We are currently doing some research about CQRS and Event Sourcing and found two major frameworks taking care of these two concerns: Axon Framework and Eventuate. Both are continuesly developed, while Eventuate is now more actively developed in the RBMH repository.
I was GIT cloning some sample projects for both and came to the conlusion that Eventuate looks a little bit more lightweight, however I assume this is due to lack of features compared to Axon.
I tried in vain to find a comparison between those 2 frameworks, also here in Stackoverflow I couldn't find any. Does someone have some input?
Being the original author of Axon Framework, this answer may be a bit biased. I am also not an expert in Eventuate, although I am familiar with its concepts.
Axon has been around for about 7 years and is being used by many systems in production since then. It appears that Eventuate has been designed as a SaaS solution (with open source local-install alternative), which is currently in Alpha.
Both frameworks seem to have been designed with different goals in mind. Where Eventuate focuses on Event Sourcing and event publication, Axon's focus is much more on the abstraction of messaging, where messaging is both Commands, Events and (in a later stage also) Queries. It provides annotations to demarcate Event and Command handlers. It also allows you to design your entities in a way that keeps it decoupled from the framework's internals.
This abstraction allows you to "plug in" any database you would like to use as Event Store. Axon provides support for JPA, JDBC and MongoDB out of the box. As far as I can tell, Eventuate provides its own event store implementation.
Related
Recently I'm learning CQRS and want to change my system to use event-sourcing patterns.
But I found that on .Net platform, there is two Event Store implementations.
NEventStore(formerly JOliver EventStore)
EventStoreDB(from EventStore.com)
The two implementations confuse me - can someone please explain the key differences between them.
NEventStore is a persistence agnostic event sourcing library for .NET with multiple storage options such as relational and document databases.
EventStoreDB is a newer and high performing event sourcing solution with its own persistence engine and API's for multiple protocols and languages. It has open source and commercial options.
As far as I know you can use EventStoreDB as a persistence engine for NEventStore in a future version of NEventStore as well.
Additionally to mentioned EventStoreDB and NEventStore there is relatively new SQLStreamStore library, that conceptually similar to NEventStore.
They have a few essential differences, but historically SQLStreamStore can be consider as a successor of NEventStore. The author of the SQLStreamStore damianh previously was a maintainer of NEventStore. The latest release of NEventStore was 15 Mar 2014, SQLStreamStore is actively developing.
For more details why SqlStreamStore was created see the answer SQLStreamStore vs NEventStore vs EventStore #108
I spent short time studing Habanero and i found it good approach for making Enterprise Application in a really short period of time.
The pattern witch Habanero use is "Active Record" as it's developers say.
My questions are:
There any similar application like Habanero witch fully support Domain
Driven Design by determining aggregate roots, entities and value objects
Is it right decision to use such tools in big organizations
Does it worth training our team on such a tool
thank you
Framework support for Domain Driven Design is quite different from frameworks supporting data driven applications. Such framework should increase the productivity of developers that works with an ubiquitous language that evolves with the business and that is learned by a domain-expert.
They should not face concepts like aggregates, root, value objects because they are just modelling concepts, conceptual tools, but ways to ease the development process. Thus a framework exposing abstract classes or interfaces named AggregateRoot, Entity or ValueObject is fundamentally broken. It doesn't provides any real value to an application, just useless indirections.
However:
There are a few frameworks designed to support domain driven design, listed here. Moreover, I'm developing one by myself based on previous experiences that worked very well
It depends, obviosly. For example we used all of the Epic's modeling patterns with success.
We used some "home made" framewoks too, and some of them proved to really increase productivity. However, such frameworks (if useful) always have steep learning curves and it depends very much on how much reliable the software have to be and what are the developers skills.
It depends on the framework, on the complexity of the business (if you don't need a domain expert to understand it, you don't need DDD) and on the developers, too. I faced successful stories and huge failures with different frameworks in different contexts. I've also had a conference that faced the topic (you can see the slides here).
Besides missing some of the benefits of Event Sourcing, are there any other drawbacks to adapting an existing architecture to CQRS without the Event Sourcing piece?
