Okay, so I've read around on the Oracal site and some questions on this site. I'm still having kind of a hard time understanding a few things about sockets so I'll see if anyone here could spend the time to explain it to my slow brain. What I'm doing is setting up a chat client and chat server (To learn swing and sockets in one swoop). Despite all the examples I've seen, I still don't quiet grasp how they work. I know how 1 socket with an input stream and 1 socket with an output stream work, but beyond that I'm having trouble understanding because that is as far as most the resources I find explain. Here is my volley of questions regarding this.
If I want to be able to handle input and output to a client at the same time what would I do? Wait for out, then if there is a change in the server switch to input stream and get the changes, then switch back to output stream? Or can I run both an input and output stream at once?
Lets say the server has to handle several clients at once. I'll have to make a socket for each client right? What would you suggest is a good way handle this?
Lets say the client wants to change the IP address or port of their current socket and connect to a different server. Would I just create a new socket, or is there some way to change the current one?
That's the main questions I have. If I can get that much understood I'm pretty sure I could figure out the rest I need on my own.
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Here's an excellent guide to sockets. It's not "Java sockets" per se, but I think you'll find it very useful:
Beej's Guide to Network Programming
To answer your questions:
Q: If I want to be able to handle input and output to a client at the
same time what would I do?
A: You don't have to do anything special. Sockets are automatically "bi-modal": you can read (if there's any data) or write at any time.
Q: Lets say the server has to handle several clients at once. I'll
have to make a socket for each client right?
A: Actually, the system gives you the socket for each new client connection. You don't "create" one - it's given to you.
Each new connection is a new socket.
Often, your server will spawn a new thread to service each new client connection.
Q: Lets say the client wants to change the IP address or port of their
current socket and connect to a different server. Would I just create
a new socket, or is there some way to change the current one?
A: The client would terminate the existing connection and open a new connection.
I'll try to do my best here, but I really don't think this is the place for that kind of questions:
First of all, you need to understand that sockets are an abstraction of the underlying operating system sockets (unix socket, win socks, etc).
These kinds of sockets are to model connection-oriented services of the transport layer (look at the OSI model). So this means that sockets offer you a stream of bytes from the client and a stream of bytes to the client, so to answer your first question, these streams are independent. Of course it is your responsibility for the design of the protocol you speak over these streams.
To answer your second question you need to know how TCP connections work, basically your server is listening over one or more network interfaces in one port (ports are the TCP addressing mechanism) and can handle a configurable backlog of incoming simultaneous connections. So the answer is, it is common that for any incoming connection a new Thread on the server gets created or obtained from a Thread pool.
To answer your third question, connections are made between hosts, so if you need to change any of them, there will be the need of creating a new connection.
Hope this helps.
Cheers
1.- If I want to be able to handle input and output to a client at the same time what would I do? Wait for out, then if there is a change in
the server switch to input stream and get the changes, then switch
back to output stream? Or can I run both an input and output stream at
once?
It depends on your protocol, if your client start the connection, then your server waits for an input before going to the output stream and sends something. Every connection, being a tcp connection or even working with files have an input stream and an output stream.
2.- Lets say the server has to handle several clients at once. I'll have to make a socket for each client right? What would you suggest is
a good way handle this?
There are different strategies for this that include multithreading so for now focus on streams.Or keep it with one server one client.
3.- Lets say the client wants to change the IP address or port of their current socket and connect to a different server. Would I just
create a new socket, or is there some way to change the current one?
Yes, the definition of a socket is a connection made by an IP address through a specific port if any of those change you need a new socket.
Related
Ok, so let´s clarify the questions...
I'm studing Sockets in Java, from my understood until now, related to this subject are:
To make multiple clients to connect to only one address in the server (port), then it is necessary to assign each client connection to another thread
Based on that I got confused about somethings AND could not find any acceptable answer here or at Google until now.
If Socket is synchronous, what happens if 2 clients try to connect AT THE SAME TIME and how the server decides who will connect first?
How the server process multiple messages from one client? I mean, does it process in order? Return ordered?
Same question above BUT with multiple messages from multiple clients?
If the messages are not ordered, how to achieve that? (in java)
Sorry about all those questions but for me all of them are related...
Edit:
As the comment said, I misunderstood the concept of synchronization, so changed that part.
Guys we ask here to LEARN not to get judged by other SO think about that before giving -1 vote ok.
what happens if 2 clients try to connect AT THE SAME TIME
It is impossible for 2 clients to connect at exactly the same time: networking infrastructure guarantees it. Two requests happening at the exact same time is called a collision (wikipedia), and the network handles it in some way: it can be through detection or through avoidance.
