking “What!?” but there isn’t enough time to get into this in any more depth here. You can read some of my previous posts on the topic of SOA for more info here.

Cross-site integration without replication

There are some domains where sensitive data cannot be allowed to “rest” just anywhere. Let’s look at a healthcare environment where we’re integrating data from multiple hospitals and care providers. While all of these partners are interested in working together to make sure that patients get the best care, which means that they need to share their data with each other, they don’t want any of THEIR data to remain at any partner sites afterwards (and are quite adamant about this).

In these cases, the decision was made that performance is less important than data ownership. Personally, I don’t agree with this mindset. The fact that data is “at rest” in a location as opposed to “in flight” does not change ownership. It could be stored in an encrypted manner so that only a certain application could use it, resulting in the same overall effect, but this is an argument that I’ve never won.

People (as physical beings) put a great deal of emphasis on the physical locations of things. It’s understandable but quite counterproductive when dealing with the more abstract domain of software.

In closing

By virtue of the fact that we don’t duplicate raw business data between services, that means that the regular data structures inside a service already look very different from what they would have looked like in a traditional layered architecture with an ORM-persisted entity model.

In fact, you probably wouldn’t see very many relationships between entities at all.

Going beyond that, you probably wouldn’t see the same entities you had before. An Order wouldn’t exist the way you expect; addresses (billing and shipping) would be stored (indexed by OrderID) in one service whereas the shipping speed (also indexed by OrderId) would be in another, and the prices may well be in yet another.

It is in this manner that data does not end up being duplicated between services, but rather is composed by many services whether that is in the UI of one system, the print-outs down by a second system, or in the integration with 3rd parties done by a third system.

If performance needs to be improved, look at having these services replicate their data from one physical system to another – in-memory caching is one way of doing this, denormalized view models might be though of as another (until you realize there isn’t very much normalization within a service to begin with).

And a word from our sponsor :-)

For those of you on “rewrite that big-ball-of-mud” projects looking to use these principles, I strongly suggest coming on one of my courses. The next one is in San Francisco and I’ve just opened up the registration for Miami.

For those of you on the other side of the Atlantic, the next courses will be in Stockholm in October and in London this December.

The schedule for next year is also coming together and it will include South Africa and Australia too.

Anyway, here’s what one attendee had to say after taking the course earlier this month:

I wanted to thank you for the excellent workshop in Toronto last week. I spent the better part of the weekend reflecting over what was presented, the insights we learned through the group exercises, and how my preconceptions of SOA have changed. By the end of the course, all the tidbits of (usually) rather ambiguous information that I’ve collected from various blogs, books, and other sources, finally coalesced into something more intelligible – one big A-HA moment if you will. Overall, I found the content of the workshop to be incredibly enlightening and it left me feeling invigorated and excited to learn more.
- Joel from Canada

Hope you’ll be able to make it.

If travel is out of the question for you, you can also look at get a recording of the course here.

One final thing

If your employer won’t foot the bill for these, please get in touch with me.
I wouldn’t want you not to be able to come just because you’re paying out of pocket.

There are very substantial discounts available.

Contact me.

UI Composition vs. Server-side Orchestration

Monday, July 9th, 2012

orchestra_compositionFollowing on my last post called UI composition techniques for correct service boundaries, one commentor didn’t seem to like the approach I described saying:

“I’m sorry, but with all due respect I must strongly disagree. You haven’t avoided any orchestration work at all, you’ve just moved it in to client side script!

How are you going to deal with the scenario that one of the service calls fails? Say a failed credit card payment, or no more rooms left? In more javascript??

I would much rather take the less brittle approach of introducing an orchestration service. Like it or not, however trivial it may be, there is a relationship between these services, if one call fails, they both fail. This should be reflected in the architecture, not hidden in javascript. With an orchestration service you also either get transactions for free provided by infrastructure, or alternatively if the underlying service doesnt support this, explicit and unit testable control over recovery.”

Since this is a common point of view, I thought I’d take the time to explain a bit more.

Let’s start at a fairly high level.

On failures

I’ve talked many times in the past about how to handle technical causes for failure like server crashes, database deadlocks, and even deserialization exceptions. Messaging and queuing solutions like NServiceBus can help overcome these issues such that things don’t actually fail – they just take a little longer to succeed.

