I have minimum 3 TCP client, each has a Thread. I'm sending out messages and waiting for the answer, but sometimes I have to wait to receive the response from all client, this is depending what kind of message sent the server out. I already made to send messages to the clients and receiving, but when I have to wait for the other client response I couldn't do that until now.
As far as you didn't mention your environment/language, I assume C#/.NET 4
You need a mechanism for each client to signal the arrival of a response. This is usually done with AutoResetEvents: Each client sends his response back to the server. The server itself can extract from the reponse (or any other property, e.g. the connection) with client has sent it. Then he sets the apporpriate AutoResetEvent.
The thread that formerly initiated sending the message can afterwards wait for all AutoResetEvents to be set.
Related
I'm creating a chat application, and one detail is that "acknowledgements" are crucial. I'll get to what that means. I'm trying to figure out what the best exchange protocol would be.
Scenario:
Alice sends Bob a message. Bob is offline, so the message is stored on the server. Bob connects to the server through a WebSocket connection. The server sends him messages that have been sent to him while he was away. This is where the problem arises. The WS API that's available for my app's ecosystem (Node.js, Nest.js specifically), has no pattern where it can wait for this message to be sent. The mechanism there seems to just be fire & forget. What if the payload is quite large and the connection drops while the message is being sent?
Now, I know socket.io has support for acknowledgements. But from what I've read, socket.io has some overhead and therefore less performance than optimal. Now whether that performance is something that I arguably need is another question, but I'm just trying to figure out how I can guarantee that the message has arrived on the other end. This means client-server and server-client directions. How can I await it? I know that one approach is to attach a unique ID to the socket event, and have the other side send you a confirmation that it received it. This is how socket.io does it if I'm not mistaken.
But my question there is how can I guarantee that the acknowledgement message was successfully sent? So then I'd need an "ack" for my "ack" and so on, so I'll always need one more acknowledgement so I don't know how that works.
What I though of as options is to use two REST endpoints to send and receive (or download) messages. You send when you send, but you receive when you receive a ping that there's messages for you to download. Now this could be done through a WebSocket connection where the server notifies the client about a new message and then the client can call this receive endpoint. This ping can also be done through a more managed solution like FCM. The pros with that approach are twofold:
First, I have the REST interface to use, which is a lot more practical
I have the Request-Response pattern to use, so I have a theoretical guarantee that things are arriving if I get a response
Now the problem with this approach is that there's a lot of overhead from opening a new HTTP connection every time I want to send or receive messages, if I'm not mistaken:
I have to wait for the initial request time to get to the server before I actually have to wait for the server to respond with messages. With the pure WebSockets case, I would theoretically then just wait for the response equivalent part there (?)
This wastes bandwidth as well.
So one more question, where can I find out which clients will actually re-use an existing HTTP connection like a WebSocket connection, if available and not create a new one? Do all clients do that? Is it only the browser? What about apps? Is it on the OS level?
So the final question is how do I solve this problem of "acknowledgements" and not waste time and bandwidth? Are any of my conclusion/questions wrong or uninformed, am I missing something?
Notes:
server is Node.js and client is Flutter
i know about the WAMP subprotocol, but for my ecosystem it doesn't have very reliable implementations
I'm not sure what your exact requirements or performance need,
but I did a project that also need reliable communication between client and server using websocket, the simplest I could think of was build request-response mechanism on top of websocket, and then build your application data on top of that.
here's high level overview how I implemented it:
implement request-response message using transaction to identify which response belongs to which request.
clients will have map storing transaction, when you send the message request wait for server to send a message response with the same transaction or wait
until timeout.
client wants to send message to server and construct the request as follow
{
"event": "sendMessage",
"type": "request",
"transaction": "<uuid/unique-value>",
"data": "<your-application-data>"
}
server parse the message and check that its a request with an event name sendMessage then call related function
server sends back response message to client
{
"event": "sendMessage",
"type": "response",
"transaction": "<uuid/unique-value>", // same unique value as in request
"data": "<your-application-data-result>"
}
because client has mapping which transaction belongs to which request, it is possible to match which request this response belongs to, if matched then complete the transaction
I want to code a synchronous programs in which cloud send mqtt message to device, then using simple to wait response to judge whether is successed. but it need a timeout, such as 5 seconds, app think it's failed. The keepalive parameter of mqtt simple API seems to lose efficacy, but the big probability is that I use or understand the error.
