I need the exact purpose of Service layer in MVC.
Can we implement without service layer..
What are the major responsibilities of these layers..
Can we implement without anyone of these layers..
Can you please someone help me to know this..
The service layer is there to provide logic to operate on the data sent to and from the DAO and the client. Very often these 2 pieces will be bundled together into the same module, and occasionally into the same code, but you'll still see them as distinct logical entities.
This may helpful to you
https://softwareengineering.stackexchange.com/questions/220909/service-layer-vs-dao-why-both
DAO is as light and exists solely to provide a connection to the DB, sometimes abstracted so different DB backends can be used.
The service layer is there to provide logic to operate on the data sent to and from the DAO and the client. Very often these 2 pieces will be bundled together into the same module, and occasionally into the same code, but you'll still see them as distinct logical entities.
Yes you can implement without service layer but will lack the security.. and without using service layer the request and response works faster...
Another reason for Service layer is security - If you provide a service layer that has no relation to the DB, then is it more difficult to gain access to the DB from the client except through the service. If the DB cannot be accessed directly from the client (and there is no trivial DAO module acting as the service) then all an attacker who has taken over the client can do is attempt to hack the service layer as well before he gets all but the most sanitised access to your data.
As these answer is already answered on StackExchange.. by gbjbaanb
I'm reading the Spring Web Flow chapter in the book Pro Spring MVC. Unfortunately there's no explicit information, where the state during a flow execution is persisted. I assume it is saved in the JVM Heap and associated with the session.
Now HTTP is a stateless protocol (REST...) and I'd like to use Spring Web Flow without saving state on the server (besides the one and only state that a session might be authenticated).
One strategy is to send all parameters of the entire flow with every HTTP request of the flow (hidden input) and thus accumulating all necessary parameters until the flow has finished.
The overhead of re-validating parameters can be avoided with signatures over already validated parameters.
Do you know, whether it might be possible to use Spring Web Flow in this way? Has anybody already done this?
Update: Why?
Persisting state is not only a violation of the principles of HTTP as a stateless protocol but has practical problems too:
If the user browses with multiple browser tabs then this can lead to inconsistent state, race conditions or data loss.
Storing state on the server makes load balancing over several servers more complicate.
Testing and debugging becomes more complicate, because requests can not be tested or analyzed in isolation but only in the context of previous requests.
Cookies must be enabled to correlate servers sessions to requests.
The server needs to synchronize access to the server-side state.
The url of a request that also depends on server state does not contain all information necessary to bookmark the state inside a flow or to make sense of it in a bug report.
I've not yet looked at the details of Web Flow but I'm confident that one could have the same programming experience and still keep all information in the request parameters.
Update: I've now learned that my requested style of flow handling has a name: Continuations. The term continuation is more common in functional programming but it's apparently not uncommon to adapt the idea to HTTP interactions.
You might be interested in checking my bean flow FSM project (restflow):
https://github.com/alfonso-presa/restflow
Although it doesn't use Spring WebFlow, I think it may help answering the spirit of the question, as it allows the implementation of a stateless server orchestrated flow. I started this project because I wanted to make almost the same as you with spring WebFlow but I found it was not possible as state is stored in session (and also REST/json serialization is not built in).
It's main purpose is making an state-machine (just like WebFlow does) but with it's state stored in a bean, which you can persist in a distributed store, or easily sign or encrypt and send back to the client as continuation for next requests.
I hope you find it useful.
edit: I created a showcase project here: https://github.com/alfonso-presa/restflow-spring-web-sample
After reading a lot about the differences between REST and SOAP, I got the impression that REST is just another word for HTTP. Can someone explain what functionality REST adds to HTTP?
Note: I'm not looking for a comparison of REST versus SOAP.
No, REST is the way HTTP should be used.
Today we only use a tiny bit of the HTTP protocol's methods – namely GET and POST. The REST way to do it is to use all of the protocol's methods.