On the logical side of things, the CQRS patterns I talk about describe an approach where aggressive client-side validation is done to prevent almost all logical causes for failure. The only thing that can’t be mitigated client-side are race conditions resulting in actions taken by other users at the same time.

In short, it really is uncommon for things to fail when being processed server-side.

Back to the specific example

The concerns raised in the comment specifically talked about a failed credit card payment or no rooms left in the hotel, so let’s start with the credit card thing:

In my last post I talked about collecting guest and credit card information from the user as a part of the “checkout” process when making a reservation for a hotel room. Just to be clear – there is a final “confirm your reservation” step that happens after all information has been collected.

What this means is that we aren’t actually charging the customer’s card when we collect that data, therefore there is no real issue with a failed credit card payment that needs to be handled by the client-side javascript. When the customer confirms their reservation, yes, there might be a failure when charging the card though there are only some specific types of rates for which the hotel charges your card when you make a reservation.

In general, failed credit card payments are handled pretty much the same way for all ecommerce – an email is sent to the customer asking for an alternative form of payment, also saying that their purchase won’t be processed until payment is made.

In any case, it is only after the reservation is placed that the responsible service would publish an event about that. The service which collected the credit card information would be subscribed to that event and initiate the charge of the card when that event arrives (or not, depending on the rate rules mentioned).

With regards to there not being any rooms left, well, first of all, there’s overbooking – hotels accept more reservations than rooms available because they know that customers sometimes need to cancel, and some just don’t show up. Secondly, there is a manual compensation process if more people show up than there are actual rooms to put them in. In some cases, a hotel will bump you up to a higher class of room (assuming there aren’t too many reservations for those), and in others they will call a “partner” hotel nearby and put you up there instead.

In summary

While arguments can be made that yes, these issues have been addressed in this specific example, there may be other domains where it is not possible to do these kinds of “tricks”. Although I do agree with that in theory, I’ve spent the better part of 5 years travelling around the world talking to hundreds of people in quite a few business domains, and every single time I’ve found it possible to apply these principles.

In short, the use of UI composition allows services to collect their own data, making it so anything outside that service doesn’t depend on those data structures which makes both development and versioning much easier. Technical failure conditions can be mitigated at infrastructure levels in most cases and other business logic concerns can be addressed asynchronously with respect to the data collection.

Give it a try.

UI Composition Techniques for Correct Service Boundires

Saturday, June 23rd, 2012

PrismOne of the things which often throws people off when looking to identify their service boundaries is the UI design. Even those who know that the screen a user is looking at is the result of multiple services working together sometimes stumble when dealing with forms that users enter data into.

Let’s take for example a screen from the Marriott.com online reservation system (below). This screen collects information about the guest staying at the hotel (name, phone number, address, etc) and credit card information.


While we might have wanted to keep guest information in a separate service from the credit card information (which may very well be the corporate card of someone responsible for travel), the above screen would seem to indicate that the data would be collected together, validated together, and would also have to be processed together.

The traditional way

In standard layered architectures you would have all the data submitted by the user passed in a single call from a controller to some “service layer” (possibly running on a different machine), which would then persist that data in one transaction.

Even if some attempt was made to separate things out, there likely would be some “orchestration service” that received the full set of data and it would make calls to the other “services”, passing in the specific data that each “service” is responsible for.

I am putting quotes around the word “service” to indicate that I don’t consider these proper services in the SOA sense (as they lack the necessary autonomy) – they are more like functions or procedures, whether or not they’re invoked XML over HTTP is besides the point.

What to do?

Like so many other things, the solution is simple but a bit counter-intuitive as it doesn’t follow the way most web development is done, i.e. one submit button => one call to the server.

Let’s say the “Red” service is responsible for guest information and the “Blue” service is responsible for credit card data. In this case, each service would have its own javascript come done with the page and that script would register itself for a callback on the click of the submit button. Each service would take the data the user entered into its part of the page and independently make a call to “the” server (could be to 2 separate servers) where the data is persisted (potentially to 2 different databases).

This raises other questions, of course.

Now that the data submitted is being processed in 2 transactions rather than just one, we may need to figure out how to correlate the data. In this specific case, it’s not such a big deal as there is no direct relationship between the guest and the credit card – both need to be independently correlated to some reservation ID.

