I´m just trying to wrap my head around CQRS(/ES). I have not done anything serious with CQRS. Probably I´m just missing something very fundamental right now. Currently, I´m reading "Exploring CQRS and Event Sourcing". There is one sentence that somehow puzzles me in regards to commands:
"A single recipient processes a command."
I´ve seen this also in the CQRS sample application from Greg Young (FakeBus.cs) where an exception is thrown when more then one command handler is registered for any command type.
For me, this is an indication that this is a fundamental principle for CQRS (or Commands?). What is the reason? For me, it is somewhat counter-intuitive.
Imagine I have two components that need to perform some action in response to a command (it doesn´t matter if I have two instances of the same component or two independent components). Then I would need to create a handler that delegates the command to these components.
In my opinion, this is introducing an unnecessary dependency. In terms of CQRS, a command is nothing more than a message that is sent. I don´t get the reason why there should be only one handler for this message.
Can someone tell me what I am missing here? There is probably a very good reason for this that I just don´t see right now.
Regards
I am by no means an expert myself with CQRS, but perhaps I can help shed some light.
"A single recipient processes a command.", What is the reason?
One of the fundamental reasons for this is transactional consistency. A command needs to be handled in one discrete (and isolated) part of the application so that it can be committed in a single transaction. As soon as you start to have multiple handlers, distributing the application beyond a single process (and maintaining transactional consistency) is nearly impossible. So, while you could design that way, it is not recommended.
Hope this helps.
Imagine I have two components that need to perform some action in response to a command (it doesn´t matter if I have two instances of the same component or two independent components). Then I would need to create a handler that delegates the command to these components.
That's the responsibility of events.
A command must be handled by one command handler and must change the state for a single aggregate root. The aggregate root then raises one or more events indicating that something happened. These events can have multiple listeners that perform desired actions.
For example, you have a PurchaseGift command. Your command handler loads the Purchase aggregate root and performs the desired operation raising a GiftPurchased event. You can have one or more listeners to the GiftPurchase event, one for sending an email to the buyer confirming the operation and another to send the gift by mail.
Related
I want to use elements of CQRS pattern in my project. I wonder if i do it right with Command and Events.
The thing that I'm not sure is if event can invoke command. To better show what i want to do I will use diagram and example.
This is an example:
User invoke TripCreateCommand. TripCreateCommandHandler do his job and after success publish TripCreatedEvent.
Now we have two listener to TripCreatedEvent (the order of listener execution does not matter)
First listener (can be execute after the second listener):
for each user in trip.author.friends invoke two Command (the order of commands is important)
PublishTripOnUserWallCommand
SendNewTripEmailNotificationCommand
SendNewTripPlatformNotification
Second listener (can be execute before the first listener):
PublishTripOnUserSocials
And this is sample diagram:
Is this a good way ? Can EventListener invoke Command, or maybe I should do it in some other way ?
Your question is about Mesage Driven Architecture which works together with but otherwise unrelated to CQRS.
Anyway, your diagram is almost correct. The event subscriber/handler (I prefer this terminology) can send new Commands via the service bus, but it's not a rule that you should always do this. I implement quite a lot of functionality directly in the event handler, although probalby would be more clean and reliable to send a new command. It really depends on what I want to do.
Note that the message handlers (commands or events) should not know about other handlers. They should know about the bus and the bus takes care of handling. This means that in your app, the event handlers would take the bus as dependency, create the command and send it via the bus. The event handler itself doesn't know what command handler generated the event and can 'reply' to it.
Usually the commands would be handled independently and you can't guarantee the order (unless they're handled synchronously) so maybe you want the second command to be issued as a result of the first command's handling. Indeed, it can be the case for a Saga.
AFAIK you are talking only about doing things synchronously, so your approach works in this case but it's probably not scalable. Moving to async handling will break this execution flow. However your application can be fine with it, not everyhting needs to be twitter.
A message driven architecture is not that straightforward and for some cases (like you want an immediate response from the backend) it's quite complicated to implement, at least more complicated than with the 'standard' approach. So maybe for those particular cases you might want to do it the 'old' way.
If you're worried about decoupling and testing, you can still design the services as they were message handlers but use them directly, instead of a service bus.
Not sure why you would need Commands for performing the updating the information on the user's wall. Why would you choose not to use a View Model Updater for that task.
Sending an email can be considered a Command but could also easily be viewed as just another View Model update.
