Archive for the ‘Databases’ Category
Monday, December 31st, 2012
Occasionally I get questions about the issue of transactional messaging – why is it so important, why does NServiceBus default to this behavior, and if we didn’t use it, what bad things could happen. I’m talking specifically about the ability to enlist a queue in a distributed transaction here.
I think the reason for this interest is the rise in popularity of cloud platforms and queuing systems like RabbitMQ (which don’t support distributed transactions) and the difficulty of setting up distributed transactions even in on-premise.
Of course, there’s also the regular scalability hand-wringing going on even though most people wouldn’t bump up against those limits anyway.
In this post, I’ll talk about the nature of the problem, explain the pitfalls in some of the common solutions, but I’ll put off the description of how to provide consistency without distributed transactions to a future post as this one is already going to be quite long.
I’ll start with the basic fault-tolerance issues and then explain how things spiral out from there.
Starting with the basics
OK, so we have a queuing system in place that dispatches messages to our business logic which does some transactional work against a database.
Let’s say that we completed the transaction against our database but before we could acknowledge to the queue that the message was processed successfully, our machine crashed. What our machine comes up again, the queue will once again dispatch us the same message. Unless we have some logic to detect that we’ve already processed it (called “idempotence” in the REST community), we will end up processing it again.
In short, the problem is duplicates.
Attempted solutions to the duplicate problem
Most queuing systems don’t do anything about duplicates, actually giving it a proper architectural name: At-least-once message delivery, as opposed to the Once-and-only-once model that a queue that supports distributed transaction provides.
The solution often suggested is to have your logic check to see if it has already processed a message with that ID before – in essence storing the ID of each message processed for some period of time. Of course, there is some performance overhead with that, but it might be a small price to pay compared to dealing with it in the logic of every use case.
On the other hand, you’ll often have some messages (like Update commands) for which it looks like you can safely process them multiple times, in which case you might want not to pay the performance overhead there. The thing is, if your logic publishes an event in addition to the regular database work (something that is quite common) and you process the same message twice, you will probably end up publishing the event twice as well.
These duplicates are different in that here we have two distinct messages with different IDs that contain the same business data. This means that recipients of these messages will not be able to filter them out at an infrastructure level anymore.
NOTE: Deduplication abilities in queues
Although the Azure Service Bus doesn’t support distributed transactions meaning you still have the issue mentioned above, Microsoft added the ability to detect and filter out duplicates based on message contents rather than just the ID. This helps quite a bit but it’s important to understand that that doesn’t cover everything for you. Let me explain:
More complex logic
In some of your most important use cases, you may have both entity updates as well as entity creation happening together in your domain model. You might be using some kind of event model (like I wrote about here) to percolate out the information that an entity was created in order to keep your service layer decoupled from the internals of the domain model.
In the callback code from these domain events, you will likely publish out an event on the queuing system containing information like the ID of the entities created as well as other business data. And there’s the rub.
You see, without distributed transactions, you can run into some problematic scenarios:
For example, if you don’t make sure that your event publishing calls to the queuing system include the same transaction object as the one you used when retrieving the original message from the queue, then those calls could “escape” before you know if the database transaction is going to succeed. Deadlocks always happen at the lousiest times. Anyway, if you’re using database generated IDs for your entities, then those IDs will get published out in events despite the database rolling back and your subscribers will now be making decisions on wrong data – not just eventually consistent data.
In this case, processing the message again doesn’t really solve the problem – it just means that you’ll be publishing events with different IDs, so an infrastructure like Azure Service Bus couldn’t really de-duplicate them.
On the other hand, if you do use the same transaction and combine in the infrastructural message ID based de-duplication described above (as identifying duplicate calls for complex business logic is damn hard), you’ll run into another problem.
Consider what would happen if your server crashes right after finishing its database work but before it completes the transaction against the queuing system. When going to retry the message, the infrastructure filtering thing would know not to call your business logic again and that message would be quietly swallowed. Unfortunately, the event publishing calls to the queuing system from the first time the message was processed were rolled back and since your business logic isn’t called again, the event publishing won’t happen again.
I hope I’ve been able to clarify what kind of scenarios distributed transactions solve for you and some of the difficulties in solving them yourself.
Now, to be clear, you could solve these problems by going in-depth on each of your use cases, analyzing the consistency needs and structuring the code differently to address those needs. But give this another thought, if our consistency is dependent on calling otherwise independent APIs in exactly the right order, and that a change in this order would not cause any visible functional effects, what would happen when developers with less expertise maintain this code?
The folks in the event sourcing community have their solution to this which is based on writing their business logic differently. As the adoption of this pattern is still pretty limited (probably still in the Innovator section of the Technology Adoption Curve), it’ll be interesting to see how it holds up with larger teams in the mainstream.
Oh, and in case it wasn’t clear from before, the guys in the REST community haven’t even begun addressing this problem when it comes to server-to-server integration.
We’re working on a solution for this with NServiceBus that won’t require you to change how you write business logic. We’ve got one big release to do before we can roll this in, and that’s coming soon (with all sorts of cool things like support for ActiveMQ and queues in the database). The solution we’ve found is architecturally sound but you’ll have to wait for my next post to hear about it.
