I'm looking for an advice related to the proper way of implementing a rollback feature in a CQRS/event-sourcing application.
This application allows to a group of editors to edit and update some editorial content, an editorial news for instance. We implemented the user interface so that each field has an auto save feature and now we would like to provide our users the possibility to undo the operations they did, so that it is possible to rollback the editorial news to a previous known state.
Basically we would like to implement something like to the undo command that you have in Microsoft Word and similar text editors. In the backend, the editorial news is an instance of an aggregate defined in our domain and called Story.
We have discussed some ideas to implement the rollback and we are looking for an advice based on real world experiences in similar projects. Here is our considerations about this feature.
How rollback works in real world business domains
First of all, we all know that in real world business domains what we are calling rollback is obtained via some form of compensation event.
Imagine a domain related to some sort of service for which it is possible to buy a subscription: we could have an aggregate representing a user subscription and an event describing that a charge has been associated to an instance of the aggregate (the particular subscription of one of the customers). A possible implementation of the event is as follows:
public class ChargeAssociatedToSubscriptionEvent: DomainEvent
{
public Guid SubscriptionId {get; set;}
public decimal Amount {get; set;}
public string Description {get; set;}
public DateTime DueDate {get; set;}
}
If a charge is wrongly associated to a subscription, it is possible to fix the error by means of an accreditation associated to the same subscription and having the same amount, so that the effect of the charge is completely balanced and the user get back its money. In other words, we could define the following compensation event:
public class AccreditationAssociatedToSubscription: DomainEvent
{
public Guid SubscriptionId {get; set;}
public decimal Amount {get; set;}
public string Description {get; set;}
public DateTime AccreditationDate {get; set;}
}
So if a user is wrongly charged for an amount of 50 dollars, we can compensate the error by means of an accreditation of 50 dollars to the user subscription: this way the state of the aggregate has been rolled back to the previous state.
Why things are not as easy as they seem
Based on the previous discussion, the rollback seems quite easy to be implemented. If you have an instance of the story aggregate at the aggregate revision B and you want to roll it back to a previous aggregate revision, say A (with A < B), you just have to do the following steps:
check the event store and get all the events between revisions A and B
compute the compensation event for each of the occurred events
apply the compensation events to the aggregate in the reverse order
Unfortunately, the second step of the previous procedure is not always possible: given a generic domain event it is not always possible to compute its compensation event, because the amount of information contained inside the event could not be enough to do that. Maybe it is possible to wisely define all the events so that they contain enough information to be able to compute the corresponding compensation event, but at the current state of our application there are several events for which computing the compensation event is not possible and we would prefer to avoid changing the shape of our events.
A possible solution based on state comparison
The first idea to overcome the issues with compensation event is computing the minimum set of events needed to roll back the aggregate by comparing the current state of the aggregate with the target state. The algorithm is basically the following:
get an instance of the aggregate at the current state (call it B)
get an instance of the aggregate at the target state (call it A) by applying only the first n events persisted inside event store (our repository allows to do that by specifying the aggregate id and the desired point in time to which materialize the aggregate)
compare the two instances and compute the minimum set of events to be applied to the aggregate in the state B in order to change its state to A
apply the computed events to the aggregate
A smarter approach based on event replay
Another way to solve the problem of rolling back to a previous state of the aggregate could be doing the same thing that the aggregate repository does when an aggregate is materialized at a specific point in time. In order to do that we should define an event, say StoryResettedEvent, whose effect is to reset the state of the aggregate by completely emptying it and do the following steps:
apply the StoryResettedEvent to our aggregate so that its state is emptied
get the first n events for the aggregate we are working on (all the events from the first saved event up to the target state A)
apply all the events to the aggregate instance
The main problem I see with this approach is the event to empty the state of the aggregate: it seems somewhat artificial, not a real domain event with a business meaning, but rather a trick to implement the rollback functionality.
The third way: persisting the compensation event each time an event is saved inside the event store
The third way we figured out to get what we need is based again on the concept of compensation event. The basic idea is that each event of the application could be enriched with a property containing the corresponding compensation event.
