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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.

CQRS.

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…



When to avoid CQRS

Friday, April 22nd, 2011

which way?It looks like that CQRS has finally “made it” as a full blown “best practice”.

Please accept my apologies for my part in the overly-complex software being created because of it.

I’ve tried to do what I could to provide a balanced view on the topic with posts like Clarified CQRS and Race Conditions Don’t Exist.

It looks like that wasn’t enough, so I’ll go right out and say it:

Most people using CQRS (and Event Sourcing too) shouldn’t have done so.

Should we really go back to N-Tier?

When not using CQRS (which is the majority of the time), you don’t need N-Tier either.

You see, if you’re not in a collaborative domain then you don’t have multiple writers to the same logical set of data as an inherent property of your domain. As such, having a single database where all data lives isn’t really necessary.

Data is inherently partitioned by who owns it.

Let’s take the online shopping cart as an example. There aren’t any use cases where users operate on each others’ carts – ergo, not collaborative, therefore not a good candidate for CQRS. Same goes for user profiles, and tons of other cases.

So why is it that we need a separate tier to run our business logic?

Originally, the application server tier was introduced for improved scalability, but specifically around managing the connection pool to the database. Increasing numbers of clients (when each had its own user/account for connecting to the database) caused problems. Luckily, most web applications side-step this problem – that is, until someone got the idea that the web server was only supposed to run the UI layer, and the Business Logic layer would be on a separate application server tier.

Rubbish – see Fowler’s First Law of Distribution: Don’t.

Keep it all on one tier. Same goes for smart clients.
No, Silverlight, you don’t count – architecturally speaking, you’re a glorified browser.

But what about scalability?

In a non-collaborative domain, where you can horizontally add more database servers to support more users/requests/data at the same time you’re adding web servers – there is no real scalability problem (caveat, until you’re Amazon/Google/Facebook scale).

Database servers can be cheap – if using MySQL/SQL Express/others.

But what about the built-in event-log CQRS/ES gives us?

Architectural gold-plating / stealing from the business.

Who put you in a position to decide that development time and resources should be diverted from short-term business-value-adding features to support a non-functional requirement that the business didn’t ask for?

If you sat down with them, explaining the long-term value of having an archive of all actions in the system, and they said OK, build this into the system from the beginning, that would be fine. Most people who ask me about CQRS and/or Event Sourcing skip this step.

Finally, you can usually implement this specific requirement with some simple interception and logging. Don’t over-engineer the solution. If using messaging, you can get this by turning on journaling, or if you want to centralize this archive, NServiceBus can forward all messages to a specific queue.

Don’t forget that this storage has a cost – including administration. Nothing is free.

What about the “proof of correctness” in Event Sourcing

I’ve heard statements made that when you use the events that flowed into/through your system AS your system’s data, rather than transforming those events to some other schema (relational or otherwise) and storing the result – you can prove that your system behaves correctly.

Let me put it this way:

No programming technique used by humans will prevent those same humans from creating bugs.
No testing technique used by humans will prevent those same humans from not catching those bugs.
* Automated tests – see programming technique.

While having a full archive of all events can allow us to roll the system back to some state, fix a bug, and roll forwards, that assumes that we’re in a closed system. We have users which are outside the system. If a user made a decision based on data influenced by the bug, there’s no automated way for us to know that, or correct for it as we roll forwards.

In short, we’re interested in the business’ behavior – as composed of user and system behavior. No proof can exist.

Umm, so where should we use it

If you’ve uncovered a scenario where you’re wondering “first-one-wins, or last-one-wins”, that’s often a good candidate for a place where CQRS could make sense. Then re-read my Race Conditions Don’t Exist post.

Also, CQRS should not be your top-level architectural pattern – that would be SOA.
CQRS, if used at all, would be used inside a service boundary only.

Given that SOA guides us away from having a given 3rd normal form entity exist in any one service, it is unlikely that the building blocks of your CQRS design will be those kinds of entities. Most 3rd normal form one-to-many and many-to-many relationships simply do not exist when doing SOA and CQRS properly.

Therefore, I’m sorry to say that most sample application you’ll see online that show CQRS are architecturally wrong. I’d also be extremely wary of frameworks that guide you towards an entity-style aggregate root CQRS model.

In Summary

So, when should you avoid CQRS?