Sunday, November 4th, 2012
Feeling a little bit rant-y today, as I just saw some more abuse of remote calls, this time on the Java side of things.
JPA is the Java Persistence API – a kind of ORM, as you’d expect. Luckily, a lot of the web services stuff was already on the way out by the time that EclipseLink DBWS came out. DBWS allowed you to expose database artifacts as web services.
I mean, it’s not like we have any other interoperable ways of accessing data, right?
Anyway, like I said, that didn’t take off, but now they’re reinventing it – this time with REST!
In case you had any doubts, REST is pure awesomeness and adding it to anything else makes it awesome too. Lest anybody take this out of context (it’s happened before), I’m being sarcastic.
Here it is.
God knows they couldn’t let Microsoft totally dominate this area with OData coming out quite some time ago. In case you were wondering, OData was designed to provide standard CRUD access of a data source over HTTP.
Of course, none of these support any transactions so if you actually wanted to do some meaningful business logic on top of this CRUD, you wouldn’t have any consistency. And, let’s face it, if you’re not doing any meaningful business logic, just basic persistence, you just do it. That problem’s been solved a long time ago.
Can we please stop reinventing SQL already?
Sunday, September 18th, 2011
One of the things that surprises some developers that I talk to is that you don’t always get consistency even with end-to-end synchronous communication and a single database. This goes beyond things like isolation levels that some developers are aware of and is particularly significant in multi-user collaborative domains.
Let’s start with an image to describe the scenario:
Image 1. 3 transactions working in parallel on 3 entities
The main issue we have here is that the values transaction 2 gets for A and B are those from T0 – before either transaction 1 or 3 completed. The reason this is an issue is that these old values (usually together with some message data) are used to calculate what the new state of C should be.
Traditional optimistic concurrency techniques won’t detect any problem if we don’t touch A or B in transaction 2.
In short, systems today are causing inconsistency.
1. Don’t have transactions which operate on multiple entities (which probably isn’t possible for some of your most important business logic).
2. Turn on multi-version concurrency control – this is called snapshot isolation in MS Sql Server.
Yes, you need to turn it on. It’s off by default.
The good news is that this will stop the writing of inconsistent data to your database.
The bad news is that it will probably cause your system many more exceptions when going to persist.
For those of you who are using transaction messaging with automatic retrying, this will end up as “just” a performance problem (unless you follow the recommendations below). For those of you who are using regular web/wcf services (over tcp/http), you’re “cross cutting” exception management will likely end up discarding all the data submitted in those requests (but since that’s what you’re doing when you run into deadlocks this shouldn’t be news to you).
The solution to the performance issues
Funny isn’t it – all those people who were afraid of eventual consistency got inconsistency instead.
Also, it’s not enough to just have eventual consistency (like between the command and query sides of CQRS). You need to drastically decrease the size of your entities. And the best way of doing that is to partition those entities across multiple business services (also known in DDD lingo as Bounded Contexts) each with its own database.
This is yet another reason why I say that CQRS shouldn’t be the top level architectural breakdown. Very useful within a given business service, yes – though sometimes as small as just some sagas.
It may seem unusual that the title of this post implies that SOA is the solution, yet the content clearly states that traditional HTTP-based web services are a problem. Even REST wouldn’t change matters as it doesn’t influence how transactions are managed against a database.
The SOA solution I’m talking about here is the one I’ve spent the last several years blogging about. It’s a different style of SOA which has services stretch up to contain parts of the UI as well as down to contain parts of the database, resulting in a composite UI and multiple databases. This is a drastically different approach than much of the literature on the topic – especially Thomas Erl’s books.
Unfortunately there isn’t a book out there with all of this in it (that I’ve found), and I’m afraid that with my schedule (and family) writing a book is pretty much out of the question. Let’s face it – I’m barely finding time to blog.
The one thing I’m trying to do more of is provide training on these topics. I’ve just finished a course in London, doing another this week in Aarhus Denmark, and another next month in San Francisco (which is now sold out). The next openings this year will be in Stockholm, London; Sydney Australia and Austin Texas will be coming in January of next year. I’ll be coming over to the US more next year so if you missed San Francisco, keep an eye out.
I wish there was more I could do, but I’m only one guy.
Hmm, maybe it’s time to change that.
Friday, July 22nd, 2011
For those of you who haven’t heard yet, the next version of NServiceBus will be making use of RavenDB as its default storage engine.
For those of you who haven’t heard about RavenDB yet – it’s a transactional document database for .NET, and it’s been/being developed by my good friend and partner in crime Ayende Rahien (a.k.a Oren Eini). Although Ayende is known for his “not invented here” tendencies, when it comes to transactional document databases, well, you’d have been hard pressed to find one – especially with a decent .NET API.
In NServiceBus we have a variety of storage needs – from things like durable subscriptions so that you get fault-tolerant pub/sub, through persistence of long-running workflow (a.k.a saga) state, and durable timeouts so that your time-bound long-running processes never get stuck. None of these actually require relations – we were just using relational databases for storage because it was the easy answer everyone was going with.
Relational vs. Document
And these relational databases came with some downsides – developers had to “beg” their DBA to create the needed tables in the production databases, when that central database was down it prevented otherwise autonomous publishers and subscribers from going about their business, and it was very difficult to version the long-running workflows especially when newer versions required different state.