In the point of the code where an event is raised it is possible to immediately compute the compensation event for the event to be raised (based on the current state of the aggregate and the shape of the event), so that the event could be enriched with this information that this way will be saved inside the event store. By doing so the compensation events events are always available, ready to be used in case of a rollback request. The downside of this solution is that each domain event must be modified and only a minimum part of the compensation events we must compute and save inside the event store will be useful for an actual rollback (most of them will never be used).
Conclusions
In my opinion the best option to solve the problem is using the algorithm based on state comparison (the first proposed solution), but we are still evaluating what to do.
Does anyone have already had a similar requirement ? Is there any other way to implement a rollback ? Are we completely missing the point and following bad approaches to the problem ?
Thanks for helping, any advice will be appreciated.
How the compensation events are generated should be the concern of the Story aggregate (after all, that's the point of an aggregate in event sourcing - it's just the validator of commands and generator of events for a particular stream).
Presumably you are following something like a typical CQRS/ES flow:
client sends an Undo command, which presumably says what version it wants to undo back to, and what story it is targetting
The Undo Command Handler loads the Story aggregate in the usual way, either possibly from a snapshot and/or by applying the aggregate's events to the aggregate.
In some way, the command is passed to the aggregate (possibly a method call with args extracted from the command, or just passing the command directly to the aggregate)
The aggregate "returns" in some way the events to persist, assuming the undo command is valid. These are the compensating events.
compute the compensation event for each of the occurred events
...
Unfortunately, the second step of the previous procedure is not always possible
Why not? The aggregate has been passed all previous events, so what does it need that it doesn't have? The aggregate doesn't just see the events you want to roll back, it necessarily processes all events for that aggregate ever.
You have two options really - reduce the book-keeping that the aggregate needs to do by having the command handler help out in some way, or the whole process is managed internally by the aggregate.
Command handler helps out:
The command handler extracts from the command the version the user wants to roll back to, and then recreates the aggregate as-of that version (applying events in the usual way), in addition to creating the current aggregate. Then the old aggregate gets passed to the aggregate's undo method along with the command, so that the aggregate can then do state comparison more easily.
You might consider this to be a bit hacky, but it seems moderately harmless, and could significantly simplify the aggregate code.
Aggregate is on its own:
As events are applied to the aggregate, it adds to its state whatever book-keeping it needs to be able to compute the compensating events if it receives an undo command. This could be a map of compensating events, pre-computed, a list of every previous state that can potentially be reverted to (to allow state comparison), the list of events the aggregate has processed (so it can compute the previous state itself in the undo method), or whatever it needs, and it just stores it in its in-memory state (and snapshot state, if applicable).
The main concern with the aggregate doing it on its own is performance - if the size of the book-keeping state is large, the simplification of allowing the command handler to pass the previous state would be worthwhile. In any case, you should be able to switch between the approaches at any time in the future without any issues (except possibly needing to rebuild your snapshots, if you have them).
My 2 cents.
For rollback operation, an orchestration class will be responsible to handle it. It will publish a aggregate_modify_generated event and a projection on the other end for this event will fetch the current state of the aggregates after receiving it. Now when any of the aggregate failed, it should generate a failure event, upon receiving it, orchestration class will generate a aggregate_modify_rollback event that will received by that projection and will set aggregate state with the previously fetched state .
One common projector can do the task, because the events will have aggregate id.
Related
I am working on a project in which we define two aggregates: "Project" and "Task". The Project, in addition to other attributes, has the points attribute. These points are distributed to the tasks as they are defined by users. In a use case, the user assigns points for some task, but the project must have these points available.
We currently model this as follows:
“task.RequestPoints(points)“, this method will create an aggregate PointsAssignment with attributes points and taskId, which in its constructor issues a PointsAssignmentRequested domain event.
The handler of the event issued will fetch the project related to the task and the aggregate PointsAssigment and call the method “project.assignPoints(pointsAssigment, service)”, that is, it will pass PointAssignment aggregate as a parameter and a service to calculate the difference between the current points of the task and the desired points.
