I would like to use NSConnection/NSDistributedObject for interprocess communication. I would like the client to be able to handle the case where the server is only occasionally reachable.
How can I determine if sending a message to the NSConnection will fail or has failed? Currently if my server (the process that has vended the remote object) dies, the client will crash if it sends a selector to the remote object.
Ideally I'd like to have a wrapper for the remote object that can lazily instantiate (or reinstantiate) the connection, and return a default value in the case where the connection could not be instantiated, or the connection has failed. I don't really know the correct way to do this using objective c.
Here's some pseudocode representing this logic:
if myConnection is null:
instantiate myConnection
if MyConnection is null:
return defaultValue
try
return [myConnection someMethod]
catch
myConnection = null
return defaultValue
Unfortunately the only way to detect a connection failure is to use an exception handler, as there is no reliable way to "ask" a remote object if the connection is still valid. Thankfully, this is simple:
//get the distributed object
id <YourDOProtocol> remoteObject = (id <YourDOProtocol>)[NSConnection rootProxyForConnectionWithRegisteredName:#"YourRegisteredName" host:yourHost];
//call a method on the distributed object
#try
{
NSString* response = [remoteObject responseMethod];
//do something with response
}
#catch(NSException* e)
{
//the receiver is invalid, which occurs if the connection cannot be made
//handle error here
}
If your server is quiting gracefully then, I'm of the understanding, that it'll post an NSConnectionDidDieNotification as it's connection closes so you could register your client like this:
[[NSNotificationCenter defaultCenter] addObserver:self selector:#selector(connectionDidDie:) name:NSConnectionDidDieNotification object:remoteObject];
Perhaps your connectionDidDie:method could set a Boolean var that you can check prior to attempting sending the message.
Your DO could post a notification to say that it's started (although I think there are also system messages for that but I've only just started learning about DO's) and you could similarly register to be notified of it's startup.
I guess Rob's answer is a definite 'catch-all' and you wouldn't need to worry about the notification centre having not got through to the server in time.
I've been using the 'did die' notification it in my first DO app and I hope it helps you.
Todd.
Related
This is my first time posting on stackoverflow, and I'm aware of the strict posting requirements. Please let me know if I'm not following any of the guidelines.
I'm currently writing an IOS (8.4) application in Xcode, using Objective-C. The goal is to use MCSessions in order to stream data between IOS devices. I'm currently struggling with the concept of sessions, despite reading numerous posts here and elsewhere that attempt to clarify the topic. Here are the resources I'm already aware of:
https://developer.apple.com/videos/play/wwdc2013-708/
https://medium.com/#phoenixy/using-multipeer-connectivity-120abacb9db
Here's my current understanding: At the most basic level, you have an advertiser, and a browser. The advertiser has a local session, which allows them to "advertise". When the browser sees an advertiser, the browser sends an invite to the advertiser to his (the browser's) local MCSession. Assuming this is all correct, here's where I'm getting confused. The advertiser can accept the invite, and in the process, passes his local session to the invitationHandler.
I have implemented the following logic in code, as shown below. However, in tracing MCSession state changes for both the advertiser and browser, a connection is attempted, but the final state is always didNotNonnect.
Code for sending invitation (Browser):
[self.broadcasterBrowser invitePeer:[broadcasterPeerIDs objectAtIndex:indexPath.row]
toSession: self.appDelegate.mpcHandler.session withContext:nil timeout:30.0 ];
Code for accepting invitation (Advertiser):
- (void)advertiser:(MCNearbyServiceAdvertiser *)advertiser
didReceiveInvitationFromPeer:(MCPeerID *)peerID withContext:(NSData *)context invitationHandler:(void(^)(BOOL accept, MCSession *session))invitationHandler
{
ArrayInvitationHandler = [NSArray arrayWithObject:[invitationHandler copy]];
// ask the user
UIAlertView *alertView = [[UIAlertView alloc]
initWithTitle:peerID.displayName
message:#"Would like to create a session with you"
delegate:self
cancelButtonTitle:#"Decline"
otherButtonTitles:#"Accept", nil];
[alertView show];
if (alertViewResult)
{
void (^invitationHandler)(BOOL, MCSession *) = [ArrayInvitationHandler objectAtIndex:0];
invitationHandler(YES, self.appDelegate.mpcHandler.session);
}
}
Any help is greatly appreciated!
