Apache Mina - 2.0.1
I have a IoHandlerAdapter which has call backs on sessionOpened and sessionClosed. When I open a session I login to the server and start sending requests, in "sessionClosed" I just restart my application.
My application makes a check
if(!session.isClosing()) {
//write to socket
} else {
//throw a runtime exception. Hopefully the sessionClosed API gets
//called soon and the next time this call will succeed.
}
However I did notice that although the session.isClosing() returns true the session actually never gets closed. i.e, I dont really get the call back in my handler. Is this possible? How can I mitigate against such a risk. Can someone explain how long it takes for the close callback to be called?
IMHO you should use a CloseFuture for this senario like so:
CloseFuture future = session.close(true);
// wait until its closed, up to 1 minute
future.awaitUninterruptibly(60000L);
if (future.isClosed()) {
System.out.println("Previous connection close completed");
}
I have seen this too. I believe this could be a race condition in the MINA layer:
http://mail-archives.apache.org/mod_mbox/mina-dev/200802.mbox/%3C15350882.post#talk.nabble.com%3E
Related
I'd like to add a service that executes some initialization operations for the system when it's first created.
I'd imagine it would be a stateless service (with cluster admin rights) that should self-destruct when it's done it's thing. I am under the impression that exiting the RunAsync function allows me to indicate that I'm finished (or in an error state). However, then it still hangs around on the application's context and annoyingly looking like it's "active" when it's not really doing anything at all.
Is it possible for a service to remove itself?
I think maybe we could try using the FabricClient.ServiceManager's DeleteServiceAsync (using parameters based on the service context) inside an OnCloseAsync override but I've not been able to prove that might work and it feels a little funky:
var client = new FabricClient();
await client.ServiceManager.DeleteServiceAsync(new DeleteServiceDescription(Context.ServiceName));
Is there a better way?
Returning from RunAsync will end the code in RunAsync (indicate completion), so SF won't start RunAsync again (It would if it returned an exception, for example). RunAsync completion doesn't cause the service to be deleted. As mentioned, for example, the service might be done with background work but still listening for incoming messages.
The best way to shut down a service is to call DeleteServiceAsync. This can be done by the service itself or another service, or from outside the cluster. Services can self-delete, so for services whose work is done we typically see await DeleteServiceAsync as the last line of RunAsync, after which the method just exits. Something like:
RunAsync(CancellationToken ct)
{
while(!workCompleted && !ct.IsCancellationRequested)
{
if(!DoneWithWork())
{
DoWork()
}
if(DoneWithWork())
{
workCompleted == true;
await DeleteServiceAsync(...)
}
}
}
The goal is to ensure that if your service is actually done doing the work it cleans itself up, but doesn't trigger its own deletion for the other reasons that a CancellationToken can get signaled, such as shutting down due to some upgrade or cluster resource balancing.
As mentioned already, returning from RunAsync will end this method only, but the service will continue to run and hence not be deleted.
DeleteServiceAsync certainly is the way to go - however it's not quite as simple as just calling it because if you're not careful it will deadlock on the current thread (especially in local developer cluster). You would also likely get a few short-lived health warnings about RunAsync taking a long time to terminate and/or target replica size not being met.
In any case - solution is quite simple - just do this:
private async Task DeleteSelf(CancellationToken cancellationToken)
{
using (var client = new FabricClient())
{
await client.ServiceManager.DeleteServiceAsync(new DeleteServiceDescription(this.Context.ServiceName), TimeSpan.FromMinutes(1), cancellationToken);
}
}
Then, in last line of my RunAsync method I call:
await DeleteSelf(cancellationToken).ConfigureAwait(false);
The ConfigureAwait(false) will help with deadlock issue as it will essentially return to a new thread synchronization context - i.e. not try to return to "caller context".
