I have an Agenda that holds many Card, and a Card has multiple DetailItem that hold a value like Email, Phone and a Label
So, I can do:
agenda = new Agenda()
oneCard = new Card()
item = new DetailItem(new Email("x#y.z"), new Label("Work")
oneCard.addItem(item)
agenda.addCard(oneCard)
By rule, there can only be one DetailItem with an Email instance with "x#y.z" value, so if you try to add a new item with that data it will raise an exception.
This seems to be ok until I try to update a DetailItem. I can't find a way that I feel comfortable with.
I try to think in terms of business model and not implementation details, but I cannot leave them off for a long, and they enter the domain I like it or not.
The thing is that I will have a REST interface, and I have two ways of doing things.
Send a PUT to /cards/<cardId> with a detailItems' array, fetch the Cardby ID, create a newCardwith the new data provided in the PUT, and sync the currentCard` with the new one.
Send a PUT to /cards//items/, fetch the Card, find the DetailItem, and update it the same way in option 1
If I go with option 1, I have to remove all DetailItem from current Card that don't exists in the new one. That leads with some INSERT queries produced by the ORM, and some DELETES. No UPDATE at all.
If I go with option 2, I will have many PUTs to change multiple items, which is not performant at all, and it will lead me to introduce an ID field to the DetailItem so I can identify them, which introduces something is not part of the domain!.
The only option I found was to go with option 1, and send a request like this:
{
...
detailItems: [
{
type: "email",
oldValue: "x#y.z",
oldLabel: "Work",
newValue: "a#b.c",
newLabel: "Work",
}
]
}
So I can do something like this:
card = agenda.getCardIdentifiedBy(the_identifier)
for(itemUpdate in jsonData.detailItems)
itemClass = ValueClassMapper.from(itemUpdate.type) // "email" -> Email
oldItem = new DetailItem(new itemClass(itemUpdate.oldValue), new Label(itemUpdate.oldLabel))
newItem = new DetailItem(new itemClass(itemUpdate.newValue), new Label(itemUpdate.newLabel))
card.updateItemWith(oldItem, newItem)
But, I don't know, maybe I have a wrong abstraction
Can anyone help?
Thanks in advance!
Part of the problem that you are running into here is that HTTP remote authoring semantics are at odds with the notion of an autonomous domain model.
HTTP PUT means "make your copy of this document look like my copy of this document". It is analogous to "save file", or "commit these rows to the database". The semantics of the request fit the model of an anemic data store.
On the other hand, our domain models are typically autonomous state machines, that expect to decide for themselves what state they should transition into based on new information. In other words, all domain model interactions roughly follow the following pattern:
domainModel.change(newInformation)
The model then changes its own data structures in response to this information (with the application code responsible for parsing the new information into a data structure that the domain model will understand).
This doesn't mean that you can't use HTTP and REST with domain models; but it does mean that the resource model looks very different from what you have described here.
The most straightforward approach would be to POST the new information to your agenda resource (autonomous domain entities really don't want you trying to directly modify their internals).
POST /agenda/12345
Content-Type: application/json
{
...
}
It would then be your application's responsibility to parse the body of the request, and then to pass that parsed representation along to your agenda domain entity, the domain entity would then decide how/if to integrate the new information with the information that it already knows about.
Another approach would be to treat the new information as a new document to be stored, where each new document would have its own URI:
PUT /messages/67890
Content-Type: application/json
{
...
}
Where the changes to /agenda/12345 are a side effect of storing the message -- see Webber 2011. Please be aware that this approach can make cache invalidation more challenging.
Related
Let's use the classic example of blog context. In our domain we have the following scenarios: Users can write Posts. Posts must be cataloged at least in one Category. Posts can be described using Tags. Users can comment on Posts.
The four entities (Post, Category, Tag, Comment) are implemented as different aggregates because of I have not detected any rule for that an entity data should interfere in another. So, for each aggregate I will have one repository that represent it. Too, each aggregate reference others by his id.