The answer is most of the time.

Here’s the strongest indication I can give you to know that you’re doing CQRS correctly: Your aggregate roots are sagas.

And the biggest caveat – the above are generalizations, and can’t necessarily be true for every specific scenario. If you’re Greg Young, then you probably can (and will) decide on your own on these matters. For everybody else, please take these warnings to heart. There have been far too many clients that have come to me all mixed up with their use CQRS in areas where it wasn’t warranted.

If you want to know everything you need to know to apply CQRS appropriately, please come to my course – there is so much unlearning to do first that just can’t happen via a series of blog posts.



Cautiously Merging IL

Sunday, August 1st, 2010

Caution, Merge AheadAs Dru mentioned on his blog, while having dinner in Kansas City, I described why NServiceBus makes use of IL Merge and some of the challenges we were facing with version management as a result. While I do think that we’ve managed to find the right balance, I must say that it wasn’t easy and I urge other developers use caution when employing it.

Mistakes Made

In the previous version of NServiceBus (1.9) we over aggresively merged 3rd party libraries like Castle into the NServiceBus binaries. That in itself, wasn’t the big problem. The problem was that we didn’t know about internalizing the libraries we merged.

This resulted in all the 3rd party types being exposed when a developer used NServiceBus. For those who happened to use the same version of those libraries as that which was merged, it wasn’t a problem. Unfortunately, developers who used libraries like Castle tended to be very particular about which version they used – which resulted in version conflicts, also known as “versioning hell”.

Internalization Challenges

As we moved to NServiceBus 2.0, we were a lot more careful about which libraries were merged and made sure to internalize them as much as possible. Yes, you read that right, “as much as possible”. You can’t always internalize all the types of a given library.

For example, NServiceBus internally uses NHibernate to persist the state of long-running processes (called sagas) and we want this implementation detail to not conflict with anything that developers want to do on top of NServiceBus. The only thing is that for NHibernate to work, it needs certain types to be exposed to the configuration environment, specifically all the types in the NHibernate.Cfg.MappingSchema namespace.

Extensibility Challenges

Once you take on an external dependency, you want to configure it to “just work” so that developers that aren’t familiar with it don’t have to learn yet another library to use your framework. So far, we’ve been able to do that quite successfully with NServiceBus. The challenge comes when developers want to customize the behavior of those 3rd party libraries, wanting to call into their APIs.

You see, once you internalize those libraries and their types, developers can’t access them. This leads to all sorts of tricky extensibility problems especially for different kinds of developers. Some are happy enough to configure things from the outside using XML – like using hbm.xml files to describe how their sagas get persisted, while others really want to use Fluent NHibernate, which is fully internalized.

Finding a Balance

What we’ve currently got for NServiceBus to try to keep everyone happy is a progressive exposure model. From the developer who’s downloading NServiceBus for the first time and wants everything to just work without changing anything, our API unfolds in multiple dimensions to allow for the highest level of extensibility and pluggability. That being said, the Fluent NHibernate issue mentioned above isn’t solvable at just an API level.

In order to address the class of developer that wants full control, we’ve got a “core only” build that doesn’t merge any assemblies into it. This class of developer usually doesn’t have a problem with referencing some more assemblies as long as they retain full control of the behaviors they want.

It ain’t easy keeping everybody happy, or, at least, not unhappy.

In Closing

I would agree that more OSS frameworks should merge and internalize 3rd party libraries that don’t need to be exposed to developers – it shortens the learning curve and increases adoption. But walk this path cautiously, it’s hard striking a balance that will work all users of your framework and it takes quite some time to get it right.

And one last thing, please, PLEASE, take care of maintaining binary compatibility from one version to the next. I know it’s a pain – we’ve been doing it with NServiceBus for the past 3 years, but your users will thank you for it.



Evolving Loosely-Coupled Frameworks & Apps

Wednesday, July 14th, 2010

This post will be less of a big-concept type posts I usually do, and more of a tip for people building and maintaining infrastructure and frameworks either open-source or internally for their companies. I’m going to illustrate this with NServiceBus as it is a large enough code base to have significant complexity and open so that you can go and take a look yourself. Trying to include some example in here would be just too small to be useful or for the point to come across.