I'm working on large application and the developers should be able to handle separating the existing architecture into Commands and Queries over the next few months, but asking them to also add in the Event Sourcing at this stage would be a HUGE problem from a resourcing perspective. Am I committing sacrilege by not including Event Sourcing?
Event Sourcing is optional and in most cases complicates things more than it helps if introduced too early. Especially when transitioning from a legacy architecture and even more when the team has no experience with CQRS.
Most of the advantages being attributed to ES can be obtained by storing your events in a simple Event Log. You don't have to drop your state-based persistence, (but in the long run you probably will, because at some point it will become the logical next step).
My recommendation: Simplicity is the key. Do one step at a time, especially when introducing such a dramatic paradigm shift. Start with simple CQRS, then introduce an Event Log when you (and your team) have become used to the new concepts. Then, if at all required, change your persistence to Event Sourcing and fire the DBA ;-)
I completely agree with Dennis, ES is no precondition for CQRS, in fact CQRS on its own is pretty easy to implement and has the potential to really simplify your design.
You can find a smooth introduction to it here
Secondly what benefits does CQRS on its own bring to the table?
Simplifies your domain objects, by sucking out all the query concerns
Makes code scalable, your queries are separated and can be easily tuned
As you iterate over your product design you can add/remove/change
individual commands/queries, instead of dealing with larger
structures as a whole (e.g. entities, aggregates, modules).
Commands and queries produce a well-known vocabulary to talk with
domain experts. Other architectural patterns (e.g. pipes and filters,
actors) use terms and concepts that may be harder to grasp by
non-programmers.
Limits the use of ORM (if you use one), I feel ORM's bring in
unwarranted complexity if you try and use them for querying, the
abstractions are leaky and heavy, trying to tune them is a nightmare :). Using an ORM only on the command side makes things much
easier, plain old SQL is the best for queries, probably using a simple library to convert result sets into DTO's is the most you need.
More on how CQRS benefits design can be found here
Also do not forget about the non tangible benefits of CQRS
If you still have your doubts, you may want to read this
We currently use CQRS for projects with medium complexity and have found it be very suitable. We started out using a custom bootstrap code and have now moved on to using the Axon Framework to give us some of the infrastructure components
Feel free to PM me in case you want to know anything more specific.
There is a fundamental problem of Event Sourcing pattern, that is the events in the Event Store may not be compatible with your event handlers in your application any more due to code change.
That being said, whenever you modify the event handler by adding new features, you need to think about the backward compatibility. You have to make sure your code can always handle the same event in different stage created by different version of your code.
When your application becomes more complex, you will find it really pain in the ass to make it backward compatible.
I think Event Sourcing is what makes people to be afraid of CQRS. And thats for a reason. Its not natural - when you interact with something in real world you don't need to get whole history about this object.
“event sourcing is a completely orthogonal concept to CQRS” (source) - technically if you don't use ES you loose nothing from CQRS features.
I have no idea why Event Sourcing is considered as the only foundation for solving of some "messaging" related problems like: duplication/missing of messages, reordering of messages and data collisions, etc. Its not true that if you don't use Event Sourcing you cant create encapsulated means to solve such problems other way.
How i see alternative ways to implement CQRS's messaging using another data-organizing principle you can read here.
I propose "signed documents" approach where you treat your data not as composition of modification events, but as composition of immutable parts signed by responsible users. Im sure there can be a lot of other solutions to implement message flow and data storage. And you need to take your business model into account when selecting which one you like to use.
The best CQRS pattern based framework in my opinion is MediatR by Jimmy Bogard, If you don't need Event Sourcing in the beginning of your application development, MediatR is the right choice. Here is the repository- https://github.com/jbogard/MediatR
In my opinion, you're making a big mistake by not using event sourcing with CQRS.
First up, you'll almost certainly have issues synchronising your Query model with the Command model. With an event store, if the query side ever gets out of synch, you simply need to replay your events to correct it. That's the theory anyway!
