I wonder what is the best practice for this business model is:
I have a User object and a Post object, the user has one or many Posts, so I want to know what's the best design:
1 - Create separate endpoints GET /users/id and GET /posts?userId=id
2 - Create only one endpoint GET /users/id and in my service layer call getPostsByUserId() then add posts to my user object and return them in the API.
could you tell me which is the right approach and the pros and cons?
If you go with the first approach, Then you are using Lazy fetching which is common in RestApis and it's being used for one-to-many relationships where you want to get instances related to one entity; Simply it has better performance because your client fetches data as specified and as much as they needed. The second approach uses Eager fetching which is good for loading a single related instance. In the end, Your selection between these two is toughly close to your usage. In RestApis the first approach is the more common and better solution. But in traditional web applications like Spring MVC, In some situations, You have to use the second approach to load all necessary things on your page.
Related
Let's assume I would like to create a blogging platform that would allow managing user accounts, therefore I came up with 2 microservices:
Blogging - managing posts, tags, etc.
Users - managing users, their roles, etc.
It's clear to me that Post contains the ID of the author (User) and User contains IDs of the Posts he wrote.
The problem is that when a client requests a Post, I would also like to return the name of the author and send it together to the client in a DTO (view model). I see 2 possible solutions for that:
Introduce the concept of the User (only ID and name) in a domain of the Blogging service. Every time a client requests a Post, all relevant data is fetched only from this microservice. Every time a user's name is modified in a Users microservice, this service is notified and updated author's names.
Keep the concerns completely separated. Every time a client asks for a Post, Blogging microservice is called to fetch Post and then Users microservice is called to fetch the author's name based on the ID.
So far I was leaning towards solution #1 as it would require only 1 call to the 1 microservice when Post is requested, but then let's say if the number of functions (microservices) starts growing and I keep adding small concepts from each of them only to limit the number of calls, I'm afraid I would end up in a spaghetti Blogging microservice... But then I also don't think the number of microservices will grow significantly ;)
Which way do you find better and why? Does any of the approaches break the microservices architecture principles and my concerns are justified?
It really depends on the way your program is going to develop.
Solution #1 (editing the Blogging model)
Here you edit your model for the sake of the performance. Most of the time I don't think altering the model itself for the sake of performance is the right way. If you don't think the concept of an User inside the Blogging context is right, you shouldn't put it there. Another problem is that you would have to deal with eventual consistency between you bounded contexts (BC). If you think the concept of User inside Blogging is not a bad idea, it might be probably the way to go.
Solution #2 (querying the data separately)
Although this might not be performance friendly, I think this would be the easiest way at the beginning. You won't have to deal with eventual consistency (+dealing with synchronizing the data between BC) and you don't have to alter your model. But if you really care about performance and keeping your model clean, you might be interested in read model.
Another solution - read model
The concept of read model is about having at least two separate models: one for reading the data and for writing/editing the data. If we want to implement this concept for your problem, we would create a read model for the Blogging BC where Posts and Users could be merged together (something like in solution #1) and then queried. We can synchronize this data with the help of events, so when the Post or User changes, it would raise an event and save the changed data into your read model. This way you can merge/alter any data you want for better performance. It might be very similar to solution #1, but the main difference is that you don't edit your main model. You create new one just for the reading. It is probably the hardest solution, but if your project is large and performance heavy, this might be a way.
I wouldn't say any solution is the best or the worst for solving your problem. It always depends on the context.
I'm doing a fairly complex emberjs application, and tying it to a backend of APIs.
The API calls are not usually tied to any particular model, but may return objects of various types in different sections of the response, e.g. a call to Events API would return events, but also return media assets and individuals involved in those events.
I've just started with the project, and I'd like to get some expert guidance on how best to separate concerns to have a clean maintainable code base.
The way I am approaching this is:
Models: essentially handle records with their fields, and other computed properties. However, models are not responsible for making requests.
e.g. Individual, Event, Picture, Post etc.
Stores: They are essentially caches. For example, an eventStore would store all events received from the server so far (from possibly different requests) in an array, and also in an hash of events indexed by id.
e.g. individualStore, eventStore etc.