Austin
I ran into a similar problem trying to use MPC. I created a custom class to handle all of the MPC connectivity details. While testing though, every time my advertiser would accept the invite, it would complain about wrong connection data and fail. I discovered that the problem was that I was vending out the MCPeerID object for my device from a class variable I created as below:
static var peerObject : MCPeerID {
return MCPeerID(displayName: deviceNameString)
}
lazy var sessionIVar = MCSession (peer: MyConnectivityClass.peerObject)
func startAdvertisingForConnectivity () {
advertiserService = MCNearbyServiceAdvertiser (peer: MyConnectivityClass.peerObject, discoveryInfo: nil, serviceType: "my-multipeer-connectivity-service-identifier")
}
Then when I got an invitation I would initialize a MCSession object using the "peerObject" computed property and return it in the invitation handler, like this:
func advertiser(_ advertiser: MCNearbyServiceAdvertiser, didReceiveInvitationFromPeer peerID: MCPeerID, withContext context: Data?, invitationHandler: #escaping (Bool, MCSession?) -> Swift.Void) {
invitationHandler(true, sessionIVar)
}
I assumed that each time I called for "MyConnectivityClass.peerObject" it would give back an identical peerID because I was always initializing it with the same display name. It turns out that's not true. So when I was advertising I was using one peerID object and then when I was responding to the invitation, I was responding with a MCSession object that contained an entirely different peerID.
So the solution was to change the "MyConnectivityClass.peerObject" computed class property to a constant, or an Ivar, in my connection handler class. Like this:
let peerObject : MCPeerID = MCPeerID(displayName: deviceNameString)
Then the rest of the code just worked because no matter how many times I called for the MCPeerID object, it was always the same. Looking back I don't know why I started out with it the way I did. :-)
Then in my connectivity class I archived and stored the MCPeerID objects for both the browser and the advertiser so that I could have the advertiser automatically accept the invitation for trusted MCPeerIDs. That's not possible if you create the MCPeerID object each time you use it, even if you always initialize it with the same DisplayName.
Before you decide its a tl:dr (too long, didnt read) post try to read at least some, since Its a question broken down in a lot of small pieces. Some of which you can probably answer and help me.
Please try to help me as much as you can. These types of problems are very common on the internet and I think you will help me and much more people after me.
I am currently researching HTTP services and the protocol itself so that I can discover if it is useful to me.
I have some basic questions as well as some code that needs to be discussed.
First I would like to know how does the communication start? I have discovered that the client sends a message in which it requests a resource (is this correct?). Then what happens? I (as a server) have to reply with what?
Do I need to append a carriage return and a line feed after every response? Somewhere it says there even need to be two (\r\n\r\n).
How can an asynchronous writing be established? (I hope this question is understandable) My primary goal is to achieve a connection between a client and a server and then a continuous data stream from server to the client. Does the client need to reply for every message it gets?
I hope I made my questions clear, since I'm not an expert in these things (yet, I am very interested in it).
And for the programming part of my problem.
I have managed to put together a simple program in Qt in C++ (server side) and a simple client in Objective C (iOS). The client connects and I can read the request header. It is like this:
Data available, incoming: "GET / HTTP/1.1
Host: localhost:9990
Connection: close
User-Agent: CFStream%20test/1.0 CFNetwork/609 Darwin/12.2.0
Should I reply to this header manually? And if so, what?
