I'm using CQRS on an air booking application. one use case is help customer cancel their tickets. But before the acutal cancellation, the customer wants to know the penalty.
The penalty is calculated based on air rules. Some of our provider could calculate the penalty through exposing an web service while the others don't. (They publish some paper explaining the algorithm instead). So I define a domain service
public interface AirTicketService {
//ticket demand method
MonetaryAmount penalty(String ticketNumber);
void cancel(String ticketNumber, MonetaryAmount penalty);
}
My question is which side(command/query) is responsible for invoking this domain service and returning result in a CQRS style application?
I want to use a Command: CalculatePenlatyCommand, In this way, it's easy to resuse the domain model, but it's a little odd because this command does not modify state.
Or should I retrieve a readmodel of ticket if this is a query? But the same DomainService is needed on both command and query side, it's odd too.
Is domain derivation a query?
There is no need to shoehorn everything in to the command-query pipeline. You could query this service independently from the UI without issuing a command or asking the read-model.
There is nothing wrong with satisfying a query using an existing model if it "fits" both the terminology and the structure of that model. No need to build up a separate read model for that purpose. It's not without risk, since the semantics and the context of the query should be closely tied to the model that is otherwise used for write purposes only. The risk I allude to is the fact that the write and read concerns could drift apart (and we're back at square one, i.e. the reason why people pick CQRS in the first place). So you must keep paying attention as new requirements come in.
Queries that fit this model really well are what I call "simulators', where you want to run a simulation using current state to e.g. to give feedback to an end user. On more than one occasion I've found that the simulation logic could be reused both as a feedback mechanism and as an execution (of a write operation/command) steering mechanism. The difference is in what we do with the outcome of the simulation. Again, this is not without risk and requires careful judgement.
You may bring arguments that Calculate Penalty Command is not odd at all.
The user asks the system to do something - command enough.
You can even have a Penalty Calculation Requested Event event in your domain, and it would feel right. Because, at some time, you may be interested in, let's say, unsure clients, ones that want to cancel tickets but they change their mind every time etc. The calculation may be performed asynchronously, too - you can provide the result (penalty cost) to the user in various ways afterwards...
Or, in some other way: on your ticket booked event, store cancellation penalty, too. Then, you can make that value accessible any time, without the need to recompute it... But this may be wrong (?) because penalty would largely depend on time, right (the late you cancel your ticket, the more you pay)?
If all this would like over-complications etc., then I guess I agree with rmac's answer, too :)
Related
Recently I faced with the following invariants in my domain Model:
An Offer treated as Expired if ExpiryAt (DateTimeOffset) < DateTimeOffset.Now.
A Director of the Company cannot be younger than 18 years old
When Document is downloaded we should set DownloadedAt field with DateTimeOffset.Now
In Application Layer to keep purity and for better testing we usually isolate System.DateTime with IDateTime interface which allow to mock Now in UnitTests.
But all these 3 scenarios belong to Domain Layer and not to Application Layer. We should not Inject external interfaces into DomainModel to keep it pure. But from other side it might be bad to use DateTime.Now or DateTimeOffset.Now directly in DomainLayer since this adds a dependency to system clock and make it harder to test sometimes since DateTime.Now will never return the same result.
So the question is - how do you deal with this dilemma?
Options I see:
Provide now as parameter to Domain Entity methods. This is viable option and simplify testing though makes code more verbose and sometimes even stupid.
Just use DateTime.Now in domain layer. I already mentioned cons of this approach.
Anything else you might suggest from your experience?
From the different options accessing the static DateTime.Now() functionality is obviously the most disadvantageous. It both does not allow for testing and also hides the domain models dependency to some non-deterministic infrastructure inside the implementation details.
The option to inject some interface to a service that can be viewed is a little better because it makes the dependency explicit and also allows for unit testing by stubbing the non-deterministic output to return some deterministic value of your choice.
But still, at runtime your domain model needs to access some infrastructure dependency. This might be a reasonable compromise in some cases, but if possible I would try to avoid that to keep the domain model pure.
If you look at the current date time in your case from a different angle it becomes more obvious that it is actually nothing else like a normal input parameter. You could see it as something like a reference date time instead of the current date time.
Referring to your first example - checking if an offer has expired - from the domain model's point-of-view it needs to check if the offer has expired at some given point in time. This given point in time just happens to be current date time in one of the use cases where the domain logic is exercised.
So bottom line, I recommend to inject the value of the (current) date time rather than an interface to some functionality in such cases. It makes explicit what data is really need in addition to the data the domain encapsulates on its own and requires for performing the business logic.