By moving to an embedded and transactional document DB, no longer do you need to bargain with the DBA, by keeping the storage together with the processing nodes you are back in parallel processing bliss, and the schema-less nature of the storage makes versioning those long-running workflows much easier.
And I’m also happy to announce that an agreement has been reached with Hibernating Rhinos (Ayende’s company) so you won’t need to license RavenDB separately to get all of these benefits. RavenDB will be bundled with NServiceBus so licensing NServiceBus covers them both (for the storage needs mentioned above).
If you’re also thinking about using RavenDB as the storage for the rest of your system, you’ll be able to get a nice discount on the license when purchasing it together with NServiceBus. This will be going into effect with the release of NServiceBus 3.0 this October.
Companies Using NServiceBus
Interestingly enough, there’s been a big uptick in companies using NServiceBus with the introduction of licensing. Most of these companies are not the kind of big, stand-up-and-take-notice names that everybody likes to have on their roster.
What makes NServiceBus particularly attractive is that you can get a lot done without requiring some kind of dedicated BizTalk/WebSphere/Tibco expert. This has brought down the barrier for thousands of developers who just want to get on with the business of getting their app to market.
And when it comes to the big names, well, once they see how much faster they can get stuff done with NServiceBus as well as how robust and scalable it is in production, they don’t want any of their competitors to know about it!
Anyway, I’m glad to say that two companies have stepped forwards: Rackspace and Reuters. Hopefully we’ll get confirmation from one of the big banks soon that we can go public with them too.
Exciting times ahead.
Friday, April 22nd, 2011
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.
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.
Saturday, March 5th, 2011
One of the things I cover early on in my course is the problem with traditional layered architecture driving people to create a business logic layer made up of a bunch of inter-related entities. I see this happening a lot, even though nowadays people are calling that bunch of inter-related entities a “domain model”.
Let me just say this upfront – most inter-related entity models are NOT a domain model.
Here’s why: most transactions don’t respect entity boundaries.
That being said, you don’t always need a domain model.
The domain model pattern’s context is “if you have complicated and everchanging business rules” – right there on page 119 of Patterns of Enterprise Application Architecture.
Persisting the customer’s first name, last name, and middle initial – and later reading and showing that data does not sound either complicated or that it is really going to change that much.
Then there are things like credit limits, that may be on the customer entity as well. It is likely that there are business requirements that expect that value to be consistent with the total value of unpaid orders – data that comes from other entities.
The problem that is created is one of throughput.
Since databases lock an entire row/entity at a time, if one transaction is changing the customer’s first name, the database would block another transaction that tried to change the same customer’s credit limit.
The bigger your entities, the more transactions will likely need to operate on them in parallel, the slower your system will get as the number of transactions increases. This feeds back in on itself as often those blocked transactions will have operated already on some other entity, leaving those locked for longer periods of times, blocking even more transactions.
And the absurd thing is that the business never demanded that the customer’s first name be consistent with the credit limit.
What if we didn’t have a single Customer entity?
What if we had one that contained first name, last name, middle initial and another that contained things like credit limit, status, and risk rating. These entities would be correlated by the same ID, but could be stored in separate tables in the database. That would do away with much of the cascading locking effects drastically improving our throughput as load increases.
And you know what? That division would still respect the 3rd normal form.
Which of these entities do you think would be classified by the business under the “complicated and everchanging rules” category?
And for those entities that are just about data persistence – do you think it’s justified to use 3 tiers? Do we really need a view model which we transform to data transfer objects which we transform to domain objects which we transform to relational tables and then all the way back? Wouldn’t some simpler 2-tier programming suffice – dare I say datasets? Ruby on Rails?
Are we ready to leave behind the assumption that all elements of a given layer must be built the same way?
Monday, September 7th, 2009
So, I’ve gotten back from a most enjoyable couple of days in Sweden where I gave two half-day tutorials, the first being the SOA and UI composition talk I gave at the European Virtual ALT.NET meeting (which you can find online here) and the other on DDD in enterprise apps (the first time I’ve done this talk).
I’ve gotten some questions about my DDD presentation there based on Aaron Jensen’s pictures:
Yes – I talk with my hands. All the time.
That slide is quite an important one – I talked about it for at least 2 hours.
Here it is again, this time in full:
You may notice that the nice clean layered abstraction that the industry has gotten so comfortable with doesn’t quite sit right when looking at it from this perspective. The reason for that is that this perspective takes into account physical distribution while layers don’t.
I’ll have some more posts on this topic as well as giving a session in TechEd Europe this November.
Oh – and please do feel free to already send your questions in.
Tuesday, September 1st, 2009
After reading Ayende’s post advocating against “soft deletes” I felt that I should add a bit more to the topic as there were some important business semantics missing. As developers discuss the pertinence of using an IsDeleted column in the database to mark deletion, and the way this relates to reporting and auditing concerns is weighed, the core domain concepts rarely get a mention. Let’s first understand the business scenarios we’re modeling, the why behind them, before delving into the how of implementation.