But with Event Sourcing, you also get to store the complete history of all entity transactions. And this means you can decide to create new queries and views after implementation. These are very often views that would not be possible with non-Event Sourced CQRS. I've heard Greg Young give the example of querying items that have been added, and then removed, from a shopping cart. With Event Sourcing this is possible. Without ES it's not possible because you only store the final state of the cart.
What is CSLA Framework and Its use ?
My Opinions From My Experience w/ a 1.7M LOC code base:
CSLA is intended for a distributed application/database environment. This is why the basic business object is and does everything, for example it's own data persistence. An object (and everything remotely associated w/ its state) is intended to be serialized, sent to a different application and/or data server and work.
If the above is not a problem you need to solve, CSLA is overkill, big time. Our development team regrets having committed to CSLA.
Juggling all the CSLA balls in a complex Windowed UI is tough. We have multi-tabbed screens (which may in turn open sub-screens) that, unless you follow the "left to right, top to bottom" flow of data entry, and click save often, ends up putting and/or fetching incomplete data to/from the database; or dropping data altogether that you just entered. Yes, our original coders are at fault, but so is CSLA... It just seems that there are so many moving parts to enable, control, and coordinate CSLA features. It's like having to deal with all the dials & switches of a fighter jet when all you really need is something more like a Cessna 152.
You will write lots of custom code to enable the CSLA features. For example CSLA will never be confused with object relational mapper (ORM) tools like Hibernate and Entity Framework. Our SAVE() methods are non trivial, so are the trivial ones.
Encouraging the use of code generators compounds problems. We used CodeSMith to generate classes from data tables. So we end up with code that has a 1-1 correspondence of table to c# class. So you must write all the code to handle dataStore to your "real" objects.
Data store/ and fetch is very inefficient w/ CSLA. Because of the Behemoth, monolithic BusinessObject-does-all-and-knows-all centric paradigm, objects end up doing a one-object-at-a-time data fetch and instantiate. Collections of composite objects significantly compound the problem. A single "get this object" always results in a cascade of separate data fetches (one or more for each individual object) to instantiate the entire inheritance & composite relationship chains. Its known as the "N+1 query problem." Oh, and fetching data ALWAYS results in a new object being created, even if we're only updating an existing one. No wonder our more complex screens are FUBAR.
It allows you to architect your application with solid object oriented principals and a good seperation of concerns.
Yes and no. Mostly no.
The BusinessObject handles it's own data storing. That is anti separation of concerns.
"It allows you..." well, yeah - so does a blank text editor screen, but does not force or encourage you like the MVC.NET framework does, for example. IMHO, CLSA provides absolutely zero benefit for ensuring that the code you develop with it follows "solid OO principles". In fact coders w/ weak OO skills (the majority, in my experience) will really stand out when using CSLA! Woe betide the maintenance programmer.
CSLA is the poster child for the solid object oriented principle favor composition over inheritance. CLSA code is untestable. Because an inherited framework BusinessObject is, does, and needs everything, all at once and every time, it's not likely that you will be able to get much test coverage. You can't get at the pieces because everything is tightly coupled.The framework is not amenable to dependency injection. It is an iron curtain of code.
Your code will be difficult to debug. Call stacks get very deep and as you get near the center of the sun so to speak, everything turns into reflection - "what *&^# methods just got called???" And you simply get lost. period.
EDIT 7 Mar 2016
What more insight can I add after the original post? Two things, perhaps:
First, It feels like CSLA has some promise. If we knew how to juggle all those moving parts together. But CSLA is so enigmatic that even things we have done right are corrupted over time. IMHO without a very strong team-wide CSLA wherewithal, any implementation is doomed. Without a vibrant "open source" of technical references, training, and community it's hopeless. In almost a decade our CSLA code, in my final analysis, is just compounding technical debt.