Also, it makes more explicit what the client code (e.g. the use case or application service) wants to tell or ask the domain model. For instance, check if the offer has expired as of now or if needed, tell me if the offer was already expired at a given point in time or even if it will be expired at an important point in time.
As further reading I recommend this great article from Vladimir Khorikov where he elaborates more on that topic.
I am new to Event Sourcing and I am considering using it for an industrial application to track events happening in a production facility.
Since the book of record is the production facility itself and not the system, and also because not everything is automated, workers will need to report at a given point in time (the recorded time) what they did at another point in time (the effective time). Therefore, I will be using events such as: TankFilledRecorded, TankOutputConnectedToPipeInputRecorded, ContainerMovedToFacilityAreaRecorded, etc. where these events refer to entities such as a tank, a pipe, or a facility area for example. These events will have both a recorded time and an effective time. Note that there is no submission or approval process for a record to be considered legit.
Domain-driven design (DDD) encourages to design events that are representative of what happens in the domain (like the ones above).
However, in my domain, I don’t care so much about how a tank, a pipe or a facility area came to existence. I just need to know that something exists from a particular point in time, and I also need to know if it is not there after a particular point in time. The main objective of the software is to track liquids and powders flowing in a circuit made of these pipes, tanks and other components. It is not an asset management system and should not become one.
Therefore, what would be the correct DDD way to design an event that represents the fact that there is a tank, a pipe or an area in the production facility?
It is a subtle question but language is important, particularly in DDD.
Here is what I came up with:
1 EntityExistenceAcknowledgmentRecorded
TankExistenceAcknowledgmentRecorded
PipeExistenceAcknowledgmentRecorded
FacilityAreaExistenceAcknowledgmentRecorded
TankDisappearanceAcknowledgmentRecorded
PipeDisappearanceAcknowledgmentRecorded
FacilityAreaDisappearanceAcknowledgmentRecorded
It seems awful to use this in the ubiquitous language. I don’t see myself talking in these terms or providing a UI with such vocabulary. But it does represent exactly what happens though.
2 EntityRegistered
TankRegistered
PipeRegistered
FacilityAreaRegistered
TankUnregistered
PipeUnregistered
FacilityAreaUnregistered
It seems much simpler and it also seems to be meaningful except for one thing. “Registered” conveys the existence of the representation of an entity in the system with immediate effect, without the possibility of saying now that the entity existed 2 days ago. Think about a UserRegistered event in a website that would indicate that the user “existed” from 10 days ago. What would that even mean?
Events are facts and you cannot change the past. However, I do need a way for my users to invalidate a record in which they made a mistake such as a typo. They can record now that they acknowledged the existence of a facility area a week ago and might realize later than there was something wrong, such as a typo in the name of the entity. They would invalidate the record and create a new one. But, invalidate something that has been “registered” does not sound right.
3 Keep looking
Try to dig more in the domain (event storming) and find the real events that brought the entities into existence even if these events are of no use in the problem that needs to be solved.
TankBuiltRecorded
PipeBuiltRecorded, PipeDeliveredRecorded
FacilityArea<something_meaningful>Recorded
TankDestroyedRecorded, TankDecommissionedRecorded
PipeDecommissionedRecorded
FacilityArea<something_meaningful>Recorded
A caution
TankFilled
TankFilledReported
TankFilledReportSubmitted
TankFilledReportSubmissionReceived
Think carefully about whether the increased precision is motivated by business value.
Therefore, what would be the correct DDD way to design an event that represents the fact that there is a tank, a pipe or an area in the production facility?
What is the business doing today? Is there already a process in place for tracking the lifetime of the hardware in the plant (a maintenance log, perhaps?) There's likely to be vocabulary in that place that gives you ideas as to what spellings would make sense in the code.
Events are facts and you cannot change the past.
That's true - but you can back date events. The effective date of the information is often distinct from the reported date of information.
I do need a way for my users to invalidate a record in which they made a mistake such as a typo.
Yes - error correction is an important part of the process that you are modeling.
You should probably review Greg Young's talk Answering a Question, which was based on this thread. It's a discussion of capturing and modeling of temporality.
Here's the good news: you are running into the right problem. Because you are capturing information about an external system, there are going to be opportunities for errors and conflicts, and you need to (a) figure out the protocols for addressing them, and then (b) model that process correctly. That might include exception reports generated by the system when it observes conflicting information, or compensating events, or even automated conflict resolution (for the easy cases -- see also Stop Over Engineering).
I would like to implement CQRS and ES using Axon framework
I've got a pretty complex HTML form which represents recruitment process with six steps.