That reservation ID would likely have been “created” on a button click on a previous screen by some other service. The reason why I put the word “created” in quotes is that this could be as simple as having the client generate a new GUID and put that in a cookie (which would cause the reservation ID to end up being submitted along with subsequent requests). Another alternative would be to put the reservation ID in the session.

It’s quite possible that the reservation ID would only be persisted much later in the service that owns it when the user actually confirms the reservation on the website.

In any case, what we can see is that each of the commands of our respective services can now be processed independently of the others in an entirely asynchronous fashion thus vastly improving the autonomy of our services.

Some words on CQRS

This style of UI composition where services leverage javascript code running in the browser isn’t technically difficult in the slightest. The rest of the implementation of each service – having a controller that takes that data and passes it on for persistence can be quite simple.

I’d say even more strongly, most of the time you shouldn’t need to use any fancy-dancy messaging to get that data persisted – that is, unless you’re still stuck with the big relational database behind 23 firewalls type data tier. Embrace NoSQL databases for the simplicity and scalability they provide – don’t try to re-invent that using messaging, CQRS, persistent view models, event-sourcing, and other crap.

There are other very valid business reasons to embrace CQRS, but they have nothing to do with persistence.

Also notice, this is all happening within a service boundary / bounded context.

In closing

If you aren’t leveraging these types of composite UI techniques, it’s quite likely that your service boundaries aren’t quite right. Do be aware of the UI design and use it to inform your choices around boundaries, but be aware of certain programming “best practices” that may lead you astray with your architecture.

Also, if you’re planning on coming to my course in Toronto to learn more about these topics, just wanted to let you know that there’s one week left for the early-bird discount.

Finally, it’s good I have a birthday that comes around once a year to remind me that my time here isn’t unlimited and that I had better get off my rear and do something meaningful with the time I do have. If you get value from these posts, leave a comment or send me a tweet to let me know – it does wonders for my motivation.

Thanks a bunch.

Why you should be using CQRS almost everywhere…

Sunday, October 2nd, 2011

grass… but differently than the way most people have been using it.

I think I’ve just about drove everybody crazy now with my apparent zigzagging on CQRS.

Some people heard about CQRS first from one of my presentations and got all excited about it. Then I did some blogging which further drove people to CQRS (as did Greg Young and some others). As CQRS was just about to hit its stride with the Early Adopters, I started pushing a more balanced view – CQRS not as an answer, but as one of many questions. More recently I’ve pushed more strongly back against CQRS saying that it should be used rarely.

So what’s the missing piece?

If you’re in the Domain-Driven Design camp (as many doing CQRS are), then it’s Bounded Contexts.

If you’re in the Event-Driven SOA camp (a much smaller camp to be sure), then it’s Services.

The problem is the naming, because the DDD guys have their kinds of services which do not fit the definition for Service of the Event-Driven SOA approach.

Let me propose the term Autonomous Business Component for the purposes of this blog post to describe that thing which is both a DDD Bounded Context (have the shared BC part of the acronym) and an SOA Autonomous Services. Resulting in the nice short form: ABC (and everyone knows you need to have a good acronym if you want something to catch on).

What does this have to do with CQRS?

Nothing just yet. Well, at least, nothing directly to do with CQRS.

Although some proponents of CQRS have stated that it can and should be used as the top-most architectural pattern, both myself and Greg Young (arguably the first two to talk about it and the two who ultimately collaborated on naming it – and now Google knows we didn’t means “cars”) always recommended it as a pattern to be used one level down.

Although Greg and I have had many long discussions on the topic and do agree very much about what the overall structure should look like, I’ll try to avoid putting words in his mouth from this point on.

Before talking more about ABCs, let’s discuss the principle upon which they rest: The Single Responsibility Principle (SRP).

What does SRP have to with CQRS?

Many developers are familiar with SRP and have seen good results from using it. What we’re going to do is take this principle to the next level.

In Object Orientation (OO), data is encapsulated in an object. A good object does not expose its data to other objects to do with as they wish. Rather, it exposes methods that other objects can invoke, and those methods operate on the internal data.