Some background

As a cohesive framework, NServiceBus makes it quite easy for developers to pick and choose which settings they want turned on and off. Being built as a loosely-coupled set of components that don’t know about each other has always kept the internal complexity low. But as the NServiceBus API has been evolving over the years, and the functionality offered has increased, some interesting challenges have popped up as the codebase has been refactored.

The challenge

The UnicastBus class has grown too large and it’s time to refactor something out. Coincidentally, users have been asking for a better “header” story for messages – the ability to specify static headers that will be appended to all messages being sent (useful for things like security tokens), as well as per message headers. So, we want to refactor all the header management out to its own component independent of the UnicastBus class.

So, here’s the issue. So far, users have specified “.UnicastBus()” as a part of the fluent code-configuration, and shouldn’t have to change that – they shouldn’t need to know that header management is now a separate component. But then how can the new component bootstrap itself into the startup, such that it gets all the dependency injection facilities of the rest of the framework? Remember that the component doesn’t know which container technology is being used (since the user can swap it out) or when the container has been set.

The solution

The only part of the framework that knows about when all DI configuration is set is the configuration component, thus it will have to be the one that invokes the new component (without knowing about it). Introduce an interface (say INeedInitialization) and scan all the types loaded looking for classes which implement that type, register them into the container, and invoke them. Have the new component implement that interface, and in its initialization have it hook into the events and/or pipelines of other parts of the system.

Other uses

One historically problematic area in NServiceBus has been people forgetting to call “.LoadMessageHandlers()”. This can now be wired in automatically by a class in the UnicastBus component via the same mechanism.

A new feature coming in the next version is the “data bus”, a component which will allow sending large quantities of data through the bus without going through the messaging pipelines. This will help people get around the 4MB limit of MSMQ and, even more importantly, the much smaller 8KB limit of Azure. We will be able to introduce the functionality transparently with the same mechanism.

As an extension point, developers can now enrich the NServiceBus framework with their own capabilities and make those available via the contrib project to the community at large. This is better than the IWantToRunAtStartup interface that was only available for those using the generic host (which excluded web apps) and gives a consistent extensibility story for all uses.

Summary

Extensibility has always been a challenge when writing object-oriented code and dependency injection techniques have helped, but sometimes you need a bit more to take things to the next level while maintaining a backwards-compatible API.

Like I said, not a ground-shaking topic but something quite necessary in creating loosely-coupled frameworks and applications. Once you know it’s there, it isn’t really a big deal. If you didn’t know to do it, you may have been contorting your codebase in all kinds of ways to try to achieve similar things.

If you want to take a look at the code, you can find the SVN repository here: https://nservicebus.svn.sourceforge.net/svnroot/nservicebus/trunk/



Server Naming and Configuration Conflicts

Saturday, June 5th, 2010

ConfigurationIn my work with clients the topic of how to handle the movement of software from one environment to another inevitably comes up. Sometimes this is in the context of NServiceBus but the problem is more generic. The faster that an organization is able to get software out the door, the more agile they can be.

Unfortunately, there is one tiny little mistake that I see almost everywhere that gets in the way, and that’s going to be the topic of this post.

The Problem

Let’s say you have a standard web app environment – some web servers, application servers, and a database server. Your web servers need to send messages to the application servers. So far, so good.

In your test environment, you have an application server called AS_01_Test, and your web servers are configured to send it messages. However, in your staging environment the application server fulfilling that same role is called AS_01_Stage. This creates a configuration problem – you need to change the config of your web servers as you move the web app from Test to Staging.

I’ve seen companies doing all sorts of creative things to get around this problem – some of them involve putting all configuration settings in a database so that they can be centrally managed and visualized. I’d like to suggest an alternative approach.

What if…

What if server names were the same across all environments?

Well, you wouldn’t need to change configuration as you moved the system between environments. That’s a good thing.

But how can that be? Wouldn’t there be a conflict if there were two machines with the same name?

The answer is that there wouldn’t be a conflict if the machines were on different networks. Not all machines have to be on the same network. We can set up as many networks / virtual networks as we like. And it is clear that we don’t need machines in one environment / network to talk to machines in another environment. I mean, under no circumstances would we want web servers in our test environment to talk to application servers in the production environment.

These separate networks provide much needed isolation, beyond solving the server naming problem.

In closing

It’s really a tiny thing when you think about – multiple networks. But that’s exactly why software developers overlook it so often – because it’s not a “software solution” to the configuration problem we perceive as a “software problem”.

