Udi Dahan   Udi Dahan – The Software Simplist
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Leveraging irrationality towards success

Thursday, September 27th, 2012

irrationalWe’ve all seen good ideas emerge in the software space – from objects, to components, to services, to domain models, and the *DD approaches. Yet, in most organizations, it is very hard for these ideas to get traction.

I’ve heard from countless developers and architects over the years about their frustration in getting everybody else to go along with them. “Can’t they see how much better [new approach] is over what we’re doing now?!” they ask, believing that things could and actually would be evaluated on their merits, especially in a rational field like IT.

The usual explanation I give has a couple of parts.

Conway’s Law

In 1968 Melvin Conway penned what later became known as Conway’s Law which stated:

“organizations which design systems … are constrained to produce designs which are copies of the communication structures of these organizations.”

An important corollary of that law is that if you wish to have a significant impact on the design of a system, you would need to have a similarly significant impact on the communication structure of the organization making that system.

The main problem is that the people that tend to be pushing for DDD, IoC, CQRS, SOA, etc are usually not as strong when it comes to the soft skills that are so necessary for bringing about organizational change. The thing is that, at a minimum, these types of changes take 3 to 5 years so it really takes a long-term commitment, both from the individual and the organization.

On the rationality of people in IT

First of all, people are a whole lot less rational than they’d like to believe – or that they’d like other people to notice. In fact, people will go to great lengths to maintain the appearance of consistency and rationality, even at the cost of harm to themselves. How’s that for irrational?

Don’t take my word for it – there’s a great book on the topic: Predictably Irrational: The Hidden Forces That Shape Our Decisions. The somewhat scary thing about it is that not only are we irrational beings, but that that irrationality can be predicted and, yes, even manipulated.

Once you can understand that the people you’re trying to convince aren’t Vulcan, you have a much better chance of being effective. I’d say that, for myself, understanding my own modes of irrationality increased my effectiveness as well, and made me quite a bit happier in life too.

Why you need to bring in a consultant

This isn’t me hawking my wares – believe me, I’m busy enough as it is, but let me know when this starts to sound familiar to you.

There’s a problem in your organization – could be that you’re not delivering software fast enough, high enough quality, whatever. Suffice it to say that Management isn’t happy. You’ve been living this pain for a while and know exactly what the source of the problem is (more often than not, management has at least a hand if not a whole arm in it). You come up with some recommendations, bring them to the higher-ups, but ultimately are ignored, dismissed, or don’t even get into the room.

Some time later, management brings in a Consultant (that’s right, with a capital ‘C’) who is there to figure out what’s wrong and come up with recommendations. In some cases, especially in larger organizations, they bring in a whole bunch of them from a brand name like McKinsey or Ernst & Young.

If these guys are smart, they listen to you, ultimately presenting your analysis and recommendations to management. Of course, those higher-ups are in awe of how quickly these guys were able to understand the inner workings of their organization. That awe lends instant credibility to their recommendations which are then adopted and given powerful political backing.

And you’re sitting there thinking, “but… but… but that’s what I was saying!!”.

It’s not the message – it’s the messenger.

Let me put it another way, explained from the perspective of management – we’re having problems, you work here, ergo you’re part of the problem. Also, you don’t make that much money (compared to management), so how smart could you be? Those brand-name consultants, well, they cost a LOT, so they MUST be good (good enough to know not to work here too).

Therefore the more the consultant costs, the more likely management is to listen, which ultimately creates the conditions for success, which makes the change happen, which proves to management that they were right to bring in an expensive consultant. A vicious (or virtuous) cycle – depending on how you look at it.

Now, it doesn’t always work this way, but it does often enough to perpetuate management’s world view.

In Closing

I do hope your organization and its leaders aren’t trapped in this kind of dysfunction, but if they are, know that you’re not alone and that you can get help – either via consulting or in some books:

Some good books include Made to Stick: Why Some Ideas Survive and Others Die and the grandfather of the field: How to Win Friends & Influence People. There are countless others and there isn’t any right place to start – the most important thing to do is to start.

