I have Java code that sets up a server on a particular port, some code is run in python that uses this server, and then the port is closed. I've set it up so that after each python run, the java loops and repeats this process (setting up a new server, serving, then closing).
I notice that when the python call is short (about <10s), that the next time Java tries to set up this port, it will fail. Are there rules about how long you must wait between successively setting up the same port on Java?
Ports can be finicky with this type of thing. They're not meant to be opened and closed all the time. You should leave the socket open and just handle each connection within, like this:
ServerSocket listener = new ServerSocket(9090);
try {
while (true) {
Socket socket = listener.accept();
try {
PrintWriter out =
new PrintWriter(socket.getOutputStream(), true);
out.println(new Date().toString());
} finally {
socket.close();
}
}
}
finally {
listener.close();
}
(credit: http://cs.lmu.edu/~ray/notes/javanetexamples/)
In general, it is better to maintain a long-running ServerSocket that serves multiple requests, as others have already answered and commented. However, I have found that sometimes there is a legitimate need to stop and start a server in rapid succession. One example is an integration test suite that involves stopping and restarting the server in different configurations to repeat test runs.
If you really have this need, then you might be interested in ServerSocket#setReuseAddress(boolean).
Enable/disable the SO_REUSEADDR socket option.
When a TCP connection is closed the connection may remain in a timeout state for a period of time after the connection is closed (typically known as the TIME_WAIT state or 2MSL wait state). For applications using a well known socket address or port it may not be possible to bind a socket to the required SocketAddress if there is a connection in the timeout state involving the socket address or port.
Enabling SO_REUSEADDR prior to binding the socket using bind(SocketAddress) allows the socket to be bound even though a previous connection is in a timeout state.
This ultimately enables the SO_REUSEADDR socket option. One source of information for more details on these socket options is the Linux socket man page.
However, please be aware that the exact perceived behavior can very greatly across different platforms. In particular, beware of the extremely different and dangerous behavior of this setting on Windows, as documented in the MSDN article Using SO_REUSEADDR and SO_EXCLUSIVEADDRUSE. Basically, SO_REUSEADDR on Windows can allow any arbitrary process to "steal" a socket already in use by another process, resulting in indeterminate behavior.
The SO_REUSEADDR socket option allows a socket to forcibly bind to a port in use by another socket. The second socket calls setsockopt with the optname parameter set to SO_REUSEADDR and the optval parameter set to a boolean value of TRUE before calling bind on the same port as the original socket. Once the second socket has successfully bound, the behavior for all sockets bound to that port is indeterminate. For example, if all of the sockets on the same port provide TCP service, any incoming TCP connection requests over the port cannot be guaranteed to be handled by the correct socket — the behavior is non-deterministic. A malicious program can use SO_REUSEADDR to forcibly bind sockets already in use for standard network protocol services in order to deny access to those service. No special privileges are required to use this option.
I advise people to think carefully and make sure you're confident with this setting rather than blindly turning it on. There is also a phenomenal prior question and answer about the relevant socket options:
Socket options SO_REUSEADDR and SO_REUSEPORT, how do they differ? Do they mean the same across all major operating systems?
Related
I have a java application which manages several socket connections to devices. I have no control over the protocol which these devices implement, and now I want my java application to send heartbeats for each device. The devices do not send data, but only respond to commands.
The javadoc for InputStream.read() states that if the end of stream is reached, it will return -1. So that seems like a reasonable way to check if the connection is open. But when I implement this solution, there are no bytes available (since the device only responds to commands), and since the connection is open, it will hang at the read call forever. Example, I peek at one byte and if that would be -1 the heartbeat would be "unhealthy":
public static void main(final String[] args) throws IOException {
try (Socket socket = new Socket()) {
socket.connect(new InetSocketAddress("192.168.30.99", 25901), 1000);
System.out.println("Connected");
final BufferedInputStream bis = new BufferedInputStream(socket.getInputStream());
bis.mark(1);
System.out.println(bis.read()); // Stalls forever here
bis.reset();
System.out.println("Done");
}
}
Is it reasonable to say that, if no byte is received within x milliseconds, the device is connected?
Is there any surefire way to check socket connectivity without heartbeats where the ip and port is important?