Not clear on what the purpose of the SendNewTripPlatformNotification is, so I cannot give any suggestions there...
Some of this could also be a candidate for a Saga. Secondly I'm missing your Domain in the diagram, that is what should be responsible for publishing any events, or do you consider the CommandHandler to be the Domain?
I am attempting to learn and apply the CQRS design approach (pattern and architecture) to a new project but seem to be missing a key piece.
My client application executes a query and retrieves a list of light-weight, read-only DTOs from the read model. The user selects an item and clicks a button to initiate some action. The action is performed by creating and sending the corresponding command object to the write model (where the command handler carries out the action, updates the data store, etc.) At some point, however, I need to update the UI to reflect changes to the state of the application resulting from the action.
How does the UI know when it is time to refresh the original list?
Additional Info
I have noticed that most articles/blogs discussing CQRS use MVC client apps in their examples. I am working on a Silverlight client right now and am beginning to wonder if the pattern simply doesn't work in that case.
Follow-Up Question
After thinking more about Bartlomiej's response and subsequent discussion, I am wondering about error handling in CQRS. Given that commands are basically fire-and-forget asynchronous operations, how do we report an error condition to the UI?
I see 'refreshing the UI' to take one of two forms:
The operation succeeds, data has changed and the UI should be updated to reflect these changes
The operation fails, data has not changed but the user should be notified of the failure and potential corrective actions.
Even with a Post-Redirect-Get pattern in an MVC, you can't really Redirect until you know the outcome of the operation. None of the examples I've seen thus far address these real-world concerns.
I've been struggling with similar issues for a WPF client. The re-query trigger for any data is dependent on the data your updating, commands tend to fall into categories:
The command is a true fire and forget method, it informs the back-end of a state change but this change does not need to be reflected in the UI, or the change simply isn't important to the UI.
The command will alter the result of a single query
The command will alter the result of multiple queries, usually (in my domain at least) in a cascading fashion, that is, changing the state of a single "high level" piece of data will likely affect many "low level" caches.
My first trigger is the page load, very few items are exempt from this as most pages must assume data has been updated since it was last visited. Though some systems may be able to escape with only updating financial and other critical data in this way.
For short commands I also update data when 'success' is returned from a command. Though this is mostly laziness as IMHO all CQRS commands should be fired asynchronously. It's still an option I couldn't live without but one you may have to if your implementation expects high latency between command and query.
One pattern I'm starting to make use of is the mediator (most MVVM frameworks come with one). When I fire a command, I also fire a message to the mediator specifying which command was launched. Each Cache (A view model property Retriever<T>) listens for commands which affect it and then updates appropriately. I try to minimise the number of messages while still minimising the number of caches that update unnecessary from a single message so I'll (hopefully) eventually end up with a shortlist of update reasons, with each 'reason' updating a list of caches.
Another approach is simple honesty, I find that by exposing graphically how the system updates itself makes users more willing to be patient with it. On firing a command show some UI indicating you're waiting for the successful response, on error you could offer to retry / show the error, on success you start the update of the relevant fields. Baring in mind that this command could have been fired from another terminal (of which you have no knowledge) so data will need to timeout eventually to avoid missing state changes invoked by other machines also.
Noting the irony that the only efficient method of updating cache's and values on a client is to un-separate the commands and queries again, be it through hardcoding or something like a hashmap.
My two cents.
I think MVVM actually fits into CQRS quite well. The ViewModel simply becomes an observable ReadModel.
1 - You initialize your ViewModel state via a query on the ReadModel.
2 - Changes on your ViewModel are automatically reflected on any Views that are bound to it.
3 - Certain changes on your ViewModel trigger a command to propegate to a message queue, an object responsible for sending those commands to the server takes those messages off the queue and sends them to the WriteModel.
4 - Clients should be well formed, meaning the ViewModel should have performed appropriate validation before it ever triggered the command. Once the command has been triggered, any event notifications can be published onto an event bus for the client to communicate changes to other ViewModels or components in the system interested in those changes. These events should carry the relevant information necessary. Typically, this means that other view models usually don't have to re-query the read model as a result of the change unless they are dependent on other data that needs to be retrieved.
5 - There is an object that connects to the message bus on the server for real-time push notifications when other clients make changes that this client is interested in knowing about, falling back to long-polling if necessary. It propagates those to the internal message bus that ties the components on the client together.