The real world doesn’t cascade
Let’s say our marketing department decides to delete an item from the catalog. Should all previous orders containing that item just disappear? And cascading farther, should all invoices for those orders be deleted as well? Going on, would we have to redo the company’s profit and loss statements?
So, is Ayende wrong? Do we really need soft deletes after all?
On the one hand, we don’t want to leave our database in an inconsistent state with invoices pointing to non-existent orders, but on the other hand, our users did ask us to delete an entity.
Or did they?
When all you have is a hammer…
We’ve been exposing users to entity-based interfaces with “create, read, update, delete” semantics in them for so long that they have started presenting us requirements using that same language, even though it’s an extremely poor fit.
Instead of accepting “delete” as a normal user action, let’s go into why users “delete” stuff, and what they actually intend to do.
The guys in marketing can’t actually make all physical instances of a product disappear – nor would they want to. In talking with these users, we might discover that their intent is quite different:
“What I mean by ‘delete’ is that the product should be discontinued. We don’t want to sell this line of product anymore. We want to get rid of the inventory we have, but not order any more from our supplier. The product shouldn’t appear any more when customers do a product search or category listing, but the guys in the warehouse will still need to manage these items in the interim. It’s much shorter to just say ‘delete’ though.”
There seem to be quite a few interesting business rules and processes there, but nothing that looks like it could be solved by a single database column.
Model the task, not the data
Looking back at the story our friend from marketing told us, his intent is to discontinue the product – not to delete it in any technical sense of the word. As such, we probably should provide a more explicit representation of this task in the user interface than just selecting a row in some grid and clicking the ‘delete’ button (and “Are you sure?” isn’t it).
As we broaden our perspective to more parts of the system, we see this same pattern repeating:
Orders aren’t deleted – they’re cancelled. There may also be fees incurred if the order is canceled too late.
Employees aren’t deleted – they’re fired (or possibly retired). A compensation package often needs to be handled.
Jobs aren’t deleted – they’re filled (or their requisition is revoked).
In all cases, the thing we should focus on is the task the user wishes to perform, rather than on the technical action to be performed on one entity or another. In almost all cases, more than one entity needs to be considered.
In all the examples above, what we see is a replacement of the technical action ‘delete’ with a relevant business action. At the entity level, instead of having a (hidden) technical WasDeleted status, we see an explicit business status that users need to be aware of.
The manager of the warehouse needs to know that a product is discontinued so that they don’t order any more stock from the supplier. In today’s world of retail with Vendor Managed Inventory, this often happens together with a modification to an agreement with the vendor, or possibly a cancellation of that agreement.
This isn’t just a case of transactional or reporting boundaries – users in different contexts need to see different things at different times as the status changes to reflect the entity’s place in the business lifecycle. Customers shouldn’t see discontinued products at all. Warehouse workers should, that is, until the corresponding Stock Keeping Unit (SKU) has been revoked (another status) after we’ve sold all the inventory we wanted (and maybe returned the rest back to the supplier).
Rules and Validation
When looking at the world through over-simplified-delete-glasses, we may consider the logic dictating when we can delete to be quite simple: do some role-based-security checks, check that the entity exists, delete. Piece of cake.
The real world is a bigger, more complicated cake.
Let’s consider deleting an order, or rather, canceling it. On top of the regular security checks, we’ve got some rules to consider:
If the order has already been delivered, check if the customer isn’t happy with what they got, and go about returning the order.
If the order contained products “made to order”, charge the customer for a portion (or all) of the order (based on other rules).
Deciding what the next status should be may very well depend on the current business status of the entity. Deciding if that change of state is allowed is context and time specific – at one point in time the task may have been allowed, but later not. The logic here is not necessarily entirely related to the entity being “deleted” – there may be other entities which need to be checked, and whose status may also need to be changed as well.
I know that some of you are thinking, “my system isn’t that complex – we can just delete and be done with it”.
My question to you would be, have you asked your users why they’re deleting things? Have you asked them about additional statuses and rules dictating how entities move as groups between them? You don’t want the success of your project to be undermined by that kind of unfounded assumption, do you?
The reason we’re given budgets to build business applications is because of the richness in business rules and statuses that ultimately provide value to users and a competitive advantage to the business. If that value wasn’t there, wouldn’t we be serving our users better by just giving them Microsoft Access?
In closing, given that you’re not giving your users MS Access, don’t think about deleting entities. Look for the reason why. Understand the different statuses that entities move between. Ask which users need to care about which status. I know it doesn’t show up as nicely on your resume as “3 years WXF”, but “saved the company $4 million in wasted inventory” does speak volumes.
One last sentence: Don’t delete. Just don’t.
Sunday, August 2nd, 2009
My article on “employing the domain model pattern” has been published in the August edition of MSDN Magazine.
Here’s a short excerpt:
“In this article, we’ll go through the reasons to (and not to) employ the domain model pattern, the benefits it brings, as well as provide some practical tips on keeping the overall solution as simple as possible.”
Saturday, January 24th, 2009
The ability to map entity relationships is broadly supported by many O/RM tools. For some reason, though, many developers run into issues when trying to map a many-to-many relationship between entities. Although much has already been written about the technological aspects of it, I thought I’d take more of an architectural / DDD perspective on it here.
