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Asynchronous call from java web service to .net application

Scenario :
There is a java library that helps asynchronously listen to some
system events. I am planning to use the already implemented asynch
functionality from a vb.net project.
I have planned to address it in the following way :
Write a web service around the java library
Add reference of that service to the .net app.
The web service will run locally on a tomcat as the .net app. The problem i am faced with is how do i make the web service communicate to the .net app asynchronously ? Should the .net app block and wait on a web service and if so how ?

I believe that firstly you need a design pattern for asynchronous services communication t better address your system requirements.
1 - Asynchronous Response Handler
The application creates a separated thread to process server response leaving main thread free for other tasks. When response arrives main thread is notified to receive the response. There are two implementation strategies: pooling and callback. With pooling, the main thread checks for response in the second thread until the response is available. With callback, the second thread notifies the main thread using a callback method.
2 - Request/Acknowledge
This patterns is based on the server participation to achieve asynchronous communication. Instead of splitting the process into two client threads, the process divided into two different transactions between client and server. This patterns needs some kind of correlation identification to associate request and response transactions. A messaging subsystem is used to achieve more scalability and availability.
A simple solution could be implemented using Asynchronous Response Handler/Callback Strategy. To do that, the service provider (java) could be a simple jax-ws web service.The service consumer(.net) implementation could use the AsyncCallback Delegate. There is a example here.
References:
http://www.servicedesignpatterns.com/WebServiceInfrastructures/AsyncResponseHandler
http://www.servicedesignpatterns.com/ClientServiceInteractions/RequestAcknowledge
http://msdn.microsoft.com/pt-br/library/system.asynccallback(v=vs.110).aspx (AsyncCallback Delegate)
http://msdn.microsoft.com/en-us/library/wyd0d1e5(v=vs.100).aspx (service consumer)
http://java.dzone.com/articles/jax-ws-hello-world (service producer)

You are planning to build a long running process, consider using some workflow engine like JBPM or Activiti. Since your calls are going to be web services which are stateless & you intend to have callbacks better would be an option to create persistent long running workflows since they would take care of most of the design & architectural issues else you would end up handling it.
e.g of issues
-Corelation between request-response
-Loss of data on reboot of system
-Application getting overwhelmed with data synchronization.
Polling & calbacks are just mechanisms for inter process communication but you need to handle req-resp correlation as well as ensure reliablity in the communication.

Server-side performing multiple requests against cloud services

I am in the process of writing a web-app that uses multiple web APIs.
For a single request of a single user, my app may need to perform up to 30 HTTP requests to other sites. The site housing the web-app can have hundreds of concurrent users.
I've been looking around trying to figure out which library should I use. I'm looking for a mature project that has detailed documentation and tested code, one that will still be around in years to come. Not sure if something like that exists (!)
Couple of questions :
In a case such as described above, should I be using an asynchronous HTTP client (without threading), or a regular (possibly pooled) HTTP client (with threading)? Asynchronicity relieves my app from using threads, but makes the code more scattered - will the above mentioned number of requests burden my server too much? (it says here that asynchronous is more scalable)
Which library is the common one to use? Is it Apache HttpComponenets HttpClient or its asynch couterpart HttpAsynchClient - which is in Alpha...)? How about jfarcand's AsyncHttpClient?
Okay, let's say I will use threads.
After digging around I realize that spawning threads from within a servlet (in my case - a Struts action), may be a big No No :
related questions:
What is recommended way for spawning threads from a servlet in Tomcat
Need help with java web app design to perform background tasks
Can i spawn a thread from a servlet ?
The way I see it, these are my options:
use my own thread pool (container doesn't manage my threads)
use a WorkManager such as CommonJ (seems like an inactive product)
use a 3rd party scheduler such as Quartz (may be an overkill ... ?)
I would appreciate any recommendations for this specific use case - aggregating lotsa data from different web services (this aggregation is invoked by a single user's single request).

Good question. I would try an asynchronous solution first to see how everything works. The asynchronous solution would be the simplest to implement.
If that doesn't work, try a more threaded model.
I would use HttpClient for making your requests. I've worked with it a lot and use it for any http work that I have to do.

