How can I make a server connect to another server in java
My program is replicated file system. So , I need the client to connect to the server in order to read or write in the text file. which is easy to implement throw TCP connection . but the problem is how can i make the server connect other servers to avoid conflicting in the text file.
I have read about multicast throw UDP connection. can I make the server have to kind of connection one using TCP with client and another connection using TCP to multicast request to other servers in the group ?
Or if there are anther way to implement that?
Thank you
I would suggest you to look at group communication frameworks, such as jgroups.
Look here for explanation on TCP transport.
Related
I want to write a program in Netty4 that should act as a server to other clients and also it itself is a client to another server. How to do this in Netty4? So far all examples I have seen are either client or server. Thanks.
There are no special difficulties here. You need to create a part that will act as a server (using ServerBootstrap), and a part that will act as a client (using Bootstrap).
If you need to establish a connection to another server while handling incoming connection from a client, you can place that logic into a ChannelHandler of the server's pipeline.
Netty provides two examples of this approach:
Hex dumping proxy
SOCKS proxy
I'm developing a chat application in Java that enables client to connect to a predefined port and when two clients are connected the server should connect those two sockets and data should be exchanged between them.
I know to create a ServerSocket and Socket that will connect and establish a communication between them through a separate port and a server socket. But, how can I connect those two connected clients?
If you are creating a chat application then Chat server will not connect client socket in order to communicate.
You can try following approach:
On server side Use a HashTable to store connected clients.
When you receive a message from client, include a an id of CLient to whom message is to be sent and simply pass on the message to other client.
There will be separate threads running for these clients, so you need to have a policy to control the load on server.
This questions sounds like a homework assignment.
If you would like to see a great contemporary demo of a chat system you may learn from Play's WebSocket chat sample application.
There is an existing service that i would like to write a dummy service (using Netty) for. It will be used for testing purposes.
The existing client code fragment for the service looks like:
Socket socket = new java.net.Socket();
socket.connect(new InetSocketAddress("localhost", 8080), 10000);
socket.setSoTimeout(20000); // set a timeout of 20 seconds
InputStreamReader ir = new InputStreamReader(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
// write some string to the server and wait for answer
out.println("SomeCommand");
// server has written some answer, read it
char[] c = new char[2];
ir.read(c, 0, 2);
String cs = new String(c);
if ("OK".equals(cs.toString())) {
// write some more string's to the server
}
// we're done, close the connection
out.println("BYE");
out.close();
socket.close();
Is Netty the right framework to create a server for java.net.Socket connections? (If not, which framework should be used, if any?)
I am trying to find a way to start with Netty using the QuoteOfTheMoment example. The QuoteOfTheMomentServerHandler does basically what i want, upon the incoming message, return some answer so that the above snippet can read the answer using the inputstream but the above socket cannot make a connection to the QuoteOfTheMomentServer. The error is "connection refused".
[EDIT] More clarification:
The problem (i think) is not connecting or the port i use. Let me try to better ask the question:
I just started with netty (no nio experience) and am not familiar with the different types of channels, pipelines and what not.
The server should, like a servlet request/response (like, not http or trying to rebuild a http servlet impl), react on a inputString written to the output-stream as in the code fragment and write back some string/bytes to the input-stream as in the code fragment, so the client only then moves on. So the connection should stay open but also be synchronous, the client waits for answer from the server. If i use the example "Writing the Server Side of a Socket" in the java tutorial i am able to get it working for the client. But i want to utilize the thread handling etc. from netty.
The QuoteOfTheMomentServerHandler seems as server side implementation what i want but can that setup handle the given client code ?
So the question is which kind of pipeline, channel or something like that should be used given the way the client works ?
Again, the client and server are existing. I want to build a dummy server implementation to work with the existing client.
Netty is a TCP/IP framework. So yes if you are developing a TCP/IP server this toolkit is good to use.
I assume you are getting a error when trying to connect the client to the server. Also the server should also be running.
When getting a the connection refused error there are a couple of thine to check. First one is the firewall(if any) on the server allowing connections to port 8080? Secondly from your client machine try open a telnet session to the server something like:
Telnet yourserverip 8080
This opens a socket connection to the server. If you get a error message Google it.