Value Objects Don’t Count
While the canonical example presented is Customer -> Address, and has a good treatment here for nHibernate, it isn’t architecturally representative.
Addresses are value objects. What this means is that if we have to instance of the Address class, and they both have the same business data, they are semantically equivalent. Customers, on the other had, are not value objects – they’re entities. If we have two customers with the same business data (both of them called Bob Smith), that does not mean they are semantically equivalent – they are not the same person.
Therefore, for our purposes here we’ll use something different. Say we have an entity called Job which is something that a company wants to hire for. It has a title, description, skill level, and a bunch of other data. Say we also have another entity called Job Board which is where the company posts jobs so that applicants can see them, like Monster.com. A job board has a name, description, web site, referral fee, and a bunch of other data.
A job can be posted to multiple job boards. And a job board can have multiple jobs posted. A regular many to many relationship. At this point, we’re not even going to complicate the association.
This is simply represented in the DB with an association table containing two columns for each of the entity tables’ ids.
In the domain model, developers can also represent this with the Job class containing a list of JobBoard instances, and the JobBoard class containing a list of jobs.
It’s intuitive. Simple. Easy to implement. And wrong.
In order to make intelligent DDD choices, we’re going to first take what may seem to be a tangential course, but I assure you that your aggregate roots depend on it.
Moving forward with our example
Let’s say the user picks a job, and then ticks off the job boards where they want the job posted, and clicks submit.
For simplicity’s sake, at this point, let’s ignore the communication with the actual job sites, assuming that if we can get the association into the DB, magic will happen later causing the job to appear on all the sites.
Our well-intentioned developer takes the job ID, and all the job board IDs, opens a transaction, gets the job object, gets the job board objects, adds all the job board objects to the job, and commits, as follows:
1: public void PostJobToBoards(Guid jobId, params Guid boardIds)
3: using (ISession s = this.SessionFactory.OpenSession())
4: using (ITransaction tx = s.BeginTransaction())
6: var job = s.Get<Job>(jobId);
7: var boards = new List<JobBoard>();
9: foreach(Guid id in boardIds)
In this code, Job is our aggregate root. You can see that is the case since Job is the entry point that the service layer code uses to interact with the domain model. Soon we’ll see why this is wrong.
** Notice that in this service layer code, our well-intentioned developer is following the rule that while you can get as many objects as you like, you are only allowed one method call on one domain object. The code called in line 12 is what you’d pretty much expect:
1: public void PostTo(IList<JobBoard> boards)
3: foreach(JobBoard jb in boards)
Only that as we were committing, someone deleted one of the job boards just then. Or that someone updated the job board causing a concurrency conflict. Or anything that would cause one single association to not be created.
That would cause the whole transaction to fail and all changes to roll back.
Rightly so, thinks our well-intentioned developer.
But users don’t think like well-intentioned developers.
If I were to go to the grocery store with the list my wife gave me, finding that they’re out of hazelnuts (the last item on the list), would NOT buy all the other groceries and go home empty handed, what do you think would happen?
Right. That’s how users look at us developers. Before running off and writing a bunch of code, we need to understand the business semantics of users actions, including asking about partial failures.
The list isn’t a unit of work that needs to succeed or rollback atomically. It’s actually many units of work. I mean, I wouldn’t want my wife to send me to the store 10 times to buy 10 items, so the list is really just a kind of user shortcut. Therefore, in the job board scenario, each job to job board connection is its own transaction.
This is more common than you might think.
Once you go looking for cases where the domain is forgiving of partial failures, you may start seeing more and more of them.
In the original transaction where we tried to connect many job boards to a single job, we saw that the single job is the aggregate root. However, once we have multiple transactions, each connecting one job and one job board, the job board is just as likely an aggregate root as the job.
We can do jobBoard.Post(job); or job.PostTo(jobBoard);
But we need just a bit more analysis to come to the right decision.
While we could just leave the bi-directional/circular dependency between them, it would be preferable if we could make it uni-directional instead. To do that, we need to understand their relationship:
If there was no such thing as “job”, would there be meaning to “job board” ? Probably not.
If there was no such thing as “job board”, would there be meaning to “job” ? Probably. Yes. Our company can handle the hiring process of a job regardless of whether the candidate came in through Monster.com or not.
From this we understand that the uni-directional relationship can be modelled as one-to-many from job board to job. The Job class would no longer have a collection of Job Board objects. In fact, it could even be in an assembly separate from Job Board and not reference Job Board in any way. Job Board, on the other hand, would still have a collection of Job objects.
Going back to the code above we see that the right choice is jobBoard.Post(job);
Job Board is the aggregate root in this case. Also, the many-to-many mapping has now dissolved, leaving behind a single one-to-many mapping.
Let that sink in a second.
While the GUI showing which jobs are posted on a given job board are well served by the above decision (simply traversing the object graph from Job Board to its collection of Jobs), that’s not the whole story. Another GUI needs to show administrative users which Job Boards a given Job has been posted to. Since we no longer have the domain-level connection, we can’t traverse myJob.JobBoards.
Our only option is to perform a query. That’s not so bad, but not as pretty as object traversal.
The real benefit is in chopping apart the Gordian M-to-N mapping knot and getting a cleaner, more well factored domain model.
