I have figured out that when my OPC UA server restarts then my Milo OPC UA Client (sdk-client 0.5.3) is automatically doing a reconnect to the server, that's nice! But subscriptions do not do a re-subscribe.
So, is there an option for the subscription or the SubscriptionManager to do this re-connect internally and automatically?
Or do I have to do this by my own? I have read about addSubscriptionListener and do this onSubscriptionTransferFaild - but this is the wrong place, because this is when the connection gets lost - the re-subscribe will fail - the reconnect should be done when the OPC UA connection comes up again. Where would be the best place to do this reconnects? Is there some listener which fires when the client reconnects to the server?
I have read about addSubscriptionListener and do this onSubscriptionTransferFaild - but this is the wrong place, because this is when the connection gets lost - the re-subscribe will fail
This actually is the right place.
When the connection is finally re-established the client will attempt to resume your old Session, which will fail, and then to transfer your previous Subscriptions to your new Session, which will fail. When these Subscription transfers fail the onSubscriptionTransferFailed callback will be invoked for each Subscription.
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I am working on a product which uses ZeroMQ (version 4.0.1).
The server and client communicate based on ZeroMQ ROUTER-socket.
To read socket events, server and client also create socket-monitor sockets (PAIR). There are three ports on which server binds and listens. Out of these three ports, one port is in a non-secured mode. Other two ports are using md5-authentication.
The issue I am facing is that, both the server and the client spontaneously receive socket disconnect for one of the secure port sockets (please see a log below). I have checked multiple times that server and client both have L3 reachability to each other.
What else I should check for?
What really triggers this error scenario?
zmq_print_callback:ZmQ: int zmq::stream_engine_t::read(void*, size_t):923
Stream engine recv():
TCP socket (187) to unknown:0 was disconnected
with error 107 [Transport endpoint is not connected]
Below sequence of events can trigger this error on server
Server receives ACCEPTED event for clientY and gets FD1.
Link-flap/network issue happens and clientY disconnects but server does not receive this disconnect.
Network recovers and clientY connects back to server.
Server receives ACCEPTED event for clientY and gets FD2. However, packets sent to this sockets does not go out of the server.
After 1 min or so, clientY receives "Transport endpoint is not connected error" for FD1.
Application can use this to treat as client disconnect.
During our development I've been running a V5.19 Kepware Server with UA access. This has been working fine until our Server machine (Windows 7) suddenly restarted. Now none of our applications can connect via UA - BadRequestTimeout error is returned. DA access is OK, it's just the UA clients that cannot connect.
I've done the usual Google and no useful information is returned other than telling me what the numerical value of the error code is, which I already knew.
The UA configuration of the server looks fine - i.e. it hasn't changed. None of the clients have changed.
Can someone please shed some light on this?
Thanks
Steve
The Server's UA Configuration had Trusted Server and Trusted Client entries (both of which were unnecessary).
I removed both of these and suddenly UA connections were allowed.
Not sure why removing these made any difference - these had been in place for a long time, when the server was initially installed.
When deploying a web application running on a traditional web server, you usually restart the web server after the code updates. Due to the nature of HTTP, this is not a problem for the users. On the next request they will get the latest updates.
But what about a WebSocket server? If I restart or kill the old process all connected users will get disconnected. So my question is, what kind of strategy have you used to deploy a WebSocket server smoothly?
You're right, every connected user will be disconnected if the server restarts.
I think the less bad solution is to tell to the client to reconnect in the onClose method of the client.
WebSockets is just a transport mechanism. Libraries like socket.io exist to build on that transport -- and provide heartbeats, browser fallbacks, graceful reconnects and handle other edge-cases found in real-time applications.
In our WebSocket-enabled application, socket.io is central to ensuring our continuous deployment setup doesn't break users' active socket connections.
If clients are connected directly to sever that does all sockets networking and application logic, then yes - they will be disconnected, due to TCP layer that holds connection.
If you have gateway that clients will be connecting to, and that gateway application is running on another server, but will communicate and forward messages to logical server, then logical server will send them back and gateway will send back to client responses. With such infrastructure, you have to implement stacking of packets on gateway until it will re-establish connection with logical server. Logical server might notify gateway server before restart. That way client will have connection, it will just wont receive any responses.
Or you can implement on client side reconnection.
With HTTP, every time you navigate away, browser actually is creating socket connection to server, transmits all data and closes it (in most cases). And then all website data is local, until you navigate away.
With WebSockets it is continuous connection, and there is no reconnection on requests. Thats why you have to implement simple mechanics when WebSockets getting closing event, you will try to reconnect periodically on client side.
It is more based on your specific needs.
If I am using MSMQ over the web, what happens if the network connection is lost between client and server?
So can you still add messages to the Queue, and if so where are they stored? In the client app, or in the clients OS etc?
For instance if I have a windows service which is adding items to a queue in a different country. What if the network connection is lost, and the windows service is restarted. Do the messages get lost forever?
The other part of the question relates to the route that a message takes, is it sent directly to the receiving queue, or is it written into a queue on the client side? Does that require MSMQ to be installed on the sending server, and how about licensing for that?
Is there any good documentation to explain the required setup?
Update: Regarding your follow up question. Yes you have to install msmq on the sending server. There aren't any licensing cost, because MSMQ is part of windows and not a separate software (just like the IIS). Here is documentation on "Setting Up a Message Queue" on windwos 2003.
Before Update: Outgoing message are stored in the outgoing queue of the sending server. They are not lost if the sending service is restarted. They will wait in the outgoing queues ( which can be inspected with the msmq manager ) for I don't know how long.
if the msmq service or the sending server are restarted. Then "express" messages will be lost. express or recoverable are properties of non-transcriptional messages.
I have a server and client program on the same machine. The server is part of an application- it can start and stop arbitrarily. When the server is up, I want the client to connect to the server's listening socket. There are win32 functions to wait on file system changes (ReadDirectoryChangesW) and registry changes (RegNotifyChangeKeyValue)- is there anything similar for network changes? I'd rather not have the client constantly polling.
There is no such Win32 API, however this can be easily accomplished by using an event. The client would wait on that event to be signaled. The server would signal the event when it starts up.
The related API that you will need to use is CreateEvent, OpenEvent, SetEvent, ResetEvent and WaitForSingleObject.
If your server will run as a service, then for Vista and up it will run in session 0 isolation. That means you will need to use an event with a name prefixed with "Global\".
You probably do have a good reason for needing this, but before you implement this please consider:
Is there some reason you need a connect right away? I see this as a non issue because if you perform an action in the client, you can at that point make a new server connection.
Is the server starting and stopping more frequently than the client? You could switch roles of who listens/connects
Consider using some form of Windows synchronization, such as semaphore. The client can wait on the synchronization primitive and the server can signal it when it starts up.
Personally I'd use a UDP broadcast from the server and have the "client" listening for it. The server could broadcast a UDP packet every X period whilst running and when the client gets one, if it's not already connected, it could connect.
This has the advantage that you can move the client onto a different machine without any issues (and since the main connection from client to server is sockets already it would be a pity to tie the client and server to the same machine simply because you selected a local IPC method for the initial bootstrap).