For example, REST dictates the usage of DELETE to erase a document (be it a file, state, etc.) behind a URI, whereas, with HTTP, you would misuse a GET or POST query like ...product/?delete_id=22.
HTTP is a protocol used for communication, usually used to communicate with internet resources or any application with a web browser client.
REST means that the main concept you are using while designing the application is the Resource: for each action you want to perform you need to define a resource on which you often do only CRUD operation, which is a simple task. For that it's very convenient to use four verbs used in HTTP protocol against the four CRUD operations (GET for Read, POST is for CREATE, PUT is for UPDATE and DELETE is for DELETE).
That's unlike the older concept of RPC (Remote Procedure Call), in which you have a set of actions you want to perform as a result of the user's call. if you think for example on how to describe a Facebook like on a post, with RPC you might create services called AddLikeToPost and RemoveLikeFromPost, and manage it along with all your other services related to FB posts, thus you won't need to create special object for Like.
With REST you will have a Like object which will be managed separately with Delete and Create functions. It also means it will describe a separate entity in your DB. That might look like a small difference, but working like that would usually yield a much simpler code and a much simpler application. With that design, most of the app's logic is obvious from the object's structure (model), unlike RPC with which you would usually have to explicitly add a lot more logic.
Designing a RESTful application is often a lot harder because it requires you to describe complicated things in a simple manner. Describing all functionalities using only CRUD functions is tricky, but after doing that your life would be a lot simpler, and you will find that you write a lot shorter methods.
One more restraint REST architecture presents is not to use a session context when communicating with a client (stateless), meaning all the information needed to understand who is the client and what he wants is passed with the web message. Each call to a function is self-descriptive, there is no previous conversation with the client which can be referenced in the message. Therefore, a client could not tell you "give me the next page" since you don't have a session to store what is the previous page and what kind of page you want, the client would have to say "my name is Yuval, get me page 2 of a specific post in a specific forum". This means a bit more data would have to transfer in the communication, but think of the difference between finding a bug reported from the "get me the next page" function in oppose to "get me page 2 of question ID 2190836 in stack overflow".
Of course there is a lot more to it, but to my humble opinion these are the main concepts in a teaspoon.
REST enforces the use of the available HTTP commands as they were meant to be used.
For example, I could do:
GET
http://example.com?method=delete&item=xxx
But with rest I would use the "DELETE" request method, removing the need for the "method" query param
DELETE
http://example.com?item=xxx
HTTP is an application protocol. REST is a set of rules, that when followed, enable you to build a distributed application that has a specific set of desirable constraints.
If you are looking for the most significant constraints of REST that distinguish a RESTful application from just any HTTP application, I would say the "self-description" constraint and the hypermedia constraint (aka Hypermedia as the Engine of Application State (HATEOAS)) are the most important.
The self-description constraint requires a RESTful request to be completely self descriptive in the users intent. This allows intermediaries (proxies and caches) to act on the message safely.
The HATEOAS constraint is about turning your application into a web of links where the client's current state is based on its place in that web. It is a tricky concept and requires more time to explain than I have right now.
HTTP is a contract, a communication protocol and REST is a concept, an architectural style which may use HTTP, FTP or other communication protocols but is widely used with HTTP.
REST implies a series of constraints about how Server and Client should interact. HTTP is a communication protocol with a given mechanism for server-client data transfer, it's most commonly used in REST API just because REST was inspired by WWW (world wide web) which largely used HTTP before REST was defined, so it's easier to implement REST API style with HTTP.
There are three major constraints in REST (but there are more):
Interaction between server and client should be described via hypertext only.
Server and client should be loosely coupled and make no assumptions about each other. Client should only know resource entry point. Interaction data should be provided by the server in the response.
Server shouldn't store any information about request context. Requests must be independent and idempotent (means if same request is repeated infinitely, exactly same result is retrieved)
And HTTP is just a communication protocol (a tool) that can help to achieve this.
