I'm developing a web service in Java EE using Apache Tomcat and so far I have written some basic server side methods and a test client. I can successfully invoke methods and get results but every time I invoke a method, the server constructor gets called again, and I also can't modify the instance variables of the server using the set methods. Is there a particular way to make my server stateful without using JAX-WS or EJB #Stateful tags?
This is a little bit of misconception here. The stateful EJB would maintain session between one client and server, so still the EJB state wouldn't be shared between various clients.
You can expose only stateless and singleton EJBs as a JAX-WS web service.
The best option is to use database for storing all bids and when the auction is finished choose the winning one.
If you want to use a file it is fine, as long as you like to play with issues like:
synchronizing access to that file from many clients
handling transactional reads and writes
resolve file corruption problems
a bunch of other problems that might happen if you are sufficiently unlucky
Sounds like a lot of work, which can be done by any sane database engine.
Related
Following is a point mentioned in a presentation slide related to SOA, and it confuses me with the concepts of service orchestration and service choreography. To enable service choreography, shouldn't a web service be able to call another web service?
SOA builds applications out of software services. Services comprise intrinsically
unassociated, loosely coupled units of functionality that have no calls to
each other embedded in them.
In theory, a service can do anything it needs to do to accomplish its job. So there doesn't seem to be a good reason to forbid using a second service to do your work. Why reinvent the wheel?
In practice, the issue is more complicated. If you start calling other services on your own web server, then you'll eventually starve it of resources. At best, "real" clients will have to wait a bit longer for their answers while your web service server plays with itself.
Another issue is recursive loops: Service A calls B calls C calls A calls B ... you get the idea. A small change in one service can introduce such a loop without anyone noticing and it can sit there for a long time until it suddenly kills your server.
That is why you should build micro services in a hierarchy inside the server (i.e. below the web service layer - this is not exposed to clients). Those micro services can use each other in a top-down manner (to avoid the loops). Unit tests then make sure they behave properly.
Lastly, such reuse is very slow. Each HTTP request takes a lot of resources to create, send, parse and process. Calling an internal method directly can be 10 - 10000 times faster.
These are the main reasons why the services exposed by a single server shouldn't reuse each other via the "public client API".
Note: There are web services which build new services by using existing ones. IFTTT - "If This Then That" is one such beast.
You could adopt every concept according to your needs. In my current project we have a separate module that is responsible for the Orchestration. This is required since in real life usage, scenarios can be very complicated. So in order to be close to the actual management of your system, you need to have such one.
Another advantage of this approach is that the Separation_of_concerns is kept. Also aligns the business request with the applications, data, and infrastructure that you have. It defines policies and service levels through automated workflows, provisioning etc.
Orchestration is critical in the delivery of Cloud services too. As they are networked to allow sharing of data-processing tasks, centralized data storage, and online access to services or resources.
My application it's C++ service. And I need to add API for it. I consider that it will be XML/JSON RPC based API. How should I design a program for reusing existing code base and provide API.
I see following options:
My application will work via RPC layer. Seems that it's bad option due to low performance;
Before starting of service I will fork it and run my application in the first process and RPC server in the second; Seems ok, but how to restart RPC server in this case?
I guess there is a well known pattern for such issues.
Thanks.
If you can use a web server, then the FastCGI concept might be what you're looking for. One of the main duties of FastCGI is to allow you to put on a public API (from the web server) that internally calls the "real" application, in your case the resident C++ service. So all work is done at the web server to create the public API using any technology you wish, and little or no code changes done in your C++ service.
Problem
We have a client-server application, server side is Glassfish 3.1.2. This app has many users, as well as many modules (e.g. View Transactions, View Banks etc). There are some long running processes invoked by client which run on server. Currently we have not found a nice solution to show the user what is going on on the server side. We want the users to get updated messages from server with given frequency. What would you suggest to use?
What we have done/tried
We (independently) used an approach with Singleton bean and a Map of client IDs similar to this, and it works of course. But then on the server side every method doSomething(Object... vars) must be converted to doSomething(Object... vars, String clientID) or whatever ID is type of. The client pulls data from server say once per second. I would like to avoid adding facades between server and client.
I was thinking about JAX-WS or JAX-RS, but I'm not familiar with these technologies deeply and not sure about what they can do.
Sockets
I should note that on the server side we have only Stateless beans (there is a reason for that), that is why I did not mention the use of Stateful bean (which is very good candidate I think).