It’s been over 40 years since Melvin Conway’s observation and, as an industry, we’re still relearning these things – usually through the school of hard knocks. But there is an upside here – I’m pretty sure that, knowing these patterns, you could pick up on some signals during the interviewing process and find a company that’s outgrown many of these issues – one that would be able to have more meritocratic discussions on technical choices.

In the worst case, you could become a consultant and make a living off of all this irrationality 🙂



Build one to throw away

Saturday, September 8th, 2012

recycleThere’s a pattern I’ve seen with companies that are starting on a large green-field project that gets them into trouble later on that I wanted to call out because the pitfall is quite subtle.

It’s something I touched on in my presentation last week about Balancing Architecture & Agile at the Agile Practitioners group in Israel but didn’t have as much time to get into.

Agile

You see, at the beginning of these projects, often our users or customers don’t exactly know what they want. Even if you go through some process of doing mock-ups, sometimes users can’t really know what they want until they can interact with working software.

This is one of the advantages of an agile approach that focuses on getting working software in the hands of users very early. It allows us to mitigate the risk of us building the wrong system.

Pitfalls

The mistake here is believing that this needs to be done in a production-ready manner, using all of the super-scalability and highly reliable infrastructural elements in our target deployment environment.

While there is value in building real functionality on top of your production infrastructure to learn more about the challenges in doing that and in that process, refining the API of your platform, baking more capabilities into it, etc, the problem is that you are slowing down the important learning of what system needs to be built.

Decouple those two things.

Build one to throw away.

Sometimes, the way to be most effective is to be somewhat inefficient.

Once you’ve gotten your users to the point that they can point at a piece of working software and say, “Yes – build me that”, then you can go about building it the right way on top of your production platform.

Let me go a step further, just to be crystal clear.

Do not unit-test that code. Do not use TDD, DDD, CQRS, Event Sourcing, SOA, etc.

Just get it done.

Architecture

You also want to have a good understanding of the use cases in your system before you make too many big architectural decisions (where that might even be just 1 decision, but you only find out after the fact). Some use cases are architecturally significant – many aren’t. This falls under the “just enough architecture up-front”, but is really very dependent on the experience and skills of the people on your team so I really can’t give you anything more prescriptive.

Of course, there’s another risk around this approach.

Organizational Dysfunction

In some organizations, there isn’t enough of an understanding or appreciation of the difference between production-ready software and a prototype. I think we’ve all been through this at one time or another. If users see “the software” works, we might be forced to deploy the prototype – all our protests for naught.

If this is the environment you’re in, don’t worry about it.

Seriously – don’t.

Growing up

You can explain to a child that fire is hot, and that they will get burned if they stick their hand in it, but all of that is theoretical – it doesn’t really sink in, because they haven’t yet had a visceral experience of “hot”.

If the organization you’re in isn’t that mature yet, they’re going to have to get burned a couple of times until they learn. The only thing you can do as a responsible adult is to *try* to get them started with smaller burns so that they’ll learn the lessons with the minimal amount of pain.

Given that the people doing the learning are higher up the food-chain, you really don’t have much influence. Accept that and move on. If necessary, cover your assets – write an email containing the “meeting minutes” in which you describe the concerns that you raised and that the decision was made against your recommendations – but phrase it gently. Depending on the state of the economy and your network, it might be a good time to start job-hunting.

In closing

I said it before but it bears repeating:

Efficiency is secondary to effectiveness.

You want to make sure your headed in the right direction before putting the pedal to the metal.

Rework is to be accepted and, yes, even embraced.

There’s even a book about it.

Have fun.



High Availability a la Dennis

Monday, September 3rd, 2012

I thought I’d give a bit of a shout-out to Dennis van der Stelt whose been applying many of the principles from my course on his project and now has something of a success story to share. Although all he put out was a little tweet about it, I think if more people bug him, we can tease some more out:

It’s not just about getting the system built quickly the first time, and it’s not just about having a maintainable code base longer term, equally important is the ability to quickly deploy major releases to the system without downtime – ‘cuz when you’re system is down, it’s not providing any business value (which is what being Agile is all about).

