ThousandIsland.Handler defines the behaviour required of the application layer of a Thousand Island server. When starting a Thousand Island server, you must pass the name of a module implementing this behaviour as the handler_module parameter. Thousand Island will then use the specified module to handle each connection that is made to the server.

The lifecycle of a Handler instance is as follows:

1. After a client connection to a Thousand Island server is made, Thousand Island will complete the initial setup of the connection (performing a TLS handshake, for example), and then call handle_connection/2.

2. A handler implementation may choose to process a client connection within the handle_connection/2 callback by calling functions against the passed ThousandIsland.Socket. In many cases, this may be all that may be required of an implementation & the value {:ok, :close, state} can be returned which will cause Thousand Island to close the connection to the client.

3. In cases where the server wishes to keep the connection open and wait for subsequent requests from the client on the same socket, it may elect to return {:ok, :continue, state}. This will cause Thousand Island to wait for client data asynchronously; handle_data/3 will be invoked when the client sends more data.

4. In the meantime, the process which is hosting connection is idle & able to receive messages sent from elsewhere in your application as needed. The implementation included in the use ThousandIsland.Handler macro uses a GenServer structure, so you may implement such behaviour via standard GenServer patterns. Note that in these cases that state is provided (and must be returned) in a {socket, state} format, where the second tuple is the same state value that is passed to the various handle_* callbacks defined on this behaviour.

It is fully supported to intermix synchronous ThousandIsland.Socket.recv calls with async return values from handle_connection/2 and handle_data/3 callbacks.

Example

A simple example of a Hello World server is as follows:

defmodule ThousandIsland.Handlers.HelloWorld do
  use ThousandIsland.Handler

  @impl ThousandIsland.Handler
  def handle_connection(socket, state) do
    ThousandIsland.Socket.send(socket, "Hello, World")
    {:ok, :close, state}
  end
end

Another example of a server that echoes back all data sent to it is as follows:

defmodule ThousandIsland.Handlers.Echo do
  use ThousandIsland.Handler

  @impl ThousandIsland.Handler
  def handle_data(data, socket, state) do
    ThousandIsland.Socket.send(socket, data)
    {:ok, :continue, state}
  end
end

Note that in this example there is no handle_connection/2 callback defined. The default implementation of this callback will simply return {:ok, :continue, state}, which is appropriate for cases where the client is the first party to communicate.

Another example of a server which can send and receive messages asynchronously is as follows:

defmodule ThousandIsland.Handlers.Messenger do
  use ThousandIsland.Handler

  @impl ThousandIsland.Handler
  def handle_data(msg, _socket, state) do
    IO.puts(msg)
    {:ok, :continue, state}
  end

  def handle_info({:send, msg}, {socket, state}) do
    ThousandIsland.Socket.send(socket, msg)
    {:noreply, {socket, state}}
  end
end

Note that in this example we make use of the fact that the handler process is really just a GenServer to send it messages which are able to make use of the underlying socket. This allows for bidirectional sending and receiving of messages in an asynchronous manner.

When Handler Isn't Enough

The use ThousandIsland.Handler implementation should be flexible enough to power just about any handler, however if this should not be the case for you, there is an escape hatch available. If you require more flexibility than the ThousandIsland.Handler behaviour provides, you are free to specify any module which implements start_link/1 as the handler_module parameter. The process of getting from this new process to a ready-to-use socket is somewhat delicate, however. The steps required are as follows:

1. Thousand Island calls start_link/1 on the configured handler_module, passing in the configured handler_options as the sole argument. This function is expected to return a conventional GenServer.on_start() style tuple. Note that this newly created process is not passed the connection socket immediately.
2. The socket will be passed to the new process via a message of the form {:thousand_island_ready, socket}.
3. Once the process receives the socket, it must call ThousandIsland.Socket.handshake/1 with the socket as the sole argument in order to finalize the setup of the socket.
4. The socket is now ready to use.

In addition to this process, there are several other considerations to be aware of:

• The underlying socket is closed automatically when the handler process ends.

• Handler processes should have a restart strategy of :temporary to ensure that Thousand Island does not attempt to restart crashed handlers.

• Handler processes should trap exit if possible so that existing connections can be given a chance to cleanly shut down when shutting down a Thousand Island server instance.

• The :handler family of telemetry events are emitted by the ThousandIsland.Handler implementation. If you use your own implementation in its place you will not see any such telemetry events.