If points are available, the project will modify its points attribute and issue a “ProjectPointsAssigned” domain event that will contain the pointsAssignmentId attribute (in addition to others)
The handler of this last event will fetch the PointsAssingment and confirm “pointsAssigment.Confirm ()”, this aggregate will issue a PointsAssigmentConfirmed domain event
The handler for this last event will bring up the associated task and call “task.AssignPoints (pointsAssignment.points)”
My question is: is it correct to pass in step 2 the aggregate PointsAssignment in the project method? That was the only way I found to be able to relate the aggregates.
Note: We have created the PointsAssignment aggregate so that in case of failure I could save the error “pointsAssignment.Reject(reasonText)” and display it to the user, since I am using eventual consistency (1 aggregate per transaction).
We think about use a Process Manager (PointsAssingmentProcess), but the same way we need the third aggregate PointsAssingment to correlate this process.
I would do it a little bit differently (it doesn´t mean more correct).
Your project doesn´t need to know anything about the PointsAssignment.
If your project is the one that has the available points for use, it can have simple methods of removing or adding points.
RemovePointsCommand -> project->removePoints(points)
AddPointsCommand -> project->addPoints(points)
Then, you would have an eventHandler that would react to the PointsAssignmentRequested (i imagine this guy has the id of the project and the number of points and maybe a status field from what you said)
This eventHandler would only do:
on(PointsAssignmentRequested) -> dispatch command (RemovePointsCommand)
// Note that, in here it would be wise to the client to send an ID for this operation, so it can do it asynchronously.
That command can either success or fail, and both of them can dispatch events:
RemovePointsSucceeded
RemovePointsFailed
// Remember that you have a correlation id from earlier persisted
Then, you would have a final eventHandler that would do:
on(RemovePointsSucceeded) -> PointsAssignment.succeed() //
Dispatches PointsAssignmentSuceeded
on(PointsAssignmentSuceeded) -> task.AssignPoints
(pointsAssignment.points)
On the fail side
on(RemovePointsFailed) -> PointsAssignment.fail() // Dispatches PointsAssignmentFailed
This way you dont have to mix aggregates together, all they know are each others id´s and they can work without knowing anything about the schema of other aggregates, avoiding undesired coupling.
I see the semantics of the this problem exactly as a bank transfer.
You have the bank account (project)
You have money in this bank account(points)
You are transferring money through a transfer process (pointsAssignment)
You are transferring money to an account (task)
The bank account only should have minimal operations, of withdrawing and depositing, it does not need to know anything about the transfer process.
The transfer process need to know from which bank it is withdrawing from and to which account it is depositing to.
I imagine your PointsAssignment being like
{
"projectId":"X",
"taskId":"Y",
"points" : 10,
"status" : ["issued", "succeeded", "failed"]
}
I have seen information on rehydrating aggregate roots in SO, but I am posting this question because I did not find any information in SO about doing so with in the context of an event sourced framework.
Has a best practice been discovered or developed for how to rehydrate aggregate roots when operating on the command side of an application using the event sourcing and CQRS pattern
OR is this still more of a “preference“ among architects?
I have read through a number of blogs and watched a number of conference presentations on you tube and I seem to get different guidance depending on who I am attending to.
On the one hand, I have found information stating fairly clearly that developers should create aggregates to hydrate themselves using “apply“ methods on events obtained directly from the event store..
On the other hand, I have also seen in several places where presenters and bloggers have recommended rehydrating aggregate roots by submitting a query to the read side of the application. Some have suggested creating specific validation “buckets“ / projections on the read side to facilitate this.
Can anyone help point me in the right direction on discovering if there is a single best practice or if the answer primarily depends upon performance issues or some other issue I am not thinking about?
Hydrating Aggregates in an event sourced framework is a well-understood problem.
On the one hand, I have found information stating fairly clearly that
developers should create aggregates to hydrate themselves using
“apply“ methods on events obtained directly from the event store..
This is the prescribed way of handling it. There are various ways of achieving this, but I would suggest keeping any persistence logic (reading or writing events) outside of your Aggregate. One simple way is to expose a constructor that accepts domain events and then applies those events.
