THis is an interesting question. I am developing a web-chat software piece and for the past couple of hours I've been trying to figure out why this happens. Basically, I add an actual chat object (the part that does communications) to the Cache collection when you start chatting. In order to detect that you closed the window, I set the sliding expiration to say 10-30 seconds. I also set the callback to let the chat client know that he needs to disconnect to end the chat session. For some odd reason, when I use the code to dispose of the chat client, whatever it is, it causes the entire w3svc process to crash (event log checked). I also tried just sending myself an email when the item is removed, which worked. I even tried to put the entire code in try-catch block but it seems to ignore that as well. Any ideas? O_o
UPD: No, i am not trying to refresh the object (in reference to this).
Adding:
HttpContext.Current.Cache.Insert("ChatClient_" + targetCid + HttpContext.Current.Session.SessionID, cl, null, Cache.NoAbsoluteExpiration,
TimeSpan.FromSeconds(15), CacheItemPriority.Normal, new CacheItemRemovedCallback(removeMyself));
Removing:
public static void removeMyself(string key, Object value, CacheItemRemovedReason reason) {
var wc = (WebClient)value;
try {
wc.Remove();
}
catch { }
}
I am in fact using the lock on HttpContext.Current.cache when adding to the cache objects.
Can you post both the cache.insert and item removed callbacks code? Are you using any kind of locking when inserting into the cache? Have you done anything to the default settings for the ASP.net cache? Are you able to reproduce this on another web server? Are you sure you are expiring the cache in ms instead of seconds...
Is your sliding expiration like this? TimeSpan.FromSeconds(30)
Related
I developing a Xamarin application, and I communicating an external custom device. My problem is very strange, firstly the application starting, and connecting automatically to device, so everything is fine. When i suddenly remove the battery from the external device, the bluetooth connection is broken, and it's working fine to, but when I turn on the external device again, my Xamarin application connecting to it very well well, but the subscriptions not working anymore.
I debugged it, but not calling anymore. I think the unsubscribe/subscribe process is wrong.
...
if (ble.GetConnectionStatus())
{
Device.BeginInvokeOnMainThread(() =>
{
...
ble.Adapter.DeviceConnectionLost -= Adapter_DeviceConnectionLost;
ble.Adapter.DeviceConnectionLost += Adapter_DeviceConnectionLost;
ble.PropertyChanged -= Ble_PropertyChanged;
ble.PropertyChanged += Ble_PropertyChanged;
data.PropertyChanged -= data_PropertyChanged;
data.PropertyChanged += data_PropertyChanged;
...
});
...
So it's so strange, because first time this working, when starting the app, but when I call it after reconnect that same subscription not working. So if its wrong, then why working this at very first time?
I have no error, just not fire the functions again after resubscribe.
So as you see, I need to "refresh" the subscription. Is there another way to solve this problem?
If that "button to recreate everything" works, then I see two alternatives.
Option 1:
Have such a button, so that user can manually "fix" the situation.
PRO: Gives the user a solution that is guaranteed to work.
CON: Requires user intervention.
Option 2:
Have a periodic timer, that decides whether/when to forcibly "fix" the situation.
PRO: Automatically recovers.
CON: Risks losing data, if forces a recovery at the same time data is arriving.
In pseudo-code, option 2 might be something like this:
// pseudo-code
static Timer timer = ..start a timer that has an event every 10 seconds.
OnTimerElapsed:
if (!eventSeenRecently)
ForceReset();
eventSeenRecently = false;
..whereever you receive data..
if (..has data..)
eventSeenRecently = true;
The concept is that you keep track of whether data continues to be received. If the device stops sending you information (but you believe it should be), then you "ForceReset" - whatever is needed to get everything going again.
DeviceConnectionLost should also set some flag, that you use to ForceReset when the device "comes back".
// pseudo-code
DeviceConnectionLost:
resetNeeded = true;
OnTimerElapsed:
if (resetNeeded && ..test that device is available again..) {
ForceReset();
resetNeeded = false;
}
Perhaps this custom device has some option or info that can help.
For example, there might be a way to query some id or other info, so you can discover that the device is now "different", in a way that requires the reset. Then the timer does that query, and uses that info to decide to reset.
I am creating a Windows service in C#. Its purpose is to consume info from a feed on the Internet. I get the data by using zeromq's pub/sub architecture (my service is a subscriber only). To debug the service I "host" it in a WPF control panel. This allows me to start, run, and stop the service without having to install it. The problem I am seeing is that when I call my stop method it appears as though the service continues to write to the database. I know this because I put a Debug.WriteLine() where the writing occurs.