That gives us much greater leverage for bigger, system-level decomposition.
We’re now all set to move up to a pub/sub solution between these aggregate roots, effectively upgrading them to Bounded Contexts. From there, we can move to full-blown internet-scale caching with REST for extra scalability on showing a job board with all its jobs.
We often look at many-to-many relationships just like any other relationship. And from a purely technical perspective, we’re not wrong. However, the business reality around these relationships is often very different – forgiving of partial failures, to the point of actually requiring them.
Since the business folks who provide us with requirements rarely think of failure scenarios, they don’t specify that “between these two entities here, I don’t want transactional atomicity” (rolling our technical eyes – the idiots [sarcasm, just to make sure you don't misread me]).
Yet, if we were to spell out what the system will do under failure conditions when transactionally atomic, those same business folks will be rolling our eyes back to us.
What I’ve found surprises some DDD practitioners is how critical this issue really is to arriving at the correct aggregate roots and bounded contexts.
It’s also simple, and practical, so you won’t be offending the YAGNI police.
From CRUD to Domain-Driven Fluency
[Podcast] Domain Models, SOA, and The Single Version of the Truth
Bryan Wheeler, Director Platform Development at msnbc.com
“Udi Dahan is the real deal.
We brought him on site to give our development staff the 5-day “Advanced Distributed System Design” training. The course profoundly changed our understanding and approach to SOA and distributed systems.
Consider some of the evidence: 1. Months later, developers still make allusions to concepts learned in the course nearly every day 2. One of our developers went home and made her husband (a developer at another company) sign up for the course at a subsequent date/venue 3. Based on what we learned, we’ve made constant improvements to our architecture that have helped us to adapt to our ever changing business domain at scale and speed If you have the opportunity to receive the training, you will make a substantial paradigm shift.
If I were to do the whole thing over again, I’d start the week by playing the clip from the Matrix where Morpheus offers Neo the choice between the red and blue pills. Once you make the intellectual leap, you’ll never look at distributed systems the same way.
Beyond the training, we were able to spend some time with Udi discussing issues unique to our business domain. Because Udi is a rare combination of a big picture thinker and a low level doer, he can quickly hone in on various issues and quickly make good (if not startling) recommendations to help solve tough technical issues.” November 11, 2010
Sam Gentile, Independent WCF & SOA Expert
“Udi, one of the great minds in this area.
A man I respect immensely.”
Ian Robinson, Principal Consultant at ThoughtWorks
"Your blog and articles have been enormously useful in shaping, testing and refining my own approach to delivering on SOA initiatives over the last few years. Over and against a certain 3-layer-application-architecture-blown-out-to- distributed-proportions school of SOA, your writing, steers a far more valuable course
Shy Cohen, Senior Program Manager at Microsoft
“Udi is a world renowned software architect and speaker. I met Udi at a conference that we were both speaking at, and immediately recognized his keen insight and razor-sharp intellect. Our shared passion for SOA and the advancement of its practice launched a discussion that lasted into the small hours of the night.
It was evident through that discussion that Udi is one of the most knowledgeable people in the SOA space. It was also clear why – Udi does not settle for mediocrity, and seeks to fully understand (or define) the logic and principles behind things.Humble yet uncompromising, Udi is a pleasure to interact with
Glenn Block, Senior Program Manager - WCF at Microsoft
“I have known Udi for many years having attended his workshops and having several personal interactions including working with him when we were building our Composite Application Guidance in patterns & practices. What impresses me about Udi is his deep insight into how to address business problems through sound architecture
. Backed by many years of building mission critical real world distributed systems it is no wonder that Udi is the best at what he does. When customers have deep issues with their system design, I point them Udi's way.”
Karl Wannenmacher, Senior Lead Expert at Frequentis AG
“I have been following Udi’s blog and podcasts since 2007. I’m convinced that he is one of the most knowledgeable and experienced people in the field of SOA, EDA and large scale systems.
Udi helped Frequentis to design a major subsystem of a large mission critical system with a nationwide deployment based on NServiceBus
. It was impressive to see how he took the initial architecture and turned it upside down leading to a very flexible and scalable yet simple system without knowing the details of the business domain.
I highly recommend consulting with Udi when it comes to large scale mission critical systems in any domain.”
Simon Segal, Independent Consultant
“Udi is one of the outstanding software development minds in the world today, his vast insights into Service Oriented Architectures and Smart Clients in particular are indeed a rare commodity. Udi is also an exceptional teacher
and can help lead teams to fall into the pit of success. I would recommend Udi to anyone considering some Architecural guidance and support in their next project.”
Ohad Israeli, Chief Architect at Hewlett-Packard, Indigo Division
“When you need a man to do the job Udi is your man! No matter if you are facing near deadline deadlock or at the early stages of your development, if you have a problem Udi is the one who will probably be able to solve it, with his large experience at the industry and his widely horizons of thinking , he is always full of just in place great architectural ideas
I am honored to have Udi as a colleague and a friend (plus having his cell phone on my speed dial).”