I wrote about related multi-environment configuration issues in this earlier post: Convention over Configuration – The Next Generation

I’m happy to say that this functionality is now in NServiceBus called “profiles” and you can read more about how they work here.

How are you handling the flow of moving software through to production? Leave your comments below.



CQRS isn’t the answer – it’s just one of the questions

Friday, May 7th, 2010

dont panicWith the growing interest in Command/Query Responsibility Segregation (CQRS), more people are starting to ask questions about how to apply it to their applications. CQRS is actually in danger of reaching “best practice” status at which point in time people will apply it indiscriminately with truly terrible results.

One of the things that I’ve been trying to do with my presentations around the world on CQRS was to explain the why behind it, just as much as the what. The problem with the format of these presentations is that they’re designed to communicate a fairly closed message: here’s the problem, here’s how that problem manifests itself, here’s a solution.

In this post, I’m going to try to go deeper.

The hitchhiker’s guide to the galaxy

In this most excellent book, one of the things that struck me was the theme that made it’s way through the whole book – starting with the answer to life, the universe, and everything: 42. By the time you get to the end of the book, you find out that the real question to life, the universe, and everything is “what do you get when you multiply 6 by 9”. And that’s how the book leaves it.

To us engineers, we can’t just accept the fact that the book would say that 6*9 = 42 when we know it’s 54. After bashing our heads on the rigid rules of math, we realize that not all math problems are necessarily in base 10, and that if we switch to base 13, the number 42 is 4*13 + 2 = 54. So, the book was right – but that’s not the point.

What’s the point?

The hitchhiker’s guide is an example of a teaching technique which presents an apparent paradox, leaving the student to dig up unspoken and unthought assumptions in order to resolve it. Key to this technique are rigid rules which do not allow any compromise or shortcuts on the student’s part.

The purpose of this technique is not for the student to learn the answer, but to gain deeper understanding, which in turn changes the way they go about thinking about problems in the future.

So, when given the problem 4*5, we do not just immediately answer 20, instead we clarify in which numeric base the question is being phrased, and only then go to solve the problem. In base 13, the answer would be 17. In hex, the answer would be 14.

The externally visible change is that we know which questions to ask in order to arrive at the right answer – not that we know the answer ahead of time.

Making an “ass” out of “u” and “me”

Let’s start at the end – one of the unspoken assumptions that has been causing problems:

All businesses can be treated the same from the perspective of software.

In our previous example, we assumed that all math problems use base 10. It turns out that different bases are useful for different domains (like base 2 for computers). We can say similar things about degrees and radians in geometry. The more we look at the real world, the more we see this repeating itself. There’s no reason that software should be any different.

Base 10 is not a ubiquitous best practice. We shouldn’t be surprised that there really aren’t best practices for software either.

Here’s another problematic assumption:

“The business” can (and do) tell us what they need in a way we can understand.

So many software fads have been built on the quicksand of this assumption. OOAD – on verbs and nouns. 4GL and other visual tools that “the business” will use directly. SOA – on IT business alignment. I expect we haven’t seen the end of this.

Some of you may be wondering why this is false, others are sagely nodding their heads in agreement.

The myth of “the business”

Unless you have a single user, who is also the CEO paying for the development, there is no “the”. It’s an amalgam of people with different backgrounds, skills, and goals – there is no homogeneity. Even if no software was involved, many business organizations are dysfunctional with conflicting goals, policies, and politics.

To some extent, we technical people have hidden ourselves away in IT to avoid the scary world of business whose rules we don’t understand. With the rise in importance of information to the world, we’ve been pulled back – being forced to talk to people, and not just computers. Luckily, we’ve been able to create a buffer to insulate ourselves – we’ve taken the less successful technical people from our heard and nominated them “business analysts”. No, not all companies do it this way, but we do need to take a minute to reflect on how information flows between the business Mars into and out of the IT Venus.

On human communication

Even if we made this insulation layer more permeable, allowing and encouraging more technical people and business people to cross its boundary, we still need to deal with the problem of two humans communicating with each other. There are enough books that have been written on this topic, so I won’t go into that beyond recommending (strongly) to technical people to read (some of) them.