More info on the service:
I am attempting to construct my service in a fashion that allows it to write to the database asynchronously. This is accomplished by using a combination of threads and the ThreadPool.
public void StartDataReceiver() // Entry point to service from WPF host
{
// setup zmq subscriber socket
receiverThread = new Tread(SpawnReceivers);
receiverThread.Start();
}
internal void SpawnReceivers()
{
while(!stopEvent.WaitOne(0))
{
ThreadPool.QueueUserWorkItem(new WaitCallback(ProcessReceivedData), subscriber.Recv()); // subscriber.Recv() blocks when there is no data to receive (according to the zmq docs) so this loop should remain under control, and threads only created in the pool when there is data to process.
}
}
internal void ProcessReceivedData(Object recvdData)
{
// cast recvdData from object -> byte[]
// convert byte[] -> JSON string
// deserialize JSON -> MyData
using (MyDataEntities context = new MyDataEntities())
{
// build up EF model object
Debug.WriteLine("Write obj to db...");
context.MyDatas.Add(myEFModel);
context.SaveChanges();
}
}
internal void QData(Object recvdData)
{
Debug.WriteLine("Queued obj in queue...");
q.Enqueue((byte[])recvdData);
}
public void StopDataReceiver()
{
stopEvent.Set();
receiverThread.Join();
subscriber.Dispose();
zmqContext.Dispose();
stopEvent.Reset();
}
The above code are the methods that I am concerned with. When I debug the WPF host, and the method ProcessReceivedData is set to be queued in the thread pool everything seems to work as expected, until I stop the service by calling StopDataReceiver. As far as I can tell the thread pool never queues any more threads (I checked this by placing a break point on that line), but I continue to see "Write obj to db..." in the output window and when I 'Break All' in the debugger a little green arrow appears on the context.SaveChanges(); line indicating that is where execution is currently halted. When I test some more, and have the thread pool queue up the method QData everything seems to work as expected. I see "Queued obj in queue..." messages in the output window until I stop the service. Once I do no more messages in the output window.
TL;DR:
I don't know how to determine if the Entity Framework is just slowing things way down and the messages I am seeing are just the thread pool clearing its backlog of work items, or if there is something larger at play. How do I go about solving something like this?
Would a better solution be to queue the incoming JSON strings as byte[] like I do in the QData method then have the thread pool queue up a different method to work on clearing the queue. I feel that that solution will only shift the problem around and not actually solve it.
Could another solution be to write a new service dedicated to clearing that queue? The problem I see with writing another service would be that I would probably have to use WCF (or possibly zmq) to communicate between the two services which would obviously add overhead and possibly become less performant.
I see the critical section in all of this being the part of getting the data off the wire fast enough because the publisher I am subscribed to is set to begin discarding messages if my subscriber can't keep up.
I am working on an assignment in asp.net to send notification email to users at specific intervals.
But the problem is that since the server is not privately owned i cannot implement a windows service on it.
Any ideas?
There's no reliable way to achieve that. If you cannot install a Windows Service on the host you could write a endpoint (.aspx or .ashx) that will send the email and then purchase on some other site a service which will ping this endpoint at regular intervals by sending it HTTP request. Obviously you should configure this endpoint to be accessible only from the IP address of the provider you purchase the service from, otherwise anyone could send an HTTP request to the endpoint and trigger the process which is probably undesirable.
Further reading: The Dangers of Implementing Recurring Background Tasks In ASP.NET.
There are several ways to get code executing on an interval that don't require a windows service.
One option is to use the Cache class - use one of the Insert overloads that takes a CacheItemRemovedCallback - this will be called when the cache item is removed. You can re-add the cache item with this callback again and again...
Though, the first thing you need to do is contact the hosting company and find out if they already have some sort of solution for you.
You could set up a scheduled task on the server to invoke a program with the desired action.
You can always use a System.Timer and create a call at specific intervals. What you need to be careful is that this must be run one time, eg on application start, but if you have more than one pools, then it may run more times, and you also need to access some database to read the data of your actions.
using System.Timers;
var oTimer = new Timer();
oTimer.Interval = 30000; // 30 second
oTimer.Elapsed += new ElapsedEventHandler(MyThreadFun);
oTimer.Start();
private static void MyThreadFun(object sender, ElapsedEventArgs e)
{
// inside here you read your query from the database
// get the next email that must be send,
// you send them, and mark them as send, log the errors and done.
}
why I select system timer:
http://msdn.microsoft.com/en-us/magazine/cc164015.aspx
more words
I use this in a more complex class and its work fine. What are the points that I have also made.
Signaling the application stop, to wait for the timer to end.
Use mutex and database for synchronize the works.
Easiest solution is to exploit global.asax application events
On application startup event, create a thread (or task) into a static singleton variable in the global class.
The thread/task/workitem will have an endless loop while(true) {...} with your "service like" code inside.