Is there any surefire way to check socket connectivity without
heartbeats where the ip and port is important?
No, you can't reliably know if the other end is alive unless you try to communicate with it.
If the other end doesn't have a no-op ping function, you're pretty much out of luck. Waiting in a blocking read() call won't help you if the connection gets cut off.
Is it reasonable to say that, if no byte is received within x
milliseconds, the device is connected?
No. It means that the device hasn't sent anything in x milliseconds. Which is normal, as it only responds to commands.
when the other end of socket do not write any byte and wait to read from socket first, blocking on read is the default behavior.
with no control over the protocol , little can be done.
it is reasonable to say, successful connect is a weaker heartbeat.
you don't have to wait for x miliseconds which makes no difference on such protocol
another tricky way , you can try to send a few bytes that most unlikely being a valid command,
for example the '\0' or '\n' ,
hoping that it will do no harm to the device and the device can close socket actively on such invalid command.
when the other end closes socket actively , read call on such socket should return -1
the better heartbeat way always have something to do with the protocol,
as the no-op ping command suggested by #Kayaman
Maybe TCP level keep-alive is solution for you:
You can turn it on by using command:
socket.setKeepAlive(true);
It sets SO_KEEPALIVE socket option. Quote from SocketOptions java-API:
When the keepalive option is set for a TCP socket and no data has been
exchanged across the socket in either direction for 2 hours (NOTE: the
actual value is implementation dependent), TCP automatically sends a
keepalive probe to the peer. This probe is a TCP segment to which the
peer must respond. One of three responses is expected: 1. The peer
responds with the expected ACK. The application is not notified (since
everything is OK). TCP will send another probe following another 2
hours of inactivity. 2. The peer responds with an RST, which tells the
local TCP that the peer host has crashed and rebooted. The socket is
closed. 3. There is no response from the peer. The socket is closed.
The purpose of this option is to detect if the peer host crashes.
Valid only for TCP socket: SocketImpl
You could also use SO_TIMEOUT by using:
socket.setSoTimeout(timeout);
Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds.
With this option set to a non-zero timeout, a read() call on the
InputStream associated with this Socket will block for only this
amount of time. If the timeout expires, a
java.net.SocketTimeoutException is raised, though the Socket is still
valid. The option must be enabled prior to entering the blocking
operation to have effect. The timeout must be > 0. A timeout of zero
is interpreted as an infinite timeout.
Call those right after connect() or accept() calls, before the program enters to
'no control of underlying protocl' -state.
I have a java socket calling a server. However, I do not know at which address I can reach the server, so I put several sockets in several threads and they try to reach the server each on one address. My probem is that I do not want to wait for the timeout but have no idea how to stop the sockets and their threads properly.
Code:
socket = new Socket();
socket.connect(endpoint, timeout); // **Blocking method**
OutputStream out = socket.getOutputStream();
//Write Data here
How can I interrupt the operation? I consider Thread.stop() a bad style and it also does not work properly. .NET Tcp Endpoints have a non-blocking pending method that allows uinsg boolean flags but I could not find something similiar
I do not know at which address I can reach the server, so I put
several sockets in several threads and they try to reach the server
each on one address.
BAD. BAD Decision. Perform some logical step to determine the server's address. Or, perform something that helps you know about the server's IP-Address.
Do this way, only if it is the last hope.
My problem is that I do not want to wait for the
timeout but have no idea how to stop the sockets and their threads
properly.
You don't have any other option that timeout. Socket.connect() is blocking. You can't do anything than waiting.
You've to wait for timeout because that is the logical way to close the socket object created. You can't just do close directly, until a timeout. Reduce the timeout to the limit when your result should come(connection should be accepted).
How can I interrupt the operation? I consider Thread.stop() a bad
style and it also does not work properly.
Yes, you should not perform Thread.stop() or Thread.interrupt(). These are bad programming styles.
If the timeout expires, make the close() operation on socket.
You should set a socket timeout for the client-socket. It is the best-practice to set a timeout for sockets. The timeout should be around 10 seconds to more depending on the needs.
You can set the timeout in your current code by calling
socket.setSoTimeout(timeout); for reading timeout, OR
for connect timeout, connect(endpoint,timeout) as you've done in your code.