The client side code looks like this (i know its not pseudo but i think its pretty self-explanatory):
- (void)setupStream
{
NSURL *url = [NSURL URLWithString:#"http://localhost:9990"];
CFHTTPMessageRef message = CFHTTPMessageCreateRequest(NULL, (CFStringRef)#"GET", (CFURLRef)url, kCFHTTPVersion1_1);
stream = CFReadStreamCreateForHTTPRequest(NULL, message);
CFRelease(message);
if (!CFReadStreamSetProperty(stream, kCFStreamPropertyHTTPShouldAutoredirect, kCFBooleanTrue))
{
NSLog(#"Some error.");
}
CFDictionaryRef proxySettings = CFNetworkCopySystemProxySettings();
CFReadStreamSetProperty(stream, kCFStreamPropertyHTTPProxy, proxySettings);
CFRelease(proxySettings);
if (!CFReadStreamOpen(stream))
{
CFRelease(stream);
NSLog(#"Error opening stream.");
}
CFStreamClientContext context = {0, self, NULL, NULL, NULL};
CFReadStreamSetClient(stream, kCFStreamEventHasBytesAvailable | kCFStreamEventErrorOccurred, readStreamCallback, &context);
CFReadStreamScheduleWithRunLoop(stream, CFRunLoopGetCurrent(), kCFRunLoopCommonModes);
NSLog(#"Done");
}
This is the setup stream method. The stream variable is a class variable of type CFReadStreamRef.
The callback looks like this:
static void readStreamCallback(CFReadStreamRef aStream, CFStreamEventType event, void *client)
{
ViewController *controller = (ViewController*)client;
[controller handleEvent:event forStream:aStream];
}
And the handle event like this:
- (void)handleEvent:(CFStreamEventType)event forStream:(CFReadStreamRef)aStream
{
if (aStream != stream)
{
return;
}
NSLog(#"Handle event callback");
switch (event)
{
case kCFStreamEventHasBytesAvailable:
NSLog(#"Work log");
UInt8 bytes[11];
CFIndex length;
length = CFReadStreamRead(stream, bytes, 11); //I know 11 bytes is hard coded, its in testing stage now. Feel free to suggest me how to do it better.
if (length == -1)
{
NSLog(#"Error, data length = -1");
return;
}
NSLog(#"Len: %li, data: %s", length, bytes);
break;
default:
NSLog(#"Other event");
break;
}
}
And thats practically all the client code that is worth mentioning. The Qt Server part (I will only post the important parts) is done like this: (this is a subclassed QTcpServer class). First the startServer(); is called:
bool Server::startServer()
{
if (!this->listen(QHostAddress::Any, 9990))
return false;
return true;
}
When there is a connection incoming the incomingConnection is fired off with the socket descriptor as a parameter:
void Server::incomingConnection(int handle)
{
qDebug("New client connected");
ServerClient *client = new ServerClient(handle, this); //The constructor takes in the socket descriptor needed to set up the socket and the parent (this)
client->setVectorLocation(clients.count()); //This is a int from a Qvector in which i append the clients, its not important for understanding right now.
connect(client, SIGNAL(clientDisconnected(int)), this, SLOT(clientDisconnected(int)), Qt::QueuedConnection); //When the client socket emits a disconnected signal the ServerClient class emits a client disconnected signal which the server uses to delete that client from the vector (thats why I use "setVectorLocation(int)") - not important right now
clients.push_back(client); //And then I append the client to the QVector - not important right now
}
The ClientServer class constructor just creates a new socket and connects the required methods:
ServerClient::ServerClient(int handle, QObject *parent) :
QObject(parent)
{
socket = new QTcpSocket(this); //Socket is a class variable
connect(socket, SIGNAL(disconnected()), this, SLOT(disconnected()));
connect(socket, SIGNAL(readyRead()), this, SLOT(readyRead()));
socket->setSocketDescriptor(handle);
}
Ready read just writes me the data incoming (it wont be much user later i think):
void ServerClient::readyRead()
{
qDebug() << "Data available, incoming: " << socket->readAll();
}
And finally the write data:
void ServerClient::writeData(QByteArray *data)
{
data->append("\r\n\r\n"); //I have read this must be appended to all outgoing data from a HTTP server
socket->write(*data);
socket->flush();
qDebug() << "Written data to client: " << *data;
}
This code however does not always work. Sometimes when I write message like "Message" the client recieves all the data and some things that shouldnt be there (the new line and a wierd symbol - can NSLog cause this?). Sometimes when I send "Hellow" the client only gets "Hel" and some other funky stuff.