On the other hand, I have also seen in several places where presenters
and bloggers have recommended rehydrating aggregate roots by
submitting a query to the read side of the application. Some have
suggested creating specific validation “buckets“ / projections on the
read side to facilitate this.
You can use the concept of snapshots as a way of optimizing your reads. This will create a memoized version of your hydrated Aggregate. You can load this snapshot and then only apply events that were generated since the snapshot was created. In this case, your Aggregate can define a constructor that takes two parameters: an existing state (snapshot) and any remaining domain events that can then be applied to that snapshot.
Snapshots are just an optimization and should be considered as such. You can create a system that does not use snapshots and apply them once read performance becomes a bottleneck.
On the other hand, I have also seen in several places where presenters
and bloggers have recommended rehydrating aggregate roots by
submitting a query to the read side of the application
Snapshots are not really part of the read side of the application. Data on the read side exists to satisfy use cases within the application. Those can change based on requirements even if the underlying domain does not change. As such, you shouldn't use read side data in your domain at all.
Event sourcing has developed different styles over the years. I could divide all o those into two big categories:
an event stream represents one entity (an aggregate in case of DDD)
one (partitioned) event stream for a (sub)system
When you deal with one stream per (sub)system, you aren't able to rehydrate the write-side on the fly, it is physically impossible due to the number of events in that stream. Therefore, you would rely on the projected read-side to retrieve the current entity state. As a consequence, this read-side must be fully consistent.
When going with the DDD-flavoured event sourcing, there's a strong consensus in the community how it should be done. The state of the aggregate (not just the root, but the whole aggregate) is restored by the command side before calling the domain model. You always restore using events. When snapshotting is enabled, snapshots are also stored as events in the aggregate snapshot stream, so you read the last one and all events from the snapshot version.
Concerning the Apply thing. You need to clearly separate the function that adds new events to the changes list (what you're going to save) and functions what mutate the aggregate state when events are applied.
The first function is the one called Apply and the second one is often called When. So you call the Apply function in your aggregate code to build up the changelist. The When function is called when restoring the aggregate state from events when you read the stream, and also from the Apply function.
You can find a simplistic example of an event-sourced aggregate in my book repo: https://github.com/alexeyzimarev/ddd-book/blob/master/chapter13/src/Marketplace.Ads.Domain/ClassifiedAds/ClassifiedAd.cs
For example:
public void Publish(UserId userId)
=> Apply(
new V1.ClassifiedAdPublished
{
Id = Id,
ApprovedBy = userId,
OwnerId = OwnerId,
PublishedAt = DateTimeOffset.Now
}
);
And for the When:
protected override void When(object #event)
{
switch (#event)
{
// more code here
case V1.ClassifiedAdPublished e:
ApprovedBy = UserId.FromGuid(e.ApprovedBy);
State = ClassifiedAdState.Active;
break;
// and more here
}
}
I am learning about DDD,CQRS and Event-sourcing and there is something I cannot figure out. Commands trigger changes in the aggregates and once the change is performed an event is fired. The event is subsequently handled by other parts of the system and preserved in the event store. However, I do not understand how replaying events would recreate the aggregate, if changes are triggered by commands.
Example: If we have a online shop.
AddItemToCardCommand -> Card Aggregate adds the item to its card -> ItemAddedToCardEvent -> The event is handled by whoever.
However, if the event is replayed, the aggregate would not add the item to its card.
To sum up, my question is how should I recreate aggregates based on the events in the event store? Also, any general advice on how to replay events the right way would be appreaciated.
For simplicity, let's assume a stateless process - our service doesn't try to keep copies of things in memory, but instead reloads aggregates as needed.
The service receives AddItemToCardCommand:{card:123, ...}. We don't have the current state of card:123 in memory, so we need to create it. We do that by loading the state of card:123 from our durable store. Because we chose to use event sourced storage, the "state" we read from the durable store is a representation of the history of events previously written by the service.
Event histories have within them all of the information you need to remember, but not necessarily in a convenient "shape" - append only lists are a great data structure for writes, but not necessarily good for reads.