What do you say, Dennis? Tell us some more!

UPDATE

Dennis gives the full story here.



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.

CQRS

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.

SOA

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.



Bandwidth, Priority, and Service Contracts

Monday, August 20th, 2012

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

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

What about bandwidth?

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

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

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

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

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

Business Priority

And now we get to the crux of the matter.

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

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

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

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

The problem with “the” network

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

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

The network.
The database.

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

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

Putting it together

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

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

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

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

In summary

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

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

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



Services are (still) not Remotely Callable Components

Sunday, July 29th, 2012

Out Of ServiceWe’re more than half-way through 2012, more than 16 years since the term SOA was introduced, and still people are treating the term “service” as if it were nothing more than a remotely-callable component (like a web service).

Take this “Bare-knuckle SOA” thing that’s started making the rounds. Not to say that the approach doesn’t have its merits (regardless of the SOA thing), but it seems to be talking about little more than regular XML and HTTP communication. Personally, I think that the functional testing done over HTTP in that post would have been much better done directly against the API of the component, bypassing HTTP entirely.

Appending the word “service” to a concept doesn’t change any of its architectural properties.

EmailService, CurrencyService, GeoLocationService – all of them just components which have methods that can be invoked remotely. If anything, the design would probably have been better if all the XML and HTTP was removed – if the components were all hosted in the same process.

In fact, in a well designed Service-Oriented Architecture, we tend to see components from many services deployed in process with each other (as I showed in my recent post UI Composition for Correct Service Boundaries).

But I’ve got to admit – the “bare knuckle” prefix, that’s some good link-bait.



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.

marriott

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 down 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.



Logically Distributed, Physically Centralized

Sunday, May 6th, 2012

centralized-distributedWhen people pull back the covers on something like MSMQ, particularly its private queues (the way NServiceBus uses it), and they see that MSMQ is storing its messages in C:WindowsSystem32, well, they’re not particularly happy.

One of the reasons they worry about these types of distributed or federated queue-based solutions has to do with physical failures. The concern is that messages would be lost if there was a hard drive failure.

The preference for centralized message broker type solutions is that we can set it up on a nice RAID infrastructure that will take care of any physical reliability concerns. (Just so that we’re clear here – I’m talking about an single datacenter, possibly connected to a disaster recovery site.)

So, here’s the thing:

Virtualization

You see, in a virtualized production environment, the C drive of a virtual machine is physically in the image file of that VM, which is sitting on a SAN (storage area network).

What that means is that when a message is sent from one processing node to another, the data of that message ends up being written to the SAN, with all of its redundant disks. Even if one of the machines has a critical failure and cannot start up again, all the VMs that were running on it can be started up on a different machine without any message loss.

In fact, most virtualized environments have monitoring and management capabilities built-in so that the VMs will be brought up automatically on another machine. Even if you aren’t using messaging, there are so many other benefits that virtualization brings that you probably should be planning on putting it in, if you haven’t already.

Databases too

In fact, many people do the same thing with databases.
The file partitions on which the database server actually stores its data are on the SAN.

Think about that for a second.

All the data in messages flowing through your queues, and the data in the database, on a SAN. This gives you the ability to do a fully consistent backup of the entire system with SAN snapshots, not to mention ship those to your disaster recovery site.

In closing

Distributed solutions are often misunderstood.
Bad experiences in the past with MSMQ can color perceptions in the present.

The thing is that today’s infrastructure is set up to handle distributed solutions much better.
Developers no longer have to turn to centralized broker or database technologies to get the centralized backup and restore capabilities administrators look for.

If you’ve been avoiding NServiceBus for these reasons, give it a try. Not only will it make your life as a developer easier, combined with this virtualization thing, it will make your administrators life easier too.



A CQRS Journey – with and without Microsoft

Thursday, March 29th, 2012

Update – clarification post here.

circlesI was on a call recently with the Advisory Board for the Microsoft Patterns & Practices (P&P) CQRS Journey project where they were showing the current state of their development. Towards the middle of the call, I mentioned that I found there to be too many concerns in one place and that I had expected there to be a division into multiple sub-domains/bounded contexts/business components (BCs). The answer was that they hadn’t gotten to the other areas yet and that’s why at that point in time there was only one BC.

