Networking and Streams

Julia provides a rich interface to deal with streaming I/O objects such as Terminals, Pipes and Tcp Sockets. This interface, though asynchronous at the system level, is presented in a synchronous manner to the programmer and it is usually unnecessary to think about the underlying asynchronous operation. This is achieved by making heavy use of Julia cooperative threading (coroutine) functionality.

So, let’s jump right in with a simple example involving Tcp Sockets. To do, let’s first create a simple server.

julia> @async begin
         server = listen(2000)
         while true
           sock = accept(server)
           println("Hello World\n")
         end
       end
Task

julia>

Those familiar with the Unix socket API, the method names will feel familiar, though their usage is somewhat simpler than the raw Unix socket API. The first call to listen will create a server waiting for incoming connections on the specified port (2000) in this case. The same function may also be used to create various other kinds of servers:

julia> listen(2000) # Listens on localhost:2000 (IPv4)
TcpServer(active)

julia> listen(ip"127.0.0.1",2000) # Equivalent to the first
TcpServer(active)

julia> listen(ip"::1",2000) # Listens on localhost:2000 (IPv6)
TcpServer(active)

julia> listen(IPv4(0),2001) # Listens on port 2001 on all IPv4 interfaces
TcpServer(active)

julia> listen(IPv6(0),2001) # Listens on port 2001 on all IPv6 interfaces
TcpServer(active)

julia> listen("testsocket") # Listens on a domain socket/named pipe
PipeServer(active)

Note that the return type of the last invocation is different. This is because this server does not listen on TCP, but rather on a Named Pipe (Windows terminology) - also called a Domain Socket (UNIX Terminology). The difference is subtle but, has to do with the accept and connect methods. The accept method retrieves a connection to the client that is connecting on the server we just created, while the connect function connects to a server using the specified method. The connect function takes the same arguments as the listen, so, assuming the environment (i.e. host, cwd, etc.) is the same you should be able to pass the same arguments to connect as you did to listen to establish the connection. So let’s try that out (after having created the server above):

julia> connect(2000)
TcpSocket(open, 0 bytes waiting)

julia> Hello World

As expected we saw “Hello World” printed. So, let’s actually analyze what happened behind the scenes. When we called connect, we connect to the server we had just created. Meanwhile, the accept function returns a server-side connection to the newly created socket and prints “Hello World” to indicate that the connection was successful.

A great strength of Julia is that since the API is exposed synchronously even though the I/O is actually happening asynchronously, we didn’t have to worry callbacks or even making sure that the server gets to run. When we called connect the current task waited for the connection to be established and only continued executing after that was done. In this pause, the server task resumed execution (because a connection request was now available), accepted the connection, printed the message and waited for the next client. Reading and writing works in the same way. To see this, consider the following simple echo server:

julia> @async begin
         server = listen(2001)
         while true
           sock = accept(server)
           @async while true
             write(sock,readline(sock))
           end
         end
       end
Task

julia> clientside=connect(2001)
TcpSocket(open, 0 bytes waiting)

julia> @async while true
          write(STDOUT,readline(clientside))
       end

julia> println(clientside,"Hello World from the Echo Server")

julia> Hello World from the Echo Server

Resolving IP Addresses

One of the connect methods that does not follow the listen methods is connect(host::ASCIIString,port), which will attempt to connect to the host given by the host parameter on the port given by the port parameter. It allows you to do thins like:

julia> connect("google.com",80)
TcpSocket(open, 0 bytes waiting)

At the base of this functionality is the getaddrinfo function which will do the appropriate address resolution:

julia> Base.getaddrinfo("google.com")
IPv4(74.125.226.225)