Second, here is a recent comment I made, below:
Our complexity often does not seem to fit in the CSLA infrastructure
so we write outside of the framework. This and cheap labor results in
rampant SRP violations and has me hitting brick walls managing dynamic
rule application, for example. Then, CSLA parent/child infrastructure
propagates composite object validation but we don't always want c/p
relationships, so we write more validation and store code. So today
our CSLA implementation is inconsistent & confusing. Refactoring to
more-better CSLA will have profound domino effects. So after that
initial injection CSLA is essentially abandoned.
end Edit
CSLA is business object framework that allows you to easily create business objects on top of a data layer. It allows you to architect your application with solid object oriented principals and a good seperation of concerns.
I would highly recommend you read the CSLA book by Rocky Lhotka called Expert C# 2008 Business Objects. That will not only teach you about the framework but also teach good software architecture principals.
You can grab the book here on Amazon
I suggest reading the What is CSLA? page, and browse through the CSLA .NET FAQ site.
For the latest published information check out the Using CSLA 4 ebook series.
In reply to #radarbob https://stackoverflow.com/a/10922373/261363, hope I won't regret this and start a flame war.
Our team has been developing a couple of LOB applications with CSLA. From my experience on writing green field apps with CSLA and maintaining existing code here are my replies to your points.
The BO is not suppose to do it's own data persistence, you will have a Factory that will handle all data persistance, for example using a ORM to map to Models that are later on saved.
Sorry to hear that, I make sure I study the framework documentation and write at least one toy application before committing to a existing code database. Furthermore you can even download and browse the CSLA code.
You have BO -> Portal -> Factories that should not be very complicated the existing CSLA examples go a long way on explaining what is happening on each level.
CLSA should never be confused with a ORM
As you should, business object are rarely mapped to one table and thus require a bit of work when saving. In the case they are mapped to one table to you use something like AutoMapper to map your BO to your POCO in 1 line.
Look into CSLA Commands, also is nothing stopping you from keeping your BO as small or big as you want as long as you keep in mind that they are not the same as the POCO's that you will persist.
In a project we worked on we where able to easily test BO to ensure that the business logic was correct. Because of the nice separation of concerns we tested our Factories in isolation to make sure that business objects will be persisted accordingly.
At one point I was able to easily persist part of my BO's in MongoDB so the application was running on a hybrid database MSSQL and MongoDB without having to even change one line of code in my business objects, all I had to do was to update the factories to use Mongo instead of the current ORM.
Hope this addresses all your points in a fair manner,
Regards
CSLA: Component-based Scalable Logical Architecture
A paragraph in a nutshell that described CSLA to me from the website was this:
CSLA .NET enables you to create an object-oriented business layer that abstracts and encapsulates your business logic and data. The framework ensures your business objects work seamlessly with all .NET interface technologies, including WinRT XAML, WPF, ASP.NET MVC, ASP.NET Web Forms, WCF, asmx services, Windows Phone 7, Silverlight, Windows Workflow and Windows Forms.
Why you might use it:
Business rule management. Once you learn the business rule system, it provides a way to enforce business logic in a tidy package. If you have an object with child objects that need to report Validation to the parent most level, there is a way to handle that. (see http://www.lhotka.net/weblog/CSLA4BusinessRulesSubsystem.aspx for more information on the rule system)
You have business objects that you need to support N-level undo? A CSLA BusinessBase has a baked in property management system (like dependency properties) for functionality like N-Level Undo (it is actually completely implemented.) (This also ties into the business rule management. You can fire validation when a primary property changes, or you can change a property based on the value of another property.)
Data portal management. This one was an interesting concept. If I need to execute data operations locally, CSLA is configured for this out of the box. I can also stand up a WCF service that references my business object libraries, and use a few lines of configuration to make a WCF endpoint to manage data operations. The WCF service is a part of the CSLA framework. It was neat to see this in action. Other scenarios? Sure! Your business object library doesn't need to change, the DataPortal class determines if it needs to execute remotely or locally, according to your configuration.
CSLA does force you to use a few mechanisms that may not feel natural at first. I think that is somewhat true of any pattern you choose to implement. However, when it comes to the challenges of implementing Service Oriented Architecture, CSLA offers a lot. Yes, you are going to have an architect level developer authoring some of your libraries. However, if you're building an enterprise class application, shouldn't you be doing that already?