The conversation got a bit derailed at that point, and I was asked how I would do it (though not quite as politely), ultimately leading to my tweeting this:

I think I got over 50 people who wanted in on this, while some of them urged me to work with P&P rather than separately. I think I’ll do both, hopefully resulting in two implementations that can be compared – one based on Azure (done by P&P) and the other based on NServiceBus (done by my guys). Who do you think is more worried 😉

But first things first

The fundamental flaw that I see happening with many software projects (including the P&P CQRS effort) is that not enough time is spent to understand the underlying business objectives – the thinking behind the use cases / user stories. Developers assume behavior is “like” that of another/similar domain – when the difference in the details matter a lot. That often leads to software boundaries that aren’t properly aligned with those of the business.

The effects of this lack of alignment may be felt only much later in the project, when we get a requirement that just doesn’t fit the architecture we’ve set up. I’ve blogged about the symptoms of this problem about 2 years ago in my post Non-functional architectural woes.

We need to get into the nitty-gritty of our problem domain to find out what makes it special.

Not all e-commerce is equal

Anytime somebody is going to make a purchase online, developers immediately create some kind of “order” entity with a bunch of “order lines”, just like they read about in all the blog posts and books. Then, all sorts of other behavior are shoe-horned around those entities and… voila, a working system.

The domain of conferences is different – we don’t actually ship products when people register so payment concerns are very different. If our company is purchasing 5 tickets to the conference, the number of people (and which specific people) that eventually go to the conference may be very different than the people we had originally registered – there doesn’t tend to be that kind of volatility in traditional B2C retail (like selling books to people online).

It’s also quite likely that if a company is sending many people to a conference that they wouldn’t be paying by credit card – invoicing and payment may happen much later. That is no reason to block registration from completing.

Not all registration systems are equal

I understand how people can look at systems like TicketMaster and use that as a model for this system but, once again, the differences in the domain matter.

First of all, most people don’t purchase movie tickets weeks in advance – conference tickets do go on sale that far in advance. Second, if the movie you want to go to is sold out this week, no big deal, you’ll see it next week – conferences are more of a one-time/yearly deal. Third, you usually go to the movies with family/friends – if you can’t get tickets for everyone, you’ll go next week. When it comes to conferences, there is no “next week”, so whoever can go, does. Also, attendees going to a conference together are usually coworkers, not family – there are less qualms about leaving someone behind.

This is already leading us to a model where we should not view a group registration as a single success or failure affair. This will have an impact on the commands, events, and transactions that flow through our system.

In any case where people are reserving something far in advance, there is a high likelihood of cancellations. This is similar to the domain of hotels/hospitality where you can cancel your reservation up to N days before your arrival at no charge. This also tends to influence the payment structure – we’d rather not have to return people’s money as there can be per-transaction charges for that, instead delaying payment can make sense.

Similar to how hotels overbook by a certain amount (to offset cancellations), our conference might look at doing something similar. The difference is that in the case of a hotel, the guest will likely just book a room in a different hotel in the case the first hotel was fully booked. This probably won’t happen with a conference.

For that reason, we want to remember who wanted to come to our conference even when we thought we were full. You see, our best chance of filling a seat that opened up due to a cancellation is by a person who wanted to register before. What we need here is a waiting list – something that doesn’t make the same kind of sense for hotels or airlines (although airlines do use waiting lists, just that that is usually exposed to travel agents and not to travelers booking online).

First-come, first-served – fairness

The traditional developer thinking about systems is rooted in synchronous and sequential processes. In attempting to give a good user experience, developers want to give the user final confirmation as quickly as possible – whether that’s success or failure.

This results in a first-come, first-served user interaction model – whichever user registers in our conference management system first, the better the chance they’ll get what they want. That sounds like a pretty fair system, the only thing is that fairness was not a requirement.

