I am analyzing some jersey 2.0 code and i have a question on how the following method works:
#Stateless
#Path("/mycoolstuff")
public class MyEjbResource {
…
#GET
#Asynchronous //does this mean the method executes on child thread ?
public void longRunningOperation(#Suspended AsyncResponse ar) {
final String result = executeLongRunningOperation();
ar.resume(result);
}
private String executeLongRunningOperation() { … }
}
Lets say im at a web browser and i type in www.mysite/mycoolstuff
this will execute the method but im not understanding what the asyncResponse is used for neither the #Asynchronous annotation. From the browser how would i notice its asychnronous ? what would be the difference in removing the annotation ? Also the suspended annotation after reading the documentation i'm not clear its purpose.
is the #Asynchronous annotation simply telling the program to execute this method on a new thread ? is it a convenience method for doing "new Thread(.....)" ?
Update: this annotation relieves the server of hanging onto the request processing thread. Throughput can be better. Anyway from the official docs:
Request processing on the server works by default in a synchronous processing mode, which means that a client connection of a request is processed in a single I/O container thread. Once the thread processing the request returns to the I/O container, the container can safely assume that the request processing is finished and that the client connection can be safely released including all the resources associated with the connection. This model is typically sufficient for processing of requests for which the processing resource method execution takes a relatively short time. However, in cases where a resource method execution is known to take a long time to compute the result, server-side asynchronous processing model should be used. In this model, the association between a request processing thread and client connection is broken. I/O container that handles incoming request may no longer assume that a client connection can be safely closed when a request processing thread returns. Instead a facility for explicitly suspending, resuming and closing client connections needs to be exposed. Note that the use of server-side asynchronous processing model will not improve the request processing time perceived by the client. It will however increase the throughput of the server, by releasing the initial request processing thread back to the I/O container while the request may still be waiting in a queue for processing or the processing may still be running on another dedicated thread. The released I/O container thread can be used to accept and process new incoming request connections.
#Suspended have more definite if you used it, else it will not make any difference of using it.
Let's talk about benefits of it:
#Suspended will pause/Suspend the current thread until it gets response,by default #NO_TIMEOUT no suspend timeout set. So it doesn't mean your request response (I/O)thread will get free and be available for other request.
Now Assume you want your service to be a response with some specific time, but the method you are calling from resource not guarantee the response time, then how will you manage your service response time? At that time, you can set suspend timeout for your service using #Suspended, and even provide a fall back response when time get exceed.
Below is some sample of code for setting suspend/pause timeout
public void longRunningOperation(#Suspended AsyncResponse ar) {
ar.setTimeoutHandler(customHandler);
ar.setTimeout(10, TimeUnit.SECONDS);
final String result = executeLongRunningOperation();
ar.resume(result);
}
for more details refer this
The #Suspended annotation is added before an AsyncResponse parameter on the resource method to tell the underlying web server not to expect this thread to return a response for the remote caller:
#POST
public void asyncPost(#Suspended final AsyncResponse ar, ... <args>) {
someAsyncMethodInYourServer(<args>, new AsyncMethodCallback() {
#Override
void completed(<results>) {
ar.complete(Response.ok(<results>).build());
}
#Override
void failed(Throwable t) {
ar.failed(t);
}
}
}
Rather, the AsyncResponse object is used by the thread that calls completed or failed on the callback object to return an 'ok' or throw an error to the client.
Consider using such asynchronous resources in conjunction with an async jersey client. If you're trying to implement a ReST service that exposes a fundamentally async api, these patterns allow you to project the async api through the ReST interface.
We don't create async interfaces because we have a process that takes a long time (minutes or hours) to run, but rather because we don't want our threads to ever sleep - we send the request and register a callback handler to be called later when the result is ready - from milliseconds to seconds later - in a synchronous interface, the calling thread would be sleeping during that time, rather than doing something useful. One of the fastest web servers ever written is single threaded and completely asynchronous. That thread never sleeps, and because there is only one thread, there's no context switching going on under the covers (at least within that process).
The #suspend annotation makes the caller actually wait until your done work. Lets say you have a lot of work to do on another thread. when you use jersey #suspend the caller just sits there and waits (so on a web browser they just see a spinner) until your AsyncResponse object returns data to it.