Following CQRS, from this scenario I have deducted typical use cases that result on commands such as WriteNewPostCommand, PublishPostCommand, DeletePostCommand etc... along with their respective queries to get data from repositories. FindPostByIdQuery, FindTagByTagNameQuery, FindPostsByAuthorIdQuery etc...
Depending on which site of the app we are (backend or fronted) we will have queries more or less complex. So, if we are on the front page maybe we need build some widgets to get last comments, latest post of a category, etc... Queries that involve a simple Query object (few search criterias) and a QueryHandler very simple (a single repository as dependency on the handler class)
But in other places this queries can be more complex. In an admin panel we require to show in a table a relation that satisfy a complex search criteria. Might be interesting search posts by: author name (no id), categories names, tags name, publish date... Criterias that belongs to different aggregates and different repositories.
In addition, in our table of post we dont want to show the post along with author ID, or categories ID. We need to show all information (name user, avatar, category name, category icon etc).
My questions are:
At infrastructure layer, when we design repositories, the search methods (findAll, findById, findByCriterias...), should have return the corresponding entity referencing to all associations id's? I mean, If a have a method findPostById(uuid) or findPostByCustomFilter(filter), should return a post instance with a reference to all categories id it has, all tags id, and author id that it has? Or should my repo have some kind of method that populates a given post instance with the associations I want?
If I want to search posts created from 12/12/2014, written by John, and categorised on "News" and "Videos" categories and tags "sci-fi" and "adventure", and get the full details of each aggregate, how should create my Query and QueryHandler?
a) Create a Query with all my parameters (authorName, categoriesNames, TagsNames, if a want retrive User, Category, Tag association full detailed) and then his QueryHandler ensamble the different read models in a only one. Or...
b) Create different Queries (FindCategoryByName, FindTagByName, FindUserByName) and then my web controller calls them for later
call to FindPostQuery but now passing him the authorid, categoryid, tagid returned from the other queries?
The b) solution appear more clean but it seems me more expensive.
On the query side, there are no entities. You are free to populate your read models in any way suits your requirements best. Whatever data you need to display on (a part of) the screen, you put it in the read model. It's not the command side repositories that return these read models but specialized query side data access objects.
You mentioned "complex search criteria" -- I recommend you model it with a corresponding SearchCriteria object. This object would be technnology agnostic, but it would be passed to your Query side data access object that would know how to combine the criteria to build a lower level query for the specific data store it's targeted at.
With simple applications like this, it's easier to not get distracted by aggregates. Do event sourcing, subscribe to the events by one set of tables that is easy to query the way you want.
Another words, it sounds like you're main goal is to be able to query easily for the scenarios you describe. Start with that end goal. Now write your event handler to adjust your tables accordingly.
Start with events and the UI. Then everything else will fit easily. Google "Event Modeling" as it will help you formulate ideas sound what and how you want to build these style of applications.
I can see three problems in your approach and they need to be solved separately:
In CQRS the Queries are completely separate from the Commands. So, don't try to solve your queries with your Commands pipelines repositories. The point of CQRS is precisely to allow you to solve the commands and queries in very different ways, as they have very different requirements.
You mention DDD in the question title, but you don't mention your Bounded Contexts in the question itself. If you follow DDD, you'll most likely have more than one BC. For example, in your question, it could be that CategoryName and AuthorName belong to two different BCs, which are also different from the BC where the blog posts are. If that is the case and each BC properly owns its own data, the data that you want to search by and show in the UI will be stored potentially in different databases, therefore implementing a query in the DB with a join might not even be possible.
Searching and Reading data are two different concerns and can/should be solved differently. When you search, you get some search criteria (including sorting and paging) and the result is basically a list of IDs (authorIds, postIds, commentIds). When you Read data, you get one or more Ids and the result is one or more DTOs with all the required data properties. It is normal that you need to read data from multiple BCs to populate a single page, that's called UI composition.