What this often means is that we will "replay" the events to create an in memory object which we can then use to answer questions about the events we will write next.
The same pattern is used when answering simple queries: we load the history of events from the store, transform the event history into a more convenient shape, and then use that shape to compute the answer.
In circumstances where query latency is more important than timeliness, we might design our query handler to read the convenient shapes from a cache, rather than trying to compute them fresh every time; a concurrently running background thread would be responsible to waking up periodically to compute new contents for the cache.
Using an async process to pull updates from an event stream is a common pattern; Greg Young discusses some of the advantages of that approach in his Polyglot Data talk.
In an ideal event scenario, you would not have an already constructed aggregate structure available in your database. You repeatedly arrive at the end data structure by running through all events stored so far.
Let me illustrate with some pseudocode of adding items to cart, and then fetching the cart data.
# Create a new cart
POST /cart/new
# Store a series of events related to the cart (in database as records, similar to array items)
POST /cart/add -> CartService.AddItem(item_data) -> ItemAddedToCart
A series of events would look like:
* ItemAddedToCart
* ItemAddedToCart
* ItemAddedToCart
* ItemRemovedFromCart
* ItemAddedToCart
When its time to fetch cart data from the DB, you construct a new cart instance (or retrieve a cart instance if persisted) and replay the events on it.
cart = Cart(id=ID1)
# Fetch contents of Cart with id ID1
for each event in ID1 cart's events:
if event is ItemAddedToCart:
cart.add_item(event.data)
else if event is ItemRemovedFromCart:
cart.remove_item(event.data)
return cart
Occasionally, when there are too many events related to the cart, you may want to generate the aggregate structure then and save it in DB. Next time, you can start with the aggregate structure savepoint, and continue applying new events. This optimization helps save time and improve performance when there are too many events to process.
What may help is to not think of the command as changing the state but rather the event as changing the state. In fact, I don't quite see how else one would go about doing so. The command handler in your aggregate would apply the invariants and, if all is OK, would immediately create the event and call some method that would apply it ([Apply|On|Do]MyEvent). The fact that you have an event after the fact does not necessarily mean other parts of your system would handle it. It is however required for event sourcing. Once you have an event you can most certainly pass that on to other parts of your system via, say, publishing on a service bus.
When you replay your events you are calling the same methods that the commands were calling to actually mutate the state of your aggregate:
public MyEvent MyCommand(string data)
{
if (string.IsNullOrWhiteSpace(data))
{
throw new ArgumentException($"Argument '{nameof(data)}' may not be empty.");
}
return On(new MyEvent
{
Data = data
});
}
private MyEvent On(MyEvent myEvent)
{
// change the relevant state
someState = myEvent.Data;
return myEvent;
}
Your event sourcing infrastructure would call On(MyEvent) for MyEvent when replaying. Since you have an event it means that it was a valid state transition and can simply be applied; else something went wrong in your initial command processing and you probably have a bug.
All events in an event store would be in chronological order for an aggregate. In addition to this the events should have a global sequence number to facilitate projection processing.
You could have a generic projection that accepts any/all events and then publishes the event on a service bus for system integration. You could also place that burden on a client of the event store to have it keep track of the position itself and then read events off the store itself. You could combine these and have the client subscribe to service bus events but ensure that it executes them in the same order by keeping track of the position (global sequence number) itself and update it as the events are processed.
I've been studying DDD for a while, and stumbled into design patterns like CQRS, and Event sourcing (ES). These patterns can be used to help achieving some concepts of DDD with less effort.
In the architecture exemplified below, the aggregates know how to handle the commands and events related to itself. In other words, the Event Handlers and Command Handlers are the Aggregates.
Then, I’ve started modeling one sample Domain just to understand how the implementation would follow the business logic. For this question here is my domain (It’s based on this):
I know this is a bad modeled example, but I’m using it just as an example.
So, using ES, at the end of the operation, we would save all the events (Green arrows) into the event store (if there were no Exceptions), each event into its given Event Stream (Aggregate Type + Aggregate Id):
Everything seems right until now. So If we want to Rebuild the internal state of an instance of any of this Aggregate, we only have to new it up (new()) and apply all the events saved in its respective Event Stream in the correct order.