Controllers: They tie to a set of related API calls, e.g. eventsController would be responsible for fetching events or a particular event, or creating a new event etc. They would 'route' the response to different stores for later retrieval. They don't keep the response once it has been sent to stores.
e.g. eventsController, userSearchController etc.
Views: They are tied to a particular view. In general, my application may have several views at different places, e.g. latestEventsView on the Dashboard in addition to having a separate events page.
Templates: are what they are.
Quite often, my templates require to be bound directly to the stores (e.g. peopleView wants to list all the individuals in the individualStore in a list, sorted by some order).
And sometimes, they bind to a computed property
alivePeople: function () { ... }.property('App.individualStore.content.#each'),
The various filtering and sorting options 'chosen' in the view, should return different lists from the store. You can see my last question at what is the right emberjs way to switch between various filtering options?
Who should do this filtering, the view themselves or the stores?
Is this kind of binding across layers okay, or a code smell? Is the separation of concerns good, or am I missing something? Shouldn't controllers be doing something more here? Should my views directly bind to stores?
Any particular special case of MVC more suited to my needs?
Update 17 April 2012
My research goes on, particularly from http://vimeo.com/user7276077/videos and http://jzajpt.github.com/2012/01/17/emberjs-app-architecture.html and http://jzajpt.github.com/2012/01/24/emberjs-app-architecture-data.html
Some issues with my design that I've figured out are:
controllers making requests (stores or models or something else should do it, not controllers)
statecharts are missing -- they are important for view-controller interactions (after sometime you realize your interactions are no more simple)
This is a good example of state charts in action: https://github.com/DominikGuzei/ember-routing-statechart-example
UPDATE 9th JANUARY 2013
Yes, it's been long but this question is lately getting lots of views, and that's why I'd like to edit it so that people may get a sense.
Ember's landscape has changed a lot since this question was framed, and the new guides are much improved. EmberJS has come up with conventions (like Rails) and the MVC is much more well defined now.
Anybody still confused should read all the guides, and watch some videos:
Seattle Ember.js Meetup
At the moment, I'm upgrading my application to Ember.js 1.0.0-pre2.
You should think of your application in terms of states. Have a look at this
Initially, only a route and a template are required to describe
something and finally display it in the browser, that's what the new
API of Emberjs tries to enforce. As your requirements get more
elaborate you can throw in a view, a controller or an object. Each
though answers a specific need.
Consider a view if you need to handle any browser events or wrap
any 3rd party javascript lib you're using for animation, styling ..
Consider an Object if you need to capture domain specific
information, most likely mimics backend information.
A controller is merely a proxy for the domain object and may encapsulate logic that doesn't pertain necessarily to the object.
That's all what's to it. If you learn how to design your application in terms of states, the rest will fall into the right place, providing you're using the latest api, enforcing the rules i mentioned previously.
Since the release of Ember 1.0.0-pre4 with the new router implementation I've seen two good references describing a standardised EmberJS app structure.
Those of you familiar with Rails would find it fairly familiar.
https://github.com/trek/ember-todos-with-build-tools-tests-and-other-modern-conveniences
http://reefpoints.dockyard.com/ember/2013/01/07/building-an-ember-app-with-rails-api-part-1.html
The ember-rails project at https://github.com/emberjs/ember-rails includes a Rails generator for creating an EmberJS application directory structure that is essentially the same as the structure described in the two links above.
The EmberJS guides also now describe the new routing structure. http://emberjs.com/guides/
UPDATE 21/08/2013
If you are using Rails then the ember-rails gem is great. I've used it with a lot of success.
There are two efforts within the ember community to assist in providing a standardised ember application layout. Apparently they are going to be merged but for now check out:
https://github.com/rpflorence/ember-tools
https://github.com/stefanpenner/ember-app-kit
See also this http://addyosmani.com/largescalejavascript/ It is not about EmberJs in particular but it's a great article that gives you an idea how to write larg scale javascript apps.
1) What are the BLL-services? What's the difference between them and Service Layer services? What goes to domain services and what goes to service layer?