So if we agree on these 3 points and especially focussing on point 3, I would suggest the following:
Figure out all the searches that you want to do and see if you can decompose them to simple searches by BC. For example, search blog posts by author name is a problem, because the author information could be in a different BC than the blog posts. So, why not implement a SearchAuthorByName in the Authors BC and then a SearchPostsByAuthorId in the Posts BC. You can do this from the Client itself or from the API. Doing it in the client gives the client a lot of flexibility because there are many ways a client can get an authorId (from a MyFavourites list, from a paginated list or from a search by name) and then get the posts by authorId is a separate operation. You can do the same by tags, categories and other things. The Post will have Ids, but not the extra details about those IDs.
Potentially, you might want more complicated searches. As long as the search criteria (including sorting fields) contain fields from a single BC, you can easily create a read model and execute the search there. Note that this is only for the search criteria. If the search result needs data from multiple BCs you can solve it with UI composition. But if the search criteria contain fields from multiple BCs, then you'll need some sort of Search engine capable of indexing data coming from multiple sources. This is especially evident if you want to do full-text search, search by categories, tags, etc. with large quantities of data. You will need to use some specialized service like Elastic Search and it won't belong to any of your existing BCs, it'll be like a supporting service.
From CQRS you will have a separeted Stack for Queries and Commands. Your query stack should represent a diferente module, namespace, dll or package at your project.
a) You will create one QueryModel and this query model will return whatever you need. If you are familiar with Entity Framework or NHibernate, you will create a Façade to hold this queries togheter, DbContext or Session.
b) You can create this separeted queries, but saying again, if you are familiar with any ORM your should return the set that represents the model, return every set as IQueryable and use LET (Linq Expression Trees) to make your Query stack more dynamic.
Using Entity Framework and C# for exemple:
public class QueryModelDatabase : DbContext, IQueryModelDatabase
{
public QueryModelDatabase() : base("dbname")
{
_products = base.Set<Product>();
_orders = base.Set<Order>();
}
private readonly DbSet<Order> _orders = null;
private readonly DbSet<Product> _products = null;
public IQueryable<Order> Orders
{
get { return this._orders.Include("Items").Include("Items.Product"); }
}
public IQueryable<Product> Products
{
get { return _products; }
}
}
Then you should do queries the way you need and return anything:
using (var db = new QueryModelDatabase())
{
var queryable = from o in db.Orders.Include(p => p.Items).Include("Details.Product")
where o.OrderId == orderId
select new OrderFoundViewModel
{
Id = o.OrderId,
State = o.State.ToString(),
Total = o.Total,
OrderDate = o.Date,
Details = o.Items
};
try
{
var o = queryable.First();
return o;
}
catch (InvalidOperationException)
{
return new OrderFoundViewModel();
}
}
In an application that uses event sourcing is it acceptable to have aggregate-wide events?
Consider a contrived example of a blog application that provides the ability to create posts and add and remove simple tags (post would be the aggregate root).
This might result in the following events:
PostCreated: postId, "title", "content"
TagAdded: postId, "Foo"
TagAdded: postId, "Bar"
TagAdded: postId, "Baz"
TagRemoved: postId, "Bar"
Replaying the above event stream would result in a post with a title, content and two tags ("Foo" & "Baz").
Now imagine the user interface only allows you to select existing tags whilst creating a post and doesn't accept free text; only privileged users have the ability to update the master list of tags.
Now when a privileged user creates a new tag, a corresponding event needs to be created so that a) the information is actually stored in the event storage and b) at some point the read model is updated so that users creating blog posts can select the new tag in the UI.
Having an event that looks like TagCreated: postId, "NewTag" doesn't seem right to me as the information does not directly apply to a single post.
Considering that in this case the information does not warrant it's own aggregate root and will only be used in this bounded context I would expect an event along the lines of:
TagCreated("NewTag")
These events would be stored in the event storage using the same aggregate id as the previous set of events for a specific post but without an id for the specific aggregate instance.
So far this sounds like a logical way to handle the problem but was wondering if I am missing anything obvious by approaching it this way.
IMHO you're complicating your life unnecessary. Domain events are usually available cross bounded context and they should be associated with an aggregate root (AR) by referencing its id.
