I am trying to implement a basic
Client(Receive Updates / Sends Request)
Server(Receives Request /
Sends Updates)
using RabbitMQ.
Here is what I want to implement
To notify client when the server is shutdown(all reasons leading to this(bug/lost connection etc....))
Notify the client when the server is up again
thanks
You want to set the mandatory flag when you publish(channel.BasicPublish...) and handle the Basic_Return event to indicate that there was a routing issue. Does not necessarily mean connection is down, but in my case I am using this as an alternative to connection is down, as when a message cannot be delivered / routed for my application purposes connection is down.
Related
I have a simple webbapp deployed in tomcat8. But some HTTP requests requires access to DB with slow queries. Sometimes HTTP client made reset connection. In the same time i'd like to handle it in my webapp for cancel slow query (which result is no longer interesting).
The main question: "How to catch reset connection from client side in phase awaiting response from server". Is it possible? Interrupting thread - the best way for it because I can easly handle it.
When connection is broken from client side tomcat does not interrupts http-nio-X thread. Why? How to do it?
I have a chat implementation working with CometD.
On front end I have a Client that has a clientId=123 and is talking to VirtualMachine-1
The longpolling connection between the VirtualMachine-1 and the Client is done through the clientId. When the connection is established during the handshake, VirtualMachine-1 registers the 123 clientId as it's own and accepts its data.
For some reason, if VM-1 is restarted or FAILS. The longpolling connection between Client and VM-1 is disconnected (since the VirtualMachine-1 is dead, the heartbeats would fail, thus it would become disconnected).
In which case, CometD loadBalancer will re-route the Client communication to a new VirtualMachine-2. However, since VirtualMachine-2 has different clientId it is not able to understand the "123" coming from the Client.
My question is - what is the cometD behavior in this case? How does it re-route the traffic from VM-1 to a new VM-2 to successfully go through handshaking process?
When a CometD client is redirected to the second server by the load balancer, the second server does not know about this client.
The client will send a /meta/connect message with clientId=123, and the second server will reply with a 402::unknown_session and advice: {reconnect: "handshake"}.
When receiving the advice to re-handshake, the client will send a /meta/handshake message and will get a new clientId=456 from the second server.
Upon handshake, a well written CometD application will subscribe (even for dynamic subscriptions) to all needed channels, and eventually be restored to function as before, almost transparently.
Messages published to the client during the switch from one server to the other are completely lost: CometD does not implement any persistent feature.
However, persisting messages until the client acknowledged them is possible: CometD offers a number of listeners that are invoked by the CometD implementation, and through these listeners an application can persist messages (or other information) into their own choice of persistent (and possibly distributed) store: Redis, RDBMS, etc.
CometD handles reconnection transparently for you - it just takes a few messages between client and the new server.
You also want to read about CometD's in-memory clustering features.
I've started working on a client-server distributed application using WCF. The clients should also send requests to the server, therefore I chose to implement duplex operations using the NetTcpBinding since all the clients will b on the same intranet.
On the server side, for the service class that implements the server contract I use these settings
[ServiceBehavior(ConcurrencyMode = ConcurrencyMode.Single, InstanceContextMode = InstanceContextMode.Single, UseSynchronizationContext = false)]
Every time a client is initialized, I'm creating an instance of the proxy class generated by adding the service using 'Add service reference' option in Visual Studio. After the proxy is initialized, I send a Connect message to the server:
_proxy.InnerChannel.Faulted += new EventHandler(InnerChannel_Faulted);
_proxy.InnerChannel.Closing += new EventHandler(InnerChannel_Closing);
//send a connect message to the server
_proxy.ClientConnected(ClientHostName, Version, ClientID, ClientIP);
Now the server has a reference to the connected client, by using
OperationContext.Current.GetCallbackChannel<IClientEvents>()
This is pretty straight forward, nothing fancy. However, I'm having a bit of trouble with making the client reconnecting to the server after the server went online. In my scenario, I will have up to 50-100 clients connected to the server and they will rarely communicate with the server, let's say in average, 1 request per hour.
What I want to achieve is to have the client "hanging" while the is offline, for this I try to reinitialize the communication channel on the client side every time the channel ends up in Faulted state, which works ok. But when I try to close the server, I get the following message
This could be because a client failed to close a sessionful channel within the required time.
I'm now struggling to find the most appropriate implementation for my scenario:
Not closing the channel after the service call. This way the client will always try to recreate the channel once the connection with the server is down (e.g. interval of 1 min). Doesn't make much sense to keep the channel opened all this time though, so I'm not that sure about this approach.
Closing the channel after each call and recreate when making a new call to the server. This works fine when making service calls, but what happens if the server wants to send a notification the client? The callback reference on the server side will not be valid anymore, I'd have to wait for the client to send a new connect message to get the new callback reference, right? In this case, should I regularly have a different call similar to a Ping() to the server in order to ensure that the server can always contact the client?
I'm still reading materials on WCF duplex operations, just can't decide which approach is better so that I don't run into problems later on.
Thanks for your advice!
I am using netTcpBinding in WCF and I would like to know, on the client side, when it disconnects or when its channel state goes to faulted, so that I can reconnect with the server.
The idea is creating a keep alive using channel events. I know it is possible to know that the client disconnected/went into faulted state in the server side using Channel.Faulted event. That way I can remove any subscriptions the client had on the server. However, I cannot reconnect the client on the server side. And that's exactly what I would like to do.
I have a lot of client programs and one service.
This Client programs communicate with the server with http channel with WCF.
The clients have dynamic IP.
They are online 24h/day.
I need the following:
The server should notify all the clients in 3 min interval. If the client is new (started in the moment), is should notify it immediately.
But because the clients have dynamic IP and they are working 24h/day and sometimes the connection is unstable, is it good idea to use wcf duplex?
What happens when the connection goes down? Will it automatically recover?
Is is good idea to use remote MSMQ for this type of notification ?
Regards,
WCF duplex is very resource hungry and per rule of thumb you should not use more than 10. There is a lot of overhead involved with duplex channels. Also there is not auto-recover.
If you know the interval of 3 minutes and you want the client to get information when it starts why not let the client poll the information from the server?
When the connection goes down the callback will throw an exception and the channel will close.
I am not sure MSMQ will work for you unless each client will create an MSMQ queue for you and you push messages to each one of them. Again with an unreliable connection it will not help. I don't think you can "push" the data if you loose the connection to a client, client goes off-line or changes an IP without notifying your system.