ES would be helpful to generate historical statistics for selected dates and track changes in form.
Admin can always perform several operations:
assign person responsible for each step
provide notes for each step
accept or reject candidate on every step
turn on/off SMS or email notifications
assign tags
Form update (difference only) is sent from UI application to backend.
Assuming I want to make changes only for servers side application, question is what should be a Command and what should be an Event, I consider three options:
Form patch is a Command which generates Form Update Event
Drawback of this solution is that each event handler needs to check if changes in form refers to this handler ex. if email about rejection should be sent
Form patch is a Command which generates several Events ex:. Interviewer Assigned, Notifications Turned Off, Rejected on technical interview
Drawback of this solution is that some events could be generated and other will not because of breaking constraints ex: Notifications Turned Off will succeed but Interviewer Assigned will fail due to assigning unauthorized user. Maybe I should check all constraints before commands generation ?
Form patch is converted to several Commands ex: Assign Interviewer, Turn Off Notifications and each command generates event ex: Interviewer Assigned, Notifications Turned Off
Drawback of this solution is that some commands can fail ex: Assign Interviewer can fail due to assigning unauthorized user. This will end up with inconsistent state because some events would be stored in repository, some will not. Maybe I should check all constraints before commands generation ?
The question I would call your attention to: are you creating an authority for the information you store, or are you just tracking information from the outside world?
Udi Dahan wrote Race Conditions Don't Exist; raising this interesting point
A microsecond difference in timing shouldn’t make a difference to core business behaviors.
If you have an unauthorized user in your system, is it really critical to the business that they be authorized before they are assigned responsibility for a particular step? Can the system really tell that the "fault" is that the responsibility was assigned to the wrong user, rather than that the user is wrongly not authorized?
Greg Young talks about exception reports in warehouse systems, noting that the responsibility of the model in that case is not to prevent data changes, but to report when a data change has produced an inconsistent state.
What's the cost to the business if you update the data anyway?
If the semantics of the message is that a Decision Has Been Made, or that Something In The Real World Has Changed, then your model shouldn't be trying to block that information from being recorded.
FormUpdated isn't a particularly satisfactory event, for the reason you mention; you have to do a bunch of extra work to cast it in domain specific terms. Given a choice, you'd prefer to do that once. It's reasonable to think in terms of translating events from domain agnostic forms to domain specific forms as you go along.
HttpRequestReceived ->
FormSubmitted ->
InterviewerAssigned
where the intermediate representations are short lived.
I can see one big drawback of the first option. One of the biggest advantage of CQRS/ES with Axon is scalability. We can add new features without worring about regression bugs. Adding new feature is the result of defining new commands, event and handlers for both of them. None of them should not iterfere with ones existing in our system.
FormUpdate as a command require adding extra logic in one of the handler. Adding new attribute to patch and in consequence to command will cause changes in current logic. Scalability is no longer advantage in that case.
VoiceOfUnreason is giving a very good explanation what you should think about when starting with such a system, so definitely take a look at his answer.
The only thing I'd like to add, is that I'd suggest you take the third option.
With the examples you gave, the more generic commands/events don't tell that much about what's happening in your domain. The more granular events far better explain what exactly has happened, as the event message its name already points it out.
Pulling Axon Framework in to the loop, I can also add a couple of pointers.
From a command message perspective, it's safe to just take a route and not over think it to much. The framework quite easily allows you to adjust the command structure later on. In Axon Framework trainings it is typically suggested to let a command message take the form of a specific action you're performing. So 'assigning a person to a step would typically be a AssignPersonToStepCommand, as that is the exact action you'd like the system to perform.
From events it's typically a bit nastier to decide later on that you want fine grained or generic events. This follows from doing Event Sourcing. Since the events are your source of truth, you'll thus be required to deal with all forms of events you've got in your system.
Due to this I'd argue that the weight of your decision should lie with how fine grained your events become. To loop back to your question: in the example you give, I'd say option 3 would fit best.
I am working on DDD project and I am currently focused on two bouned contexts, Orders and Warehouse.
What confuses me is the following situation:
Order keep track of all the placed orders, and warehouse keeps track about all the available inventory. If user places one order for certain product item, that would mean one less item of that product in a warehouse. I am oversimplifying this process, so please bear with me.
Since two domain models are placed inside of a different BC, i am currently relying on eventual consistency ie. after one item has been sold, it would eventually be removed from the warehouse.
That situation unfortunately leads to the problem scenario where another user could simultaneously make another order of the same item, and it would appear as available until eventual consistency kicks is. That is something it is unacceptable by the domain expert.
