As design-pattern if i am correct we use strings ( normally called business logic status ) to convey the message between layers. But my problem is if a method return string then how to transfer other data objects . For this situation i am currently using http request object (which is global) to transfer data, which makes all the 3 layers dependent on presentation layer so it is not good.
Is there any other way to transfer data between layers?
Passing messages around using strings is considered a bad idea (google "Stringly typed") and definitely not a design pattern. You should create proper objects and pass them between the layers.
Related
Why should I use DTOs/Domain objects when I can just put all business classes in a class library, use them in the business logic, then also pass those same business objects on to boundary classes?
UPDATE:
All are great points, thanks for the help. A follow up question:
Where do you typically place these DTOs? Alongside the Domain objects i.e. in the same namespace?
namespace MedSched.Medical
{
public class MedicalGroup
{
//...
}
public class MedicalGroupDTO
{
//...
}
}
The DTO's provide an abstraction layer for your domain model. You can therefore change the model and not break the contract you have for your service clients. This is akin to a common practice in database design where views and procedures become the abstraction layer for the underlying data model.
Serialization - you may over expose data and send bloated messages over the wire. This may be mitigated by using serialization attributes, but you may still have extra information.
Implicit vs. Explicit Contracts - by exposing domain objects you leave it up to the client to interpret their usage since they do not have the full model at their disposal. You will often make updates to domain objects implicitly based on the presence or removal of associations, or blindly accept all changes. DTOs explicitly express the usage of the service and the desired operation.
Disconnected Scenarios - having explicit messages or DTOs would make it easier for you to implement messaging and messaging patterns such as Message Broker, etc.
DDD - pure DDD demands that domain objects are externally immutable, and therefore you must offload this to another object, typically a DTO.
I can think of 2 basic scenarios for using DTOs:
You're creating your business object from data that's incomplete and will fail validation. For example, you're parsing a CSV file or an Excel file from where your business object is created. If you use data directly from these objects to create your business object, it is quite possible to fail several validation rules within the object, because data from such files are prone to errors. They also tend to have a different structure that you have in your final business object: having a placeholder for that incomplete data will be useful.
You're transporting your business object through a medium that is bandwidth intensive. If you are using a web service, you will need to use DTOs to simplify your object before transport; otherwise the CLR will have a hard time trying to serialize all your data.
DTOs are Data Transfer Objects, with Transfer being the key word. They are great when you want to pass your objects across the wire and potentially communicate with a different language because they are "light weight" (ie no business logic.)
If your application isn't web service based, DTOs aren't really going to buy you anything.
So deciding to use or not use DTOs should be based on the architecture of your application.
There are times when the data you want to pass around doesn't map exactly to the structure the business objects, in which case you use DTOs.
Ex: when you want to pass a subset of the data or a projection.
You know that for an API project there are many patterns to retrieve and serve data to client. Controller <-> Service <-> Dao pattern is one of the these patterns.
Is it good to return any type of object from service layer or service layer return objects must be related entity?
For example, we have User, UserService, UserDao.
Should UserService return types be User or any type of object is suitable?
Having logical operations in service layer force me to use other objects as return type.
What is the best practice for that case?
I've found that I need 3 types of objects to keep things manageable (I'm still in a battle trying to come up with a decent naming strategy). It's sometimes beneficial to have:
Objects returned from your data layer - These objects will be dictated by your data implementation. For example, they could change if you decide to store your data differently. Perhaps you decide to serialise an object and store it as JSON rather than storing it in a separate table, or you move from SQL to NOSQL.
Objects returned from your domain layer - These are application objects and should model your domain. They shouldn't contain anything that is specific to your data implementation.
Objects sent to and from you API - It's useful to have separate serialisation objects to shape your API. It means you can tweak your API without having to touch the business logic. On some projects, I have multiple versions of the API objects for v1, v2 etc.
The widely accepted practise involve the service to have all business logic in it and return the Data Transfer Object(DTO)/ Business Object like User in your case. The Service may call DAO, any other datasources to fetch the entities, and a mapper/converter util can be used to convert the entities to DTO objects.