What are the problems? What should I pay more attention about? Anything that will help me will be MUCH appreciated. And please dont paste in some links that contain a book with a few hundred pages, Im sure this can be solved just by explaining things to me.
THANKS A LOT!
Jan.
You asked many questions ... and that's a perfectly legitimate thing to do :)
I confess - it was too long, I didn't read :(
BUT ...
1) Yes, the HTTP protocol does expect na "CRLF" ("\r\n"). Many servers and many clients are "forgiving", but strictly speaking - yes, you need them.
REFERENCE: RFC 2616
2) Wanting to understand HTTP "internals" is also perfectly legitimate - I applaud you.
One good way is to read the RFC(s).
Another is to use a "telnet" client: http://blog.tonycode.com/tech-stuff/http-notes/making-http-requests-via-telnet
Yet another is to study requests and responses in FF Firebug
3) Socket programming is another issue - which explains why sometimes you might read "hello world", and other times you might just get "hel".
Strong recommendation: Beej's Guide to Network Programming
4) Finally, no way would I actually write a server in Qt with C++ (except maybe as a toy "science experiment", or for some really off-the-wall requirement)
I would definitely write server code in C# (for Windows servers), Java (for everything else) or a scripting language I felt comfortable with (Perl, Ruby/RoR, Python and Lua all come to mind).
IMHO .. and hope that helps!
Your questions pretty much amount to "how does HTTP work", and the full answer lies in the specification.
We have a simple wpf application that connects to a service running on the local machine. We use a named pipe for the connection and then register a callback so that later the service can send updates to the client.
The problem is that with each call of the callback we get a build up of memory in the client application.
This is how the client connects to the service.
const string url = "net.pipe://localhost/radal";
_channelFactory = new DuplexChannelFactory<IRadalService>(this, new NetNamedPipeBinding(),url);
and then in a threadpool thread we loop doing the following until we are connected
var service = _channelFactory.CreateChannel();
service.Register();
service.Register looks like this on the server side
public void Register()
{
_callback = OperationContext.Current.GetCallbackChannel<IRadalCallback>();
OperationContext.Current.Channel.Faulted += (sender, args) => Dispose();
OperationContext.Current.Channel.Closed += (sender, args) => Dispose();
}
This callback is stored and when new data arrives we invoke the following on the server side.
void Sensors_OnSensorReading(object sender, SensorReadingEventArgs e)
{
_callback.OnReadingReceived(e.SensorId, e.Count);
}
Where the parameters are an int and a double. On the client this is handled as follows.
public void OnReadingReceived(int sensorId, double count)
{
_events.Publish(new SensorReadingEvent(sensorId, count));
}
But we have found that commenting out _event.Publish... makes no difference to the memory usage. Does anyone see any logical reason why this might be leaking memory. We have used a profiler to track the problem to this point but cannot find what type of object is building up.
Well I can partially answer this now. The problem is partially caused by us trying to be clever and getting the connection to be opened on another thread and then passing it back to the main gui thread. The solution was to not use a thread but instead use a dispatch timer. It does have the downside that the initial data load is now on the GUI thread but we are not loading all that much anyway.
However this was not the entire solution (actually we don't have an entire solution). Once we moved over to a better profiler we found out that the objects building up were timeout handlers so we disabled that feature. That's OK for us as we are running against the localhost always but I can imagine for people working with remote services it would be an issue.
we have more than dozon of wcf services and being called using TCP binding. There are a lots of calls to same wcf service at various places in code.
AdminServiceClient client = FactoryS.AdminServiceClient();// it takes significant time. and
client.GetSomeThing(param1);
client.Close();
i want to cache the client or produce it from singleton. so that i can save some time, Is it possible?
Thx
Yes, this is possible. You can make the proxy object visible to the entire application, or wrap it in a singleton class for neatness (my preferred option). However, if you are going to reuse a proxy for a service, you will have to handle channel faults.
First create your singleton class / cache / global variable that holds an instance of the proxy (or proxies) that you want to reuse.