Ward Bell, VP Product Development at IdeaBlade
“Everyone will tell you how smart and knowledgable Udi is ... and they are oh-so-right. Let me add that Udi is a smart LISTENER. He's always calibrating what he has to offer with your needs and your experience ... looking for the fit. He has strongly held views ... and the ability to temper them with the nuances of the situation.I trust Udi to tell me what I need to hear, even if I don't want to hear it, ... in a way that I can hear it.
That's a rare skill to go along with his command and intelligence.”
Eli Brin, Program Manager at RISCO Group
“We hired Udi as a SOA specialist for a large scale project. The development is outsourced to India. SOA is a buzzword used almost for anything today. We wanted to understand what SOA really is, and what is the meaning and practice to develop a SOA based system.
We identified Udi as the one that can put some sense and order in our minds. We started with a private customized SOA training for the entire team in Israel. After that I had several focused sessions regarding our architecture and design.
I will summarize it simply (as he is the software simplist): We are very happy to have Udi in our project. It has a great benefit. We feel good and assured with the knowledge and practice he brings. He doesn’t talk over our heads
. We assimilated nServicebus as the ESB of the project. I highly recommend you to bring Udi into your project.”
Catherine Hole, Senior Project Manager at the Norwegian Health Network
“My colleagues and I have spent five interesting days with Udi - diving into the many aspects of SOA. Udi has shown impressive abilities of understanding organizational challenges, and has brought the business perspective into our way of looking at services. He has an excellent understanding of the many layers from business at the top to the technical infrstructure at the bottom. He is a great listener, and manages to simplify challenges in a way that is understandable both for developers and CEOs, and all the specialists in between
Yoel Arnon, MSMQ Expert
“Udi has a unique, in depth understanding of service oriented architecture and how it should be used in the real world
, combined with excellent presentation skills. I think Udi should be a premier choice for a consultant or architect of distributed systems.”
Vadim Mesonzhnik, Development Project Lead at Polycom
“When we were faced with a task of creating a high performance server for a video-tele conferencing domain we decided to opt for a stateless cluster with SQL server approach. In order to confirm our decision we invited Udi.
After carefully listening for 2 hours he said: "With your kind of high availability and performance requirements you don’t want to go with stateless architecture."
One simple sentence saved us from implementing a wrong product and finding that out after years of development. No matter whether our former decisions were confirmed or altered, it gave us great confidence to move forward relying on the experience, industry best-practices and time-proven techniques that Udi shared with us.
It was a distinct pleasure and a unique opportunity to learn from someone who is among the best at what he does.”
Jack Van Hoof, Enterprise Integration Architect at Dutch Railways
“Udi is a respected visionary on SOA and EDA, whose opinion I most of the time (if not always) highly agree with. The nice thing about Udi is that he is able to explain architectural concepts in terms of practical code-level examples
Neil Robbins, Applications Architect at Brit Insurance
“Having followed Udi's blog and other writings for a number of years I attended Udi's two day course on 'Loosely Coupled Messaging with NServiceBus' at SkillsMatter, London.
I would strongly recommend this course to anyone with an interest in how to develop IT systems which provide immediate and future fitness for purpose. An influential and innovative thought leader and practitioner in his field, Udi demonstrates and shares a phenomenally in depth knowledge that proves his position as one of the premier experts in his field globally.
The course has enhanced my knowledge and skills in ways that I am able to immediately apply to provide benefits to my employer. Additionally though I will be able to build upon what I learned in my 2 days with Udi and have no doubt that it will only enhance my future career.
I cannot recommend Udi, and his courses, highly enough
Nick Malik, Enterprise Architect at Microsoft Corporation
“You are an excellent speaker and trainer, Udi
, and I've had the fortunate experience of having attended one of your presentations. I believe that you are a knowledgable and intelligent man.”
Sean Farmar, Chief Technical Architect at Candidate Manager Ltd
“Udi has provided us with guidance in system architecture and supports our implementation of NServiceBus in our core business application.
He accompanied us in all stages of our development cycle and helped us put vision into real life distributed scalable software. He brought fresh thinking, great in depth of understanding software, and ongoing support that proved as valuable and cost effective.
Udi has the unique ability to analyze the business problem and come up with a simple and elegant solution for the code and the business alike.With Udi's attention to details, and knowledge we avoided pit falls that would cost us dearly
Børge Hansen, Architect Advisor at Microsoft
“Udi delivered a 5 hour long workshop on SOA for aspiring architects in Norway. While keeping everyone awake and excited
Udi gave us some great insights and really delivered on making complex software challenges simple. Truly the software simplist.”
Motty Cohen, SW Manager at KorenTec Technologies
“I know Udi very well from our mutual work at KorenTec. During the analysis and design of a complex, distributed C4I system - where the basic concepts of NServiceBus start to emerge - I gained a lot of "Udi's hours" so I can surely say that he is a professional, skilled architect with fresh ideas and unique perspective
for solving complex architecture challenges. His ideas, concepts and parts of the artifacts are the basis of several state-of-the-art C4I systems that I was involved in their architecture design.”
Aaron Jensen, VP of Engineering at Eleutian Technology
“Awesome. Just awesome.
We’d been meaning to delve into messaging at Eleutian after multiple discussions with and blog posts from Greg Young and Udi Dahan in the past. We weren’t entirely sure where to start, how to start, what tools to use, how to use them, etc. Being able to sit in a room with Udi for an entire week while he described exactly how, why and what he does to tackle a massive enterprise system was invaluable to say the least.