2) Howcome I refactor BBL model to give it a behavior: Post entity holds a collection of feedbacks which already makes it possible to add another Feedback thru feedbacks.Add(feedback). Obviosly there are no calculations in a plain blog application. Should I define a method to add a Feedback inside Post entity? Or should that behavior be mantained by a corresponing service?
3) Should I use Unit-Of-Work (and UnitOfWork-Repositories) pattern like it's described in http://www.amazon.com/Professional-ASP-NET-Design-Patterns-Millett/dp/0470292784 or it would be enough to use NHibernate ISession?
1) Business Layer and Service Layer are actually synonyms. The 'official' DDD term is an Application Layer.
The role of an Application Layer is to coordinate work between Domain Services and the Domain Model. This could mean for example that an Application function first loads an entity trough a Repository and then calls a method on the entity that will do the actual work.
2) Sometimes when your application is mostly data-driven, building a full featured Domain Model can seem like overkill. However, in my opinion, when you get used to a Domain Model it's the only way you want to go.
In the Post and Feedback case, you want an AddFeedback(Feedback) method from the beginning because it leads to less coupling (you don't have to know if the FeedBack items are stored in a List or in a Hashtable for example) and it will offer you a nice extension point. What if you ever want to add a check that no more then 10 Feedback items are allowed. If you have an AddFeedback method, you can easily add the check in one single point.
3) The UnitOfWork and Repository pattern are a fundamental part of DDD. I'm no NHibernate expert but it's always a good idea to hide infrastructure specific details behind an interface. This will reduce coupling and improves testability.
I suggest you first read the DDD book or its short version to get a basic comprehension of the building blocks of DDD. There's no such thing as a BLL-Service or a Service layer Service. In DDD you've got
the Domain layer (the heart of your software where the domain objects reside)
the Application layer (orchestrates your application)
the Infrastructure layer (for persistence, message sending...)
the Presentation layer.
There can be Services in all these layers. A Service is just there to provide behaviour to a number of other objects, it has no state. For instance, a Domain layer Service is where you'd put cohesive business behaviour that does not belong in any particular domain entity and/or is required by many other objects. The inputs and ouputs of the operations it provides would typically be domain objects.
Anyway, whenever an operation seems to fit perfectly into an entity from a domain perspective (such as adding feedback to a post, which translates into Post.AddFeedback() or Post.Feedbacks.Add()), I always go for that rather than adding a Service that would only scatter the behaviour in different places and gradually lead to an anemic domain model. There can be exceptions, like when adding feedback to a post requires making connections between many different objects, but that is obviously not the case here.
You don't need a unit-of-work pattern on top on the NHibernate session:
Why would I use the Unit of Work pattern on top of an NHibernate session?
Using Unit of Work design pattern / NHibernate Sessions in an MVVM WPF
I have an application with about 20 models and controllers and am not using any particular framework. What is the best practice for using multiple remote objects in Flex performance-wise?
1) Method 1 - One per Component - Each component instantiates a RemoteObject for itself
2) Method 2 - Multiple in Application Root - Each controller is handled by a RemoteObject in the root
3) Method 3 - One in Application Root - Combine all controllers into one class and handle them with one RemoteObject
I'm guessing 3 will have the best performance but will be too messy to maintain and 1 would be the cleanest but would take a performance hit. What do you think?
Best practice would be "none of the above." Your Views should dispatch events that a controller or Command component would use to call your service(s) and then update your model on return of the data. Your Views would be bound to the data, and then the Views would automatically be updated with the new data.
My preference is to have one service Class per different piece or type of data I am retrieving--this makes it easier to build mock services that can be swapped for real services as needed depending on what you're doing (for instance if you have a complicated server setup, a developer who is working on skinning would use the mocks). But really, how you do that is a matter of personal preference.
So, where do your services live, so that a controller or command can reach them? If you use a Dependency Injection framework such as Robotlegs or Swiz, it will have a separate object that handles instantiating, storing, and and returning instances of model and service objects (in the case of Robotlegs, it also will create your Command objects for you and can create view management objects called Mediators). If you don't use one of these frameworks, you'll need to "roll your own," which can be a bit difficult if you're not architecturally minded.