So IMO I am stuck with two options
merge order and warehouse (at least the part regarding product
inventory, units available in warehouse) into one BC
have Order BC (or microservice if you wish) to be dependent of Warehouse BC (microservice) in order to pull a live product units
available
Which option does actually follows DDD patern? Is there another way out?
You could use a reservation system with a timeout.
Using a messaging analogy: With a broker-style queuing mechanism (such as RabbitMQ) you get a message from the queue and you have control over it until you either acknowledge that it can be removed from the queue or you release it back to the queue.
You could do the same thing in your ordering process. You reserve any items on your order. SO when you add them they have a status of, say, reserving and upon sending some message to reserve the items. If the response comes back you can decide how to proceed. Perhaps you could add any items that cannot be reserved onto a back order or try again later.
There are going to be different ways to approach this. Depending on your business case it may be acceptable to only check availability when someone really accepts the order.
If you domain expert reckons it is totally unacceptable that having this resolved at the end of the process then you could move it to the start. The issue is of course that user A could reserve and never buy thereby losing user B as a customer; whereas only applying the real "taking" of the item at the end of the process is closer to ensuring a purchase. But that is a business decision.
This issue is a really great example of where reality actually is eventually consistent. Is it really the best thing to decline an order if there is no inventory currently in the warehouse - even if there was a replenishment due in the next 20 minutes?
What if the item was actually on the shelf, but the operator hadn't yet keyed it into the system?
Sometimes designers and domain experts assume that people want 100% consistency, when really, users might be willing to accept a delay in confirmation of their order, if it increased the chance that their order would be accepted rather than rejected.
In the case above, why make it the user's job to retry their order N minutes later? In an eventually consistent system, you can accommodate such timing flexibility by including a timeout to retry the attempt to fulfill the order for a period of time before confirming to the client that it really wasn't possible.
There are technical solutions that will give you 100% consistency, but I think really this is not a technical challenge but a cultural/mindset one, changing people's minds about what is possible & acceptable to achieve an what is actually a better outcome.
IMO you can build a PlaceOrderSaga which will ask for inventory availability before placing the order.
I wonder how to model Calendar using DDD and CQRS. My problem consist in increasing number of events. I consider Calendar as Aggregate Root which contains Events (Calendar Events). I dont want to use ReadSide in my Commands but I need way to check events collisions at domain level.
I wonder how to model Calendar using DDD and CQRS. My problem consist in increasing number of events.
The most common answer to "long lived" aggregates is to break that lifetime into episodes. An example of this would be the temporary accounts that an accountant will close at the end of the fiscal year.
In your specific case, probably not "the Calendar" so much as "the February calendar", the "the March calendar", and so on, at whatever grain is appropriate in your domain.
Im not sure if Im right about DDD aproach in terms of validation. I believe the point is not to allow the model to enter into invalid state
Yes, but invalid state is a tricky thing to define. Udi Dahan offered this observation
A microsecond difference in timing shouldn’t make a difference to core business behaviors.
More succinctly, processing command A followed by processing command B produces a valid state, then it should also be true that you end up processing command B first, and then A.
Let's choose your "event collisions" example. Suppose we handle two commands scheduleMeeting(A) and scheduleMeeting(B), and the domain model understands that A and B collide. Riddle: how do we make sure the calendar stays in a valid state?
Without loss of generality, we can flip a coin to decide which command arrives first. My coin came up tails, so command B arrives first.
on scheduleMeeting(B):
publish MeetingScheduled(B)
Now the command for meeting A arrives. If your valid calendars do not permit conflicts, then your implementation needs to look something like
on scheduleMeeting(A):
throw DomainException(A conflicts with B)
On the other hand, if you embrace the idea that the commands arrive shouldn't influence the outcome, then you need to consider another approach. Perhaps
on scheduleMeeting(A)
publish MeetingScheduled(A)
publish ConflictDetected(A,B)
That is, the Calendar aggregate is modeled to track not only the scheduled events, but also the conflicts that have arisen.
See also: aggregates and RFC 2119
Event could also an be an Aggregate root. I don't know your business constraint but I think that if two Events colide you could notify the user somehow to take manual actions. Otherwise, if you really really need them not to colide you could use snapshots to speed up the enormous Calendar AR.
I dont want to use ReadSide in my Commands but I need way to check events collisions at domain level.
You cannot query the read model inside the aggregate command handler. For the colision detection I whould create a special DetectColisionSaga that subscribes to the EventScheduled event and that whould check (possible async if are many Events) if a colision had occurred and notify the user somehow.