For more info check these links:
https://martinfowler.com/articles/richardsonMaturityModel.html
http://roy.gbiv.com/untangled/2008/rest-apis-must-be-hypertext-driven
REST = Representational State Transfer
REST is a set of rules, that when followed, enable you to build a distributed application that has a specific set of desirable constraints.
REST is a protocol to exchange any(XML, JSON etc ) messages that can use HTTP to transport those messages.
Features:
It is stateless which means that ideally no connection should be maintained between the client and server.
It is the responsibility of the client to pass its context to the server and then the server can store this context to process the client's further request. For example, session maintained by server is identified by session identifier passed by the client.
Advantages of Statelessness:
Web Services can treat each method calls separately.
Web Services need not maintain the client's previous interaction.
This in turn simplifies application design.
HTTP is itself a stateless protocol unlike TCP and thus RESTful Web Services work seamlessly with the HTTP protocols.
Disadvantages of Statelessness:
One extra layer in the form of heading needs to be added to every request to preserve the client's state.
For security we need to add a header info to every request.
HTTP Methods supported by REST:
GET: /string/someotherstring
It is idempotent and should ideally return the same results every time a call is made
PUT:
Same like GET. Idempotent and is used to update resources.
POST: should contain a url and body
Used for creating resources. Multiple calls should ideally return different results and should create multiple products.
DELETE:
Used to delete resources on the server.
HEAD:
The HEAD method is identical to GET except that the server MUST NOT return a message-body in the response. The meta information contained in the HTTP headers in response to a HEAD request SHOULD be identical to the information sent in response to a GET request.
OPTIONS:
This method allows the client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.
HTTP Responses
Go here for all the responses.
Here are a few important ones:
200 - OK
3XX - Additional information needed from the client and url redirection
400 - Bad request
401 - Unauthorized to access
403 - Forbidden
The request was valid, but the server is refusing action. The user might not have the necessary permissions for a resource, or may need an account of some sort.
404 - Not Found
The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible.
405 - Method Not Allowed
A request method is not supported for the requested resource; for example, a GET request on a form that requires data to be presented via POST, or a PUT request on a read-only resource.
404 - Request not found
500 - Internal Server Failure
502 - Bad Gateway Error
Not quite...
http://en.wikipedia.org/wiki/Representational_State_Transfer
REST was initially described in the
context of HTTP, but is not limited to
that protocol. RESTful architectures
can be based on other Application
Layer protocols if they already
provide a rich and uniform vocabulary
for applications based on the transfer
of meaningful representational state.
RESTful applications maximise the use
of the pre-existing, well-defined
interface and other built-in
capabilities provided by the chosen
network protocol, and minimise the
addition of new application-specific
features on top of it.
http://www.looselycoupled.com/glossary/SOAP
(Simple Object Access Protocol) The
standard for web services messages.
Based on XML, SOAP defines an envelope
format and various rules for
describing its contents. Seen (with
WSDL and UDDI) as one of the three
foundation standards of web services,
it is the preferred protocol for
exchanging web services, but by no
means the only one; proponents of REST
say that it adds unnecessary
complexity.
REST is a specific way of approaching the design of big systems (like the web).
It's a set of 'rules' (or 'constraints').
HTTP is a protocol that tries to obey those rules.
From You don't know the difference between HTTP and REST
So REST architecture and HTTP 1.1 protocol are independent from each
other, but the HTTP 1.1 protocol was built to be the ideal protocol to
follow the principles and constraints of REST. One way to look at the
relationship between HTTP and REST is, that REST is the design, and
HTTP 1.1 is an implementation of that design.
HTTP is a communications protocol that transports messages over a network.
SOAP is a protocol to exchange XML-based messages that can use HTTP to transport those messages.
Rest is a protocol to exchange any(XML or JSON) messages that can use HTTP to transport those messages.
REST is not necessarily tied to HTTP. RESTful web services are just web services that follow a RESTful architecture.
