I want to apologize for my bad english first. I am trying to implement DDD in my project, but I have some problems I will try to describe. I am developing a chat application. Front-end is quite simple, no rooms, just one window showing last n messages.
Here comes the first problem. I don't see any entity here (ChatRoom would be ok, but I have only one room). Message seems like value object to me. So I don't know how to save state of chat (I think that having repositories for value objects is a bad practice).
This chat should have one specific feature, it should group and store related messages together (create something like ChatSegment). These segments will be divided by blocks of m off-topic messages (off-topic messages mean messages unrelated to messages in current segment).
I cannot imagine the way to make this work, without using stateful service. This behavior does not fit into any Entity (not even into the hypothetical ChatRoom entity). Segmenter entity does not seem right either. How would you solve this problem?
Maybe my thoughts are totally incorrect, but I would need to make them clear.
Thank you,
Brano.
In an IM application I'd (OTTOMH) treat Meassage as an Entity with a unique Id.
Combining DDD with DNC could be very powerful here if you treat your Message type as a «moment» archetype, and perhaps also use «role» archetypes (e.g. Chatter) in order for the users of the system to take on more than one «role» of needed.
Furthermore; the concept of "next/prior moment (i.e. message)" seems to be a perfect match.
Related
I am working on a cooking recipe app for google home and I need a way to string several GoogleResponses (SimpleResponse etc..) together without requiring user interaction between them.
I have searched for other answers pertaining to this, and while I have found a few similar questions to mine, the replies tend to be along the lines of "the system was designed for dialogues so what would be the point?".
I fully understand this point of view, however because of the nature and behaviour requirements of the app that I am developing I find myself in need of this particular possibility.
The recipes are divided into steps (revolutionary, I know..) and there is roughly a 1 to 1 correspondence between steps and GoogleResponses.
To give an example of how a typical recipe unfolds it is usually like this (this is a simplification of course):
main content -> question -> main content -> question -> etc..
With each instance of "main content" being a step of the recipe and each "question" requiring user input.
If if was just like this all the time then there would not be a problem, I could just bundle each "main content -> question" section into one GoogleResponse and be done.
However there are often times where the recipe flows more like:
main content -> main content -> main content -> question
With each "main content" being a step in the recipe, it does not make sense in this context to bundle them together into the same response (there is a system for the user to move back and forth between steps).
I was originally using MediaResponses for the "main content" sections as those do not require user input to move onto the next step, but due to various reasons I won't go into here as this is already getting quite long, the project manager has decided that MediaResponses should not be used in this project.
The short answer is the one you already encountered - trying to make conversational actions not-so-conversational doesn't work very well. However, there are a few things you can look into.
Recipe Structured Data
Since you're working on a recipe action, specifically, it may be worthwhile to use the standard recipe support that comes with the Assistant.
On the upside - people will be familiar with it, and you don't need to do much code, just provide markup on a webpage.
On the downside - if you have other requirements for how you want the interaction to go, it isn't that flexible. (For example, if you're asking questions at some of the recipe points, or if you want to offer measurement adjustments based on number of people to serve.)
Misuse the "No Input" event
You can configured dynamic reprompts so you get an event if the user doesn't say anything after a few seconds. If they want to speed a reply, they could ask for the next context specifically, or you can catch the actions_intent_NO_INPUT event in Dialogflow and advance yourself.
There are a few downsides here:
Not all devices support no-input. In particular, for example, mobile devices won't generate this.
This may only be valid for two no-input events in a row. On the third event, the Assistant may automatically close the conversation. (The documentation is unclear on this, and the exact behavior has changed over time.)
Media Response
You're not clear why using Media Response "shouldn't be used", but this is one of the only ways way to trigger an event when speaking is completed.
There are several downsides, however:
There are a number of bugs with Media Response around quitting
On devices with screens, there is a media player. Since the media itself is incidental to what you're doing, having the player doesn't make sense
It isn't supported on all surfaces
Interactive Canvas
A similar approach, however, would be to use the Interactive Canvas. This gives you an HTML page with JavaScript that you control, including being able to generate responses to the server as if the user spoke them (or as if they touched a suggestion chip). You can also listen to events for when the generated speech has finished.