SRP would guide us to not have the same data exist in two objects. For example, if we saw the customer’s first name as an internal data member of two objects, we’d be right to question that kind of duplication and move to refactor it away. However, when we see two systems doing the exact same thing – somehow that gets excused.

“Of course we need to be able to see the customer’s first name in the front-end website as well as in the back-end fulfillment system. How could we NOT have the customer’s first name in both those code-bases?”

And there’s the catch.

Who said that a system should be a single code-base?

But what about integration?

Although many times we do need to integrate existing systems together, sometimes we have the ability to change those systems. More importantly, when going to create a new solution, we can avoid getting ourselves into the problems that integration tries to solve.

Integrating with a system that cannot be changed can be done also by composing multiple ABCs, but that’s a topic for another post.

It is better to think of integration as a necessary evil – kind of like regular expressions and multi-threading; things to be avoided unless absolutely necessary.

“If you have a problem that you decide to use a regular expression to solve, you now have 2 problems.” Or so the saying goes. With multi-threading, you have a non-deterministic number of problems to solve.

If you thought you had duplicate responsibilities with 2 systems operating on the same data, how will introducing a 3rd code base (also known as “integration”) help? Remember that Single Responsibility Principle – our goal is to get it down to one.

OK, so how do ABCs do that?

In order for us to get back into alignment with SRP, that would require us to have responsibility for a single piece of data exist in one code base. Note that SRP makes no statements about how many physical places a given code base can be deployed to. Nor does it state that only a single technology can be in play – code that emits HTML can be packaged at design time together with rich-client code in the same solution.

If an ABC is responsible for a piece of data, it is responsible for it everywhere, and forever. No other ABC should see that data. That data should not travel between ABCs via remote procedure call (RPC) or via publish/subscribe. It is the ultimate level of encapsulation – SRP applied at the highest level of granularity.

This results in systems which are the result of deploying the components of multiple ABCs to the same physical place. The ABC which owns the customer name would have the necessary web code to render it in the e-commerce front-end and in the shipping back-end for printing on labels. This would mean that practically every screen in any UI is a composite of widgets owned by their respective ABCs.

This is ultimately what keeps the complexity of each ABC’s code base to a minimum.

But why not just use CQRS as the top-level pattern? ABCs are weird.

Imagine trying to create a single denormalized view model for the entire Amazon.com product page – product name, price, inventory, editorial review, customer comments, other products that customers viewed, other products that customers bought, etc.

Pretty complex, right?

How much duplication would you have for the page shown after you add an item to a cart? Once again, you need to show other products that customers bought, their names, images, prices, and inventory.

And then on the home page – items you might be interested in, names, images, prices.

And that’s only in the front-end system.

It’s not just the duplication, but how complex the code is for each one.

Instead of the duplication that top-level CQRS would bring you, consider an ABC responsible for products names and images that has just about the same view model composed on each of the above screens. The same with another ABC responsible for price.

You may be thinking that this would result in more queries to get the data to show on a page, and you’d be right. But it isn’t necessarily a classical N+1 Select problem, as the queries are bounded to the number of ABCs. Secondly, consider the ability to have well-tuned caching at the granularity of an ABC – something that would be much more difficult when dealing with everything as a single monolithic view model. In short, not only will it not be a performance problem, often it will actually improve performance.

OK – that explains “everywhere”, what about “forever”?

Forever is where things get interesting – or more accurately, when they get interesting.

Let’s talk about things like invoices.

One of the requirements in this area is that immutability. If the customer’s name was Jane Smith when they made their purchase, it doesn’t matter that they’ve since changed their name to Jane Jones, the invoice should still show Jane Smith.

Often developers push these types of requirements on the data warehouse guys – that’s where history gets handled. The only thing is that if your ABC owns the customer’s name, then no other code base can deal with it. If it’s your data, you have to handle all historical representations of it.

On the one hand, this would seem to kill the data warehouse. On the other hand, it means that the principles of data warehouses are now core to every code-base.

This means you don’t ever delete data (see my previous blog post on the subject), and you definitely don’t overwrite it with an update – even if you think you’re in a simple CRUD domain. The only case where you can get away with traditional CRUD is if we’re talking about private data – data that is only ever acted on by a single actor.

This sounds like the collaboration you talk about with CQRS

It’s similar in principle but different in practice.