We now have a much better direction and, more importantly, have the confidence we need to start introducing these powerful concepts into production at Eleutian.”
Gad Rosenthal, Department Manager at Retalix
“A thinking person. Brought fresh and valuable ideas that helped us in architecting our product. When recommending a solution he supports it with evidence and detail so you can successfully act based on it
. Udi's support "comes on all levels" - As the solution architect through to the detailed class design. Trustworthy!”
Chris Bilson, Developer at Russell Investment Group
“I had the pleasure of attending a workshop Udi led at the Seattle ALT.NET conference in February 2009. I have been reading Udi's articles and listening to his podcasts for a long time and have always looked to him as a source of advice on software architecture.
When I actually met him and talked to him I was even more impressed. Not only is Udi an extremely likable person, he's got that rare gift of being able to explain complex concepts and ideas in a way that is easy to understand
All the attendees of the workshop greatly appreciate the time he spent with us and the amazing insights into service oriented architecture he shared with us.”
Alexey Shestialtynov, Senior .Net Developer at Candidate Manager
“I met Udi at Candidate Manager where he was brought in part-time as a consultant to help the company make its flagship product more scalable. For me, even after 30 years in software development, working with Udi was a great learning experience
. I simply love his fresh ideas and architecture insights.
As we all know it is not enough to be armed with best tools and technologies to be successful in software - there is still human factor involved. When, as it happens, the project got in trouble, management asked Udi to step into a leadership role and bring it back on track. This he did in the span of a month. I can only wish that things had been done this way from the very beginning.
I look forward to working with Udi again in the future.”
Christopher Bennage, President at Blue Spire Consulting, Inc.
“My company was hired to be the primary development team for a large scale and highly distributed application. Since these are not necessarily everyday requirements, we wanted to bring in some additional expertise. We chose Udi because of his blogging, podcasting, and speaking. We asked him to to review our architectural strategy as well as the overall viability of project.
I was very impressed, as Udi demonstrated a broad understanding of the sorts of problems we would face. His advice was honest and unbiased and very pragmatic. Whenever I questioned him on particular points, he was able to backup his opinion with real life examples.
I was also impressed with his clarity and precision. He was very careful to untangle the meaning of words that might be overloaded or otherwise confusing. While Udi's hourly rate may not be the cheapest, the ROI is undoubtedly a deal
I would highly recommend consulting with Udi.”
Robert Lewkovich, Product / Development Manager at Eggs Overnight
“Udi's advice and consulting were a huge time saver for the project I'm responsible for. The $ spent were well worth it
and provided me with a more complete understanding of nServiceBus and most importantly in helping make the correct architectural decisions earlier thereby reducing later, and more expensive, rework.”
Ray Houston, Director of Development at TOPAZ Technologies
“Udi's SOA class made me smart - it was awesome.
The class was very well put together
. The materials were clear and concise and Udi did a fantastic job presenting it. It was a good mixture of lecture, coding, and question and answer. I fully expected that I would be taking notes like crazy, but it was so well laid out that the only thing I wrote down the entire course was what I wanted for lunch. Udi provided us with all the lecture materials and everyone has access to all of the samples which are in the nServiceBus trunk.
Now I know why Udi is the "Software Simplist." I was amazed to find that all the code and solutions were indeed very simple. The patterns that Udi presented keep things simple by isolating complexity so that it doesn't creep into your day to day code. The domain code looks the same if it's running in a single process or if it's running in 100 processes.”
Ian Cooper, Team Lead at Beazley
“Udi is one of the leaders in the .Net development community, one of the truly smart guys who do not just get best architectural practice well enough to educate others but drives innovation. Udi consistently challenges my thinking in ways that make me better at what I do
Liron Levy, Team Leader at Rafael
“I've met Udi when I worked as a team leader in Rafael. One of the most senior managers there knew Udi because he was doing superb architecture job in another Rafael project and he recommended bringing him on board to help the project I was leading.
Udi brought with him fresh solutions and invaluable deep architecture insights. He is an authority on SOA (service oriented architecture) and this was a tremendous help in our project.
On the personal level - Udi is a great communicator and can persuade even the most difficult audiences
(I was part of such an audience myself..) by bringing sound explanations that draw on his extensive knowledge in the software business. Working with Udi was a great learning experience for me, and I'll be happy to work with him again in the future.”
Adam Dymitruk, Director of IT at Apara Systems
“I met Udi for the first time at DevTeach in Montreal back in early 2007. While Udi is usually involved in SOA subjects, his knowledge spans all of a software development company's concerns
. I would not hesitate to recommend Udi for any company that needs excellent leadership, mentoring, problem solving, application of patterns, implementation of methodologies and straight out solution development.
There are very few people in the world that are as dedicated to their craft as Udi is to his. At ALT.NET Seattle, Udi explained many core ideas about SOA. The team that I brought with me found his workshop and other talks the highlight of the event and provided the most value to us and our organization. I am thrilled to have the opportunity to recommend him.”
Eytan Michaeli, 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, .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, 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, 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, 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, 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, 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, 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, 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, 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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