was trying to encapsulate a partial view to show feedback that i can push back to the client.
This Article shows a method of pushing back data using HTML5 Server-Sent events (SSE).
I noticed that if i opened up several browser tabs and then closed one i got exceptions as the logic didn't remove the respective stream from the ConcurrentQueue. I amended the code as below
private static void TimerCallback(object state)
{
StreamWriter data;
Random randNum = new Random();
// foreach (var data in _streammessage)
for (int x = 0; x < _streammessage.Count; x++)
{
_streammessage.TryDequeue(out data);
data.WriteLine("data:" + randNum.Next(30, 100) + "\n");
try
{
data.Flush();
_streammessage.Enqueue(data);
}
catch (Exception ex)
{
// dont re-add the stream as an error ocurred presumable the client has lost connection
}
}
//To set timer with random interval
_timer.Value.Change(TimeSpan.FromMilliseconds(randNum.Next(1, 3) * 500), TimeSpan.FromMilliseconds(-1));
}
I also had to amend the OnStreamAvailable member as the framework syntax had changed to the second parameter being a HttpContent rather than HttpContentHeaders
public static void OnStreamAvailable(Stream stream, HttpContent headers, TransportContext context)
The problem now is i am still getting inconsistant behaviour if i add or remove clients i.e it times out when trying to initialise a new client. Does anyone have any ideas or more examples of using SSE with WinAPI and the correct "framework of methods" to handle disconnected clients
Cheers
Tim
This article is actually an adaptation of my original article from May - http://www.strathweb.com/2012/05/native-html5-push-notifications-with-asp-net-web-api-and-knockout-js/ (notice even variable names and port numbers are the same :-).
It is a very valid point that you are raising, and detecting a broken connection is something that's not very easy with this setup. The main reason is that while ASP.NET (the host) allows you to check a broken connection, there is no notification mechanism between ASP.NET (host) and Web API informing about that.
That is why in order to detect a broken connection (disconnected client) you should really try writing to the stream, and catch any error - this would mean the client has been disconnected.
I asked the same question to Brad Wilson/Marcin Dobosz/Damien Edwards at aspconf, and Damien suggested using HttpContext.Current.Response.IsClientConnected - so basically bypassing Web API and obtaining the connectivity info from the underlying host directly (however there is still a race condition involved anyway). That is really .NET 4. He also pointed an interesting way in which this problem could be avoided in .NET 4.5 using an async cancellation token. Frankly, I have never got around to test it, but perhaps this is something you should explore.
You can see their response to this problem in this video - http://channel9.msdn.com/Events/aspConf/aspConf/Ask-The-Experts - fast forward to 48:00
Related
I want my data access layer to handle exceptions thrown by DocumentDB API provided via Microsoft.Azure.Documents.Client.DocumentClient class. For example, the optimistic concurrency check implemented using AccessCondition class, but others as well.
By looking at the exception thrown, the best way to recognize different DocumentClient-specific exceptions seems to be something like this:
try { ... }
catch (DocumentClientException exception)
when (exception.Error.Code == "Some magic here")
{
//let the user know how to recover from this..
}
I don't like such magic strings as they are not verifiable compile-time. It may contract a typo, or it may change on random moment with DocumentDB client/server changes, etc. Also, it is not clear which such magic codes I could/should be handling since I don't see the Microsoft.Azure.DocumentDB .net API containing any ErrorCodes enum or constants, nor find any list in documentation.
Where can I find a list of possible Error.Code values DocumentClient API can throw?
To make it even more confusing, the XmlDoc for DocumentClient.CreateDocumentAsync method suggest working instead on http status codes.
UPDATE: This question is not about Http status codes but DocumentClientException.Error.Code field as I assume the latter is more precise.
Where can I find a list of possible error codes values DocumentClient API can throw?
It's hard to find the completely list of error code that DocumentClinet API throw. The exception is depend on what your request.
For example, the optimistic concurrency check
Azure Cosmos DB uses ETags for handling optimistic concurrency.
When we retrieve a document from Azure Cosmos DB, it always contains an ETag property as apart of our document.