One thing people who don't know how to roll their own (such as the people who wrote the older versions of Cairngorm) tend to fall back on is Singletons. These are not considered good practice in this day and age, especially if you are at all interested in unit testing your work. http://misko.hevery.com/code-reviewers-guide/flaw-brittle-global-state-singletons/
A lot depends on how much data you have, how many times it gets refreshed from the server, and of you have to support update as well as query.
Number 3 (and 2) are basically a singletons - which tends to work best for large applications and large datasets. Yes, it would be complex to maintain yourself, but that's why people tend to use frameworks (puremvc, cairgorm, etc). much of the complexity is handled for you. Caching data within the frameworks also enhances performance and response time.
The problem with 1 is if you have to coordinate data updates per component, you basically need to write a stateless UI, always retrieving the data from the server on each component visibility.
edit: I'm using cairgorm - have ~ 30 domain models (200 or so remote calls) and also use view models. some of my models (remote object) have 10's of thousands of object instances (records), I keep a cache with/write back. All of the complexity is encapsulated in the controller/commands. Performance is acceptable.
In terms of pure performance, all three of those should perform roughly the same. You'll of course use slightly more memory by having more instances of RemoteObject and there are a couple of extra bytes that get sent along with the first request that you've made with a given RemoteObject instance to your server (part of the AMF protocol). However, the effect of these things is negligible. As such, Amy is right that you should make a choice based on ease of maintainability and not performance.
If I expose IQueryable from my service layer, wouldn't the database calls be less if I need to grab information from multiple services?
For example, I'd like to display 2 separate lists on a page, Posts and Users. I have 2 separate services that provides a list of these. If both provides IQueryable, will they be joint in 1 database call? Each repository creates a context for itself.
It's best to think of an IQueryable<T> as a single query waiting to be run. So if you return 2 IQueryable<T> instances and run them in the controller, it wouldn't be any different than running them separably in their own service methods. Each time you execute the IQuerable<T> to get results, it will run the query by itself independent of other IQuerable<T> objects.
The only time (as far as I know) it will make an impact if there is enough time between the two service calls that the database connection might close, but you would need a considerable amount of time in between the service calls for that to be the case.
Returning IQuerable<T> to the controller still has some usefulness, such as easier handling of paging and sorting (so sorting is done on the controller and is not done on the service layer which doesn't necessarily care about how data is sorted or paged). This isn't a performance concern though, and people will disagree about if it's best to do this in the controller or not (I've seen reputable developers do this and give well thought out reasons why).
No. The best an IQueryable can do is reduce the number of calls within a singular database context. An IQueryable will not cross contexts.
Personally, I don't use IQueryables past the repositories for a number of reasons:
1) I don't use the same domain objects as database objects, and seeing "no translation to SQL" pisses me off ;)
2) I don't like the necessary structure for IQueryables in views: foreach (var item in collection){var tempItem = item; code on tempItem}
3) I've come up with a method of passing generic filters to the data layer (LinqKit and PredicateBuilder are gods)
If these reasons don't apply to you, of course you should feel free to use IQueryables to whichever layer you desire.
Not with two different contexts.
Definitely NO. It's a leaky abstraction.
It allows abominations like this:
q.Where(x=>{Console.WriteLine("fail");return true;});
Thing is - when exposing IQueryable, You are saying that Your data layer fully supports linq to objects.
If you make two method calls you will make two queries.
You can combine the methods into a single method which gets all the data at once.
If you are implementing the repository pattern you will have an easier time if you instantiate one database context per request.
Your service layer is exactly that, a layer which serves up what you need. Often times my service layers are named things like SearchService which has methods for returning every packaged collection I will ever need (the actual view models themselves). And if I ever need a new search, my service layer gets a new method. The backing for your service layer can then contain any data backing or persistence model you would like, be it a repository or Entity Framework provider, etc.
To answer your question though, the line needs to be drawn at the service layer, all queries need to be contained within it and only data returned.