I am trying to write a control panel to
Inform about certain KPIS
Enable the user to init certain requests / jobs by pressing a button that then runs a stored proc on the DB or sets a specific setting etc
So far, so good, except I would like to run some bigger jobs where the length of time that the job is running for is unknown and could run over both the script timeout period AND the time the user is willing to wait for a response.
What I want is a "fire and forget" process so the user hits the button and even if they kill the page or turn off their phone they know the job has been initiated and WILL complete.
I was looking into C# BeginExecuteNonQuery which is an async call to the query so the proc is fired but the control doesn't have to wait for a response from it to carry on. However I don't know what happens when the page/app that fired it is shut.
Also I was thinking of some sort of Ajax command that fires the code in a page behind the scenes so the user doesn't know about it running but then again I believe if the user shuts the page down the script will die and the command will die on the server as well.
The only way for certain I know of is a "queue" table where jobs are inserted into this table then an MS Agent job comes along every minute or two checking for new inserts and then runs the code if there is any. That way it is all on the DB and only a DB crash will destroy it. It won't help with multiple jobs waiting to be run concurrently that both take a long time but it's the only thing I can be sure of that will ensure the code is run at all.
Any ideas?
Any language is okay.
Since web browsers are unconnected, requests from them always take the full amount of time. The governing factor isn't what the browser does, but how long the web site itself will allow an action to continue.
IIS (and in general, web servers) have a timeout period for requests, where if the work being done takes simply too long, the request is terminated. This would involve abruptly stopping whatever is taking so long, such as a database call, running code, and so on.
Simply making your long-running actions asynchronous may seem like a good idea, however I would recommend against that. The reason is that in ASP and ASP.Net, asynchronously-called code still consumes a thread in a way that blocks other legitimate request from getting through (in some cases you can end up consuming two threads!). This could have performance implications in non-obvious ways. It's better to just increase the timeout and allow the synchronously blocking task to complete. There's nothing special you have to do to make such a request complete fully, it will occur even if the sender closes his browser or turns off his phone immediately after (presuming the entire request was received).
If you're still concerned about making certain work finish, no matter what is going on with the web request, then it's probably better to create an out-of-process server/service that does the work and to which such tasks can be handed off. Your web site then invokes a method that, inside the service, starts its own async thread to do the work and then immediately returns. Perhaps it also returns a request ID, so that the web page can check on the status of the requested work later through other methods.
You may use asynchronous method and call the query from this method.
Your simple method can be changed in to a asynch method in the following manner.
Consider that you have a Test method to be called asynchronously -
Class AsynchDemo
{
public string TestMethod(out int threadId)
{
//call your query here
}
//create a asynch handler delegate:
public delegate string AsyncMethodCaller(out int threadId);
}
In your main program /or where you have to call the Test Method:
public static void Main()
{
// The asynchronous method puts the thread id here.
int threadId;
// Create an instance of the test class.
AsyncDemo ad = new AsyncDemo();
// Create the delegate.
AsyncMethodCaller caller = new AsyncMethodCaller(ad.TestMethod);
// Initiate the asychronous call.
IAsyncResult result = caller.BeginInvoke(
out threadId, null, null);
// Call EndInvoke to wait for the asynchronous call to complete,
// and to retrieve the results.
string returnValue = caller.EndInvoke(out threadId, result);
Console.WriteLine("The call executed on thread {0}, with return value \"{1}\".",
threadId, returnValue);
}
From my experience a Classic ASP or ASP.NET page will run until complete, even if the client disconnects, unless you have something in place for checking that the client is still connected and do something if they are not, or a timeout is reached.
However, it would probably be better practice to run these sorts of jobs as scheduled tasks.
On submitting your web page could record in a database that the task needs to be run and then when the scheduled task runs it checks for this and starts the job.
Many web hosts and/or web control panels allow you to create scheduled tasks that call a URL on schedule.
Alternately if you have direct access to the web server you could create a scheduled task on the server to call a URL on schedule.
Or, if ASP.NET, you can put some code in global.asax to run on a schedule. Be aware though, if your website is set to stop after a certain period of inactivity then this will not work unlesss there is frequent continuous activity.