6 - The last part to handle is the fact that clients can be occasionally connected, which should be the only reason a command fails (they don't have internet access at the moment), which is when the client should be notified of problems.
In my ASP.NET MVC 3 I use 2 techniques depending on use case:
already well-known Post-Redirect-Get pattern which fits nicely with CQRS. Your MVC action that triggers the command returns a redirection to action that performs a query.
in some cases, like real-time updates of other clients, I rely on domain events/messages. I create an event handler that uses singlarR to push changes to all connected and interested clients.
There are two major ways you can take as far as I know :
1) design your UI , so that the user does not see its changes right away. Like for instance a message to tell him his action is a success, and offering him different choices to continue his work. this should buy you enough time to have updated your readmodel.
2) more complex, but you might keep the information you have send to the server and shows them in the interface.
The most important I guess, educate your user if you can so that they know why the data is not here... yet!
I am thinking about it only now, but these are for sync command handling, not async, in async things go really harder on the brain...the client interface becomes an event eater too..
I am currently writing my first saga and I am a bit puzzled with the read model. Let's explain it with an example :
I have three bounded context : programming, contractor and control. Each of them has its specific read model.
worflow :
programming send an event "JobScheduled"
Saga receives this event and tell contractor to "schedule the Work".
When done the Contractor send an event "JobDone".
The Saga receives this event and tell Control to "Start Control Period".
Everything turns out to be fine here. We are on the write side so we are passing vital information for the process to go on.
My question comes with unnecessary information. Let's say that the event "JobScheduled"
has a note field : "test note", and before this job is done this field is changed to "test note important". This change is of no importance to the workflow as described, but it is important that the contractor might see the change in the field when looking at the read model of the contractor bounded context.
Am I to give to the saga the event NoteChanged and process it or should I create a projection which directly listens to this event in my contractor bounded context?
To give it to the saga looks to me like unnecessary work because I am only updating readmodel here there is no domain involved in the change.
On the other hand, making a direct coupling between the two bounded context removes one of the assets of sagas which is the possibilty of modifying the interactions the bounded context have between each other in the workflow.
Thanks for your reading,
If changing the note is important, it should be modelled explicitly. This can be accomplished by introducing an Event just like you already did.
If said Event has any relevancy to the process, it can be handled by a Saga. If it only needs to be represented in different Read Models, then just handling it in their respective projections should be fine.
One context may very well listen to and handle the events of another one, even across application boundaries. At least this is how cross-context integration should work in an event centric architecture.
Personally I would send a command "Change note", because I see it like an information that have to be saved in the event stream of your aggregate. Than if your saga don't "feel" to tell anyone about this command, or simply give the information to an handler that quietly update your read model I guess is fine.
Event sourcing is pitched as a bonus for a number of things, e.g. event history / audit trail, complete and consistent view regeneration, etc. Sounds great. I am a fan. But those are read-side implementation details, and you could accomplish the same by moving the event store completely to the read side as another subscriber.. so why not?
Here's some thoughts:
The views/denormalizers themselves don't care about an event store. They just handle events from the domain.
Moving the event store to the read side still gives you event history / audit
You can still regenerate your views from the event store. Except now it need not be a write model leak. Give him read model citizenship!
Here seems to be one technical argument for keeping it on the write side. This from Greg Young at http://codebetter.com/gregyoung/2010/02/20/why-use-event-sourcing/:
There are however some issues that exist with using something that is storing a snapshot of current state. The largest issue revolves around the fact that you have introduced two models to your data. You have an event model and a model representing current state.
The thing I find interesting about this is the term "snapshot", which more recently has become a distinguished term in event sourcing as well. Introducing an event store on the write side adds some overhead to loading aggregates. You can debate just how much overhead, but it's apparently a perceived or anticipated problem, since there is now the concept of loading aggregates from a snapshot and all events since the snapshot. So now we have... two models again. And not only that, but the snapshotting suggestions I've seen are intended to be implemented as an infrastructure leak, with a background process going over your entire data store to keep things performant.
And after a snapshot is taken, events before the snapshot become 100% useless from the write perspective, except... to rebuild the read side! That seems wrong.