The last one is that you might be running a server like tomcat, glassfish, IIS which uses port 8080 already. Try a non standard port like 10810 for example.
UPDATES:
If you are new to netty please read the users guide found here http://netty.io/docs/stable/guide/html/.
I had a look at the Quote of the moment service and I do believe I found part of the problem. The Quote of the moment service is a broadcast UDP/IP client and server. UDP is a much more lightweight "version" of TCP IP. It does not guarantee delivery to the client or server and it is broadcast. UDP is sort of like a radio broadcast as it is generally not targeted to a specific IP but broadcast over the entire network. Thus you normal TCP IP connection will not be able to work on the UDP server.
See this link on how to write a UDP Client http://systembash.com/content/a-simple-java-udp-server-and-udp-client/.
I would suggest that you convert the Quote of the moment server from UDP to TCP/IP server as this will give you some practise in creating a TCP/IP server without getting into too much detail. Once you are comfortable with that you should be able to start once from scratch.
Just remember that Netty handles the NIO part for you. It is a higher level framework based on NIO thus hiding a lot of the detail from you. You dont need to know NIO that well to use netty but you need to understand the Netty concepts well.
I have created this game bot where it connects to the game, and starts playing.
My problem is that i can't start more than one of these as the other then won't work.
Is is possible that if i run 2 instances of the same program the sockets are interfering with each other ? After all, they do connect to the same IP with the same port ?
And sometimes after i close(just closing cmd) the program is unable to connect again. Is that cause i didn't close the connections right ?.
I hope this is enough else i'll just have to post my source code
Best regards.
It's possible to connect to the same socket/port several times. Actually a socket is a double peer: {client ip/ client port}{server ip/server port}. When you connect to a server, your client port is assigned dynamically. You will have a new and different client port per client. So it should work unless the server side forbid it.
You should have a server that listens for multiple connections. A server is bound to a port and once that port is in use another application cannot use it. So for the server just have one instance. Multiple clients can connect to this IP/Port as long as the Server accepts multiple connections.
If a client connects to the server and the other clients stop working this may be because the server does not support multiple clients. To do this you need to use multi threading in the server. The server should accept a client socket and create a new instance of a client with it's own StreamReader/Writer objects.
http://tutorials.jenkov.com/java-multithreaded-servers/multithreaded-server.html
if you are working with a specific TCP port, then there is a close-wait period that this port cannot be claimed temporariliy for some time. also multiple programs cannot listen the same TCP port. Use threads.
I have something like a proxy server (written in java) running between my clients and the actual video server (made in c++). Everything the clients send goes through this proxy and is then redirected to the server.
It is working fine, but I have some issues and think it would be better if I could make this proxy server only to listen to the clients requests and then somehow tell the server that a request has been made from the client side, and that it is supposed to create a connection with the client directly.
Basically in the TCP level what I want to happen is something like this:
1- whenever a client sends a SYN to my proxy, the proxy just sends a message to the real server telling the ip and port of the client.
2- The server would then send the corresponding SYN-ACK to the specified client creating a direct connection between client and server.
The proxy would then be just relaying the initial requests (but not the later data transfer) to the actual server. I just don't know if that is possible.
Thank you very much
Nelson R. Perez
That's very much the way some games (and Fog Creek CoPilot) do it, but it requires support on both the server and the client. Basically the proxy has to say to the client and server "try communicating with the directly on this ip and this port" and if they can't get through (because one or both is behind a NAT or firewall), they fall back to going through the proxy.
I found this good description of "peer to peer tcp hole punching" at http://www.brynosaurus.com/pub/net/p2pnat/
Does the proxy and server lives on the same machine? If so, you can pass the connection to the server using Socket Transfer or File Descriptor Passing. You can find examples in C here,
http://www.wsinnovations.com/softeng/articles/uds.html
If they are on the different machines, there is no way to pass connection to the server. However, it's possible to proxy the IP packets to server using VIP (Virtual IP). This is below socket so you have to use Link layer interface, like DLPI.
You don't have control of TCP handshake in userland like that. This is what firewalls/routers do but it all happens in the kernel. Take a look at the firewalling software for your platform - you might not even have to code anything.