There are, however, a number of downsides which probably prevent you from using this right now:
The biggest is that the Interactive Canvas can only be used for games right now. (But this seems to be a policy decision, rather than a technical one. So perhaps it will be lifted in the future.)
It does not work on smart speakers - only some devices with screens.
Combining the above approaches
One way to get around the device limitations of the Interactive Canvas and the poor visuals that accompany Media Response might be to mix the two. For devices that support IC, use that. If not, try using Media Response. (You may even wish to consider the no-input reprompt for some platforms.)
But this still won't work on all devices, and still has the limitation that Interactive Canvas is only for games right now.
Summary
There is no one, clear, way to handle this... and this isn't a feature they are likely to add given the conversational nature of the platform. However, there may be some workarounds which might work for your scenario.
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.
When you receive arguments in string format from the UI inside you controller, do you pass strings to application service (or to command) directly ?
Or, do you create value objects from the strings inside the controller ?
new Command(new SomeId("id"), Weight.create("80 kg"), new Date())
or
new Command("id", "80 kg", new Date())
new Command("id", "80", "kg", new Date())
Maybe it is not important, but it bothers me.
The question is, should we couple value objects from the domain to (inside) the controller ?
Imagine you don't have the web between you application layer and the presentation layer (like android activity or swing), would you push the use of value objects in the UI ?
Another thing, do you serialize/unserialize value objects into/from string like this ?
Weight weight = Weight.create("80 kg");
weight.getValue().equals(80.0);
weight.getUnit().equals(Unit.KILOGRAMS);
weight.toString().equals("80 kg");
In the case of passing strings into commands, I would rather pass "80 kg" instead of "80" and "kg".
Sorry if the question is not relevant or funny.
Thank you.
UPDATE
I came across that post while I was searching information about a totally different topic : Value Objects in CQRS - where to use
They seem to prefer primitives or DTOs, and keep VOs inside the domain.
I've also taken a look at the book of V. Vernon (Implementing DDD), and it talks about (exactly -_-) that in chapter 14 (p. 522)
I've noticed he's using commands without any DTOs.
someCommand.setId("id");
someCommand.setWeightValue("80");
someCommand.setWeightUnit("kg");
someCommand.setOtherWeight("80 kg");
someCommand.setDate("17/03/2015 17:28:35");
someCommand.setUserName("...");
someCommand.setUserAttribute("...");
someCommand.setUserOtherAttributePartA("...");
someCommand.setUserOtherAttributePartB("...");
It is the command object that would be mapped by the controller. Value objects initialization would appeare in the command handler method, and they would throw something in case of bad value (self validation in initialization).
I think I'm starting to be less bothered, but some other opinions would be welcomed.
As an introduction, this is highly opinionated and I'm sure everyone has different ideas on how it should work. But my endeavor here is to outline a strategy with some good reasons behind it so you can make your own evaluation.
Pass Strings or Parse?
My personal preference here is to parse everything in the Controller and send down the results to the Service. There are two main phases to this approach, each of which can spit back error conditions:
1. Attempt to Parse
When a bunch of strings come in from the UI, I think it makes sense to attempt to interpret them immediately. For easy targets like ints and bools, these conversions are trivial and model binders for many web frameworks handle them automatically.
For more complex objects like custom classes, it still makes sense to handle it in this location so that all parsing occurs in the same location. If you're in a framework which provides model binding, much of this parsing is probably done automatically; if not - or you're assembling a more complex object to be sent to a service - you can do it manually in the Controller.
Failure Condition
When parsing fails ("hello" is entered in an int field or 7 is entered for a bool) it's pretty easy to send feedback to the user before you even have to call the service.
2. Validate and Commit
Even though parsing has succeeded, there's still the necessity to validate that the entry is legitimate and then commit it. I prefer to handle validation in the service level immediately prior to committing. This leaves the Controller responsible for parsing and makes it very clear in the code that validation is occurring for every piece of data that gets committed.