In a collaborative domain, an inherent property of the domain is that multiple actors operate in parallel on the same set of data. A reservation system for concerts would be a good example of a collaborative domain – everyone wants the “good seats” (although it might be better call that competitive rather than collaborative, it is effectively the same principle).

A customer’s name would not fall under that category. It isn’t an inherent property of the domain for multiple actors to operate on that data. While there can be multiple readers, one can easily enforce a single writer without any adverse effects. Doing that with a reservation system would cause the online system to behave as if users were lining up in front of a box office – not a desirable outcome.

Private data would be something like a user’s shopping cart. Until they make a purchase, that data doesn’t need to be visible anywhere. Here you could theoretically do simple CRUD – that is, until the business realizes that there’s extremely valuable information to be extracted from the historical record of things people do with their carts.

I think you’re ready to make your point, so just make it already

OK – so we now realize that Update and Delete don’t exist in their traditional form. Delete is really just a kind of update, and update is effectively an “upsert” – a combination of update and insert to retain history. This can be done by having ValidFrom and ValidTo columns for our data.

In which case, Create is really just a special case of Upsert, which looks like this:

UPDATE Something SET ValidTo = NOW() WHERE Id=@Id AND ValidTo = NULL; INSERT INTO Something SET { regular values }, Id=@Id, ValidTo = NULL;

And then we’d have 2 forms of Read – reading the current state (ValidTo = NULL), and reading history (ValidFrom <= Instant AND (ValidTo >= Instant OR ValidTo = NULL))

Here we don’t need fancy N-Tier architectures, data transfer objects, service layers, or domain models. A simple 2-Tier approach could probably suffice. We don’t need a task-based UI, events, denormalized view models, or any of that CQRS stuff. This was at the crux of my previous anti-CQRS post.

The only thing is that this is exactly CQRS.

Say what?

Have we not effectively separated the responsibility of commands/upserts and queries/reads?

As Greg Young has said before, “the creation of 2 objects where there previously was one”.

Effectively 2 paths through our ABC.


Let me give you a second to gather your thoughts.


You see, CQRS is an approach, a mind-set – not a cookie cutter solution. Frameworks that guide you to applying CQRS exactly the same way everywhere are taking you in the wrong direction. The fact is that you couldn’t possibly know what your Aggregate Roots were before you figured out how to break your system down into ABCs. Attempting to create commands and events for everything will make you overcomplicate your solution.

So the built-in history of this model is event-sourcing?

Well, it’s not event-sourcing in the sense that we don’t necessarily have events. It achieves many of the benefits of event-sourcing by giving us the full history of what happened.

On the whole issue of replaying events to fix bugs – that’s a bit problematic, logically, unless we have a closed system. A closed system is one that doesn’t interact with anything else – no other systems, no users, nothing. As such, closed systems aren’t that common.

In an open system, one with users, let’s say there was a bug. This bug could have caused the wrong data to be written and/or shown to users. As such, users could have submitted subsequent commands based on that erroneous data that they would not have submitted otherwise. There’s no way for us to know.

The problem with replaying events when we fix the bug is that we’re in essence rewriting history – making it as if the user didn’t see the wrong data. The only problem is that we can’t know which events not to replay – we can’t automatically come up with the right events that should have come afterwards. We could try to sit together with our users and have them try to revise history manually, but our organization often isn’t in a bubble. Our users interacted with customers and suppliers. It isn’t feasible to try to undo the real-world impacts of this situation.

Why didn’t you just tell us this from the very beginning?

I did, you just weren’t listening.

You wanted a cookie cutter, and until you tried CQRS out as cookie cutter (and saw it create a bunch of complexity) you wouldn’t listen to anything else.

As developers, we’re trained to solve problems – the faster the better. Unfortunately, this causes us to be blind to things that don’t immediately present themselves as solutions.

When applying CQRS with ABCs, the solutions you end up with are very simple, but the process of getting there is quite hard and takes practice. Finding the boundaries of ABCs such that data isn’t duplicated between them and that data doesn’t travel between them either via RPC or publish/subscribe – it may feel impossible the first several times you try. Keep at it – it is almost always possible.

We haven’t touched on the whole saga/aggregate-root thing yet, but that isn’t as important until you can successfully apply the principles described here.