In your example, I'd consider Tag to be a value object, so it would require a post id. But if you want the Tag to be available as itself, then it would be an AR and so, the event would have a TagId property.
Btw, a domain event is a DTO, meant to be available everywhere, they're not a domain detail that needs to be encapsulated in an aggregate.
I think you missed the concept "tag catalog" or something like that. It could have a single aggregate (or perhaps you will sometimes have several catalogs for different user groups or something like that) with the catalog as the root, containing all the tags as value objects.
I'm developing an application with Domain Drive Design approach. in a special case I have to retrieve the list of value objects of an aggregate and present them. to do that I've created a read only repository like this:
public interface IBlogTagReadOnlyRepository : IReadOnlyRepository<BlogTag, string>
{
IEnumerable<BlogTag> GetAllBlogTagsQuery(string tagName);
}
BlogTag is a value object in Blog aggregate, now it works fine but when I think about this way of handling and the future of the project, my concerns grow! it's not a good idea to create a separate read only repository for every value object included in those cases, is it?
anybody knows a better solution?
You should not keep value objects in their own repository since only aggregate roots belong there. Instead you should review your domain model carefully.
If you need to keep track of value objects spanning multiple aggregates, then maybe they belong to another aggregate (e.g. a tag cloud) that could even serve as sort of a factory for the tags.
This doesn't mean you don't need a BlogTag value object in your Blog aggregate. A value object in one aggregate could be an entity in another or even an aggregate root by itself.
Maybe you should take a look at this question. It addresses a similar problem.
I think you just need a query service as this method serves the user interface, it's just for presentation (reporting), do something like..
public IEnumerable<BlogTagViewModel> GetDistinctListOfBlogTagsForPublishedPosts()
{
var tags = new List<BlogTagViewModel>();
// Go to database and run query
// transform to collection of BlogTagViewModel
return tags;
}
This code would be at the application layer level not the domain layer.
And notice the language I use in the method name, it makes it a bit more explicit and tells people using the query exactly what the method does (if this is your intent - I am guessing a little, but hopefully you get what I mean).
Cheers
Scott
Background (ie what the heck is a relative complement?)
Relative Complement
What I'm trying to do
Let's say I've got a custom Vehicle entity that has a VehicleType option set that is either "Car", or "Truck". There is a 1 to many relationship between Contact and Vehicle (ie. ContactId is on the vehicle entity). How do I write an XRM query (Linq To CRM, QueryExpression, fetch Xml, whatever) that returns the contacts with only cars?
Option 1:
I’d prefer a modification of the proposal that AdamV makes above. I can’t think of a way that you’d get this particular query answered using Linq to CRM, Query Expressions, FetchXML alone. Daryl doesn’t offer what the client is, but I would suppose if Linq and Query Expressions were acceptable offerings, .NET is on the table. Creating aggregate fields containing the count of the related entity on the parent entity (contact in this case) offers more than the Boolean option. If the query requirements ever changed to a threshold (more than X cars, less than Y trucks, between X and Y total vehicles) the Boolean options fails to deliver. The client in this question isn’t known, but I can’t think of many (any?) cases where pulling all the records to the client on a set of 500K+ rows is more efficient than a the SQL query that CRM would make on your behalf against several integer fields with range clauses.
Upside:
Maintains client purity in Query approach
Simple client query
Probably as performant as possible
Downside:
Setups for Aggregate fields
Workflow or plugin to manage the increment and decrement of the aggregate fields
SQL Script for initial load of the aggregates.
Risk that aggregate fields get out of sync (workflow or plugin fails)
Option 2:
If purity within the client isn’t essential, and .NET is on the table – skip the aggregate fields and the setup and just run SQL against the Views. If you don’t want to work with the ADO.NET, a thin ORM like Dapper, Massive, or PetaPOCO can still give you an object model. As Andreas offers in his comment on the OP’s first answer, it seems like something fairly trivial to do in SQL.