Rather, I’d like to focus on the environment in which these discussions take place. IT has been around long enough, and users have used computers long enough, that a certain amount of tainting has taken place. If the world was a trial, the evidence would have been thrown out as untrustworthy.

When users tell you what they want, they’re usually framing that with respect to the current system that they’re using. “Like the old system – but faster, and with better search, and more information on that screen, and…”

At this point, business analysts write down and formalize these “requirements” into some IT-sanctioned structure (use cases, user stories, whatever), at which point developers are told to build it. Users only know what they didn’t want when developers deliver exactly what was asked.

How can that be?

These are not the “requirements” you are looking for

Users ultimately dictate solutions to us, as a delta from the previous set of solutions we’ve delivered them. That’s just human psychology – writer’s block when looking at a blank page, as compared to the ease with which we provide “constructive criticism” on somebody else’s work.

We need to get the real requirements. We need to probe beyond the veneer:

  • Why do you need this additional screen?
  • What real-world trigger will cause you to open it?
  • Is there more than one trigger?
  • How are they different?
  • etc, etc, etc…

This is real work – different work than programming. It requires different skills. And that’s not even getting into the political navigation between competing organizational forces.

But let’s say that you don’t have (enough) people with these skills in your organization. What then?

Enter CQRS

CQRS gives us a set of questions to ask, and some rigid rules that our answers must conform to. If our answers don’t fit, we need to go back to the drawing board and move things around and/or go back to “the business” and seek deeper understanding there.

For each screen/task/piece of data:

Will multiple users be collaborating on data related to this task?
Look at every shred of raw data, not just at the entity level.
Are there business consistency requirements around groups of raw data?

If “the business” answers no – ask them if they see that answer changing, and if so, in what time frame, and why. What changing conditions in the business environment would cause that to change – what other parts of the system would need to be re-examined under those conditions.

After understanding all that and you find a true single-user-only-thing, then you can use standard “CRUD” techniques and technologies. There are no inherent time-propagation problems in a single-user environment – so eventual consistency is beyond pointless, it actually makes matters worse.

On the other hand, if the business-data-space is collaborative, the inherent time-propagation of information between actors means they will be making decisions on data that isn’t up-to-the-millisecond-accurate anyway. This is physics, gravity – you can’t fight it (and win).

The rule for collaboration

Actors must be able submit one-way commands that will fail only under exceptional business circumstances.

The challenge we have is how to achieve the real business objectives uncovered in our previous “requirements excavation” activities and follow this rule at the same time. This will likely involve a different user-system interaction than those implemented in the past. UI design is part of the solution domain – it shouldn’t be dictated by the business (otherwise it’s like someone asking you to run a marathon, but also dictating how you do so, like by tying your shoelaces together).

Many of the technical patterns I described in my previous blog post describe the tools involved. BTW, hackers can be considered “exceptional actors” – the business actually wants their commands to fail.

In Summary

The hard and fast rule of CQRS about one-way commands is relevant for collaborative domains only. This domain has inherent eventual consistency – in the real world. Taking that and baking it into our solution domain is how we align with the business.

The process we go through, until ultimately arriving at one-way-almost-always-successful-commands is business analysis. Rejecting pre-formulated solutions, truly understanding the business drivers, and then representing those as directly as possible in our solution domain – that’s our job.

After doing this enough times and/or in more than one business domain, we may gain the insight that there is no cookie-cutter, one-size-fits-all, best-practice solution architecture for everything. Each problem domain is distinct and different – and we need to understand the details, because they should shape the resulting software structure.

The next time the business tell us to implement 42, we’ll use CQRS along with other questioning techniques until we can get “6 x 9” out of them, learning from the exercise what are the significant and stable parts of the business – ultimately helping us to “build the right system, and to build the system right”.

Don’t Panic :-)



Projects, Assemblies, and Namespaces – oh my

Sunday, May 3rd, 2009

Every once in a while this topic pops up, and since the nServiceBus code base doesn’t follow the apparently accepted practice, and I do get asked about it, here goes.

First of all, the conventional wisdom:

“If you don’t choose assembly to represent component, the natural artifact candidate is then namespace.”

There’s only one minor assumption here that deserves being dragged out into the light.

While Visual Studio creates an assembly from every project by default, you can take those assemblies and merge them together into a single assembly using this nice little utility from Microsoft. It is likely that each project would have its own namespace too, so we should still be aligned with the conventional wisdom.