What is Rest -
1- Client-server
2- Stateless
3- Cacheable
4- Layered system
5- Code on demand
6- Uniform interface
REST APIs must be hypertext-driven
From Roy Fielding's blog here's a set of ways to check if you're building a HTTP API or a REST API:
API designers, please note the following rules before calling your creation a REST API:
A REST API should not be dependent on any single communication protocol, though its successful mapping to a given protocol may be dependent on the availability of metadata, choice of methods, etc. In general, any protocol element that uses a URI for identification must allow any URI scheme to be used for the sake of that identification. [Failure here implies that identification is not separated from interaction.]
A REST API should not contain any changes to the communication protocols aside from filling-out or fixing the details of underspecified bits of standard protocols, such as HTTP’s PATCH method or Link header field. Workarounds for broken implementations (such as those browsers stupid enough to believe that HTML defines HTTP’s method set) should be defined separately, or at least in appendices, with an expectation that the workaround will eventually be obsolete. [Failure here implies that the resource interfaces are object-specific, not generic.]
A REST API should spend almost all of its descriptive effort in defining the media type(s) used for representing resources and driving application state, or in defining extended relation names and/or hypertext-enabled mark-up for existing standard media types. Any effort spent describing what methods to use on what URIs of interest should be entirely defined within the scope of the processing rules for a media type (and, in most cases, already defined by existing media types). [Failure here implies that out-of-band information is driving interaction instead of hypertext.]
A REST API must not define fixed resource names or hierarchies (an obvious coupling of client and server). Servers must have the freedom to control their own namespace. Instead, allow servers to instruct clients on how to construct appropriate URIs, such as is done in HTML forms and URI templates, by defining those instructions within media types and link relations. [Failure here implies that clients are assuming a resource structure due to out-of band information, such as a domain-specific standard, which is the data-oriented equivalent to RPC’s functional coupling].
A REST API should never have “typed” resources that are significant to the client. Specification authors may use resource types for describing server implementation behind the interface, but those types must be irrelevant and invisible to the client. The only types that are significant to a client are the current representation’s media type and standardized relation names. [ditto]
A REST API should be entered with no prior knowledge beyond the initial URI (bookmark) and set of standardized media types that are appropriate for the intended audience (i.e., expected to be understood by any client that might use the API). From that point on, all application state transitions must be driven by client selection of server-provided choices that are present in the received representations or implied by the user’s manipulation of those representations. The transitions may be determined (or limited by) the client’s knowledge of media types and resource communication mechanisms, both of which may be improved on-the-fly (e.g., code-on-demand). [Failure here implies that out-of-band information is driving interaction instead of hypertext.]
REST is a light version of SOAP, with no sugars or colors agents added.
The goal of both SOAP and REST is to allow the communication between Information Systems that may be written in different languages and use different communiation protocols.
While SOAP uses API contracts to expose it's services and define the way a client can call a service, what Parameters should be sent and what results are to be expected, REST on the other hand has uses no API contracts, for a client to know what services exist and how to call them it should look into the Rest API documentation (this can be defined in a yml file with a OpenAPI or Swagger).
Second SOAP is verbose, it reyes on XML to send a request and describe the services, parameteres and the results returned. On the other hand REST relyed on simple JSON Objects to send requests and receive results. JSON is simple to undersand, lightweight and does not use too much bandwith when sending requests or receiving results.
Does Astoria Service Model only support
ATOM,JSON,XML,XML+HTTP
Are formats like SOAP,WSDL,ASMX outdated? .So when i wish to develop SOA can i ignore SOAP,ASMX,WSDL formats?
I would add to the above answer and say there is in-fact a way to discover the metadata about the Data Services (REST) endpoint. Every endpoint includes a service document (just do a GET on the root of the endpoint) that describes the sets exposed by the service. Further, going to the $metadata endpoint from the root of the service (i.e. http://mydomain/myservice.svc/$metadata) returns an XML metadata document that fully describes the service (the sets, types, properties on types, relationships between sets, and service operations).
No, most definitely not!