The DTO objects returned from Service can be embedded in a ResponseEntity(In the case of Spring MVC) and returned from the controller.
This makes your application into 3 separated layers like web, service, dataaccess. This supports design principles like Separation of Concerns, Single Responsibility. This also makes your unit testing and code management easier.
I am implementing a SOAP-Webservice which receives different Requests. Should my Manager-class transform this Request objects into an internal representation before delegating them to implementation classes?
I think this would be a good idea concerning decoupling. But doing this I have to create a copy of each RequestObject class and name it InternalRequestObject which stores the same data as the original Request.
Does this make sense?
It makes sense if you intend to reuse those implementation classes which I would call your business layer.
In your current setup, you have the business layer exposed as a web service. The skeleton of the web service is - if you will - a client for your business layer.
Now the question that arises is: should your business layer care what kind of clients it will have? Should the data contract of the business layer be dictated by the clients, or should the clients respect the data contract exposed by the business layer?
The obvious response is the clients should respect the data contract of the business layer, so the answer to your question would be: Yes, you should map the SOAP requests to an internal request type used by the implementation classes in order to obtain a better decoupling between the two.
There is only one case in which I would consider using the request types directly all the way down into my business layer: If I would be absolutely (101%) sure that I will never have to expose my business layer as anything else than a SOAP web service.
The idea is that you only have two main options at this:
1. keep the same request type all over the place. The disadvantage to this is that you will suffer a lot of rewrites ripping through your business layer if at some point you have to add other (non SOAP) clients.
OR
2. map the SOAP request type to an internal type. The disadvantage to this will be that you risk duplicating code and working extra just to find out at the end that all was for nothing and you didn't need to add other clients after all.
Think about your situation and choose carefully! But I must tell you that, personally, I haven't encountered the disadvantage of number 2 so far. I always ended up adding new clients to the thing and at that point it helped having mapped the types from the very beginning.
I am exposing a couple domain objects via a SOAP based web service. Some of my domain objects have a large number of fields. I do not want to include values in my web service request/response unless they are needed.
For example, if I have a Book domain object with fields title, genre, and isbn, if I wanted to use my web service to update the name of a book, I want my request to only include the title field (omitting the other two fields that aren't being updated).
Likewise, I want my web service clients to be able to specify which fields they want to be returned when they load books.
This would allow clients to load the title field thereby reducing the size of the data going across the wire because the fields that aren't needed would not be included in the response.
Does anyone know of any patterns or best practices to deal with this type of requirement?
You touched multiple problems where each deserves separate explanation:
Reducing traffic - reducing traffic usually means reducing roundtrips not reducing payload. Reducing traffic is achieved by implement better operations which do multiple actions instead of exposing CRUD operations.
Reducing payload - if you don't want to transfer whole entity you should use Data transfer objects. Special object transferring only data required for given operation.
Dynamic response - web services are not supposed to do that. Web service has fixed interface defined by WSDL where each message payload is specified by XSD. If you want dynamically change returned data structure you will break this. It doesn't mean it is not possible - you can define that your service operation returns xsd:any = any XML and it will be your duty to prepare returned XML and duty of your client to parse XML.
You can either make the fields optional in the XSD data type, or you can specify that in the changeTitle request you don't expect a Book, but only an ID and a string.
When you invent the changeAttributes request and you have optional fields, you have to decide what a missing field means. It could be clear this field or leave this field untouched.
I've recently overheard people saying that data transfer objects (DTOs) are an anti-pattern.
Why? What are the alternatives?
Some projects have all data twice. Once as domain objects, and once as data transfer objects.
This duplication has a huge cost, so the architecture needs to get a huge benefit from this separation to be worth it.
DTOs are not an anti-pattern. When you're sending some data across the wire (say, to an web page in an Ajax call), you want to be sure that you conserve bandwidth by only sending data that the destination will use. Also, often it is convenient for the presentation layer to have the data in a slightly different format than a native business object.