I have a prototype that works well with SSE and WebSockets, but crashes when using LongPolling in the moment that I put a little bit of stress in the browser.
My app can create games, and each game generate its own events, and those events must be sent to the browser. I have a button to create one, ten and a hundred games at once. Create each game requires a POST call to a WebAPI, so the x10 button creates 10 requests and x100 creates 100 requests to the server.
When I use SSE or WS, it works nicely, I can call the x100 button and create a hundred games, all the games gets its respective events. I can see the 100 HTTP POST request being all successful.
But if I switch to LongPolling mode, I can create games one by one, as long I do not click too fast, and it works well. In the moment I click fast or click the "ten" or "hundred" button, all WebAPI calls but one get stuck and eventually fails with this message:
{"Message":"Anerrorhasoccurred.","ExceptionMessage":"Ataskwascanceled.","ExceptionType":"System.Threading.Tasks.TaskCanceledException","StackTrace":"atSystem.Web.Http.ApiController.<InvokeActionWithExceptionFilters>d__1.MoveNext()\r\n---Endofstacktracefrompreviouslocationwhereexceptionwasthrown---\r\natSystem.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()\r\natSystem.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Tasktask)\r\natSystem.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Tasktask)\r\natSystem.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__0.MoveNext()"}
Even running in debug, I cannot see that exception happening anywhere.
And SignalR disconnects:
[14:24:37 GMT+0000 (GMT Standard Time)] SignalR: Long poll complete. jquery.signalR-2.0.1.js:75
[14:24:37 GMT+0000 (GMT Standard Time)] SignalR: Disconnect command received from server. jquery.signalR-2.0.1.js:75
[14:24:37 GMT+0000 (GMT Standard Time)] SignalR: Stopping connection.
And it is true, the server actually sends a D:1 in the last response, but I don't know why is that happening, I have nothing in my code that disconnects SignalR connections.
It happens in, at least, Google Chrome and IE 10.
I have no clue about what could be the problem. Any idea?
Cheers.
UPDATE:
I have created a small project that reproduces the issue. It have shared it here.
Accessing: http://localhost/LongPollingLoadTest/ we can add one game, ten or a hundred without problems, because it will use SSE or WebSockets if available.
Now, open http://localhost/LongPollingLoadTest/?transport=longPolling. You will see how most of calls gets stuck and also most of the times, the SignalR connection crashes.
I think the problem is somehow related with the groups management:
[AcceptVerbs("POST")]
public async Task<GameInfo> Post([FromBody]GameRequest request)
{
var game =new GameInfo() { Id = Guid.NewGuid(), Name = request.Name };
if (_games.TryAdd(game.Id, game))
{
var context = GlobalHost.ConnectionManager.GetConnectionContext<MyPersistentConnection>();
await context.Groups.Add(request.ConnectionId, game.Id.ToString());
await context.Connection.Broadcast(game);
Thread.Sleep(100);
return game;
}
else
throw new ArgumentException("Already exists");
}
In your case looks like the longPolling connection became not alive over DisconnectTimeout (which default value is 30 seconds), then server sent disconnect command to client.
Could you also add SignalR trace in your repro?
Issue already reported here as #dfowler indicated: https://github.com/SignalR/SignalR/issues/2456
Cheers.
I do asynchronous requests in LoadState method of a certain Page. I use HttpClient to make a request and I expect the splashscreen to go away while I await the result.
If I am not connected to any networks, the splashscreen immediately goes away and I get a blank page because the request obviously didn't happen.
But if I am connected to a network but have connectivity issues (for example, I set a wrong IP address) it seems to start a request and just block.
My expectation was that the HttpClient would realize that it cannot send a request and either throw an exception or just return something.
I managed to solve the issue of blocking by setting a timeout of around 800 milliseconds, but now it doesn't work properly when the Internet connection is ok. Is this the best solution, should I be setting the timeout at all? What is the timeout that's appropriate which would enable me to differentiate between an indefinitely blocking call and a proper call that's just on a slower network?