A single thread for each remote http connection, and using a synchronous http client will probably be easier. I would try this approach first, and see if it is fast/scalable enough. For the synchronous approach, apache http client is a good choice.
If a synchronous solution is not good enough, something like netty may be a good fit. It uses NIO so you won't get thousands of threads.

I do not know of any existing software to do this for you that will not be overkill. But you might try splitting things up. That is, separate the fetching of the data of the showing of the result. Since you do not provide no further details on the problem at hand I cannot say for you whether that would be feasible or not.
Basically the idea is to create a service that will fetch those 30 subsequent requests for you and if possible process it into a request. The client of this service is the service that is running on the web. It will receive a request from a user and subsequently put it's own request trough to your data service. When the data service is ready it will return it's response. Either synchronously or asynchronously.
You could program your data service in any language you want, even Java, without being bound to servlets ensuring that fetching the subsequent 30 request and combining them in a response isn't being done by the webserver. This could also enhance the responsiveness of your web server itself.
Nutshell: branch of the ``difficult" tasks to a specialised service where you can transparently handle parallelism.

I would use Jetty and in the servlets I'd use the continuation mechanism to free up the thread while waiting for the third party web request to complete. This will allow maximum concurrency on your server, as you can have a lot more suspended requests than threads.
You can use either continuations or the servlet 3.0 asynchronous API, the end result is the same.

How to : Async Callbacks using SOAP/REST Web Services with Java

We have a Java API that needs to be supplemented/fronted with a SOAP/REST Web service Layer.
What does it take to implement Async Calls across process/server boundaries using
a) SOAP Webservices
b) RESTful webservices
Some of the methods might need multiple calls to respond to the request.
We will be using Java/J2ee to implement the SOAP/restful service using a library like CXF or Axis or Jax-RS/WS.
Any examples ? Gotchas ?
Thank you,

The Async Http Client is an open source library that was specifically designed for this type of problem. It utilizes futures and wraps up a lot of the detail and hassle out of making async calls.
The author has a very good getting started guide and there is an active discussion group. The author is a very talented developer and the project is under continuous development.
From the documentation:
The library uses Java non blocking I/O
for supporting asynchronous
operations. The default asynchronous
provider is build on top of Netty
(http://www.jboss.org/netty), the Java
NIO Client Server Socket Framework
from JBoss, but the library exposes a
configurable provider SPI which allows
to easily plug in other frameworks.

Your question is not clear. I am interpreting your question as you want your serverside code to call a remote REST web services in an Async manner. If so then your best bet is to use the Futures feature of java.util.concurrent it will do exactly what you want. If my interpretation of the question is wrong then please update your question with exactly where the async operations need to happen.

Akka http://akka.io/
Great framework, great performance - Here are their claims:
"""
Simpler Concurrency
Write simpler correct concurrent applications using Actors, STM & Transactors.
Event-driven Architecture
The perfect platform for asynchronous event-driven architectures. Never block.
True Scalability
Scale out on multi-core or multiple nodes using asynchronous message passing.
Fault-tolerance
Embrace failure. Write applications that self-heal using Erlang-style Actor supervisor hierarchies.
Transparent Remoting
Remote Actors gives you a high-performance transparent distributed programming model.
Scala & Java API
Scala and Java API as well as Spring and Guice integration. Deploy in your application server or run stand-alone.
"""

#Vivek
GET is async and other HTTP methods
are not.
This isn't true. Please go ahead and read about AJAX :-)

For REST web services (apart from GET) everything else (POST/PUT..) is Async, it returns you the HTTP status code of the opeeration.
If you want to make GET also Async then you will have to use Threads, We did it once in Spring framework using #Async annotation (which internally spawns a thread).
From get return the callback URL as the response and use threads to process the request asynchronously.
Apart from that, For both SOAP / REST once you get the request you can publish it on a JMS queue to make the service Async.

One of the best ways to implement asynch ops is to use callbacks. For REST APIs, design of APIs and Rest client should support this. For instance , client class should pass itself or it's inner class as listner. Rest API on server side should maintain request id and call back listener in map . Once processing is done , it can call method on listener based on request id from map.