Imagine you had a really long operation you had to do and you want to do it on another thread (or multiple threads). Now we can have the user wait until we are done. Don't forget in jersey you'll need to add the " true" right in the jersey servlet definition in web.xml to get it to work.
Related
I'm having a #RestController webservice method that might block the response thread with a long running service call. As follows:
#RestController
public class MyRestController {
//could be another webservice api call, a long running database query, whatever
#Autowired
private SomeSlowService service;
#GetMapping()
public Response get() {
return service.slow();
}
#PostMapping()
public Response get() {
return service.slow();
}
}
Problem: what if X users are calling my service here? The executing threads will all block until the response is returned. Thus eating up "max-connections", max threads etc.
I remember some time ago a read an article on how to solve this issue, by parking threads somehow until the slow service response is received. So that those threads won't block eg the tomcat max connection/pool.
But I cannot find it anymore. Maybe somebody knows how to solve this?
there are a few solutions, such as working with asynchronous requests. In those cases, a thread will become free again as soon as the CompletableFuture, DeferredResult, Callable, ... is returned (and not necessarily completed).
For example, let's say we configure Tomcat like this:
server.tomcat.max-threads=5 # Default = 200
And we have the following controller:
#GetMapping("/bar")
public CompletableFuture<String> getSlowBar() {
return CompletableFuture.supplyAsync(() -> {
silentSleep(10000L);
return "Bar";
});
}
#GetMapping("/baz")
public String getSlowBaz() {
logger.info("Baz");
silentSleep(10000L);
return "Baz";
}
If we would fire 100 requests at once, you would have to wait at least 200 seconds before all the getSlowBar() calls are handled, since only 5 can be handled at a given time. With the asynchronous request on the other hand, you would have to wait at least 10 seconds, because all requests will likely be handled at once, and then the thread is available for others to use.
Is there a difference between CompletableFuture, Callable and DeferredResult? There isn't any difference result-wise, they all behave the similarly.
The way you have to handle threading is a bit different though:
With Callable, you rely on Spring executing the Callable using a TaskExecutor
With DeferredResult you have to to he thread-handling by yourself. For example by executing the logic within the ForkJoinPool.commonPool().
With CompletableFuture, you can either rely on the default thread pool (ForkJoinPool.commonPool()) or you can specify your own thread pool.
Other than that, CompletableFuture and Callable are part of the Java specification, while DeferredResult is a part of the Spring framework.
Be aware though, even though threads are released, connections are still kept open to the client. This means that with both approaches, the maximum amount of requests that can be handled at once is limited by 10000, and can be configured with:
server.tomcat.max-connections=100 # Default = 10000
in my opinion.the async may be better for the sever.for this particular api, async not works well.the clients also hold the connections. finally it will eating up "max-connections".you can send the request to messagequeue(kafka)and return success to clients. then you get the request and pass it to the slow sevice.
I'm having an issue with a web service with users trying to guess application IDs by looping over random IDs.
The bad requests are coming from random IPs, so I cannot just ban their IP (unless I do it dynamically, but I'm not looking into that yet).
Currently when I detect a client that has made 10 bad app ID attempts I put them on a block list in my app, and reject further requests from that IP for the day.
I want to minimize the amount of work my server needs to do, as the bad client will continue to send 1000s of requests even though they get rejected. I know there are dynamic Firewall solutions, but want something easy to implement in my app for now. Currently I am sleeping for 5 seconds to reduce the calls, but what I want to do is just not send a response to the client, so it has to timeout.
Anyone know how to do this in Java, in JAX-RS?
My service is like,
#Path("/api")
public class MyServer {
#GET
#Consumes(MediaType.APPLICATION_XML)
#Produces(MediaType.APPLICATION_XML)
#Path("/my-request")
public String myRequest(String type,
#Context HttpServletRequest requestContext,
#Context HttpServletResponse response) {
...
}
See:
How to stop hack/DOS attack on web API
You are looking for asynchronous responses which are supported by JAX-RS. The tutorial for Jersey contains some examples of how to implement an asynchronous response to a request.