In other words, we could choose a Visual Studio project to represent a logical component and still be in the same camp as Jeremy:

“I’m very firmly in the camp that says you should only split assemblies by deployment targets”

What everyone agrees about seems to be that coupling hurts, and should be managed.

Where does coupling come from? Well, from references between two pieces of code. If we were to represent our logical components as Visual Studio projects, we could easily see those references without the help of any 3rd party tools. The compiler would even yell at us if we were to (accidentally) create an evil circular reference.

While some might complain about the long compile time when we have many projects in a single solution, good componentization often doesn’t require us to put all projects in a single solution. In fact, each component could theoretically have its own solution – since it’s reasonable to assume we’d really only be working on one component at a time. In which case, compile time per developer task would be a non-issue.

Going through the whole code base is usually only needed when doing a full-system debug when trying to track down a problem. This wouldn’t need to be done against a solution with all projects. We’d do this using PDBs of the merged projects (as that’s what actually got delivered, and where the bug was found). After spelunking through those PDBs, we’d eventually find the problematic component (or 2, or 3, or …), and open up developer tasks for each component.

Regardless of if we’re putting out a patch for an existing customer or rolling these changes into a release with other tasks, all the logical components would be built into a physical system (merged as necessary) and the system would be put through QA.

In short, it looks like just a bit of unconventional wisdom gets us a nice balance.



Backwards-Compatibility: Why Most Versioning Problems Aren’t

Friday, April 10th, 2009

image

I’ve been to too many clients where I’ve been brought in to help them with their problems around service versioning when the solution I propose is simply to have version N+1 of the system be backwards-compatible with version N. If two adjacent versions of a given system aren’t compatible with each other, it is practically impossible to solve versioning issues.

Here’s what happens when versions aren’t compatible:

Admins stop the system from accepting any new requests, and wait until all current requests are done processing. They take a backup/snapshot of all relevant parts of the system (like data in the DB). Then, bring down the system – all of it. Install the new version on all machines. Bring everything back up. Let the users back in.

If, heaven-forbid, problems were uncovered with the new version (since some problems only appear in production), the admins have to roll back to the previous version – once again bringing everything down.

This scenario is fairly catastrophic for any company that requires not-even high availability, but pretty continuous availability – like public facing web apps.

If adjacent versions were compatible with each other, we could upgrade the system piece-meal – machine by machine, where both the old and new versions will be running side by side, communicating with each other. While the system’s performance may be sub-optimal, it will continue to be available throughout upgrades as well as downgrades.

This isn’t trivial to do.

It impacts how you decide what is (and more importantly, what isn’t) nullable.

It may force you to spread certain changes to features across more versions (aka releases).

As such, you can expect this to affect how you do release and feature planning.

However, if you do not take these factors into account, it’s almost a certainty that your versioning problems will persist and no technology (new or old) will be able to solve them.

Coming next… Units of versioning – inside and outside a service.



External Value Configuration with IoC

Friday, June 13th, 2008

One of the things I haven’t like about using IoC containers, AKA dependency injection frameworks, was the string-based configuration model they exposed. In order to set these values, developers had 2 options: either use XML config (usually without the benefit of intellisense or refactoring support), or use code (still quoting property names – again, no intellisense or refactoring support).

In short, there seemed to be a hole in the development model.

Here’s an example from how nServiceBus used to do this:

builder.ConfigureComponent(typeof(HttpTransport), ComponentCallModelEnum.Singleton)
  .ConfigureProperty(“DefaultNumberOfWorkerThreads”, 10)
  .ConfigureProperty(“DefaultNumberOfSenderThreads”, 10);

The problem was that if a developer got the case of the property wrong, misspelled it in some way, or somebody later refactored/renamed that property, the system would break. It would also be very difficult to figure out why.

Then, a couple of weeks ago, it dawned on me.

This was the same problem we used to have with testing using mock objects – before we had today’s more advanced frameworks. So, the solution must be to use the same techniques. The container should give the developer an object that looks just like their class, but that would intercept all calls. Then, that interceptor could turn those into the config calls shown above. Here’s what the new config model looks like: 

HttpTransport transport = builder.ConfigureComponent<HttpTransport>
                                                   (ComponentCallModelEnum.Singleton);

transport.DefaultNumberOfSenderThreads = 10;
transport.DefaultNumberOfWorkerThreads = 10; 

Granted, you’re not going to have tons of code like this. However, for all those parameters which are factory-configured and that customers/integrators shouldn’t tinker with, it makes a difference. The biggest difference is during that time of development where you’ve gotten into preliminary integration tests but the systems components are still being “polished”.

