I would have to design a job scheduling system which works like - users (clients) would deposit jobs (executables) to a server. I have three files - client.c, jobQ.c and server.c. The jobQ would take client requests, and send them to the server on specific timestamps (if a user wants to run job X on server Y at 07-29-2010 3:34 AM, then the jobQ would store it in a stack, and when time comes and if the server is free, it sends the job to the server).
jobQ.c would act as a server for client.c and as a client for server.c.
I have used TCP/IP sockets to program these, and the problem I am facing is while creating multiple sockets in jobQ.c. Is it possible for the same file to have client and server sockets? The error points to this line in jobQ.c:
sockSer = socket(AF_INET, SOCK_STREAM, 0);
error: lvalue required as decrement operand
...when I am opening a second socket to talk to the server.
My idea is that jobQ would open different ports to listen to clients and connect to the server.
Thanks,
Sayan
Related
I'm building a message broker which communicates with clients over ZeroMQ PUSH/PULL sockets and has the ability to exclude clients from messages they're not subscribed to from the server side (unlike ZeroMQ pub/sub which excludes messages on the client side).
Currently, I implement it in the following way:
Server: Binds ZeroMQ PULL socket on a fixed port
Client: Binds a ZeroMQ PULL socket on a random or fixed port
Client: Connects to the server's PULL socket and sends a handshake message containing the new client's address and port.
Server: Recieves handshake from client and connects a PUSH socket to the client's PULL server. Sends handshake response to the client's socket.
Client: Recieves handshake. Connected!
Now the client and server can communicate bidirectionally and the server can send messages to only a certain subset of clients. It works great!
However, this model doesn't work if the clients binding PULL sockets are unable to open a port in their firewall so the server can connect to them. How can I resolve this with minimal re-architecting (as the current model works very well when the firewall can be configured correctly)
I've considered the following:
Router/dealer pattern? I'm fairly ignorant on this and documentation I found was sparse.
Some sort of transport bridging? The linked example provides an example for PUB/SUB.
I was hoping to get some advice from someone who knows more about ZeroMQ than me.
tl;dr: I implemented a message broker that communicates with clients via bidirectional push/pull sockets. Each client binds a PULL socket and the server keeps a map of PUSH sockets so that it can address specific subscribers. How do I deal with a firewall blocking the client ports?
You can use the router/dealer to do this like you say. By default the ROUTER socket tracks every connection it has. The way it does this is by having the caller stick the connection identity information in front of each message it recieves. This makes things like pub/sub fairly trivial as all you need to do is handle a few messages server side that the DEALER socket sends it. In the past I have done something like
1.) Server side is a ROUTER socket. The ROUTER handles 2 messages from DEALER sockets SUB/UNSUB. This alongside the identity info sent as the first part of a frame allows the router to know the messages that a client is interested in.
2.) The server checks the mapping to see which clients should be sent a particular type of data using the map and then forwards the message to the correct client by appending the identity again to the start of the message.
This is nice in that it allows a single port to be exposed on the server. Client side we do not need to expose ports, simply just connect to the server ROUTER socket.
See https://zguide.zeromq.org/docs/chapter3/ for more info.
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.
I've read a lot of theory about sockets and Client-Server connection on this forums but some points remains blurred or some answers does not satisfy me completely.
Also, i'd like my words to be confirmed, completed or corrected :
1)_ A socket is made out of IP source (IP of the Client), Port source (a port automatically and randomly choosen by the OS between 1024 and 65535), IP destination (127.0.0.1 ? Something i don't get here), Port destination (developper defined-by port for the server) and protocol type.
There may be something wrong in those lines already.
But considering it is true, how can the server differenciate two processes accessing the server from the same machine ? (Understand, how the developper can make any difference if he wants to prevent multiple access from the same machine).
The only difference would be the source port which is auto-filled by the OS. In this case, it would act like it was a totally different machine, right ?
2)_ I heard there was actually a pair of sockets. One generated by the Client, and one by the server.
Is there really a need for the server to have a second socket ? Is this socket a simple replica to keep a copy in the "Client currently connected"-list or is it a different socket, with different values ?