With asynchronous responses, the thread that is responsible for answering a request is freed for handling another request already before a request is completed. This feature is activated by adding a parameter with the #Suspended annotation. What you would need to do additionally, would be to register a dedicated scheduler that is responsible for waking up your requests after a given time-out like in this example:
#Path("/api")
public class MyServer {
private ScheduledExecutorService scheduler = ...;
#GET
#Consumes(MediaType.APPLICATION_XML)
#Produces(MediaType.APPLICATION_XML)
#Path("/my-request")
public String myRequest(String type,
#Context HttpServletRequest requestContext,
#Context HttpServletResponse response,
#Suspended AsyncResponse asyncResponse) {
scheduler.schedule(new Runnable() {
#Override
public void run() {
asyncResponse.resume(...)
}
}, 5, TimeUnit.SECOND);
}
}
This way, no thread is blocked for the waiting time of five seconds what gives an opportunity for handling other requests in the meantime.
JAX-RS does not offer a way of completely discarding a request without an answer. You will need to keep the connection open to produce a time out, if you terminate the connection, a user is notified of the termination. Best you could do would be to never answer an asynchronous request but this will still consume some ressources. If you wanted to avoid this, you would have to solve the problem outside of JAX-RS, for example by proxying the request by another server.
One way to do that would be to set up mod_proxy where you could answer the proxy with an error code for the mallicious requests and set up a very large retry limit for such requests.
I may suggest move IP deny logic from REST to plain HTTP Filter:
#WebFilter(urlPatterns = "/*", asyncSupported = true)
#WebListener
public class HttpFilter implements Filter {
#Override
public void init(FilterConfig filterConfig) throws ServletException { }
#Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
if(denyIP(servletRequest.getRemoteAddr())) {
AsyncContext asyncContext = servletRequest.startAsync();
asyncContext.setTimeout(100000);
}else{
filterChain.doFilter(servletRequest, servletResponse);
}
}
#Override
public void destroy() { }
private boolean denyIP(String address){
//todo
return true;
}
}
It is cheaper for application server: no XML/JSON deserialization, no call to REST classes. Also you may notice that I never call asyncContext.start. I check Wildfly 8.2 server. In this case Wildfly does not use thread per request. I sent a lot of requests, but amount of threads was constant.
PS
trying to guess application IDs by looping over random IDs
It is not DOS attack. It is brute force attack.
There are many possible solutions, with the restrictions you have given 2 possible solutions come to my mind:
1) Use a forward proxy that already has support for limiting requests. I personally have used Nginx and can recommend it partly because it is simple to set up. Relevant rate limiting config: Limit Req Module
2) Use Asynchronous JAX-RS to let timeout the malicious request that you detected. Sane request can be processed directly. But beware of the consequences, either way such an approach will consume resources on the server!
You can try asyncContext supported from Tomcat 3.0.
This feature decouples web request handler and processer. In your case, the thread which accepts the request has to wait/sleep more than timeout configured. Making the same thread sleep for such a long time would starve them and it would drastically affect the performance of servers. So, asynchronous processing is the right way to go.
I have used asyncContext with Java single thread executor http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html
It worked well for me. I had similar business case, where I had to mock my application.
Refer this for implementation http://peter-braun.org/2013/04/asynchronous-processing-from-servlet-3-0-to-jax-rs-2-0/
Single thread executor would not eat resources and its ideal for this use case.
I have not tried this.... just a shot in the dark here, so take it with a grain of salt. So the problem is that once you detect something fishy and put the IP in block-mode you do not want to waste another iota of resources on this request and also cause them to waste time timingout. But your framework will respond if you throw an exception. How about interrupting your current thread? You can do this by Thread.currentThread().interrupt();. The hope is that the Java container processing the request will check the interrupt status. It might not be doing that. I know I have seen IO related classes not process requests because the interrupted flag was set.
EDIT: If interrupting your thread does not work, you can also try throwing an InterruptedException. It might accomplish the desired affect.
As far as I know, in the Servlet specification there is no such mechanism. As JAX-RS implementations use servlets (at least Jersey and Resteasy), you do not have a standard way to achieve that in Java.
The idea of using the aynchronous JAX-RS is better than Thread.sleep(5000), but it will still use some resources and is nothing but a way to process the request later, instead of ignoring the request for always.
I once solved a similar problem by creating a TCP/IP tunnel application.