If the timeout expires, a java.net.SocketTimeoutException is raised, though the Socket is still valid. The timeout must be > 0. A timeout of zero is interpreted as an infinite timeout.
You're probably not using a try-catch-finally in your code. That'd be a better design here.
As you're doing a connect timeout, so your code can be amended to exit the blocking method like as shown below :
try{
socket = new Socket();
socket.connect(endpoint,timeout); // **Blocking method**
OutputStream out = socket.getOutputStream();
//Write Data here
}
catch(Exception e){
e.printStackTrace();
}
finally{
socket.close();
}
Using the App Engine Trusted Tester Sockets to connect to APNS. Writing to socket works fine.
But the problem is that the Socket gets reclaimed after 2 minutes of inactivity. It says in the Trusted Tester Website that any socket operation keeps the socket alive for further 2 minutes. It is nicer to keep the socket open until APNS decides to close the connection.
After trying pretty much all of the Socket API methods short of writing to the Output Stream, Socket gets closed after 2 minutes no matter what. What have I missed?
Deployed on java backend.
You can't keep a socket connected to APNS artifically open; without sending actual push notifications. The only way to keep it open is to send some arbitrary data/bytes but that would result in an immediate closure of the socket; APNS closes the connection as soon as it detects something that does not conform to the protocol, i.e. something that is not an actual push notification.
SO_KEEPALIVE
What about SO_KEEPALIVE? App Engine explicitly says it is supported. I think it just means it won't throw an exception when you call Socket.setKeepAlive(true); calls wanted to set socket options raised Not Implemented exceptions before. Even if you enable keep-alive your socket will be reclaimed (closed) if you don't send something for more than 2 minutes; at least on App Engine as of now.
Actually, it's not a big surprise. RFC1122 that specifies TCP Keep Alive explicitly states that TCP Keep Alives are not to be sent more than once every two hours, and then, it is only necessary if there was no other traffic. Although, it also says that this interval must be also configurable, there is no API on java.net.Socket you could use to configure that (most probably because it's highly OS dependent) and I doubt it would be set to 2 minutes on App Engine.
SO_TIMEOUT
What about SO_TIMEOUT? It is for something completely else. The javadoc of Socket.setSoTimeout() states:
Enable/disable SO_TIMEOUT with the specified timeout, in milliseconds. With this option set to a non-zero timeout, a read() call on the InputStream associated with this Socket will block for only this amount of time. If the timeout expires, a java.net.SocketTimeoutException is raised, though the Socket is still valid. The option must be enabled prior to entering the blocking operation to have effect. The timeout must be > 0. A timeout of zero is interpreted as an infinite timeout.
That is, when read() is blocking for too long because there's nothing to read you can say "ok, I don't want to wait (block) anymore; let's do something else instead". It's not going to help with our "2 minutes" problem.
What then?
The only way you can work around this problem is this: detect when a connection is reclaimed/closed then throw it away and open a new connection. And there is a library which supports exactly that.
Check out java-apns-gae.
It's an open-source Java APNS library that was specifically designed to work (and be used) on Google App Engine.
https://github.com/ZsoltSafrany/java-apns-gae
Did you try getSoLinger()? That may be the getSocketOpt that works (kind of) currently and it may reset the 2 minute timeout. In theory, also doing a zero byte read would as well but I'm not sure that would, if you try that, use this method on the inputstream.
public int read(byte b[], int off, int len)
If these suggestions don't work, please file an issue with the App Engine issue tracker.
There will be some other fixes coming, e.g. using socket options etc.
Use getpeername().
From https://developers.google.com/appengine/docs/java/sockets/overview ...
Sockets may be reclaimed after 2 minutes of inactivity; any socket
operation (e.g. getpeername) keeps the socket alive for a further 2
minutes. (Notice that you cannot Select between multiple available
sockets because that requires java.nio.SocketChannel which is not
currently supported.)
We have a simple client server architecture between our mobile device and our server both written in Java. An extremely simple ServerSocket and Socket implementation. However one problem is that when the client terminates abruptly (without closing the socket properly) the server does not know that it is disconnected. Furthermore, the server can continue to write to this socket without getting any exceptions. Why?