Sketching something from top of mind:
SELECT c.*
FROM Contact
WHERE C.Contactid in (
Select contactid
FROM Vehicle v
group by v.contactid , v.type
having v.type = ‘Car’ and count(contactid) > 1
)
AND NOT IN (
Select contactid
FROM Vehicle v
group by v.contactid , v.type
having v.type <> ‘Car’ and count(contactid) > 1
)
Upside:
Much less work
CRM Entities get left alone
Downside:
Depending on the client and/or the application mixing DataAccess methods is a bit kludgy.
Likely less performant than Option 1
Option 3:
Mix and Match: Take the aggregate fields from Option 1. But update them using a scheduled SQL job (or something similar) with a query similar to the initial load job you’d need to write in Option 1
Upside:
Takes most of the work and risk out of Option 1
Keeps all of the performance of Option 1
Downside:
Some will see this as an unsupported feature.
In order to order to perform a true Relative Complement Query you need to be able to perform a subquery.
Your query would basically say give me all the contacts with cars, and then, within those results, remove any contacts that have a vehicle that isn't a car. This is what the SQL in #JasonKoopmans answer does. Unfortunetly, CRM does not support SubQueries.
Therefore, the only way to achieve this is to either perform the sub query on the client side, as I resorted to doing, or storing the results of what would be the subquery in a manner that can be accessed through the main query (ie storing counts on the contact entity).
You could theoretically do this "on the fly" by making a SubQueryResult entity that stores a ContactId, and SubQueryId. You'd first pull back the contacts that have at least 1 car, and create a SubQueryResult record for each record, with it's contactId, and a single SubQueryId that is generated client side to tie them all together.
Then you'd do another query that says give me all the contacts that are in this SubQueryResult with this SubQueryId, that do not have any vehicles that aren't cars.
I could only assume that this wouldn't be any more efficient than performing the two separate queries and performing the filter client side. Although with the new ExecuteMultipleRequests in the new CRM release, it may be close.
I have resorted to pulling back all of my records in CRM, and performing the check on the client side since CRM 2011 doesn't support this via Query Expressions.
You could write two Fetch XML statements, one to return all contacts and the count of their vehicles, and another to return all contacts and the count of their cars, then compare the list on the client side. But once again, you're having to return every contact and filter it client side.
It's not tested but how about this query expression? I'm linking in the Vehicle entity as an inner join, requiring that it's a Car. I'm assuming that the field VehicleType is a String because I'm a bit lazy and don't want to test it (I'm typing this hardcore style, no compilation - pure brain work).
Optionally, you might want to add a Criteria section as well to control which of the Contact instances that actually get retrieved. Do tell how it went!
Sorry for the verbosity. I know you like it short. My brains work better when circumlocutory.
new QueryExpression
{
EntityName = "contact",
ColumnSet = new ColumnSet("fullname"),
LinkEntities =
{
new LinkEntity
{
JoinOperator = JoinOperator.Inner,
LinkFromEntityName = "contact",
LinkFromAttributeName = "contactid",
LinkToEntityName = "vehicle",
LinkToAttributeName = "contactid",
Columns = new ColumnSet("vehicletype"),
EntityAlias = "Vroom",
//LinkCriteria = { Conditions =
//{
// new ConditionExpression(
// "vehicletype", ConditionOperator.Equal, "car")
//} }
LinkCriteria = { Conditions =
{
new ConditionExpression(
"vehicletype", ConditionOperator.NotEqual, "truck")
} }
}
}
};
EDIT:
I've talk to my MVP Gustaf Westerlund and he's suggested the following work-around. Let me stress that it's not an answer to your original question. It's just a way to solve it. And it's cumbersome. :)
So, the hint is to add a flag in the Contact or Person entity. Then, every time you create a new instance of Vehicle, you need to fire a message and using a plugin, update the information on the first about the creation of the latter.
This has several drawbacks.
It requires us to do stuff.
It's not the straight-forward do-this-and-that type of approach.
Maintenance is higher for every new type of Vehicle one adds.
Buggibility is elevated since there are many cases to regard (what happens to the flagification when a Vehicle instance is reasigned, deleted etc.).