It's a small application that listens to an external port (e.g. the http port 80). Under normal conditions, all received calls are accepted, creating dedicated socket objects. These individual sockets then call the real (hidden) webserver, which runs on the same physical server, or any server within your local network.
The tunnel itself is like a dispatcher, but can also act like a loadbalancer. It can be multifunctional.
The thing is that you're working low-level with sockets at this point. If you hand a list of banned ip-addresses to this application, then it can shut-out applications on a very low-level (not even accepting their socket calls at all).
You could integrate this in the very same java application as well. But I think it is more flexible to regard this as a seperate application.
(Let me know if you need some source code, I may have some code laying around to get you started)
Alright, I've already asked one question regarding this, but needed a bit more info. I'll try to be coherent with my question as much as I can. (Since I am not sure of the concepts).
Background
I have a java web project(dynamic). I am writing Restful webservices. Below is a snippet from my class
/services
class Services{
static DataSource ds;
static{
ds = createNewDataSource;
}
/serviceFirst
#Consumes(Something)
#produces(Something)
public List<Data> doFirst(){
Connection con = ds.getConnection();
ResultSet res = con.execute(preparedStatement);
//iterate over res, and create list of Data.
return list;
}
}
This is a very basic functionality that I have stated here.
I've got tomcat server where I've deployed this. I've heard that Tomcat has a threadpool, of size 200 (by default). Now my question is that, how exactly does the threadpool work here.
Say I have two requests coming in at the same time. That means that 2 of the threads from the threadpool will get to work. Does this mean that both the threads will have an instance of my class Services? Because below is how I understand the threads and concurrency.
public class myThread extends Thread(){
public void run(){
//do whatever you wan to do here;
}
}
In the above, when I call start on my Thread it will execute the code in run() method and all the objects that it creates in there, will belong to it.
now, coming back to the Tomcat, is there somewhere a run() method written that instantiates the Services class, and that is how the threadpool handles 200 concurrent requests. (Obviously, I understant they will require 200 cores for them to execute concurrently, so ignore that).
Because otherwise, if tomcat does not have 200 different threads having the same path of execution (i.e. my Services class), then how exactly will it handle the 200 concurrent requests.
Thanks
Tomcat's thread pool works, more or less, like what you would get from an ExecutorService (see Executors).
YMMV. Tomcat listens for requests. When it receives a request, it puts this request in a queue. In parallel, it maintains X threads which will continuously attempt to take from this queue. They will prepare the ServletRequest and ServletResponse objects, as well as the FilterChain and appropriate Servlet to invoke.
In pseudocode, this would look like
public void run() {
while (true) {
socket = queue.take();
ServletRequest request = getRequest(socket.getInputStream());
ServletResponse response = generateResponse(socket.getOutputStream());
Servlet servletInstance = determineServletInstance(request);
FilterChain chain = determineFilterChainWithServlet(request, servletInstance);
chain.doFilter(request,response); // down the line invokes the servlet#service method
// do some cleanup, close streams, etc.
}
}
Determining the appropriate Servlet and Filter instances depends on the URL path in the request and the url-mappings you've configured. Tomcat (and every Servlet container) will only ever manage a single instance of a Servlet or Filter for each declared <servlet> or <filter> declaration in your deployment descriptor.
As such, every thread is potentially executing the service(..) method on the same Servlet instance.
That's what the Servlet Specification and the Servlet API, and therefore Tomcat, guarantee.
As for your Restful webservices, read this. It describes how a resource is typically an application scoped singleton, similar to the Servlet instance managed by a Servlet container. That is, every thread is using the same instance to handle requests.
Servlet 3.0 allows the 'request' thread (or 'main' thread) to delegate long-running processing to some other thread, so as to free itself to receive more requests. Agreed.
That is, we are achieving scalability (of requests) by utilizing multi-threading.
But this requires that my 'Servlet container JVM' is capable of such processing.
What if I have a multi-tiered architecture where the 'Servlet container JVM' is only the entry point, while the logic for servicing requests lies somewhere else in some other JVM (henceforth called as 'Service JVM' in this post).
What if I want to post the incoming 'request' (or atleast the relevant attributes of the request) to a JMS queue and let the 'request' be grabbed and processed by one out of a pool of 'Service JVMs' ? Wouldnt it be better to delegate the responsibility of sending the 'response' (say as JSON) also to this Service JVM ?