When we then want to send our request to replace a document, we can specify an AccessCondition with the ETag we received when we fetched out our document.
If the ETag we send is not current, the server will return a 412 Precondition Failed status code. In our .NET SDK, this is wrapped up in a DocumentClientException.
Here is an example that show the possible problems when the concurrency occurred.
By decompile the version 1.22.0 client, the code is set as a HttpStatusCode enum. I think all the possible values can be found here https://msdn.microsoft.com/en-us/library/system.net.httpstatuscode(v=vs.110).aspx then.
However, what really contains richer information for debug is the Error.Message. Might need to decompile the whole library to figure out, or wait for Microsoft to release the source codes, which is unlikely to happen since the latest update in github was 2 or 3 years ago.
public Error Error
{
get
{
if (this.error == null)
{
this.error = new Error()
{
Code = this.StatusCode.ToString(),
Message = this.Message
};
}
return this.error;
}
}
There is a list of the HTTP Status Codes for Azure Cosmos DB
I use the following code in my catch blocks
catch (DocumentClientException e)
{
var resp = new HttpResponseMessage
{
StatusCode = (HttpStatusCode) e.StatusCode,
Content = new StringContent(e.Message)
};
return resp;
}
Letting the user know how to handle the exception should be done on the client application.
I have a Sparkjava app which I have deployed on a Tomcat server. It uses SQL2O to interface with the MySQL-database. After some time I start to have trouble connecting to the database. I've tried connecting directly from SQL2O, connecting through HikariCP and connecting through JNDI. They all work for about a day, before I start getting Communications link failure. This app gets hit a handful of times a day at best, so performance is a complete non issue. I want to configure the app to use one database connection per request. How do I go about that?
The app doesn't come online again afterwards until I redeploy it (overwrite ROOT.war again). Restarting tomcat or the entire server does nothing.
Currently every request creates a new Sql2o object and executes the query using withConnection. I'd be highly surprised if I was leaking any connections.
Here's some example code (simplified).
public class UserRepositry {
static {
try {
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
protected Sql2o sql2o = new Sql2o("jdbc:mysql://mysql.server.name/dbname?serverTimezone=UTC", "username", "password");
public List<Users> getUsers() {
return sql2o.withConnection((c, o) -> {
return c.createQuery(
"SELECT\n" +
" id,\n" +
" name\n" +
"FROM users"
)
.executeAndFetch(User.class);
});
}
}
public class Main {
public static void main(String[] args) {
val gson = new Gson();
port(8080);
get("/users", (req, res) -> {
return new UserRepository().getUsers();
}, gson::toJson);
}
}
If you rely on Tomcat to provide the connection to you: It's coming from a pool. Just go with plain old JDBC and open that connection yourself (and make sure to close it as well) if you don't like that.
So much for the answer to your question, to the letter. Now for the spirit: There's nothing wrong with connections coming from a pool. In all cases, it's your responsibility to handle it properly: Get access to a connection and free it up (close) when you're done with it. It doesn't make a difference if the connection is coming from a pool or has been created manually.
As you say performance is not an issue: Note that the creation of a connection may take some time, so even if the computer is largely idle, creating a new connection per request may have a notable effect on the performance. Your server won't overheat, but it might add a second or two to the request turnaround time.
Check configurations for your pool - e.g. validationQuery (to detect communication failures) or limits for use per connection. And make sure that you don't run into those issues because of bugs in your code. You'll need to handle communication errors anyways. And, again, that handling doesn't differ whether you use pools or not.
Edit: And finally: Are you extra extra sure that there indeed is no communication link failure? Like: Database or router unplugged every night to connect the vacuum cleaner? (no pun intended), Firewall dropping/resetting connections etc?