Also, this post has already gotten long enough, so it looks like now would be a good time to stop.

Until next time…

Inconsistent data, poor performance, or SOA – pick one

Sunday, September 18th, 2011

One of the things that surprises some developers that I talk to is that you don’t always get consistency even with end-to-end synchronous communication and a single database. This goes beyond things like isolation levels that some developers are aware of and is particularly significant in multi-user collaborative domains.

The problem

Let’s start with an image to describe the scenario:


Image 1. 3 transactions working in parallel on 3 entities

The main issue we have here is that the values transaction 2 gets for A and B are those from T0 – before either transaction 1 or 3 completed. The reason this is an issue is that these old values (usually together with some message data) are used to calculate what the new state of C should be.

Traditional optimistic concurrency techniques won’t detect any problem if we don’t touch A or B in transaction 2.

In short, systems today are causing inconsistency.

Some solutions

1. Don’t have transactions which operate on multiple entities (which probably isn’t possible for some of your most important business logic).

2. Turn on multi-version concurrency control – this is called snapshot isolation in MS Sql Server.

Yes, you need to turn it on. It’s off by default.

The good news is that this will stop the writing of inconsistent data to your database.
The bad news is that it will probably cause your system many more exceptions when going to persist.

For those of you who are using transaction messaging with automatic retrying, this will end up as “just” a performance problem (unless you follow the recommendations below). For those of you who are using regular web/wcf services (over tcp/http), you’re “cross cutting” exception management will likely end up discarding all the data submitted in those requests (but since that’s what you’re doing when you run into deadlocks this shouldn’t be news to you).

The solution to the performance issues

Eventual consistency.

Funny isn’t it – all those people who were afraid of eventual consistency got inconsistency instead.

Also, it’s not enough to just have eventual consistency (like between the command and query sides of CQRS). You need to drastically decrease the size of your entities. And the best way of doing that is to partition those entities across multiple business services (also known in DDD lingo as Bounded Contexts) each with its own database.

This is yet another reason why I say that CQRS shouldn’t be the top level architectural breakdown. Very useful within a given business service, yes – though sometimes as small as just some sagas.

Next steps

It may seem unusual that the title of this post implies that SOA is the solution, yet the content clearly states that traditional HTTP-based web services are a problem. Even REST wouldn’t change matters as it doesn’t influence how transactions are managed against a database.

The SOA solution I’m talking about here is the one I’ve spent the last several years blogging about. It’s a different style of SOA which has services stretch up to contain parts of the UI as well as down to contain parts of the database, resulting in a composite UI and multiple databases. This is a drastically different approach than much of the literature on the topic – especially Thomas Erl’s books.

Unfortunately there isn’t a book out there with all of this in it (that I’ve found), and I’m afraid that with my schedule (and family) writing a book is pretty much out of the question. Let’s face it – I’m barely finding time to blog.

The one thing I’m trying to do more of is provide training on these topics. I’ve just finished a course in London, doing another this week in Aarhus Denmark, and another next month in San Francisco (which is now sold out). The next openings this year will be in Stockholm, London; Sydney Australia and Austin Texas will be coming in January of next year. I’ll be coming over to the US more next year so if you missed San Francisco, keep an eye out.

I wish there was more I could do, but I’m only one guy.

Hmm, maybe it’s time to change that.

The Danger of Centralized Workflows

Wednesday, July 13th, 2011

It isn’t uncommon for me to have a client or student at one of my courses ask me about some kind of workflow tool. This could be Microsoft Workflow Foundation, BizTalk, K2, or some kind of BPEL/orchestration engine. The question usually revolves around using this tool for all workflows in the system as opposed to the SOA-EDA-style publish/subscribe approach I espouse.

The question

The main touted benefit of these workflow-centric architectures is that we don’t have to change the code of the system in order to change its behavior resulting in ultimate flexibility!

Some of you may have already gone down this path and are shaking your heads remembering how your particular road to hell was paved with the exact same good intentions.

Let me explain why these things tend to go horribly wrong.

What’s behind the curtain

It starts with the very nature of workflow – a flow chart, is procedural in nature. First do this, then that, if this, then that, etc. As we’ve experienced first hand in our industry, procedural programming is fine for smaller problems but isn’t powerful enough to handle larger problems. That’s why we’ve come up with object-oriented programming.