How the server process multiple messages from one client? I mean, does it process in order?
Yes. The Socket class API uses the TCP/IP protocol, which includes sequence numbers in every segment, and re-orders segments so that they are processed in the order they are sent, which may be different from the order they are received.
If you used DatagramSocket instead, that would use UDP, which does not guarantee ordering.
Same question above BUT with multiple messages from multiple clients?
There are no guarantees of the relative ordering of segments sent from multiple sources.
I'm working on a Java client-server program. I use a socket to make the connection but I'm not sure about the efficiency. So, this is my question:
every time that I need a connection with a server, is it more efficient to create a new socket or to use the one created for the previous connection?
I think I need a method to verify if the client is already connected.
In terms of performance is more efficient re-using a previously created socket. But this don't need to be the best solution, it depends on each case.
The idiom case is having a ServerSocket in the server side that returns a Socket each time a new client connects to to it. If you require multiple clients to work against the server you could create a Thread to treat each client passing it the returned Socket.
Moreover, you don't need a method to verify you have a client already connected, ServerSocket returns a new Socket each time it happens.
Please, read more about this topic in Oracle tutorials.
I've been thinking about this all day, i dont really think if the Title is the correct one but here it goes, let me explain my situation: Im working on a project, a server made in Java for clients made in Delphi. Conections are good, multiple clients with its own threads, i/o working good. The clients send Strings to the server which i read with BufferedReader. Depending on the reserved words the server receives, it makes an action. Before the client sends the string, it inserts information to a SQL Server database so the server can go and check it after getting the order/command via socket. The server obtains the information in the database, process it, and send it to... let's call it "The Dark Side".
At the moment that the transaction is done, and the info is sent to the dark side, the server inserts the information... cough cough, dark information into a database table so the client can go and take what it requested. BUT, i need to report that to the client! ("Yo, check again the database bro, what you want is there :3").
The conection, the socket is made in other class. Not the one that i want to use to answer to the client, so if i dont have the socket, i dont have the OutputStream, which i need to talk back. That class, the one processing and sending information to the dark side, is going to be working with hundred of transactions in group.
My Issue is here: I can't report to the client that is done because i dont have the sockets references in that class. I instance the clients thread like:
new Client(socket).start();
Objects without references variables, but, i have an option i can take: Store the Sockets and their ip's in a HashMap object at the moment that a new connection is made, like this:
sockets.put(newSocket.getInetAddress().getHostAddress(), newSocket);
Then i can get the socket(so i can get the OutputStream and answer) calling an static method like this:
public static Socket getSocket(String IP) {
Socket RequestedSocket;
RequestedSocket = sockets.get(IP);
return RequestedSocket;
}
But i want you to tell me if there is a better way of doing this, better than storing all of those sockets in a list/hashmap. How can i get those objects without reference variables ? Or maybe thats a good way of doing it and im just trying to overpass the limits.
P.S.: I tried to store the Client objects in the database, serializing them, but the sockets can't be serialized.
Thanks.
This is a design issue for you. You will need to keep track of them somewhere, one solution might be to simply create a singleton class [SocketMapManager] for instance that holds the hashmap, so that you can access it statically from other classes. http://www.javaworld.com/javaworld/jw-04-2003/jw-0425-designpatterns.html
Any solution that tells you to keep a reference to the socket/ connection/ stream is bad -> as that means your connections are going to be held up while the server does its work.
You have a couple of options open
1. have the clients act as servers too. when they connect, they give the server their IP, port and some secret string as part of the hand shake. This means you have control over client code to make this happen.
the servers have a protocol to either take new jobs or check status of old jobs. Client pools the server periodically.
clients connect to database or other application (web service or plain socket like the original app) that connects to data base to get the status of the job. Meaning server gives client a job id.
a socket is open then it one OS resource open. can read up Network Programming: to maintain sockets or not?
All depends on
1. how many client connect at a time/ in 5 minutes.
2. how many seconds/ minutes does one client's request take to process
if number of clients in 5 minutes is maximum (in next 3 years) 300 at a time/ in any 5 minute duration and each request takes at a max 50 seconds to process then a dedicated server with max 50,000 sockets should suffice. Else you need async or more servers (and a DNS/ web server/ port forwarding or other method for load balance)
I'm having a bit of a problem trying to understand what is the flow of the operations, and what exactly you have at disposition. Is this sequence correct?
1. client writes to database (delphi)
2. client writes to server (delphi)
3. server writes to database (java)
4. server writes to client (java)
5. client reads database (delphi)
And the problem is pass 4?