I know this is a Java-oriented question, but in .NET languages anonymous types, serialization, and LINQ allow DTOs to be constructed on-the-fly, which reduces the setup and overhead of using them.
"DTO an AntiPattern in EJB 3.0" (original link currently offline) says:
The heavy weight nature of Entity
Beans in EJB specifications prior to
EJB 3.0, resulted in the usage of
design patterns like Data Transfer
Objects (DTO). DTOs became the
lightweight objects (which should have
been the entity beans themselves in
the first place), used for sending the
data across the tiers... now EJB 3.0
spec makes the Entity bean model same
as Plain old Java object (POJO). With
this new POJO model, you will no
longer need to create a DTO for each
entity or for a set of entities... If
you want to send the EJB 3.0 entities
across the tier make them just
implement java.io.Serialiazable
OO purists would say that DTO is anti-pattern because objects become data table representations instead of real domain objects.
I don't think DTOs are an anti-pattern per se, but there are antipatterns associated with the use of DTOs. Bill Dudney refers to DTO explosion as an example:
http://www.softwaresummit.com/2003/speakers/DudneyJ2EEAntiPatterns.pdf
There are also a number of abuses of DTOs mentioned here:
http://anirudhvyas.com/root/2008/04/19/abuses-of-dto-pattern-in-java-world/
They originated because of three tier systems (typically using EJB as technology) as a means to pass data between tiers. Most modern day Java systems based on frameworks such as Spring take a alternative simplified view using POJOs as domain objects (often annotated with JPA etc...) in a single tier... The use of DTOs here is unnecessary.
Some consider DTOs an anti-pattern due to their possible abuses. They're often used when they shouldn't be/don't need to be.
This article vaguely describes some abuses.
The question should not be "why", but "when".
Definitively it's anti-pattern when only result of using it is higher cost - run-time or maintenance. I worked on projects having hundreds of DTOs identical to database entity classes. Each time you wanted to add a single field you ad to add id like four times - to DTO, to entity, to conversion from DTO to domain classes or entities, the inverse conversion, ... You forgot some of the places and data got inconsistent.
It's not anti-pattern when you really need different representation of domain classes - flatter, richer, ...
Personally I start with a domain class and pass it around, with proper checks at the right places. I can annotate and/or add some "helper" classes to make mappings to database, to serialization formats like JSON or XML ... I can always split a class to two if I feel the need.
It's about your point of view - I prefer to look at a domain object as a single object playing various roles, instead of multiple objects created from each other. If the only role an object is to transport data, then it's DTO.
If you're building a distributed system, then DTOs are certainly not an anti pattern. Not everyone will develop in that sense, but if you have a (for example) Open Social app all running off JavaScript.
It will post a load of data to your API. This is then deserialized into some form of object, typically a DTO/Request object. This can then be validated to ensure the data entered is correct before being converted into a model object.
In my opinion, it's seen as an anti-pattern because it's mis-used. If you're not build a distributed system, chances are you don't need them.
DTO becomes a necessity and not an ANTI-PATTERN when you have all your domain objects load associated objects EAGERly.
If you don't make DTOs, you will have unnecessary transferred objects from your business layer to your client/web layer.
To limit overhead for this case, rather transfer DTOs.
I think the people mean it could be an anti-pattern if you implement all remote objects as DTOs. A DTO is merely just a set of attributes and if you have big objects you would always transfer all the attributes even if you do not need or use them. In the latter case prefer using a Proxy pattern.
The intention of a Data Transfer Object is to store data from different sources and then transfer it into a database (or Remote Facade) at once.
However, the DTO pattern violates the Single Responsibility Principle, since the DTO not only stores data, but also transfers it from or to the database/facade.
The need to separate data objects from business objects is not an antipattern, since it is probably required to separate the database layer anyway.
Instead of DTOs you should use the Aggregate and Repository Patterns, which separates the collection of objects (Aggregate) and the data transfer (Repository).
To transfer a group of objects you can use the Unit Of Work pattern, that holds a set of Repositories and a transaction context; in order to transfer each object in the aggregate separately within the transaction.