You'll also want to put a Thread.Sleep(60000) in the loop so it doesn't eat unnecessary CPU cycles.
static void FakeService(object obj) {
while(true) {
try {
// - get a list of users to send emails to
// - check the current time and compare it to the interval to send a new email
// - send emails
// - update the last_email_sent time for the users
} catch (Exception ex) {
// - log any exceptions
// - choose to keep the loop (fake service) running or end it (return)
}
Thread.Sleep(60000); //run the code in this loop every ~60 seconds
}
}
EDIT Because your task is more or less a simple timer job any of the ACID type concerns from an app pool reset or other error don't really apply, because it can just start up again and keep trucking along with any data corruption. But you could also use the thread to simply execute a request to an aspx or ashx that would hold your logic.
new WebClient().DownloadString("http://localhost/EmailJob.aspx");
I'm working on a Web app using ASP.NET. I have a class called "Sistema" that uses the Singleton pattern.
When the instance of Sistema is created, the database connection is opened and a process runs that loads some static information for later use. This lasts almost 2 minutes.
private static Sistema instance;
private Sistema()
{
OpenDataBase();
LoadStaticInformation();
}
public static Sistema GetInstance()
{
if (instance == null)
{
instance = new Sistema();
}
return instance;
}
The reason why I keep the connection to the database open is because I'm using db4o, that strongly suggests this. Here are some references:
db4o best practice to query objects from db
Is it OK to open a DB4o file for query, insert, update multiple times?
Query regarding database connectivity in db4o
On my Web App I have a Master Page that controls if the user is logged in by checking a Session variable. If this Session is null, then the user is sent to the Login Page.
On the Login Page, the first thing I do is to check if the instance of "Sistema" is null. If it is, then when the user hits the Submit button, a message is shown saying "Login can take up to two minutes. Please wait". If it is not null, then no message is shown as the login action takes only a couple of seconds.
I have been told by the users, that when going through the System, they are sometimes sent back to the login page, and when they try to login, the message saying "Login can take up to two minutes" is displayed and login indeed takes a while.
The fact that they are sent back to the login page means that the Session variable is lost, and the message being displayed means that the instance of "Sistema" is also null.
In order to determine why this is happening, I created a web page that sends an email to me when the instance of Sistema detected to be null. I thought that if I was able to know when this occurred, I might discover what is going on.
This web page is really simple. It runs every 10 minutes and checks if the instance of Sistema is null. If it is, then an email is sent and the instance of Sistema is created.
bool isInstanceNull = Sistema.IsInstanceNull();
if (isInstanceNull)
{
String emailTo = "...";
String emailContent = "...";
Functions.SendMail(emailTo, "Sistema is null", emailContent, "");
Sistema.GetInstance();
Functions.SendMail(emailTo, "Sistema has been created", emailContent, "");
}
The only thing I discovered is that it's not happening at a specific time. For example, last week it happened around 7pm, but today it happened at 2 am.
Regarding the Session timeout, I'm using a solution in the code behind: http://www.beansoftware.com/ASP.NET-Tutorials/Keep-Session-Alive.aspx.
Any suggestions to why is this happening?
The application pool has a property that causes it to be automatically recycled every N minutes (defaults to 1740, or every 29 hours.) Make this zero to disable recycling. The propertry is (on IIS7) under the "Recycling" heading and is called "Regular Time Interval (minutes)"
Apart from that, you should always close connections immediately and dont use static connections at all in ASP.NET (when Connection-Pooling is enabled which is default).
I mention it because of:
private static Sistema instance;
private Sistema()
{
OpenDataBase();
LoadStaticInformation();
}
You should not keep a connection to the database open. Typically a new connection is opened for every request. Maybe the database sometimes decides it has too many users or the connection is open for too long, as a result it closes the connection and your object crashes. An open connection to the database is also a security risk.
I'm not 100% sure about this though...
You should probably move connecting to the database down to some kind of query execute method. Also it seems unwise to load such a blob of data all at once, can't you do it on the background or only load the information a user needs to see at the time?
Long post.. sorry
I've been reading up on this and tried back and forth with different solutions for a couple of days now but I can't find the most obvious choice for my predicament.
About my situation; I am presenting to the user a page that will contain a couple of different repeaters showing some info based on the result from a couple of webservice calls. I'd like to have the data brought in with an updatepanel (that would be querying the result table once per every two or three seconds until it found results) so I'd actually like to render the page and then when the data is "ready" it gets shown.
The page asks a controller for the info to render and the controller checks in a result table to see if there's anything to be found. If the specific data is not found it calls a method GetData() in WebServiceName.cs. GetData does not return anything but is supposed to start an async operation that gets the data from the webservice. The controller returns null and UpdatePanel waits for the next query.