I could perhaps check for Internet connectivity before each request, but that sounds like an unpredictable solution...
EDIT: Now, it's really interesting. I have tried again, and it blocks at this point:
var rd = await httpClient.SendAsync(requestMsg);
If I use Task.Run() as suggested in the comments and get a new Thread, then it's always fine.
BUT it's also fine without Task.Run() if there is no Internet access but the network access is not "Limited" (it says that the IPv4 connectivity is "Internet access" although I cannot open a single website in a browser and no data is returned from the web service. It just throws System.Net.Http.HttpRequestException which was something I was expecting in the first place) Only blocks when the network connection is Limited.
What if instead of setting a timeout, you checked the connection status using
public static bool IsConnected
{
get
{
return NetworkInformation.GetInternetConnectionProfile() != null;
}
}
This way if IsConnected, then you make the call; otherwise, ignore it.
I'm not sure if you are running this in App.xaml.cs? I've found requests made in that class can be fickle and it may be best to move the functionality to an extended splash screen to ensure the application makes it all the way through the activation process.
http://msdn.microsoft.com/en-us/library/windows/apps/xaml/Hh868191(v=win.10).aspx
I'm trying to get to grips with Server-Side Events as they fit my requirements perfectly and seem like they should be simple to implement, however I can't get past a vague error and what looks like the connection repeatedly being closed and re-opened. Everything I have tried is based on this and other tutorials.
The PHP is a single script:
<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
function sendMsg($id, $msg) {
echo "id: $id" . PHP_EOL;
echo "data: $msg" . PHP_EOL;
echo PHP_EOL;
ob_flush();
flush();
}
$serverTime = time();
sendMsg($serverTime, 'server time: ' . date("h:i:s", time()));
?>
and the JavaScript looks like this (run on body load):
function init() {
var source;
if (!!window.EventSource) {
source = new EventSource('events.php');
source.addEventListener('message', function(e) {
document.getElementById('output').innerHTML += e.data + '<br />';
}, false);
source.addEventListener('open', function(e) {
document.getElementById('output').innerHTML += 'connection opened<br />';
}, false);
source.addEventListener('error', function(e) {
document.getElementById('output').innerHTML += 'error<br />';
}, false);
}
else {
alert("Browser doesn't support Server-Sent Events");
}
}
I have searched around a bit but can't find information on
If Apache needs any special configuration to support server-sent events, and
How I can initiate a push from the server with this kind of setup (e.g. can I simply execute the PHP script from CLI to give a push to the already-connected-browser?)
If I run this JS in Chrome (16.0.912.77) it opens the connection, receives the time, then errors (with no useful information in the error object), then reconnects in 3 seconds and goes through the same process. In Firefox (10.0) I get the same behaviour.
EDIT 1: I thought the issue could be related to the server I was using, so I tested on a vanilla XAMPP install and the same error comes up. Should a basic server configuration be able to handle this without modification / extra configuration?
EDIT 2: The following is an example of output from the browser:
connection opened
server time: 01:47:20
error
connection opened
server time: 01:47:23
error
connection opened
server time: 01:47:26
error
Can anyone tell me where this is going wrong? The tutorials I have seen make it look like SSE is very straightforward. Also any answers to my two numbered questions above would be really helpful.
Thanks.
The problem is your php.
With the way your php script is written, only one message is sent per execution. That's how it works if you access the php file directly, and that's how it works if you access the file with an EventSource. So in order to make your php script send multiple messages, you need a loop.
<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
function sendMsg($id, $msg) {
echo "id: $id" . PHP_EOL;
echo "data: $msg" . PHP_EOL;
echo PHP_EOL;
ob_flush();
flush();
}
while(true) {
$serverTime = time();
sendMsg($serverTime, 'server time: ' . date("h:i:s", time()));
sleep(1);
}
?>
I have altered your code to include an infinite loop that waits 1 second after every message sent (following an example found here: Using server-sent events).