According to documentation Java sockets should throw exceptions if you try to write to a socket that is not reachable on the other end!
The connection will eventually be timed out by Retransmit Timeout (RTO). However, the RTO is calculated using a complicated algorithm based on network latency (RTT), see this RFC,
http://www.ietf.org/rfc/rfc2988.txt
So on a mobile network, this can be minutes. Wait 10 minutes to see if you can get a timeout.
The solution to this kind of problem is to add a heart-beat in your own application protocol and tear down connection when you don't get ACK for the heartbeat.
The key word here (without closing the socket properly).
Sockets should always be acquired and disposed of in this way:
final Socket socket = ...; // connect code
try
{
use( socket ); // use socket
}
finally
{
socket.close( ); // dispose
}
Even with this precautions you should specify application timeouts, specific to your protocol.
My experience had shown, that unfortunately you cannot use any of the Socket timeout functionality reliably ( e.g. there is no timeout for write operations and even read operations may, sometimes, hang forever ).
That's why you need a watchdog thread that enforces your application timeouts and disposes of sockets that have been unresponsive for a while.
One convenient way of doing this is by initializing Socket and ServerSocket through corresponding channels in java.nio. The main advantage of such sockets is that they are Interruptible, that way you can simply interrupt the thread that does socket protocol and be sure that socket is properly disposed off.
Notice that you should enforce application timeouts on both sides, as it is only a matter of time and bad luck when you may experience unresponsive sockets.
TCP/IP communications can be very strange. TCP will retry for quite a while at the bottom layers of the stack without ever letting the upper layers know that anything happened.
I would fully expect that after some time period (30 seconds to a few minutes) you should see an error, but I haven't tested this I'm just going off how TCP apps tend to work.
You might be able to tighten the TCP specs (retry, timeout, etc) but again, haven't messed with it much.
Also, it may be that I'm totally wrong and the implementation of Java you are using is just flaky.
To answer the first part of the question (about not knowing that the client has disconnected abruptly), in TCP, you can't know whether a connection has ended until you try to use it.
The notion of guaranteed delivery in TCP is quite subtle: delivery isn't actually guaranteed to the application at the other end (it depends on what guaranteed means really). Section 2.6 of RFC 793 (TCP) gives more details on this topic. This thread on the Restlet-discuss list and this thread on the Linux kernel list might also be of interest.
For the second part (not detecting when you write to this socket), this is probably a question of buffer and timeout (as others have already suggested).
I am facing the same problem.
I think when you register the socket with a selector it doesn't throw any exception.
Are you using a selector with your socket?
My chat application connects to a server and information is sent/received by the user. When the connection changes, such as 3g->wifi, wifi->3g, losing a data connection, etc, the socket sometimes stays connected for ages before disconnecting. During this time, it's impossible to tell if the connection is still active, it seems as if messages are being sent just fine. Other times, when sending a message, it will throw an IO error and disconnect.
Apart from implementing code to detect connection changes and reconnecting appropriately, is it possible to have the socket immediately throw an IO exception when connectivity changes?
Edit: I'm connecting using the following code:
Socket sock = new Socket();
sock.connect(new InetSocketAddress(getAddress(), getPort())), getTimeout());
//get bufferedReader and read until BufferedReader#readLine() returns null
I'm not using setSoTimeout as data may not be transferred for long periods of time depending on the remote server's configuration.
Are you talking about a java.net.Socket connection? Then try setSoTimeout(). Otherwise specify how you're connecting.
This is an old problem that I've seen a few times before in the database world.
The solution I used there was to manage the connection at the application level. I'd explicitly send a no-op message of some sort (i.e. SELECT 1 WHERE FALSE) over the connection every so often as a ping, and if this failed I would tear down and re-establish the connection, possibly to a failover server if the original wasn't accepting connections.
As previous answers already pointed out, this is a common problem. Even after sending a custom "ping" it might need some time until the socket realizes that the underlying connection is broken. Plus, regular pings are quite energy-demanding using 3-4G mobile networks, due to their tail states. Don't do that!
What you can do, however, is requesting to get informed when the connectivity changes (last section), and close/reconnect the socket manually in the according broadcast receiver. (EDIT: I see you already found out about this; just keeping it here for completeness)