Another performance related topic: file storage. Sometimes we need to attach large binary files to entities. Conceptually, sometimes these are associated with entities, but sometimes they ARE the entities. Putting these in the event store means you have to physically load that data each and every time you load the entity. That's bad enough, but imagine several or hundreds of these in a large aggregate. Every answer I have seen to this is to basically bite the bullet and pass a uri to the file. That is a cop-out, and undermines the distributed system.
Then there's maintenance. Rebuilding views requires a process involving the event store. So now every view maintenance task you ever write further binds your write model into using the event store.. forever.
Isn't the whole point of CQRS that the use cases around the read model and write model are fundamentally incompatible? So why should we put read model stuff on the write side, sacrificing flexibility and performance, and coupling them back up again. Why spend the time?
So all in all, I am confused. In all respects from where I sit, the event store makes more sense as a read model detail. You still achieve the many benefits of keeping an event store, but you don't over-abstract write side persistence, possibly reducing flexibility and performance. And you don't couple your read/write side back up by leaky abstractions and maintenance tasks.
So could someone please explain to me one or more compelling reasons to keep it on the write side? Or alternatively, why it should NOT go on the read side as a maintenance/reporting concern? Again, I'm not questioning the usefulness of the store. Just where it should go :)
This is a long dead question that someone pointed me to. There are quite a few reasons why its better to store events on the write side.
From my understanding the architecture you are talking about is a very common one that I see ... fail. We will store our domain model in a relational database then put out events. You add the twist of them saving the events on the read side in an event store. This will likely lead to a mess.
The first issue you will run into is in the publishing of your events. What happens when I save to the database and publish to say MSMQ (I die in the middle). So DTC gets introduced between them. This is a huge thing to bring in, distributed transactions should be avoided like the plague. It is also quite inefficient as I am probably making the data durable twice (once to queue once to database). This will limit system throughput by a lot (DTC benchmarks of 200-300 messages/second are common, with events only 20-30k/second is common).
Some work around the need for DTC by putting a table in their database that has the events and operates as a queue. This will avoid the need for DTC however this will still run into the next issue.
What happens when you have a bug? I know you would never write buggy code but one of the Jrs/maintenance developers later working with the project. As an example what happens when the domain object change and the event raised do not match? Say you set State on your domain object to "LA" (hardcoded) but you properly set State on the event to cmd.State ("CT").
How will you detect such errors are occurring? The biggest problem with what is being discussed is that there are now two sources of "truth" there is the database on the write side and the event stream coming out. There is no way to prove that they are equivalent. This will cause all sorts of weird bugs down the line.
I think this is really an excellent question. Treating your aggregate as a sequence of events is useful in its own right on the write side, making command retries and the like easier. But I agree that it seems upsetting to work to create your events, then have to make yet another model of your object for persistence if you need this snapshotting performance improvement.
A system where your aggregates only stored snapshots, but sent events to the read-model for projection into read models would I think be called "CQRS", just not "Event Sourcing". If you kept the events around for re-projection, I guess you'd have a system that was very much both.
But then wouldn't you have three definitions? One for persisting your aggregates, one for communicating state changes, and any number more for answering queries?
In such a system it would be tempting to start answering queries by loading your aggregates and asking them questions directly. While this isn't forbidden by any means, it does tend to start causing those aggregates to accrete functionality they might not otherwise need, not to mention complicating threading and transactions.
One reason for having the event store on the write-side might be for resolving concurrency issues before events become "facts" and get distributed/dispatched, e.g. through optimistic locking on committing to event streams. That way, on the write side you can make sure that concurrent "commits" to the same event stream (aggregate) are resolved, one of them gets through, the other one has to resolve the conflicts in a smart way through comparing events or propagating the conflict to the client, thus rejecting the command.
Getting into CQRS and I understand that you have commands (app layer) and events (from the domain).
In the simple case where events are to update the read model, do read model updates fail? If there is no "bug" then I cannot see them failing and as I am using EventStore, I know there is a commit flag which will retry failures.
So my question is do I have to do anything in addition to EventStore to handle failures?
Coming from a world where you do everything in one transaction and now things are done separately is worrying me.
Of course there may be cases where a published event will fail in the read models.
You have to make sure you can detect that and solve it.
The nice thing is that you can replay all the events again and again so you have the chance not only to fix the error. You can also test the fix by replaying every single event if you want.
I use NServiceBus as my publishing mechanism which allows me to use an error queue. Using my other logging tools together with the error queue I can easily determine what happened since I have the error log and the actual message that caused the error in the first place.