Real question: why do you want to call it Async? Having looked at solutions for parallel processing on the Java EE side, it's not recommended that you spawn child threads within a container on your own.
In your case, it looks like the following:
1. you're looking to create a wrapper contract in WSDL (REST or SOAP or both) and if you clients are not just browsers (AJAX)(i mean you'd have adopters from the server-side), then, for JAX-WS -> you could create a #CallBack end-ponint (http://docs.oracle.com/cd/E15051_01/wls/docs103/webserv_adv/callback.html)
or
if it's REST end-point that requires being called from an adopter (server-side), use jAX_RS 2.0 feature
Note: Above assumes it's point to point but in an Async way
Here are a few options:
if you're looking to call REST Or SOAP or some other function asynchronously, you can use workManager API (JSR )
or
use JMS and use a request-reply model if you need it (short running) and calling multiple end-points in parallel
use WS-BPEL (only WSDL end-points) - this is a OASIS standard as well
use SCA (any component with any technology) that can contain assemblies of WS-BPEL component (stateless or stateful) running in BPEL engine like Apache ODE or IBM process server and other components and collaborates. This is a standard

Pattern for designing a scalable web service

I am writing a web service in Java which needs to handle a large number of requests / second. The general flow will be:
Web service receives a request from client
Returns a 'keep polling me' response to client
Calls another web service (or
services), and waits for them to
respond (with a timeout)
Client polls our web service, until
it receives a response (with a
timeout)
Researching on the Internet, I have found two general approaches to writing web services:
Spawn a thread for each request
Use the Reactor pattern (central dispatcher thread responds to IO events)
Do you have a recommendation for which approach is generally better, and what are the pros/cons of each approach? I would also appreciate pointers to examples.

Don't think multi-threading. Think asynchronously. I happened to have just coded an async handler that ran 2,000 RPS with <10 threads in IIS. Not sure how java works since I'm a .net guy but I gotta believe they have similar BeginXXX/EndXXX methods. If you ever spawn a thread then you're not considering all the places your code can block: data base IO, File I/O, web services, etc. These are the places your performance will cause your site to be slow.
Async, Async, Async.
Chant and repeat.

In addition to "No Refunds No Returns" response, I'd say yeah "Think Asynchronously" as you should be allowing your container to manage the multi-threading/scalability and high-availability issues of the web services it has deployed, this allows you to set-up things like clustering and so forth using your application container.
EDIT: So in conclusion, there isn't a pattern as such, maybe you should explore the scalability/availability features of your application container...

Asynchronism is indeed the right approach but don't manage this yourself, use something that supports asynchronous web service invocation like JAX-WS 2.0 (which uses the Future interface and/or the Executor framework from java.util.concurrent). See Asynchronous Web Service Invocation with JAX-WS 2.0.

What kind of application would serve as a dedicated application server?

In a very popular ecommerce store, I'd imagine the actual processing of the credit card would be moved to some sort of dedicated application server, and made into more of a asynchronous process.
What sort of java application type would that be? i.e. a service that would take a message of the queue, and start processing the request and update some db table once finished.
In .net, I guess one would use a windows service. What would you use in the java world?

It is typically a J2EE application that uses a HTTP web service interface or a JMS messaging interface. HTTP interfaces are accessible via a URL, and JMS connects to a queue to pick up messages that are sent to it. The app can run on any one of the major commercial (WebSphere, Weblogic, Oracle) or free (Glassfish, JBoss) servers.

In Java you already have great open source projects that do all this for you like Glassfish, Tomcat etc.

For a mission critical system, you might want something like IBM MQ series as the middleware, and a straight Java application that uses the MQ interface to process the requests.
At a few banks that I know of, this is their architecture. Originally the application servers were written in C, as was the middleware. They were able to switch to java because the code that was actually doing the critical work (sending and receiving messages, assuring guaranteed delivery, protecting against interruptions if a component went down) were the IBM MQ's.

In our case we use an application server from Sybase that can house Java components. They are pretty much standard Java classes that have public methods that are exposed for calling via CORBA. Components can also be scheduled to run constantly or on a schedule (like a service) to look for work to do (via items in a database table, an Oracle AQ queue, or a JMS queue). All of this is contained in the app server and the app server provides transaction management, resource management, and database connection pooling for us.

Or use an OSGI environment.