In the real world, if people are standing in line for tickets, they’d get really upset if the tellers decided somewhat arbitrarily to serve people in the back of the line before those in the front. The great thing about online systems is that nobody can see the “virtual line” – the system can be as unfair as we like and there isn’t a real way for the users to know that this is happening.

Why be unfair?

While conferences, theaters, and airlines all want to have all seats filled, the difference between the ongoing models of airlines and theaters and the once-a-year model of the conference influence how sales are done. Some companies send a lot of employees to our conference so we want to give them preference in registration. This is area that we have the most leverage over – when it comes to the masses who arrive in ones and twos, there’s not very much we can do. It makes sense to bend over backwards for a large group, but not for a small one. A commitment from a large company tends to mean more than that from a small one.

If Boeing has already registered 70 people to your conference and now wants to send 5 more, are you really going to tell them “sorry, we’re fully booked”, or are you going to do everything in your power to keep them happy so that next year they’ll want to keep working with you? Wouldn’t it be nice if you could “unregister” some people to make room for the Beoing guys.

Now, you can’t necessarily do this up until the last minute, but potentially 2 weeks (or whatever) before the event could be reasonable, leaving people the ability to cancel flights and hotels without charges (assuming we tell them during registration that they should buy refundable plane tickets).

The easiest way to “unregister” someone is to not tell them that their registration was confirmed. In short, 2 weeks before the start of the event we finalize all registrations deciding (based on our internal priority) who gets in and who doesn’t. We may have logic that decides to immediately finalize registration from Boeing (and other select customers) without waiting until 2 weeks before the event.

Just don’t look TOO unfair

Appearance is everything. Perception matters. You don’t want to get a reputation for being unfair.

So when we open registration, we can allow the first N people to bypass our waiting list and get accepted right away (payment still needing to be handled later). At that point, you can start moving new registrations through the waiting list.

The thing is, nobody knows that you aren’t actually full at that point 🙂

Influences on architecture

I hope you’re getting the impression that this collection of scenarios is going to have a big impact on the design. It indicates to us which parts of the business need to be 100% consistent with each other and which parts can be eventually consistent – ultimately defining where one bounded context stops and another begins. This has a direct impact on the events that we’d end up with – who would publish what, and how many others would subscribe to it.

I know some people will look at the above scenarios and say “but what if the requirements were different?”. The thing is that not all requirements are created equal. In working with our business stakeholders, we need to identify which elements are stable and which are potentially volatile and, yes, that’ll be different in each project. We want to align the main boundaries of our software with the stable business elements.

And don’t even try to create a system so flexible that it could handle any new requirement without any architectural changes – down that path lies madness. User-defined custom fields used in user-defined custom workflows, all of it appearing in reports with sorting, filtering, and grouping. You might as well give your users Visual Studio.

Back to P&P

I don’t know if P&P will adopt this set of requirements for their CQRS Journey. The thing here is that we can see the collaborative nature of the domain quite clearly – multiple actors working in parallel where the decisions of one affect the outcomes of another.

The requirements that I’ve seen being handled in the CQRS Journey so far don’t seem complicated enough to justify anything more than a 2-tier architecture – it’s feeling somewhat over-engineered right now. I know that people in the community see other benefits to CQRS but I’ll have to put up a separate blog post describing why there are other better solutions than CQRS most of the time.

Anyway, I’m willing to see how things progress and tweak these requirements (up to a point) so that both the NServiceBus solution and the Azure solution are addressing the same problem.

In closing

Occasionally I hear people still raising the agile mantra against Big Design/Requirements Up Front. The thing is that Agile Manifesto never said to intentionally bury your head in the sand with regards to the purpose of the system. It was a push-back against spending months in analysis without anything but documents coming out, but the goal was to reach a middle ground. Nobody ever said “no design up-front” or “no requirements up front”.

I’m going to try to work with both P&P and the alumni of my Advanced Distributed Systems Design course to come up with simplest possible solution that addresses the requirements (functional and non-functional).

Hope you’ll find this journey interesting.

Update – clarification post here.



   


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“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.”

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