So, my answer to your question is changed to: "can't be done". This remains effective until (gladly) proven wrong by presented alternative solution. Duck!
Personally, I'd fetch (almost) everything and unleash the hounds of LINQ onto it. But I'd do that without smiling nor proud. :)
I'm trying to figure out how to accomplish the following:
User can have many Websites
What I need to do before adding a new website to a user, is to take the website URL and pass it to a method which will check whether the Website already exist in the database (another User has the same website associated), or whether to create a new record. <= The reason for this is whether to create a new thumbnail or use an existing.
The problem is that the repository should be per aggregate root, which means I Cant do what I've Explained above? - I could first get ALL users in the database and then foreach look with if statement that checks where the user has a website record with same URL, but that would result in an endless and slow process.
Whatever repository approach you're using, you should be able to specify criteria in some fashion. Therefore, search for a user associated with the website in question - if the search returns no users, the website is not in use.
For example, you might add a method with the following signature (or you'd pass a query object as described in this article):
User GetUser(string hasUrl);
That method should generate SQL more or less like this:
select u.userId
from User u
join Website w
on w.UserId = u.UserId
where w.Url = #url
This should be nearly as efficient as querying the Website table directly; there's no need to load all the users and website records into memory. Let your relational database do the heavy lifting and let your repository implementation (or object-relational mapper) handle the translation.
I think there is a fundamental problem with your model. Websites are part of a User aggregate group if I understand correctly. Which means a website instance does not have global scope, it is meaningful only in the context of belonging to a user.
But now when a user wants to add a new website, you first want to check to see if the "website exists in the database" before you create a new one. Which means websites in fact do have a global scope. Otherwise anytime a user requested a new website, you would create a new website for that specific user with that website being meaningful in the scope of that user. Here you have websites which are shared and therefore meaningful in the scope of many users and therefore not part of the user aggregate.
Fix your model and you will fix your query difficulties.
One strategy is to implement a service that can verify the constraint.
public interface IWebsiteUniquenessValidator
{
bool IsWebsiteUnique(string websiteUrl);
}
You will then have to implement it, how you do that will depend on factors I don't know, but I suggest not worrying about going through the domain. Make it simple, it's just a query (* - I'll add to this at the bottom).
public class WebsiteUniquenessValidator : IWebsiteUniquenessValidator
{
//.....
}
Then, "inject" it into the method where it is needed. I say "inject" because we will provide it to the domain object from outside the domain, but .. we will do so with a method parameter rather than a constructor parameter (in order to avoid requiring our entities to be instantiated by our IoC container).
public class User
{
public void AddWebsite(string websiteUrl, IWebsiteUniquenessValidator uniquenessValidator)
{
if (!uniquenessValidator.IsWebsiteUnique(websiteUrl) {
throw new ValidationException(...);
}
//....
}
}
Whatever the consumer of your User and its Repository is - if that's a Service class or a CommandHandler - can provide that uniqueness validator dependency. This consumer should already by wired up through IoC, since it will be consuming the UserRepository:
public class UserService
{
private readonly IUserRepository _repo;
private readonly IWebsiteUniquenessValidator _validator;
public UserService(IUserRepository repo, IWebsiteUniquenessValidator validator)
{
_repo = repo;
_validator = validator;
}
public Result AddWebsiteToUser(Guid userId, string websiteUrl)
{
try {
var user = _repo.Get(userId);
user.AddWebsite(websiteUrl, _validator);
}
catch (AggregateNotFoundException ex) {
//....
}
catch (ValidationException ex) {
//....
}
}
}
*I mentioned making the validation simple and avoiding the Domain.
We build Domains to encapsulate the often complex behavior that is involved with modifying data.
What experience shows, is that the requirements around changing data are very different from those around querying data.
This seems like a pain point you are experiencing because you are trying to force a read to go through a write system.
It is possible to separate the reading of data from the Domain, from the write side, in order to alleviate these pain points.
CQRS is the name given to this technique. I'll just say that a whole bunch of lightbulbs went click once I viewed DDD in the context of CQRS. I highly recommend trying to understand the concepts of CQRS.