Aside: On the current project that has adopted this model, we’ve probably saved (conservatively) about 3 months of effort with this tiny (?) thing, and this isn’t a huge project. If that’s more than you would’ve thought, well, I was surprised myself. First, understand that in the old config model, everything still compiles and unit tests pass, even though its broken.

Just consider what happens in the lab when this occurs. You have N testers that can’t test the new version, waiting. You have the person who installed the version, trying to figure out what’s wrong. They then call in one of the developers where most of the new development occurred since the previous version. They fiddle around with it, looking at exception traces and whatnot. In the best case, we’re talking about 2 hours from noticing its broken until a new version comes out fixed. Multiply that by N+3 people. Then multiply by the number of versions you do integration tests on in the lab.

Caveat: In the current version, properties must be virtual in order for this to work.

For those of you who want just this feature without nServiceBus, I’ve put up all the binaries here. For the source, you’ll need to go to here.

Let me know what you think – especially if you can take the implementation to the point where it won’t need virtual properties to work :)



WCF, Smart Clients, and Deadlocks

Friday, April 11th, 2008

There’s a new article up on MSDN describing how to write Smart Clients using WCF. The author is none other than WCF-Master Lowy and he goes over the multitude of ways you can deadlock yourself.

Here’s a taste:

UI Thread and Concurrency Management

Whenever you use hosting on the UI thread, deadlocks are possible. For example, the following setup is guaranteed to result with a deadlock: A Windows Forms application is hosting a service with UseSynchronizationContext set to true, and UI thread affinity is established. The Windows Forms application then calls the service over one of its endpoints. The call to the service blocks the UI thread, while WCF posts a message to the UI thread to invoke the service. That message is never processed, because of the blocking UI thread—hence, the deadlock.

Another possible case for a deadlock occurs when a Windows Forms application is hosting a service with UseSynchronizationContext set to true and UI thread affinity is established. The service receives a call from a remote client. That call is marshaled to the UI thread and is eventually executed on that thread. If the service is allowed to call out to another service, that can result in a deadlock if the callout causality tries somehow to update the UI or call back to the service’s endpoint, because all of the service instances that are associated with any endpoint (regardless of the service-instancing mode) share the same UI thread.

Similarly, you risk a deadlock if the service is configured for reentrancy and it calls back to its client. You risk a deadlock if the callback causality tries to update the UI or enter the service, because that reentrance must be marshaled to the blocked UI thread.

Actually, I have difficulty believing that Juval would go so far as to suggest that even the forms should be services, but he does:

Form as a Service

The main motivation for hosting a WCF service on the UI thread is if the service must update the UI or the form. The problem is always: How does the service reach out and obtain a reference to the form? While the techniques and ideas that appear thus far in the listings certainly work, it would be simpler yet if the form were the service and hosted itself. For this to work, the form (or any window) must be a singleton service. The reason is that singleton is the only instancing mode that enables you to provide WCF with a live instance to host. In addition, you would not want a per-call form that exists only during a client call (which is usually very brief), nor would you want a per-session form that only a single client can establish a session with and update.

When a form is also a service, having that form as a singleton service is the best instancing mode all around.

I think that this article serves as a great treatise leading to only one conclusion – you’d have to be crazy to try to do this without some higher level framework, preferably with a different low-level framework too :-) . Sucks Microsoft didn’t put one out – nor is there a pending beta, CTP, or even word about some project with a codename handling this. From what I know about Prism, it doesn’t intend to handle this issue either.

One thing that isn’t covered in the article is that if you do choose not to tie the client-side service to the UI thread, you open yourself up to race conditions. Reasons you’d want to handle messages on a different thread center around UI responsiveness. I’ve written about these things before:

The more I read things like this, the more I feel that I have to get going with my nServiceBus based solution. I’m fairly swamped as it is, so if anyone is interested in helping get this project off the ground, I’d be most grateful – as I think anyone else that had to build a smart client would.



   


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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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