This type of loop is what I'm currently using and it eliminated the constant connection drop and reconnect every 3 seconds. However (and I've only tested this in chrome), the connections are now only kept alive for 30 seconds. I will be continuing to figure out why this is the case and I'll post a solution when I find one, but until then this should at least get you closer to your goal.
Hope that helps,
Edit:
In order to keep the connection open for ridiculously long times with php, you need to set the max_execution_time (Thanks to tomfumb for this). This can be accomplished in at least three ways:
If you can alter your php.ini, change the value for "max_execution_time." This will allow all of your scripts to run for the time you specify though.
In the script you wish to run for a long time, use the function ini_set(key, value), where key is 'max_execution_time' and value is the time in seconds you wish your script to run for.
In the script you wish to run for a long time, use the function set_time_limit(n) where n is the number of seconds that you wish your script to run.
Server Sent Events are easy only when it comes to the Javascript part. First of all a lot of tutorials on SSE in the internet are closing their connections in the server part. Be it PHP or Java examples. This is really astonishing because what you get then is just a different way of implementing a "Ajax Polling" system with a strictly defined payload structure (and some minor features like client retry values set by server side). You can easily implement that with a few lines of jQuery. No need for SSE then.
According to the spec of SSE, i would say that the retry shouldnt be the normal way of implementing a client side loop. For me SSE is a one way streaming method which relies on a server backend which does not close the connection after pushing the first data to the client.
In Java its useful to use Servlet3 Async spec in order to free the request thread immediately and do the processing / streaming in a different thread. This works so far but still i dont like the 30 seconds connection lifetime for the EventSource request. Even i am pushing data every 5 seconds, the connection will be terminated after 30 seconds (chrome, firefox). Of course SSE will reconnect per default after 3 seconds but still i dont think this is the way it should be.
One problem is that some Java MVC frameworks dont have the ability to keep the connection open after data sending, so that you end up coding to the bare Servlet API. After on 24hours on coding prototypes in Java, i am more or less dissapointed because the gain over a traditional jQuery-Ajax-loop is not THAT much. And the problem with polyfilling the SSE feature is also existant.
The problem is not a server side issue, this all happens on the client and is part of the spec (I know it sounds weird).
http://dev.w3.org/html5/eventsource/
"When a user agent is to reestablish the connection, the user agent must run the following steps. These steps are run asynchronously, not as part of a task. (The tasks that it queues, of course, are run like normal tasks and not asynchronously.)"
Queue a task to run the following steps:
If the readyState attribute is set to CLOSED, abort the task.
Set the readyState attribute to CONNECTING.
Fire a simple event named error at the EventSource object.
I can't see any need to have an error here, so I have modified your Init function to filter out the error event fired whilst connecting.
function init() {
var CONNECTING = 0;
var source;
if (!!window.EventSource) {
source = new EventSource('events.php');
source.addEventListener('message', function (e) {
document.getElementById('output').innerHTML += e.data + '';
}, false);
source.addEventListener('open', function (e) {
document.getElementById('output').innerHTML += 'connection opened';
}, false);
source.addEventListener('error', function (e) {
if (source.readyState != CONNECTING) {
document.getElementById('output').innerHTML += 'error';
}
}, false);
}
else {
alert("Browser doesn't support Server-Sent Events");
}
}
There is no actual issue with the code, that I can see. The answer selected as correct, is then, incorrect.
This sums up the behavior mentioned in the question (http://www.w3.org/TR/2009/WD-html5-20090212/comms.html):
"If such a resource (with the correct MIME type) completes loading (i.e. the entire HTTP response body is received or the connection itself closes), the user agent should request the event source resource again after a delay equal to the reconnection time of the event source. This doesn't apply for the error cases that are listed below."