My question is related to changes in the model. Because, software development is a process where we never stop learning about our domain, and we always come with new ideas. So, let’s analyze some change scenarios:
Change Scenario 1:
Let´s pretend that now, if the Reservation Aggregate check’s that the seat is not available, it should send an event (Seat not reserved) and this event should be handled by one new Aggregate that will store all people that got their seat not reserved:
In the hypothesis where the old system already handled the initial command (Place order) correctly, and saved all the events to its respective event streams:
When we want to Rebuild the internal state of an instance of any of this Aggregate, we only have to new it up (new()) and apply all the events saved in its respective Event Stream in the correct order. (Nothing changed). The only thing, is that the new Use case didn’t exist back in the old model.
Change Scenario 2:
Let’s pretend that now, when the payment is accepted we handle this event (Payment Accepted) in a new Aggregate (Finance Aggregate) and not in the Order Aggregate anymore. And It send a new Event (Payment Received) to the Order Aggregate. I know this scenario is not well structured, but something like this could happen.
In the hypothesis where the old system already handled the initial command (Place order) correctly, and saved all the events to its respective event streams:
When we want to Rebuild the internal state of an instance of any of this Aggregate, we have a problem when applying the events from the Aggregate Event Stream to itself:
Now, the order doesn’t know anymore how to handle Payment Accepted Event.
Problems
So as the examples showed, whenever a system change reflects in an event being handled by a different event handler (Aggregate), there are some major problems. Because, we cannot rebuild the internal state anymore.
So, this problem can have some solutions:
Possible Solution
When an event is not handled by the aggregate in which Event Stream it is stored, we can find the new handler and create a new instance and send the event to it. But to maintain the internal state correct, we need the last event (Payment Received) to be handled by the Order Aggregate. So, we let it dispatch the event (and possible commands):
This solution can have some problems. Let’s imagine that a new command (Place Order) arrives and it has to create this order instance and save the new state. Now we would have:
In gray are the events that were already saved in the last call when the system hadn’t already gone through model changes.
We can see that a new Event Stream is created for the new aggregate (Finance W). And we can see that Event Streams are append-only, so the Payment Accepted event in the Order Y Event Stream is still there.
The first Payment Accepted event in Finance W Event Stream is the one that was supposed to be handled by the Order but had to find a new handler.
The Yellow payment received event in Order’s Event Stream is the event that was generated by the new handler of the Payment Accepted when the Payment Accepted event from the Order’s Event Stream was handled by the Finance.
All the other Green Events are new events that were generated by handling the Place Order Command in the new model.
Problem With the Solution
The next time the aggregate needs to be rebuild, there will be a Payment Accepted event in the stream (because it is append-only), and it will again call the new handler, but this have already been done and the Payment Received event have already been saved to the stream. So, it is not necessary to go through this again, we could ignore this event and continue.
Question
So, my question is how can we handle with model changes that impact who handle each event? How can we rebuild the internal state of an Aggregate after a change like this?
Will we need to build some event Stream migration that changes the events from one stream to the new schema (one or more streams)? Just like we would need in a Relational database?
Will we never be allowed to remove one handler, so we can only add new handlers? This would lead to unmanageable system…
You got almost all right, except one thing: Aggregates should not handle events from other Aggregates. It's like a non-event-sourced Aggregate shares a table with another Aggregate: they should not.
In event-driven DDD, Aggregates are the system's building blocks that receive Commands (things that express the intent) and return Events (things that had happened). For every Command type must exist one and only one Aggregate type that handle it. Before executing a Command, the Aggregate is fed with all its own previously emitted Events, that is, every Event that was emitted in the past by this Aggregate instance is applied to this Aggregate instance, in the chronological order.
So, if you want to correctly model your system, you are not allowed to send events from one Aggregate as events to another Aggregate (a different type or instance).