I have a Line-of-Business (LoB) Windows 8.1 Store application I developed for a client. The client side-loads it on several Windows 10 tablets. They use it in an environment where WiFi is spotty at best and they would like to get some sort of notification inside the app, regardless of what page they are on, notification that will let them know that they've lost connectivity to the network. I have created a method on my Web API that is not hitting the repository (database). Instead, it quickly returns some static information regarding my Web API, such as version, date and time of the invocation and some trademark stuff that I'm required to return. I thought of calling this method at precise intervals of time and when there's no response, assume that the Web API connectivity is lost. In my main page, the first one displayed when the application is started, I have the following stuff in the constructor of my view model:
_webApiStatusTimer = new DispatcherTimer();
_webApiStatusTimer.Tick += OnCheckWebApiStatusEvent;
_webApiStatusTimer.Interval = new TimeSpan(0, 0, 30);
_webApiStatusTimer.Start();
Then, the event handler is implemented like this:
private async void OnCheckWebApiStatusEvent(object sender, object e)
{
// stop the timer
_webApiStatusTimer.Stop();
// refresh the search
var webApiInfo = await _webApiClient.GetWebApiInfo();
// add all returned records in the list
if (webApiInfo == null)
{
var messageDialog = new MessageDialog(#"The application has lost connection with the back-end Web API!");
await messageDialog.ShowAsync();
// restart the timer
_webApiStatusTimer.Start();
}
}
When the Web API connection is lost, I get a nice popup message that informs me that the Web API is no longer available. The problem I have is that after a while, especially if I navigate away from the first page but not necessary, I get an UnauthorizedAccessException in my application.
I use the DispatcherTimer since my understanding is that this is compatible with
UI threads, but obviously, I still do something wrong. Anyone cares to set me on the right path?
Also, if you did something similar and found a much better approach, I'd love to hear about your solution.
Thanks in advance,
Eddie
First, If you are using Windows Store Apps, then you could possibly use a Background task to check poll for the status of the web api instead of putting this responsibility on your view model, its not the viewmodels concern
Second, if you are connecting from your Windows store app to your API then one successful authentication/ authorization for the first time, how and where do you store the token (assuming you are using token authentication). If you are (and ideally you should), is there a timer that you start which is set to the token expiration time? Is your local storage getting flushed somehow and loosing the aurthorization data?
Need more information.
i am trying to implement a Server Sent Events (SSE) webpage which is powered by Spring. My test code does the following:
Browser uses EventSource(url) to connect to server. Spring accepts the request with the following controller code:
#RequestMapping(value="myurl", method = RequestMethod.GET, produces = "text/event-stream")
#ResponseBody
public DeferredResult<String> subscribe() throws Exception {
final DeferredResult<String> deferredResult = new DeferredResult<>();
resultList.add(deferredResult);
deferredResult.onCompletion(() -> {
logTimer.info("deferedResult "+deferredResult+" completion");
resultList.remove(deferredResult);
});
return deferredResult;
}
So mainly it puts the DeferredResult in a List and register a completion callback so that i can remove this thing from the List in case of completion.
Now i have a timer method, that will periodically output current timestamp to all registered "browser" via their DeferredResults.
#Scheduled(fixedRate=10000)
public void processQueues() {
Date d = new Date();
log.info("outputting to "+ LoginController.resultList.size()+ " connections");
LoginController.resultList.forEach(deferredResult -> deferredResult.setResult("data: "+d.getTime()+"\n\n"));
}
The data is sent to the browser and the following client code works:
var source = new EventSource('/myurl');
source.addEventListener('message', function (e) {
console.log(e.data);
$("#content").append(e.data).append("<br>");
});
Now the problem:
The completion callback on the DeferredResult is called on every setResult() call in the timer thread. So for some reason the connection is closed after the setResult() call. SSE in the browser reconnects as per spec and then same thing again. So on client side i have a polling behavior, but i want an kept open request where i can push data on the same DeferredResult over and over again.
Do i miss something here? Is DeferredResult not capable of sending multiple results? i put in a 10 seconds delay in the timer thread to see if the request only terminates after setResult(). So in the browser the request is kept open until the timer pushes the data but then its closed.
Thanks for any hint on that. One more note: I added async-supported to all filters/servlets in tomcat.
Indeed DeferredResult can be set only once (notice that setResult returns a boolean). It completes processing with the full range of Spring MVC processing options, i.e. meaning that all you know about what happens during a Spring MVC request remains more or less the same, except for the asynchronously produced return value.