In doing this, we can eliminate an ancillary responsibility from the Service layer. There's no need to make it parse objects - its single purpose is to commit information.
Failure Condition
When validation fails (someone enters an address on the moon, or enters a date of birth 300 years in the past), the failure should be reported back up to the caller (Controller, in this case). While the user probably makes no distinction between failure to parse and failure to validate, it's an important difference for the software.
Push Value Objects to UI?
I would accept parsed objects as far up the stack as possible, every time. If you can have someone else's framework handle that bit of transformation, why not do it? Additionally, the closer to the UI that the objects can live, the easier it is to give good, quick feedback to the user about what they're doing.
A Note on Coupling
Overall, pushing objects up the stack does result in greater coupling. However, writing software for a particular domain does involve being tightly coupled to that domain, whatever it is. If a few more components are tightly coupled to some concepts that are ubiquitous throughout the domain - or at least to the API touchpoints of the service being called - I don't see any real reduction in architectural integrity or flexibility occurring.
Parse One Big String or Components?
In general, it tends to be easiest to just pass the entire string into the Parse() method to get sorted through. Take your example of "80 kg":
"80 kg" and "120 lbs" may both be valid weight inputs
If you're passing in strings to a Parse() method, it's probably doing some fairly heavy lifting anyway. Expecting it to split a string based on a space is not overbearing.
It's far easier to call Weight.create(inputString) than it is to split inputString by " ", then call Weight.create(split[0], split[1]).
It's easier to maintain a single-string-input Parse() function as well. If some new requirement comes in that the Weight class has to support pounds and ounces, a new valid input may be "120 lbs 6 oz". If you're splitting up the input, you now need four arguments. Whereas if it's entirely encapsulated within the Parse() logic, there's no burden to outside consumers. This makes the code more extensible and flexible.
The difference between a DTO and a VO is that a DTO has no behavior, it's a simple container designed to pass data around from component to component. Besides, you rarely need to compare two DTO's and they are generally transient.
A Value Object can have behavior. Two VO's are compared by value rather than reference, which means for instance two Address value objects with the same data but that are different object instances will be equal. This is useful because they are generally persisted in one form or another and there are more occasions to compare them.
It turns out that in a DDD application, VO's will be declared and used in your Domain layer more often than not since they belong to the domain's Ubiquitous Language and because of separation of concerns. They can sometimes be manipulated in the Application layer but typically won't be sent between the UI layer and Application. We use DTO's for that instead.
Of course, this is debatable and depends a lot on the layers you choose to build your application out of. There might be cases when crunching your layered architecture down to 2 layers will be beneficial, and when using business objects directly in the UI won't be that bad.
One of the basic tenets of CQRS, as I understand it, is that commands should be behaviour-centric, and have a value in the business or the UL, and not data-centric, ie., CRUD. Instead of focusing on updating a customer, we have commands like CustomerHasMoved. What if you have CRUD screens which are there to correct certain data. For example, we need to change the name of a customer which is misspelled. This doesn't really have much value in the business. Should this just be under the umbrella of an UpdateCustomer command?
I just want to put a comment on this quickly as it popped up.
It is important to note that some objects are actually CRUD and thats ok. I may not really care why a name is changing in my domain where I ship products to people and only need that data to print mailing labels. The trick is in making behavior the default and THEN reverting to a CRUD interface once you are sure you really don't care about the reasons as opposed to vice versa.
Greg
Actually, there could be various reasons to update the name of a customer. As you were saying, it could be misspelled or... you could get married and change your name to your husband's.
If you had only an UpdateCustomer command, you would loose the original intent and you would not be able to have different behaviours for each of them. If the name was misselled it could be as simple as updating the database, whereas if your customer got married you might need to notify the marketing departement so tthat they can offer a discount.
In the case that your entity is purely CRUD, that is there is no intent that you can associate with modifying the properties, then it's OK to have an UpdateEntityCommand. You can then transition slowly to something more task based
CustomerHasMoved is the event that is fired after you have updated the customers location. This event updates the read databases/cache databases. The command from the gui should be MoveCustomer or something like that. I think I would put the update of the customer name in a command like UpdateCustomer.