When that operation is complete it'll store the data in it's relevant place in the db where the controller will find it the next time the page asks for it.
The solution I have in place now is to fire up another thread. I will host the page on a shared webserver and I don't know if this will cause any problems..
So the current code which resides on page.aspx:
Thread t = new Thread(new ThreadStart(CreateService));
t.Start();
}
void CreateService()
{
ServiceName serviceName = new ServiceName(user, "12345", "MOVING", "Apartment", "5100", "0", "72", "Bill", "rate_total", "1", "103", "serviceHost", "password");
}
At first I thought the solution was to use Begin[Method] and End[Method] but these don't seem to have been generated. I thought this seemed like a good solution so I was a little frustrated when they didn't show up.. is there a chance I might have missed a checkbox or something when adding the web references?
I do not want to use the [Method]Async since this stops the page from rendering until [Method]AsyncCompleted gets called from what I've understood.
The call I'm going to do is not CPU-intensive, I'm just waiting on a webService sitting on a slow server, so what I understood from this article: http://msdn.microsoft.com/en-us/magazine/cc164128.aspx making the threadpool bigger is not a choice as this will actually impair the performance instead (since I can't throw in a mountain of hardware).
What do you think is the best solution for my current situation? I don't really like the current one (only by gut feeling but anyway)
Thanks for reading this awfully long post..
Interesting. Until your question, I wasn't aware that VS changed from using Begin/End to Async/Completed when adding web references. I assumed that they would also include Begin/End, but apparently they did not.
You state "GetData does not return anything but is supposed to start an async operation that gets the data from the webservice," so I'm assuming that GetData actually blocks until the "async operation" completes. Otherwise, you could just call it synchronously.
Anyway, there are easy ways to get this working (asynchronous delegates, etc), but they consume a thread for each async operation, which doesn't scale.
You are correct that Async/Completed will block an asynchronous page. (side note: I believe that they will not block a synchronous page - but I've never tried that - so if you're using a non-async page, then you could try that). The method by which they "block" the asynchronous page is wrapped up in SynchronizationContext; in particular, each asynchronous page has a pending operation count which is incremented by Async and decremented after Completed.
You should be able to fake out this count (note: I haven't tried this either ;) ). Just substitute the default SynchronizationContext, which ignores the count:
var oldSyncContext = SynchronizationContext.Current;
try
{
SynchronizationContext.SetSynchronizationContext(new SynchronizationContext());
var serviceName = new ServiceName(..);
// Note: MyMethodCompleted will be invoked in a ThreadPool thread
// but WITHOUT an associated ASP.NET page, so some global state
// might be missing. Be careful with what code goes in there...
serviceName.MethodCompleted += MyMethodCompleted;
serviceName.MethodAsync(..);
}
finally
{
SynchronizationContext.SetSynchronizationContext(oldSyncContext);
}
I wrote a class that handles the temporary replacement of SynchronizationContext.Current as part of the Nito.Async library. Using that class simplifies the code to:
using (new ScopedSynchronizationContext(new SynchronizationContext()))
{
var serviceName = new ServiceName(..);
// Note: MyMethodCompleted will be invoked in a ThreadPool thread
// but WITHOUT an associated ASP.NET page, so some global state
// might be missing. Be careful with what code goes in there...
serviceName.MethodCompleted += MyMethodCompleted;
serviceName.MethodAsync(..);
}
This solution does not consume a thread that just waits for the operation to complete. It just registers a callback and keeps the connection open until the response arrives.
You can do this:
var action = new Action(CreateService);
action.BeginInvoke(action.EndInvoke, action);
or use ThreadPool.QueueUserWorkItem.
If using a Thread, make sure to set IsBackground=true.
There's a great post about fire and forget threads at http://consultingblogs.emc.com/jonathangeorge/archive/2009/09/10/make-methods-fire-and-forget-with-postsharp.aspx
try using below settings
[WebMethod]
[SoapDocumentMethod(OneWay = true)]
void MyAsyncMethod(parameters)
{
}
in your web service
but be careful if you use impersonation, we had problems on our side.
I'd encourage a different approach - one that doesn't use update panels. Update panels require an entire page to be loaded, and transferred over the wire - you only want the contents for a single control.
Consider doing a slightly more customized & optimized approach, using the MVC platform. Your data flow could look like:
Have the original request to your web page spawn a thread that goes out and warms your data.
Have a "skeleton" page returned to your client
In said page, have a javascript thread that calls your server asking for the data.
Using MVC, have a controller action that returns a partial view, which is limited to just the control you're interested in.
This will reduce your server load (can have a backoff algorithm), reduce the amount of info sent over the wire, and still give a great experience to the client.