What you need for SSE is something more focused, i.e. write each value to the response using an HttpMessageConverter. I've created a ticket for that https://jira.spring.io/browse/SPR-12212.
Note that Spring's SockJS support does have an SSE transport which takes care of a few extras such as cross-domain requests with cookies (important for IE). It's also used on top of a WebSocket API and WebSocket-style messaging (even if WebSocket is not available on either the client or the server side) which fully abstracts the details of HTTP long polling.
As a workaround you can also write directly to the Servlet response using an HttpMessageConverter.
I am at a complete loss as to why I am experiencing this problem. I am new to NServiceBus and have so far set up a dead simple 'server' which listens for messages sent by a web application. The server asks for custom initialisation (IWantCustomInitialization) and uses a custom builder for Castle Windsor 2.5.1. This custom builder is basically a copy of the one that comes with the NServiceBus source code, with two minor changes to move away from methods deprecated in Windsor 2.5.
Note that my code shares the container instance with NServiceBus.
The problem I experience is that every message sent by the web application is processed five (5) times by the server. The log files have five entries for each attempt, with the fifth attempt looking like this:
2011-03-28 16:04:10,326 [Worker.8] DEBUG NServiceBus.Unicast.UnicastBus [] - Calling 'HandleEndMessage' on NServiceBus.SagaPersisters.NHibernate.NHibernateMessageModule
2011-03-28 16:04:10,327 [Worker.8] DEBUG NServiceBus.Unicast.UnicastBus [] - Calling 'HandleEndMessage' on Server.NHibernateSessionMessageModule
2011-03-28 16:04:10,341 [Worker.8] DEBUG NServiceBus.Unicast.UnicastBus [] - Calling 'HandleError' on NServiceBus.SagaPersisters.NHibernate.NHibernateMessageModule
2011-03-28 16:04:10,342 [Worker.8] DEBUG NServiceBus.Unicast.UnicastBus [] - Calling 'HandleError' on Server.NHibernateSessionMessageModule
2011-03-28 16:04:10,344 [Worker.8] ERROR NServiceBus.Unicast.Transport.Msmq.MsmqTransport [] - Message has failed the maximum number of times allowed, ID=80cffd98-a5bd-43e0-a482-a2d96ca42b22\20677.
I have no indication why the message fails, and I don't know where to dig for more information/output.
The configuration 'endpoint' looks like this:
public void Init()
{
container = Windsor.Container;
NServiceBus.Configure.With().CastleWindsor251Builder(container).XmlSerializer().MsmqTransport().IsolationLevel(System.Transactions.IsolationLevel.Unspecified);
var masterInstaller = new NotificationServerInstaller();
masterInstaller.Install(container, null);
}
The message handler is, at this stage, really contrived, and looks like this:
public class NewUserMessageHandler : IHandleMessages<NotifyNewUserMessage>
{
private readonly IGetUserQuery _getUserQuery;
public NewUserMessageHandler(IGetUserQuery getUserQuery)
{
_getUserQuery = getUserQuery;
}
public void Handle(NotifyNewUserMessage message)
{
var result = _getUserQuery.Invoke(new GetUserRequest { Id = new Guid("C10D0684-D25F-4E5E-A347-16F85DB7BFBF") });
Console.WriteLine("New message received: {0}", message.UserSystemId);
}
}
If the first line in the handler method is commented out, the message is processed only once.
I have found some posts/threads on the web (including StackOverflow) which talk about similar issues, notably http://tech.groups.yahoo.com/group/nservicebus/message/5977 and Anyone using Ninject 2.0 as the nServiceBus ObjectBuilder? - but I haven't had any success in making my problem go away.
I'd be most obliged for any help. I'm a n00b at NServiceBus!
NServiceBus isn't handling it multiple times by default it will retry 5 times if an exception occurs, you can set this in a config file. Have you got distributed transactions turned on? Because you are committing to a database and you have an open transaction (the queue transaction) when you open another transaction it will try and upgrade it to a distributed transaction, I think that may be the issue. Have you run with the console app? You should see some out put on there.
I would recommend wrapping the body of the Handle method in a try/catch and add a break point to the catch and see what is wrong.
Once you work it out, remove the try/catch.