Erlang server, Java client - TCP messages get split?

Question

As the title says, I have a server written in Erlang, a client written in Java and they are communicating through TCP. The problem that I am facing is the fact that gen_tcp:

Answer 1

As Jon said, TCP is a streaming protocol and has no concept of a message in the sense that you are looking for. It is often broken up based on your rate of reading, kernerl buffer size, MTU of network, etc... There are no guarantees that you don't get your data 1 byte at a time.

The easiest change to make to your app to get what you want is to change the erlang server side's TCP_OPTIONS {packet,0} to {packet,4}

and change the java writer code to:

while(true) {
   byte[] data = sc.nextLine().getBytes(StandardCharsets.UTF_8); // or leave out the UTF_8 for default platform encoding
   output.writeInt(data.length);
   output.write(data,0,data.length);
}

you should find that you receive exactly the right message.

You also should add {packet,4} to the erlang client if you make this change on the server side as the server now expects a 4 byte header indicating the size of the message.

note: the {packet,N} syntax is transparent in erlang code, the client doesn't need to send the int, and the server doesn't see the int. Java doesn't have the equivalent of size framing in the standard library, so you have to write the int size yourself.