More important: you are saying that there isn't a socket in the Client class, and that you don't have a list of Client too?
Are you able to use the reflection to search/obtain a socket reference from Client?
If you say you don't have the socket, how could it be that you can add that socket in a HashMap?
Last but not least: why do you need to store the socket? Maybe every client opens one connection which is used for multiple requests?
It could be beautiful if all the answers could be conveyed to just one ip:port...
I have an application where I have to send Signals and Data between two entities.
Which way is best practice and why?
Open 4 sockets, 2 for signals in both directions and 2 for data in both directions
Open 2 sockets each for data and signals
Just open 1 socket and filter for signals and data
First of all sockets are full-duplex, i.e. you don't need a separate socket (connection) for sending and receiving.
Secondly, it's hard to tell without going into more details. However knowing the history of HTTP (one connection for commands/headers and data) and FTP (separate connections for commands and data) seems like HTTP designers made a better choice. Two connections is more code to maintain, also some firewalls don't like hanging, idle FTP command connection when large portions of data are transferred.
So, go for one connection, it's easy to distinguish between signals and data (flag, header, etc.) Also incoming and outgoing channel is completely orthogonal.
Why can't you just have 1 socket for all data and 1 socket for all signals. You can send signals to the remote entity while reading signals from the remote entity from the same socket. This is sort of like how FTP (non-passive) works, you have a control connection and then one used for the actual data transfer.
Having 4 sockets seems overkill, but I don't know the context of what the signals or data are.
If you don't mind more code to decode the network communication, you can definitely do it all in one socket. It'll require more thought into the actual communication, a better "protocol" if you will. Both the 2 socket and 1 socket model are fine.
There is a possible "middle way" - use JBoss remoting or similar to multiplex several virtual sockets on top of one real one. That way you can keep the data and control channels separate at the application level but still require only one TCP port (and thus at most one firewall hole) at the operating system level.
Another way: use a dedicated middleware, built for realtime application like 0MQ.
To instantiate ØMQ:
Jzmq obj = new Jzmq (hostname);
Where hostname is name or IP address of the box where zmq_server is running.
To create wiring, createExchange, createQueue and bind functions can be used. For detailed description of how wiring mechanism works have a look here.
int eid = obj.createExchange ("E", Jzmq.SCOPE_GLOBAL, "10.0.0.1:5555");
obj.createQueue ("Q", Jzmq.SCOPE_GLOBAL, "10.0.0.1:5556");
obj.bind ("E", "Q");
Sending a message is pretty straightforward. Message is supplied in form of byte array:
byte msg [] = {1, 2, 3, 4, 5, 6};
obj.send (eid, msg);
Receiving a message is even more simple:
byte [] msg = obj.receive ();
Full sample available here.
I am writing the code for a server that would help two different applications in different platforms communicate with each other. To visualize it a bit it would be something like this :
App1<------>Server<------>App2
What server does is rear var1 from app2, write it to app1, then read var2 from app1 and write it to app2. Like this :
while(true){
var1 = app2stream.readInt();
app1stream.writeInt(var1);
var2 = app1stream.readDouble();
app2stream.writeDouble(var2);
}
My problem is that at some point i have this code at my server :
app1.accept();
app2.accept();
What this means is that no matter what, and given the fact that the server is always running, app1 is the one that should connect first since app1.accept() is a blocking method.
Is there any way around this? It would be great to allow the two applications to connect to the server regardless of who "came" first and then wait for the server to proceed with the above code. Can i use threads just for the accept() part and then pass the streams to the other thread? I read a bit about channels but got a bit buffled, any examples would be great.
Use NIO
It allows you to do non-blocking sockets (including accept) using the Selector class.
Basically, it gives you much more native access to the system libraries and the ability to handle your task without having to multi-thread.
Only have one accept call, and one server socket. You can make the determination which app has connected once they connect. If you can't get it from the connection details, have them send an authcode (probably a good idea anyway) which you can map to your app.
You should probably treat them both the same unless they say otherwise.
For example when the each socket connects send a "what client?" message.
Then check whether the client responds with 1 or 2.
If both respond with 1 or something just disconnect both.
I think the "standard" way to do this is to have the server listening on a port, and when a message comes in, immediately spin off a new thread to handle it, then go back to listening for another message. Then, as Glowcoder says, make all the connections in the same loop and make it figure out which is which after connecting.
I suppose the alternative is to have multiple threads, each listening on different ports. I've never tried to do that, I'm not sure if one would block until a connection was made and so you'd never get to the other thread.