If you need to model business processes that involve multiple Aggregates, the correct way of doing it is by using a Saga/Process manager. This is a different component. It is the opposite of an Aggregate.
It receive Events emitted by Aggregates and sends Commands to other Aggregates.
In simplest cases, a Saga manager simply takes properties from one Event and creates+populates a Command with those properties. Then it sends the Command to the destination Aggregate.
In more complicated cases, the Saga waits for multiple Events and when all are received only then it creates and sends a Command.
The Saga may also deduplicate or reorder events.
In your case, a Saga could be Sale, whose purpose would be to coordinate the entire sales process, from ordering to product dispatching.
In conclusion, you have that problem because you have not modeled correctly your system. If your Aggregates would have handled only their specific Commands (and not somebody else's Events) then even if you must create a new Saga when a new Business process emerges, it would send the same Command to the Same Aggregate.
Answering briefly
my question is how can we handle with model changes that impact who handle each event?
Handling events is generally an easy thing to change, because the handling part is ephemeral. Events have a single writer, but they can have many readers. You just need to arrange for the plumbing to notify each subscriber of the event.
So in scenario #1, its the PaymentAggregate that writes down the PaymentAccepted event (in its own stream), and then your plumbing notifies the OrderAggregate that the PaymentAccepted event happened, and it does the next thing in its own logic.
To change to scenario #2, we'd leave the Payment Aggregate unchanged, but we'd arrange the plumbing so that it tells the FinanceAggregate about PaymentAccepted, and that it tells the OrderAggregate about PaymentReceived.
Your pictures make it hard to see this; I think you aren't being careful to track that each change of state is stored in the stream of the aggregate that changed. Not your fault - the Microsoft picture is really awful.
In other words, your arrow #3 "Seats Reserved" isn't a SeatsReserved event, it's a Handle(SeatsReserved) command.
So, I trigger command on aggregate root and it has some 10 events happened as a result of the command. This events are internal ones, and since outer systems need aggregation of this events, I decided to make projection (read projection basically). In order to make this projection from 10 events (internal) TO 1 event (external), I have to apply some business rules (business rules concerning merging of events). Where should I put this rules, since it seems like part of domain but I'm creating projections of internal events?
Basically since projection logic is part of domain, should I keep it inside aggregate and call it in code where projection is made?
UPDATE
So, inside one aggregate root, I have e.g. 3 events (internal) as response to one Command (aggregate.createPaintandwashatsametime(id, red)) that is sent to aggregate root and that are spreading through all the aggregate root entities like: CarCreated(Id), CarSeatColored(Red), CarWashed() etc. (all this 3 events are happened because of single command). External system expects to receive one external event as CarMaintainenceDone(Id, repainted=true, washed=true, somevalue=22);
Now, if i have some complex logic to make this CarMaintainenceDone event (like if(color==red then in projection somevalue==22 otherwise 44) - should this go in projection code or be part of domain?
UPDATE 2
Let me try to give you new example. Just ignore how domain is modeled since this is just example:
As you can see we have AggregateRoot that contains Multiplier which is there just to call things with the right name. When we do multiplication we first send integer 1 to ObjectA which has some logic to set internal state and emit ObjectAHasSetParam event. The same thing goes with ObjectB. Finally, ObjectC listens to all of this events, and on paramsHasBeenSet will do actual multiplication.
In event store in this case I would preserve list of events:
[ObjectAHasSetParam , ObjectBHasSetParam , ObjectCHasMultiplied ]
My point here was: if I emit all of this events one by one out of process - the state that somebody else updates will possibly be inconsistent, since this 3 events make sense only together. That is why I wanted to make something like projection, but I think in this case I just need to publish list of this events together instead of event by event.