The problem lies with the stream. I've successfully kept a single EventStream open before in perl; just send the appropriate HTTP headers, and start sending stream data; never shutdown the stream server side. The issue is that it seems most HTTP libraries attempt to close the stream after its been opened. This will cause the client to attempt to reconnect to the server, which is fully standard compliant.
This means that it will appear that the problem is solved by running a while loop, for a couple of reasons:
A) The code will continue to send data, as if it were pushing out a large file
B) The code (php server) will never have the chance to attempt to close the connection
However, the problem here is obvious: to keep the stream alive, a constant stream of data must be sent. This results in wasteful utilization of resources, and negates any benefits the SSE stream is supposed to provide.
I'm not enough of a php guru to know, but I'd imagine that something in the php server/later in the code is prematurely closing the stream; I had to manipulate the stream at Socket level with Perl to keep it open, since HTTP::Response was closing the connection, and causing the client browser to attempt to re-open the connection. In Mojolicious (another Perl web framework), this can be done by opening a Stream object and setting the timeout to zero, so that the stream never times out.
So, the proper solution here is not to use a while loop; it is to call the appropriate php functions for opening, and keeping open, a php stream.
I was able to do it by implementing a custom event loop. It seems that this html5 feature is not ready at all and has compatibility issues even with the latest version of google chrome. Here it is, working on firefox (can't get the message sent correctly on chrome) :
var source;
function Body_Load(event) {
loopEvent();
}
function loopEvent() {
if (source == undefined) {
source = new EventSource("event/message.php");
}
source.onmessage = function(event) {
_e("out").value = event.data;
loopEvent();
}
}
P.S. : _e is a function that calls document.getElementById(id);
According to the Spec, the 3 second reconnection is by design when the connection is closed. PHP with a loop should theoretically stop this but the PHP script will be running indefinitely and wasting resources. You should try to avoid using apache and php for SSE because of this issue.
The standard http response should close a connection once the response is sent. You can change this with the header "connection: keep-alive" which should tell the browser that the connection is meant to stay open although this can cause problems if you're using proxies.
node.js or something similar is a better engine to use for SSE rather than apache/php and since it's basically JavaScript, its pretty easy to get to grips with.
Server Sent Event as name suggests the data should be traveling from server to client if it has to reconnect every three seconds to retrieve data from server then it is no different than other polling mechanisms.The purpose of SSE is to alert client as soon as there is new data which client is unaware of.Since server closes connection even if header is keep-alive there is no other way than to run php script in infinite loop but with considerable thread sleep to prevent burden on server.Till now i don't see any other way out and its better than spamming server every 3 seconds for new data.
I'm trying the same thing. With varying degrees of success.
Had the same problem with Firefox, running the same js code as mentioned.
Using the Nginx server and some PHP that exited(ie no continual loop), I could get messages back to a "Request" from firefox only once the PHP exited.
Run the PHP as a script in PHP.exe and all is good on the concole, stings are printed when flushed. However, Nginx doesn't send the data until the PHP has completed. Tried adding extra \r\n\r\n and flush() or ob_flush() did not help.
There is no pushing of data, as shown in Wireshark logs, just a delayed response packet to the GET.
Read that I need a "push" module for Nginx that requires a re-build from source.
So this is definitely an Nginx problem.
Using a socket in 'C' I was able to push data to Firefox as expected, and the socket was kept open, and no messages were missed. However this has the disadvantage that I need to server the page.html and the events/stream from the same socket or firefox will not connect due to Cross Site Url problems. There are some ways around this in certain situations, but not for a iframe in a menu system. This approach did prove the point that the SSE does work with firefox and there are pushed packets in the wireshark log. Where option 1 only had request/reply packets.
All this said, I still don't have a solution. I've tried to remove the buffering on the PHP and Nginx. But still nothing until PHP finishes. Tried different header options, eg chunks didn't help either.
I don't feel like writing a full blown http server in 'C' but this seems to be the only option that is working for me at the moment.
I'm about to try Apache, but most write ups suggest that this is worse than Nginx at this job.