I have yet to see an object-oriented workflow drag-and-drop engine. Yes, it works great for simple demo-ware apps. But if you try to through your most complex and volatile business logic at it, it will become a big tangled ball of spaghetti – just like if you were using text rather than pictures to code it.

And that’s one of the fundamental fallacies about these tools – you are still writing code. The fact that it doesn’t look like the rest of your code doesn’t change that fact. Changing the definition of your workflow in the tool IS changing your code.

On productivity

Sometimes people mention how much more productive it would be to use these tools than to write the code “by hand”. Occasionally I hear about an attempt to have “the business” use these tools to change the workflows themselves – without the involvement of developers (”imagine how much faster we could go without those pesky developers!”).

For those of us who have experienced this first-hand, we know that’s all wrong.

If “the business” is changing the workflows without developer involvement, invariably something breaks, and then they don’t know what to do. They haven’t been trained to think the way that developers have – they don’t really know how to debug. So the developers are brought back in anyway and from that point on, the business is once again giving requirements and the devs are the one implementing it.

Now when it comes to developer productivity, I can tell you that the keyboard is at least 10x more productive than the mouse. I can bang out an if statement in code much faster than draggy-dropping a diamond on the canvas, and two other activities for each side of the clause.

On maintainability

Sometimes the visualization of the workflow is presented as being much more maintainable than “regular code”.

When these workflows get to be to big/nested/reused, it ends up looking like the wiring diagram of an Intel chip (or worse). Check out the following diagram taken from the DailyWTF on a customer friendly system:


The bigger these get, the less maintainable they are.

Now, some would push back on this saying that a method with 10,000 lines of code in it may be just as bad, if not worse. The thing is that these workflow tools guide developers down a path where it is very likely to end up with big, monolithic, procedural, nested code. When working in real code, we know we need to take responsibility for the cleanliness of our code using object-orientation, patterns, etc and refactoring things when they get too messy.

Here is where I’d bring up the SOA/pub-sub approach as an alternative – there is no longer this idea of a centralized anything. You have small pieces of code, each encapsulating a single business responsibility, working in concert with each other – reacting to each others events.

Productivity take 2: testing and version control

If you’re going to take your most complex and volatile business logic and put it into these workflow tools, have you thought about how your going to test it? How do you know that it works correctly? It tends to be VERY difficult to unit-test these kinds of workflows.

When a developer is implementing a change request, how do they know what other workflows might have been broken? Do they have to manually go through each and every scenario in the system to find out? How’s that for productivity?

Assuming something did break and the developer wants to see a diff – what’s different in the new workflow from the old one, what would that look like? When working with a team, the ability to diff and merge code is at the base of the overall team productivity.

What would happen to your team if you couldn’t diff or merge code anymore?
In this day and age, it should be considered irresponsible to develop without these version control basics.

In closing

There are some cases where these tools might make sense, but those tend to be much more rare than you’d expect (and there are usually better alternatives anyway). Regardless, the architectural analysis should start without the assumption of centralized workflow, database, or centralized anything for that matter.

If someone tries to push one of these tools/architectures on you, don’t walk away – run!

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.


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I was very impressed, as Udi demonstrated a broad understanding of the sorts of problems we would face. His advice was honest and unbiased and very pragmatic. Whenever I questioned him on particular points, he was able to backup his opinion with real life examples. I was also impressed with his clarity and precision. He was very careful to untangle the meaning of words that might be overloaded or otherwise confusing. While Udi's hourly rate may not be the cheapest, the ROI is undoubtedly a deal. I would highly recommend consulting with Udi.”

Robert Lewkovich, Product / Development Manager at Eggs Overnight
“Udi's advice and consulting were a huge time saver for the project I'm responsible for. The $ spent were well worth it and provided me with a more complete understanding of nServiceBus and most importantly in helping make the correct architectural decisions earlier thereby reducing later, and more expensive, rework.”

Ray Houston 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.”

Ian Cooper Ian Cooper, Team Lead at Beazley
“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do.”

Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading.
Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project.
On the personal level - Udi is a great communicator and can persuade even the most difficult audiences (I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Consult with Udi

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

97 Things Every Software Architect Should Know

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