I'm implementing a bounded context using event sourcing but have come across a problem. Say I'm modelling a game of soccer, and I'm interested in both the individual goals scored (who scored them etc) and the overall score. So if I have a Match aggregate root I ideally want events raised called GoalScored and ScoreChanged. The reason I want the score explicitly stated from the domain like this is that I don't want lots of different listeners and possibly other bounded contexts all computing the same thing.
This seems simple, but: the Match object has a Goal() method that adds a new goal. In the spirit of event sourcing this doesn't directly mutate Match state, but raises a GoalScored event that is handled within the Match which then mutates the state (as well as the event being pushed out to denormalizers). So in terms of raising a ScoreChanged, the score hasn't actually changed until the GoalScored event has been handled, so do I raise another event in response to that event (ScoreChanged), effectively chaining the events? I don't think so, for a start when an aggregate root is reloaded from the event store lots of extra events are going to be created each time in response to each GoalScored.
I also thought about working out what the score would in the command handler that raises GoalScored, sort of a 'what if' situation. Then I could raise both events in the command handler. I'd really rather not do that though - it just doesn't seem 'right'. Working out the score is simple enough for soccer, but other games (cricket for example) requires more work.
I could put both the goal and the score in the GoalScored event, which is fair enough, but again it doesn't seem right - the score has got nothing to do with a GoalScored event per se.
All the examples used when discussing event sourcing seem to use the eCommerce Customer/Order domain, and I've never seen a similar case as this.
Does anyone have any experience dealing with situations like this?
Thanks
The choosing of clean domain events, just like other modeling, should result in concepts that are mirrored in the domain. You say "the score has got nothing to do with a GoalScored event per se". But it does. In soccer, the only way a score can change is if a goal is scored. Though, goals can be taken back, e.g. via offsides calls or other penalties. It's not clear whether you want to model this or not.
It is common for a domain method to emit multiple events at once. A good framework will make it easy to consider them as a bunch, e.g. as a single commit. Why not emit both a GoalScored and ScoreChanged event?
You may also want to consider whether this domain has any commands at all. The soccer match itself is the system of record. The events that come from the match are a record of history already. Are you perhaps just writing a system which processes streams of events into streams of more events?
One thing that often helps when thinking about event sourcing is to notice your tense - you say StartMatch, but in an event sense it's actually the event MatchStarted.
As for the ScoreChanged, are you consuming this event outside of the Match? If not, the Score should be the only part publically accessible, and the GoalScored event simply mutates this. This holds CQRS in a minor way (how I get the score doesn't depend on how I change it). Score can then internally hold a state, or can replay all the GoalScored events to arrive at a number each time it's called. The 'feel' of properly designed event-sourced solutions can always regenerate any state from the events.
Now, if the ScoreChanged needs to alert other parts of your system (a player ranking, for instance), you can either multi-cast the event to different roots, or you might need to refactor your design. In this case, do you want to update players in real time, or only after a match is complete?
I have this open source project for event sourcing in scala which contains examples of how a basket works using it. Just in case is useful for you https://github.com/politrons/Scalaydrated
Related
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).
We are using an event store that stores a single aggregate - a user's order (imagine an Amazon order than can be updated at any moment by both a client or someone in the e-commerce company before it actually gets dispatched).
For the first time we're going to allow our company's employees to see the order's history, as until now they could only see its current state.
We are now realizing that the events that form up the aggregate root don't really show the intent or what the user actually did. They only serve to build the current state of the order when applied sequencially to an empty order. The question is: should they?
Imagine a user that initially had one copy of book X and then removed it and added 2 again. Should we consider this as an event "User added 1 book" or events "User removed 1 book" + "User added 2 books" (we seem to have followed this approach)?