I would very appreciate it if you guys can suggest me some advice
print("----before simple")
msg = subscribe.simple("paho/test/simple", hostname="39.100.79.76",port=1883,keepalive=5,will = {'topic': "paho/test/disconnect", 'payload':"network or device anomaly", 'qos':2, 'retain':0})
print("----after simple")
then run it, simple API cannot to end
----before simple
infinite...
Correctly determine if it is successful to synchronize the edge cloud application
You have miss understood what the keepalive property for a MQTT client is for.
The keepalive is used by the Broker to check if the client is still functioning. It does this by keeping a timer since the last MQTT packet was received from the client. If it does not receive a packet when the timer reaches the keepalive time it sends a MQTT Ping request to the client. If it doesn't receive a response to that packet with in half the keepalive time then is will disconnect the client and publish any Last Will & Testament message that the client may have set.
The Paho client library handles MQTT Ping messages in the background with no need for the user to be involved.
The code sample you have provided will wait indefinitely for a response.
My HTTP server needs to make sure that HTTP responses are delivered to the client.
I feel (and I will implement it so) that the most correctly way is to send the "I received the data" request from the client.
But I have a bit academic question:
Has the server some other means to detect whether the response delivered to the client or not (say due to the client's computer was evaporated by a direct hit of an H-bomb during data transmission)?
According to the node.js http documentation:
Class: http.ServerResponse
...
Event: 'finish'
Emitted when the response has been sent. More specifically, this event
is emitted when the last segment of the response headers and body have
been handed off to the operating system for transmission over the
network. It does not imply that the client has received anything yet.
After this event, no more events will be emitted on the response
object.
Also I did not find nothing better in the 'net' module documentation.
So it seems I can only know that my data is in the kernel buffer for transmisstion over the network.
Can I detect some errors at this transmission by Node.js means?
Can I detect some errors at this transmission by some easy-to-implement non-Node.js means?
Any links and thoughts that this is not an HTTP way or even not a TCP way are also appreciated.
The thing about TCP is that the receiver must acknowledge the fact that it receives every single bit of data. If that's not the case, sender keeps sending the pieces of data that receiver has not yet acknowledged to receive.
I checked out the Net module and I saw this interesting piece:
Event: 'end'
#
Added in: v0.1.90
Emitted when the other end of the socket sends a FIN packet.
I have no implementation idea for this but still, this might be useful for your case.
I am writing a Client, Server-based chat. The Server is the central component and handles all the incoming messages and outgoing messages. The clients are that chat users. They see the chat in a frame and can also write chat messages. These messages are sent over to the server. The server in turn updates all clients.
My problem is synchronisation of the clients. Since the server is multi-threaded, both messages can be received from clients and updates (in form of messages) have to be sent out aswell. Since each client is getting updated in in its own thread, there is no guarantee that all clients will receive the same messages. We have a snychronisation problem.
How do I solve it?
I have messed with timestamps and a buffer. But this is not a good solution again because there is no guarantee that after assigning a timestamp the message will be put into the buffer immediately afterwards.
I shall add that I do not know the clients. That is, I only have one open connection in each thread on the server. I do not have an array of clients or something like that to keep track of all the clients.
I suggest that you implement a queue for each client proxy (that's the object that manages the communication with each client).
Each iteration of your server object's (on its own thread) work:
1. It reads messages from the queues of all client proxies first
2. Decides if it needs to send out any messages based on its internal logic and incoming messages
3. Prepares and puts any outgoing messages to the queues of all its client proxies.
The client proxy thread work schedule is this:
1. Read from the communication.
2. Write to the queue from client proxy to server (if received any messages).
3. Read from the queue from server to client proxy.
4. Write to communication channel to client (if needed).
You may have to have a mutex on each queue.
Hope that helps
For example we have a basic node.js server <-> client comunicaciton.
A basic node.js server who sends each 500ms a message to the only o every one client connected with their respective socket initiated, the client is responding correctly to the heratbeat and receiving all the messages in time. But, imagine the client has a temporal connection lag (without closing socket), cpu overload, etc.. And cannot process nothing during 2secs or more.
In this situation, where goes all those the messages that are not yet received by the client??
They are stored in node? in any buffer or similar?
And viceversa? The client is sending every 500ms a message to the server (the server only listens without responding), but the server has a temporary connection issue or cpu overhead during 2 or 3 secs..
Thanks in advice!! any information or aclaration will be welcomed
Javier
Yes, they are stored in buffers, primarily in buffers provided by the OS kernel. Same thing on the receiving end for connections incoming to a node server.