3)_ When does a Client should "disconnect" ? At each query ? At the end of some process ? Other ?
Thanks for enlightenment !
1)_ A socket is made out of IP source (IP of the Client), Port source
(a port automatically and randomly choosen by the OS between 1024 and
65535), IP destination (127.0.0.1 ? Something i don't get here), Port
destination (developper defined-by port for the server) and protocol
type.
I wouldn't say a "socket is made" out of those data points; rather a TCP-connection can be uniquely identified using just those data points:
1. Source IP - the IP address of the client computer
2. Source Port - the port number (on the client computer) that the client is sending packets from and receiving packets on
3. Destination IP - the IP address of the server computer
4. Destination Port - the port number (on the server computer) that the server is sending packets from and receivign packets on
5. Protocol type - what communications-protocol is in use (i.e. either TCP or UDP)
But considering it is true, how can the server differenciate two
processes accessing the server from the same machine ?
It can differentiate them because the 5-tuple (above) will be unique for each of the two connections. In particular, in the TCP packets the server receives from process #2, field #2 (Source Port) will be different from the value it has in the packets received from process #1.
The only difference would be the source port which is auto-filled by
the OS. In this case, it would act like it was a totally different
machine, right ?
The server can act however it was programmed to act -- but in most cases the server will be programmed not to care whether two client connections come from the same physical machine or not. To most servers, a client is a client, and a client's physical location is not that important.
Is there really a need for the server to have a second socket ? Is
this socket a simple replica to keep a copy in the "Client currently
connected"-list or is it a different socket, with different values ?
A socket is a just data structure that lives in a computer's memory to help it keep track of the current state associated with a particular network connection. Since both the client and the server need to keep track of their end of the connection, both the client the server will have their own socket representing their endpoint. (Note the difference between a "TCP-connection", which you can imagine as an imaginary/virtual wire running from one computer to another, and the two "sockets", which would be the imaginary/virtual connectors at the ends of that wire, that attach the wire to the client-program on one end, and the server-program on the other end)
3)_ When does a Client should "disconnect" ? At each query ? At the
end of some process ? Other ?
Whenever it wants to; it's up to the programmer(*). There are startup/shutdown costs to opening and connecting a new socket, but there is also some ongoing memory and CPU overhead to keeping a socket open indefinitely, so the programmer will have to make a design decision about whether he wants to keep sockets open over extended periods, or not.
(*) Note that in a modern OS, if the client program exits or crashes, the socket will be automatically closed and the connection automatically disconnected by the OS.
I have a simple node.js client and server programs running on one machine and when I try to connect to the server with second instance of client program simultaneously I get EADDRINUSE, Address already in use error. Is it possible to have two or more TCP based socket client connections (created with createConnection) to one server (created with createServer) on the same machine, or only one client program can be connected to the server at the same time?
Yes, it's possible. Infact, very common. Many applications open dozens, or hundreds of connections to the same server. It sounds like your client program is binding on a port. Only the server should be binding on a port. You should verify.
The client will usually use a random port between 1024-65535, as assigned by your OS. You don't need to worry about it. Since the client is initiating a connection to the server, the server port must unique to one program. Which is why your problem signifies you are trying to start the server twice. Please see http://www.tcpipguide.com/free/t_TCPIPClientEphemeralPortsandClientServerApplicatio.htm
I have 2 sip clients on the same computer.
Both of them is registering to a server that is running on port 5060.
For the first client the UDP is on port 5060 and for the other is 5061. When I come from one client to another, after the ringing part i receive the error:
only one usage of each socket address is normally permited.
Got any ideas why I got this error?
Your server and client are both trying to use port 5060, hence the error message. Change the first client to use 5062 or something else.
Also, 5061 is normally used for secured SIP (normal listening port + 1 in the proxy/server). Do not use it for the second client.
It means you're clients are both trying to claim the same socket for the communication channel, or the server is trying to reclaim the socket given to client A, to reuse it for client B.
The software handeling the socket, should be smart enough to rely on the OS to assign port numbers instead of hardcoding the port numbers in the code, this is a 100% guarantee for socket issues.