In some cases we have one initial event that then is followed by other events. I, developer, know for sure that all these events were triggered by a single command, but it seems incredibly brittle for me to make that kind of assumptions when generating on the fly this "order history" functionality for the user to see. But if I don't treat them, at least in the order history feature as a single action, it will seem like there were lots of order amendments when in fact there was just one, big one.
Should I have "macro" events that contain "micro events" inside? Should I just attach the command's id to the event so I can then easily inferr what event happened at the same and which ones not (an alternative would be relying on timestamps.. but that's disgusting).
What's the standard approch to deal with this kind of situations? I would like to be able to look at any time to the aggregate's history and generate this report (I don't want to build the report incrementally every time the order is updated).
Thanks
Command names should ideally be descriptive of intent. Which should mean it's possible to create event names which make the original intent clear. As a rule of thumb, the events in the event stream should be understandable to the relevant members of the business. It's a good rule of thumb. It should contain stuff like 'cartUpdated' etc.
Given the above, I would have expected that the showing the event stream should be fine. But I totally get why it may not be ideal in some circumstances. I.e. it may be too detailed. In which case maybe create a 'summeriser' read model fed the events.
It is common to include the command’s ID in the resulting events’ metadata, along with an optional correlation ID (useful for long running processes). This then makes it easier to build the order history projection. Alternatively, you could just use the event time stamps to correlate batches in whatever way you want (perhaps you might only want one entry even for multiple commands, if they happened in a short window).
Events (past tense) do not always capture human - or system - user intent. Commands (imperative mood) do. As all command data cannot always be easily retraced from the events it generated, keeping a structured log of commands looks like a good option here.
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 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.
I'm new to the CQRS/ES world and I have a question. I'm working on an invoicing web application which uses event sourcing and CQRS.
My question is this - to my understanding, a new command coming into the system (let's say ChangeLineItemPrice) should pass through the domain model so it can be validated as a legal command (for example, to check if this line item actually exists, the price doesn't violate any business rules, etc). If all goes well (the command is not rejected) - then the appropriate event is created and stored (for example LineItemPriceChanged)
The thing I didn't quite get is how do I keep this aggregate in memory to begin with, before trying to apply the command. If I have a million invoices in the system, should I playback the whole history every time I want to apply a command? Do I always save the event without any validations and do the validations when constructing the view models / projections?
If I misunderstood any part of the process I would appreciate your feedback.
Thanks for your help!
You are not alone, this is a common misunderstanding. Let me answer the validation part first:
There are 2 types of validation which take place in this kind of system. The first is the kind where you look for valid email addresses, numeric only or required fields. This type is done before the command is even issued. A command which contains these sorts of problems should not be raised as commands (for belt and braces you can check at the domain side but this is not a domain concern and you are better off just preventing this scenario).
The next type of validation is when it is a domain concern. It could be the kind of thing you mention where you check prices are within a set of specified parameters. This is a domain concept the business people would understand, do and be able to articulate.
The next phase is for the domain to apply the state change and raise the associated events. These are then persisted and on success, published for the rest of the app.
All of this is can be done with the aggregate in memory. The actions are coordinated with a domain service which handles the command. It loads the aggregate, apply's all it's past events (or loads a snapshot) then issues the command. On success of the command it requests all the new uncommitted events and tries to persist them. On success it publishes the new events.
As you see it only loads the events for that specific aggregate. Even with a lot of events this process is lightning fast. If performance is a problem there are strategies such as keeping aggregates in memory or snapshotting which you can apply.
To your last point about validating events. As they can only be generated by your aggregate they are trustworthy.
If you want more detail check out my overview of CQRS and ES here. And take a look at my post about how to build aggregate roots here.
Good luck - I hope they help!
It is right that you have to replay the event to 'rehydrate' the domain aggregate. But you don't have to replay all events for all invoices. If you store the entity id of the root aggregate in the events, you can just select and replay the events that with the relevant id.
Then, how do you find the relevant aggregate root id? One of the read repositories should contain the relevant information to get the id, based on a set of search criteria.