When you create the proxy, you need to subscribe to the Faulted event on the inner channel
proxyInstance.InnerChannel.Faulted += new EventHandler(ProxyFaulted);
and then put some reconnect code inside the ProxyFaulted event handler. The Faulted event will fire if the service drops, or the connection times out because it was idle. The faulted event will only fire if you have reliableSession enabled on your binding in the config file (if unspecified this defaults to enabled on the netTcpBinding).
Edit: If you don't want to keep your proxy channel open all the time, you will have to test the state of the channel before every time you use it, and recreate the proxy if it is faulted. Once the channel has faulted there is no option but to create a new one.
Edit2: The only real difference in load between keeping the channel open and closing it every time is a keep-alive packet being sent to the service and acknowledged every so often (which is what is behind your channel fault event). With 100 users I don't think this will be a problem.
The other option is to put your proxy creation inside a using block where it will be closed / disposed at the end of the block (which is considered bad practice). Closing the channel after a call may result in your application hanging because the service is not yet finished processing. In fact, even if your call to the service was async or the service contract for the method was one-way, the channel close code will block until the service is finished.
Here is a simple singleton class that should have the bare bones of what you need:
public static class SingletonProxy
{
private CupidClientServiceClient proxyInstance = null;
public CupidClientServiceClient ProxyInstance
{
get
{
if (proxyInstance == null)
{
AttemptToConnect();
}
return this.proxyInstance;
}
}
private void ProxyChannelFaulted(object sender, EventArgs e)
{
bool connected = false;
while (!connected)
{
// you may want to put timer code around this, or
// other code to limit the number of retrys if
// the connection keeps failing
AttemptToConnect();
}
}
public bool AttemptToConnect()
{
// this whole process needs to be thread safe
lock (proxyInstance)
{
try
{
if (proxyInstance != null)
{
// deregister the event handler from the old instance
proxyInstance.InnerChannel.Faulted -= new EventHandler(ProxyChannelFaulted);
}
//(re)create the instance
proxyInstance = new CupidClientServiceClient();
// always open the connection
proxyInstance.Open();
// add the event handler for the new instance
// the client faulted is needed to be inserted here (after the open)
// because we don't want the service instance to keep faulting (throwing faulted event)
// as soon as the open function call.
proxyInstance.InnerChannel.Faulted += new EventHandler(ProxyChannelFaulted);
return true;
}
catch (EndpointNotFoundException)
{
// do something here (log, show user message etc.)
return false;
}
catch (TimeoutException)
{
// do something here (log, show user message etc.)
return false;
}
}
}
}
I hope that helps :)
In my experience, creating/closing the channel on a per call basis adds very little overhead. Take a look at this Stackoverflow question. It's not a Singleton question per se, but related to your issue. Typically you don't want to leave the channel open once you're finished with it.
I would encourage you to use a reusable ChannelFactory implementation if you're not already and see if you still are having performance problems.
I am finding the doc for CFStreamCreatePairWithSocketToCFHost confusing:
Specifically, its not clear to me how the function can set the readStream pointer to null on error.
as far as I understand, the pointer is passed by value - so the function can only change the objected pointed to by the pointer.
Right now I can't figure out how to detect connection errors.
Relevant doc snippet:
Creates readable and writable streams connected to a given CFHost object.
void CFStreamCreatePairWithSocketToCFHost (
CFAllocatorRef alloc,
CFHostRef host,
SInt32 port,
CFReadStreamRef *readStream,
CFWriteStreamRef *writeStream
);
readStream
Upon return, contains a CFReadStream object connected to the host host on port port, or NULL if there is a failure during creation. If you pass NULL, the function will not create a readable stream. Ownership follows the Create Rule.
This is my connecting code, it goes all the way to NSLog(#"Connected") even when the server is down.