ASMX = ASP.NET webservices - this is outdated, it was introduced in .NET 1.0 and basically replaced with WCF in .NET 3.0.
BUT: WCF is definitely NOT outdated! WCF is the Microsoft standard way of communicating between two systems. It uses SOAP (including WSDL and XSD) by default, and this is mature and reliable technology which works well in enterprise scenarios where you need things like data integrity, (human and machine readable) service description through WSDL and service metadata, and so forth. SOAP also offers more advanced features like reliable messaging and transactional support.
REST / ADO.NET Data Services is a more lightweight, easier-to-get-at approach at exposing services, but it's lacking in many ways: there's no unified service description available, so you cannot really "discover" what methods and what datatypes the service offer; either you have knowledge yourself, or the service provider gives you a documentation in plain English, but there's no standard way of describing a REST service to the outside world (yet). Also, you don't really know ahead of time what kind of data that service might return - there's no XML schema to stick to - it's more of a "let's hit the service and see what comes back" approach which might work quite OK in some cases, but not really in larger scale, enterprise-style environments.
So to sum up: the SOAP (WSDL,XSD) vs. REST debate is ongoing, both have their reasons to be, and I don't see one of them replacing the other - they're supplanting one another.
I was wondering if any of you had successfully implemented DDD in a Client/Server app and would like to share some experiences.
We are currently working on a smart client in Flex and a backend in Java. On the server we have a service layer exposed to the client that offers CRUD operations amongst some other service methods. I understand that in DDD these services should be repositories and services should be used to handle use cases that do not fit inside a repository. Right now, we mimic these services on the client behind an interface and inject implementations (Webservices, RMI, etc) via an IoC container.
So some questions arise:
should the server expose repositories to the client or do we need to have some sort of a facade (that is able to handle security for instance)
should the client implement repositories (and DDD in general?) knowing that in the client, most of the logic is view related and real business logic lives on the server. All communication with the server happens asynchronously and we have a single threaded programming model on the client.
how about mapping client to server objects and vice versa? We tried DTO's but reverted back to exposing the state of our objects and mapping directly to them. I know this is considered bad practice, but it saves us an incredible amount of time)
In general I think a new generation of applications is coming with the growth of Flex, Silverlight, JavaFX and I'm curious how DDD fits into this.
I would not expose repositories directly to the client. The first big problem as you mention is security: you can't trust the client, so you cannot expose your data access API to potentially hostile clients.
Wrap your repositories with services on the server and create a thin delegate layer in the client that handles the remote communication.
Exposing your Entities is not necessarily a bad practice it's just that it becomes problematic when you start to factor in things like lazy loading, sending data over the wire the client doesn't need, etc. If you write a DTO class which wraps one or more entities and delegates get/set calls you can actually build up a DTO layer pretty quickly, especially using code generation available in most IDEs.
The key to all of this is that a set of patterns should really only apply to a part of your application, not to the whole thing. The fact that you have rich logic in your domain model and use repositories for data access as part of DDD should not influence the client in any way. Conceptually the RIAs that I build have three layers:
Client uses something like MVC, MVP or MVVM to present the UI. The Model layer eventually calls into...
What I might call the "Integration Layer." This is a contract of services and data objects that exist on both the client and server to allow the two to coordinate. Usually the UI design drives this layer so that (A) only the data that the client needs is passed to it and (B) data access can be coarse-grained, i.e. "make one method call for all the state needed for this set of UI.
Server using whatever it wants to handle business logic and data access. This might be DDD or something a little more old school, like a data layer built using stored procs in the DB and a lot of "ResultSet" or "DataTable" objects.
The point really is that the client and server are both very different animals and they need to vary independently. In order to do so, you need a layer inbetween that is a fair compromise between the needs of the UI and the reality of how things might need to be on the server.
The one big advantage that Silverlight/WPF and JavaFX have over Flex + anything is that you can use a lot of logic in the first two because you have the same VM on both sides of the app. Flex is the best UI technology hands down but it lacks a server component where code could be shared and re-used more effectively.