class AggregateRoot{
Multiplier ml;
void handle(MultiplyCommand(1,2)){
ml.multiply(1,2);
}
}
class Multiplier{
ObjectA a;
ObjectB b;
ObjectC res;
void multiply(1,2){
a.setParam(1);
b.setParam(2);
publish(paramsHaveBeenSet());
}
}
class ObjectA{
int p;
void setParam(1){
p = 1 + 11;
publish(ObjectAHasSetParam(12));
}
}
class ObjectB{
int p;
void setParam(2){
p = 2 + 22;
publish(ObjectBHasSetParam(24));
}
}
class ObjectC{
int p1; int p2;
int res;
listen(ObjectAHasSetParam e1){
p1 = e1.par;
}
listen(ObjectBHasSetParam e2){
p2 = e2.par;
}
listen(paramsHaveBeenSet e3){
res = p1 * p2;
publish(ObjectCHasMultiplied(288));
}
}
External system expects to receive one external event as CarMaintainenceDone(Id, repainted=true, washed=true, somevalue=22);
A ha! The short answer is process manager.
The longer answer is that you (should) have two aggregates right now. One of them is tracking the state of the car. The other is tracking the process of maintaining the car.
The big hint that there is another aggregate hidden somewhere: you've got this CarMaintenanceDone event, with no aggregate responsible for generating it. All events have an "aggregate" somewhere that produces them. The aggregate might be the real world, or a proxy for the real world (HttpRequestReceived), or a digital thing in some other bounded context; but the event is telling you that something, somewhere, changed state.
That is to say, you have some aggregate that knows the rule of when the maintenance is done. It's an information resource, a log of work. When CarWashed is published (by the Car, or the washing machine, or whatever), an event handler subscribed to the CarWashed event sends a command to the Maintenance aggregate to inform it. The Maintenance aggregate updates its own state, runs its logic, and publishes a MaintenanceCompleted event when all of the individual steps have been accounted for.
Most things that are process like can be implemented as Aggregates; the weird bit is that the "commands" tend to look like event handlers. But they have their own history (based on what they have observed), which describes how the state machine changed in response to each event observed.
It might be more than two, depending on the complexity of the processes.
Rinat Abdullin wrote a good introduction to process managers, that I reference frequently.
Isn't there a clear distinction between an aggregate and a process manager though? I thought process managers would only coordinate and live in the application service world, sending appropriate commands to aggregates based on the events received.
From what I've seen -- no, there isn't. The literature doesn't make that very clear.
For example, Udi Dahan wrote
Here’s the strongest indication I can give you to know that you’re doing CQRS correctly: Your aggregate roots are sagas.
Saga, here, being equivalent to a process.
There's often 2 event models, internal events (only visible within a BC) and external events (published to the outside world). You could decide to make everything external but then you have to version everything.
You can read more about internal vs external events in the Patterns, Principles, and Practices of Domain-Driven Design book p.408 (scroll up a bit in the link).
Projections shouldn't be responsible to publish external events. One common practice would be to register an internal event handler from the application service layer which is responsible for publishing external events on a messaging infrastructure. You could leverage that process to aggregate these events together and publish a single external event from them.
How the aggregation is performed would be up to you, but since internal events can be raised synchronously and handlers are usually single-threaded you can just setup a state machine in the handler that kicks-in when it receives the first event of the batch and aggregates them until it receives the last, then publish on the message bus.
If your messaging infrastructure cannot participate in the same transaction as your event store you could just have an additional process that reads the committed events in order and does the same thing as above.
An alternative would be to let the consumer deal with the aggregation. That could be the right approach if the consumer should be able to veto what "CarMaintenanceDone" means.
Finally, you could also publish an extra event from the aggregate itself. The event may not be leveraged by the AR itself, but sometimes it's better to just do what's more practical (just like enriching events with data only consumed by the read model). This approach would also have the advantage of not having to change the logic if more events are added.
There should not be a notion of a external event. Events are generated by the Aggregates and consumed by synchronous read-models, sagas or published to the outside world where other systems and microservices use them whatever they want.
So, in your case, the consumer (implemented as a saga for example) should aggregate those events by its business rules and then do something (a saga can create a new command for example) and not the Aggregate.
UPDATE (in response to question being updated)
If you think that car maintenance is a responsibility of the Car Aggregate, then Car aggregate should raise the event. It depends on how the future behavior of the Car Aggregate is influenced by that CarMaintainenceDone event. In this particular context, I would generate the event from the Car aggregate, to make code simpler.