NSLog(#"Attempting to (re)connect to %#:%d", m_host, m_port);
while(TRUE)
{
CFHostRef host = CFHostCreateWithName(kCFAllocatorDefault, (CFStringRef)m_host);
if (!host)
{
NSLog(#"Error resolving host %#", m_host);
[NSThread sleepForTimeInterval:5.0];
continue;
}
CFStreamCreatePairWithSocketToCFHost(kCFAllocatorDefault, host , m_port, &m_in, &m_out);
CFRelease(host);
if (!m_in)
{
NSLog(#"Error");
}
CFStreamClientContext context = {0, self,nil,nil,nil};
if (CFReadStreamSetClient(m_in, kCFStreamEventHasBytesAvailable | kCFStreamEventErrorOccurred | kCFStreamEventEndEncountered, networkReadEvent, &context))
{
CFReadStreamScheduleWithRunLoop(m_in, CFRunLoopGetCurrent(),kCFRunLoopCommonModes);
}
if (CFWriteStreamSetClient(m_out, kCFStreamEventErrorOccurred | kCFStreamEventEndEncountered, networkWriteEvent, &context))
{
CFWriteStreamScheduleWithRunLoop(m_out, CFRunLoopGetCurrent(),kCFRunLoopCommonModes);
}
BOOL success = CFReadStreamOpen(m_in);
CFErrorRef error = CFReadStreamCopyError(m_in);
if (!success || (error && CFErrorGetCode(error) != 0))
{
NSLog(#"Connect error %s : %d", CFErrorGetDomain(error), CFErrorGetCode(error));
[NSThread sleepForTimeInterval:5.0];
}
else
{
NSLog(#"Connected");
break;
}
}
From the "CFNetwork Programming Guide":
Opening a stream can be a lengthy process, so the CFReadStreamOpen and CFWriteStreamOpen functions avoid blocking by returning TRUE to
indicate that the process of opening the stream has begun. To check
the status of the open, call the functions CFReadStreamGetStatus and
CFWriteStreamGetStatus, which returnkCFStreamStatusOpening if the open
is still in progress, kCFStreamStatusOpen if the open is complete,
orkCFStreamStatusErrorOccurred if the open has completed but failed.
In most cases, it doesn’t matter whether the open is complete because
the CFStream functions that read and write will block until the stream
is open.
Also check out the kCFStreamEventOpenCompleted,
(http://developer.apple.com/library/ios/#documentation/CoreFoundation/Reference/CFStreamConstants/Reference/reference.html)
: a stream event that reports the successful completion of the opening
process. So to conclude, after calling CFReadStreamOpen (or Write),
which will probably succeed, register to listen to the "OpenCompleted"
event to identify a "real" success.
Surely after you call CFStreamCreatePairWithSocketToCFHost() just test readstream to see if it's NULL?
As you're passing in the memory location of the readstream pointer, the function can easily set that to whatever value it chooses (either a reference to a created object, or alternatively NULL).
Edit
I've tried your code, and I agree, it's very confusing. It appears that the CFReadStreamRef is readily created and opened, even for a nonsense host (I literally used "nonsense"). I don't believe this function will return NULL pointers for an unreachable host.
I suppose this makes sense, in as far as until one tries to open the stream, whether it will work or not is unknown.
So, the readStream param is a pointer to the CFReadStreamRef and, as such, can definitely be set to NULL by the function. &foo means "address of foo" and if you have the address you can set the value.
My reading of the documentation for CFStreamCreatePairWithSocketToCFHost is that they will be set to NULL on failure, but that failure is not about connection failure, but other kinds of failure (memory, etc). So not likely you'll get an error there.
Looks to me like the issue is that CFReadStreamOpen can return immediately with true when it can open the stream in the background and so this code is not really opening the stream or testing that it's been opened, merely queuing it for opening). From the documentation for CFReadStreamOpen:
" If the stream can open in the background without blocking, this function always returns true."
So I think you will need to follow the rest of the instructions for CFReadStreamOpen and schedule the stream on a run loop, or perhaps poll (though obviously polling in a tight loop isn't likely to work).
In the documentation for CFReadStreamOpen we see:
Opening a stream causes it to reserve all the system resources it requires. If the stream can open in the background without blocking, this function always returns true.
I suspect that the stream is opening in the background, and thus you are saying "Connected" before it actually opens. You've already scheduled the stream with a runloop, so if you let the run loop run, you'll probably get a callback with the event type set to kCFStreamEventErrorOccurred, and from there you can process the error appropriately.