CSLA, when architected correctly, is testable. We use the repository pattern to replace out the actual dal with a mock layer and test both by specification (using NUnit/SpecFlow) and in a unit fashion when appropriate.
As far as support, including Rocky himself, there is a community of contributors that ensure things like CSLA.Net for Xamarin become a reality. There are consultancies that know CSLA and use it on a regular basis depending on the scope of work (and no, not just the consultancy for which I work.)
All things considered, CSLA may not be for you. Like others have indicated, read the site and the books (especially Expert C# 2008 Business Objects.) Ask questions, as the CSLA community tends to give quality advice.
CSLA is described in detail here. The new book is a great starting point. As a great compliment to the book I would recommend checking out our CSLA 3.8 templates. Rocky recommends using a Code Generator, and we have the leading set of templates, that will get you up and running in no time.
Thanks
-Blake Niemyjski (Author of the CodeSmith CSLA Templates)
Update: This question was inspired by my larger quest for mapping ontologically the whole software systems architecture enchilada. I've written a blog post about it, and hopefully it will help clarify what I'm after.
Many, many, many frameworks and stacks that's event-driven have too much variation for my little head to get around. Is there somewhere some resources that defines the outline of a reasonable Application Event Model, what events there are, and what triggers are most common?
I've got my own framework with a plugin and event-driven architecture, but I want to open-source it, and as such would like to make it closer to some common ground as not to alienate people.
So to clarify; this is for an application, meaning setting up the environment, the dependencies, the data sources (like databases), and being a MVC framework setting up the model, the view, launching controllers / actions, and in the GUI various stages of the interface (header, content, columns, etc.).
Ideas? Thoughts? Pointers? (And I've made it language and platform neutral at this point)
I read your blog entry, which btw I found an extremely interesting read, but... this question does not seem to reflect the broadness of the issue you are presenting there.
What you are after is very abstract and theoretical. What I mean to say is that if you tie any of those ideas to actual technology you will find yourself 'stuck' with it. This is why many of us are reluctant to use any framework. Especially the 'relabeled' products suddenly claiming to conform to the trend. We choose mainly on the basis of what appears to be needed to reach a predetermined result.
Frameworks (or tools in general) that target the application architecture domain distinguish themselves primarily by the amount of responsibility they are designed to take on. Spring for example only deals with the concept of decoupling and is therefore easily adopted and useable in many situations. The quality of any framework is expressed in terms of how well the designers of such frameworks were able to keep their products within the boundaries of that responsibility. Some front-to-end products will do exactly the opposite, code generators being among the 'worst' of them.
To answer your question at the top of this page, I do not think there is a framework that does what you want at this time and I do not think there is a single model of how applications (should) work. Keep in mind though that the application architecture domain deals with technology more than it does with concepts. In other words: If it works and meets the requirements, then you're pretty much done.
That said, you might find something of value in agent-based systems.
Heh. Most developers pick the major framework they like the tools for and stick with it. That's usually the winning strategy. I sympathize with your desire not to marry a single vendor.
Keep in mind however, that in developing your own framework, you're going to end up tied to a single vendor anyway. :-)
Is there somewhere some resources that defines the outline of a reasonable
Application Event Model, what events there are, and what triggers are most common?
I don't think so.
From what I see, there are two kinds of models out there: those with a real framework with which you can make a working data entry dialog, and abstract meta-meta-models that are optimized for modeling themselves.
Try surveying a few current frameworks that have good documentation online and cross-reference the major terminology in a spreadsheet. It's an interesting exercise.
I'd have a look at Spring for Java, and the XT Framework Spring module (http://springmodules.dev.java.net/docs/reference/0.9/html/xt.html), which apparently supports event-driven architecture, as starting points. Spring has an MVC framework (inc. convention-based routing to controllers), db configuration (for Hibernate, particularly), plus full dependency injection support. There's also a mechanism in Spring for modularising your web apps, called Spring Slices. And it can be integrated with Jersey for building RESTful apps.
(Unfortunately, I tried to provide links to everything, but this place only lets new users post a single link. So you'll have to do some googling :) )