I dont think 'AsyncContext' can be passed meaningfully outside of the Servlet container JVM. So, how to really delegate request-processing and response-sending, to be done by distributed services (JVMs) ?
In terms of code/pseudo-code, my question is:
#WebServlet(urlPatterns = "/AsyncServlet", asyncSupported=true)
public class AsyncServlet extends HttpServlet {
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
AsyncContext asyncCtx = request.startAsync();
// Put the asyncCtx in a JMS queue so as to be picked by another
// service JVM that can really service this request.
// return back to receiving requests and dont worry abt sending a response
// The service JVM will take care of sending the appropriate response
// as it has the data necessary for the response.
}
}
One option seems to be to have Worker threads (in the Servlet container JVM) wait for responses from the Service JVMs. After a Service JVM does the actual processing, it can communicate the results (thru messages or otherwise) to the respective Worker thread, and have the Worker thread send the GET response.
I want to know if there is (am sure there shd be !) a better alternative than this, as this seems so convoluted !
Set context as async
Store context inside singleton bean
Send a jms request
Process jms request
Send jms reply
Get context for reply from the singleton bean
Send reply to client
You might want to set a timer for cleanup and you can replace jms with async one-way ejb calls
I'm trying to set up a servlet that I can use to call webservices asynchronously. For the most part, it is working fine. I have a servlet with a doGet method and a js that calls it. I have a callback method in the js that the servlet correctly calls when it has finished doing its thing.
The complication is that one of the web services I am calling is also asynchronous, and I would like to be able to essentially call the js callback method a second time after the asynchronous ws callback has finished. For example, if you have a status field, when you call the synchronous web service, it immediately updates to "Beginning Synchronous Call" and then when the servlet callback arrives it changes to the callback value, which is the result of the web service.
When you call the asynchronous web service, the update field immediately updates to "Beginning Asynchronous Call", and shortly receives the first callback from the servlet indicating that the web service has been requested, so we update the field to "Processing Web Service" or whatever. The problem is that once the web service finishes and calls back to the servlet, I can't seem to figure out how to send the result to the js callback method.
I'm pretty new at AJAX and servlets, so maybe this is a horrible way to accomplish what I want.
The web services are both being called in the Servlet, mostly using Netbeans auto-generated WS calls. The WS calls themselves work fine, but once I get the result of the asynchronous WS, I am stuck inside of the handleResponse method of the webservice callback and no longer have any reference to the response element for the document I want to update.
I tried to store the original response variable as a static member variable and use it in the handleResponse method like so:
javax.xml.ws.AsyncHandler<WsClients.Op11Response> asyncHandler = new javax.xml.ws.AsyncHandler<WsClients.Op11Response>() {
public void handleResponse(javax.xml.ws.Response<WsClients.Op11Response> asyncResponse) {
try {
storedResponse.setContentType("text/xml");
String returnString = asyncResponse.get().getReturn();
storedResponse.getWriter().write("<returnData><content>"
+ returnString + "</content></returnData>");
} catch (Exception ex) {
}
}
};
This will not compile with a debugger attached and does not seem to be able to assign a reference anyway.
Is there a better way to do this?
The nature of HTTP is that you cannot send anything back to the client unless client requested this information either by polling or by keeping the connection open.
The operation to start the asynchronous call ends immediately and you need to return from the servlet doGet method (while technically you can stay in the servlet call until your async call finishes I wouldn't recommend that as it ties up the server resources. It is generally a good practice to return from the servlet as soon as you can).
The best course of action would be:
Have internal data structure (e.g. HashMap with appropriate synchronization) to hold the asynchronous calls that are executing.
When you start a new call, assign it pseudo-random key and return it from the initial call.
Using the above key, have browser-side javascript AJAX calls periodically poll the status of the call and display the results.
Do not forget to clean up finished or stale calls (for example by running a timer thread).
When you comfortable with the polling implementation in step 3 above, you may want to consider Comet, a.k.a. long poll to replace client-side polling.
Servlet cannot send response again. HTTP protocol is synchronous, and only client can initiate a request-response exchange.
For async updates you need to perform polling from the client side to the server side, and accumulate messages on the server side (in the sessions) until client picks them up or they expire.