Regards, Oleg
WebSocket could be a suitable choice, it allows the server to send unsolicited data to clients with no strong coupling, you just have to store a client id to map client connections to running tasks and be able to push updates to the right connection.
The client id/socket connection mapping can be maintained in a singleton bean using an in-memory structure, i.e. a hash map, or a permanent datastore for scalability purposes or in case you need a robust solution.
Some useful links to better understand WebSocket technology are this and this.
I have an application running in a Java EE App Server and it needs to call a web service of a partner company.
Using wsimport.exe from my JDK (1.6) I have generated the client classes. I instantiate the service and get the port in order to call the web service.
I noticed that the first call to the web service is slow, and I am led to believe this is because it is validating the WSDL. Subsequent calls are fast.
I could keep the WSDL locally, and apparently that will speed up the first call.
In order to optimise my app, I was thinking I could create a pool of the clients. This has the added advantage that I have some throttling in the app - lets say I have a pool of 5 clients, then at most I will be using memory for 5 clients. If the load increased suddenly on my server, I don't have to worry that an unlimited number of clients would cause an out of memory error. I am assuming, based on past experience, that the web service clients use a lot of memory...
Would you bother with a pool?
How would you get over the first call to the web service being slow?
What is the best way to create that pool, so that I have to do the least amount of programming (i.e. I'd like to use a library / API / whatever, so that I don't have to reinvent the wheel and code some hairy bugs).
The Apache Commons Pool might be exactly what I am after.
It is configurable and seems to have thought of everything.
A colleague of mine suggested that you can use the #WebServiceRef annotation on a field in an EJB. The idea is that the server would inject a reference to a client, from which one can create a port for each thread that calls the EJB.
I assume that injected references come from a pool, although the specification doesn't appear to talk about this. The Javadoc for the annotation explicitly mentions that:
"the injected references are not thread safe"
AKKA with a master/slave setup as shown in the link could work well, albeit a little more complex than the Apache Commons Pool listed in another answer. AKKA also uses an execution pool, with its own threads, which isn't strictly allowed in the Java EE world, although I'd argue that because a well tested framework is in charge of the threads, there is no danger, and it shouldn't interfere with the app servers control of threads anyway as the number of threads being handled by AKKA is minimal.
I am looking for a clean way to add service oriented access to an existing GWT application (client + RemoteService based server). The thing is that all the services are already in place, described by the #RemoteServiceRelativePath notation. It would be nice to be able to actually add the #WebService notation and have access to them both with RPC and XML/JSON/..
The real problem is that extending a current application to support other clients than the existing GWT one is a bit hard because of the GWT obfuscation. This also leads to an unneeded coupling between client and server since they both need to be deployed at the same time, because of the .gwt.rpc generated files.
I would like to reuse the existing RemoteService interfaces to define web services and connect to them with new clients via a plain-text protocol. Additionally, I would like to port the existing GWT client to the same protocol.
Is it possible to do this while using the same interfaces and implementation just by annotation?
What would be the best way to port the existing client to use a plain text protocol, RequestBuilder? Or just inject a new serialization implementation that does xml / json?
I don't even know where to start with this, this is why I'm asking. Maybe it is better to rewrite all the services and port everything at once but it will break everything until this is finished.
We've had a different approach since GWT the coupling of GWT between server and client side is not all bad but gives you a nice integration and you don't have to think too much about communication issues etc.
For that, our application had a frontend tier which consisted of the full gwt stack (client + server-coupling) and on the server-side, we connected via spring and RPC to the service layer.
On that way you can use on the benefits of spring and you don't loose the comfort of GWT.
But I Would like to hear if somebody already has gone other ways ;)
This is rather late and GWT is not the wonderchild it once was. However, for the sake of tying loose ends here's the solution I went for:
create a Java generator that parses all model (shared client/server classes) files through reflection and generates a Java file that reads/writes SOAP objects
bootstrap the above into a generic Java handler that handles native objects + array, sets, maps
write the service that can deal with the generated XML from the files above
It sounds a bit terse and a bit complicated but it 'only' took ~1 month to write the code to reliably convert >200 objects to their XML representation, automatically. The added benefit is that it allows mocking and cross-platform clients/servers.
As a summary, the generated code creates new methods 'fromXML' and 'toXML' that feed the fields that are public members (get/set) in the given class. So, given MyClass it would generate the MyClassSerializer and MyClassDeserializer Java classes that implement those SOAP-specific methods and also publish themselves to a 'dispatcher'. So whenever that dispatcher sees 'MyClass' it would know where to get the ser/deser functions from.