File Activation

NPL can communicate with remote NPL script using the NPL.activate function. It is a powerful function, and is available in C++, mono plugin too.

Basic Syntax

Like in neural network, all communications are asynchronous and uni-directional without callbacks. Although the function does return a integer value. Please see NPL Reference for details.

NPL.activate(url, {any_pure_data_table_here, })
  • @param url: a globally unique name of a NPL file name instance. The string format of an NPL file name is like below. [(sRuntimeStateName|gl)][sNID:]sRelativePath[] the following is a list of all valid file name combinations:
    • – a file of user001 in its default gaming thread
    • (world1) – a file of server001 in its thread world1
    • (worker1)script/hello.lua – a local file in the thread worker1
    • (gl)script/hello.lua – a glia (local) file in the current runtime state’s thread
    • script/hello.lua – a file in the current thread. For a single threaded application, this is usually enough.
    • (worker1)NPLRouter.dll – activate a C++ file. Please note that, in windows, it looks for NPLRonter.dll; in linux, it looks for ./
    • (worker1)DBServer.dll/DBServer.DBServer.cs – for C# file, the class must have a static activate function defined in the CS file.
  • @param msg: it is a chunk of pure data table that will be transmitted to the destination file.

Neuron File

Only files associated with an activate function can be activated. This is done differently in NPL/C++/C# plugin

  • In NPL, it is most flexible by using the build-in NPL.this function
local function activate()
   -- input is passed in a global "msg" variable
  • msg is passed in a global msg variable which is visible to all files, and the msg variable will last until the thread receives the next activation message.
  • In NPL’s C++ plugin, you need to define a C function. Please see ParacraftSDK’s example folder for details.
  • In NPL’s mono C# plugin, you simply define a class with a static activate function. Please see ParacraftSDK’s example folder for details.

Input Message, msg.nid and msg.tid

In above code, msg contains the information received from the sender, plus the source id of the sender. For unauthenticated senders, the source id is stored in msg.tid, which is an auto-generated number string like “~1”. The receiver can keep using this temporary id msg.tid to send message back, such as

local function activate()
   -- input is passed in a global "msg" variable
   NPL.activate(format("%s:some_reply_file.lua", msg.tid or msg.nid), {"replied"});

The receiver can also rename the temporary msg.tid by calling NPL.accept(msg.tid, nid_name), so the next time if the receiver got a message from the same sender (i.e. the same TCP connection), the msg.nid contains the last assigned name and msg.tid field no longer exists. We usually use NPL.accept to distinguish between authenticated and unauthenticated senders, and reject unauthenticated messages by calling NPL.reject(msg.tid) as early as possible to save CPU cycles.

See following example:

local function activate()
   -- input is passed in a global "msg" variable
   if(msg.tid) then
      -- unauthenticated? reject as early as possible or accept it. 
      if(msg.password=="123") then
         NPL.accept(msg.tid, msg.username or "default_user");
   elseif(msg.nid) then
      -- only respond to authenticated messages. 
      NPL.activate(format("%s:some_reply_file.lua", msg.nid), {"replied"});

Please note, msg.tid or msg.nid always match to a single low-level TCP connection, hence their names are shared to all neuron files in the process. For example, if you accept in one neuron file, all other neuron files will receive messages in msg.nid form.

Please see

Neuron File Visibility

For security reasons, all neuron files can be activated by other files in the same process. This includes scripts in other local threads of the same process. See also MultiThreading.

To expose script to remote computers, one needs to do two things.

  • one is to start NPL server by listening to an IP and port: NPL uses TCP protocol for all communications.
  • second is to tell NPL runtime that a given file is a public neuron file.

See below:

NPL.StartNetServer("", 8080);
NPL.AddPublicFile(filename, id);
  • “” means all IP addresses, one can also use “”, “localhost” or whatever IP addresses.
  • 8080: is the port number. Pick any one you like.

The second parameter to NPL.AddPublicFile is an integer, which is transmitted on behalf of the long filename to save bandwidth. So it must be unique if you add multiple public files.

Please note, that file name must be relative to working directory. such as NPL.AddPublicFile("script/test/test.lua", 1). Absolute path is not supported at the moment.

Activating remote file

Once a server NPL runtime exposes a public file, other client NPL runtime can activate it with the NPL.activate function. Please note that, an NPL runtime can be both server and client. The one who started the connection is usually called server. Pure client must also call NPL.StartNetServer in order to activate the server. But it can specify port="0" to indicate that it will not listen for incoming connections.

However, on the client, we need to assign a local name to the remote server using the NPL.AddNPLRuntimeAddress, so that we can refer to this server by name in all subsequent NPL.activate call.

NPL.AddNPLRuntimeAddress({host = "", port = "8099", nid = "server1"})

Usually we do this during initialization time only once. After that we can activate public files on the remote server like below:

NPL.activate("server1:helloworld.lua", {})

Please note the name specified by nid is arbitrary and used only on the client computer to refer to a computer. In other words, different clients can name the same remote computer differently.

Message Delivery Guarantees

Please note that the first time that a computer activate a remote file, a TCP connection is automatically established, but the first message is NOT delivered. This is because NPL.activate() is asynchronous, it must return a value before the new connection is established. It always returns 0 when your message is delivered via an existing path, and non-zero in case of first message to a remote system.

If there is no already established path (i.e no TCP connection), NPL will immediately try to establish it. However, please note, the message that returns non-zero is NOT delivered, even if NPL successfully established a path to the remote system soon afterwards. Thus it is the programmer’s job to activate again until NPL.activate returns 0. This guarantees that a message with 0 return value is always delivered at least in the viewpoint of NPL runtime.

The same mechanism can be used to recover lost-connections.

To write fault-tolerant message passing code, consider following.

  • When a NPL runtime process start, ping remote process with NPL.activate until it returns 0 to establish TCP connections. This ensures that all subsequent NPL.activate across these two systems can be delivered.
  • Discover Lost Connections:
    • Method1: Use a timer to ping or listen for disconnect system event and reconnect in case connection is lost. However, individual messages may be lost during these time.
    • Method2: Use a wrapper function to call NPL.activate, which checks its return value. If it is non-zero, either reconnect with timeout or put message to a pending queue in case connection can be recovered shortly and resend queued messages.

We leave it to the programmer to handle all occasions when NPL.activate returns non-zero values, since different business logic may use a different approach.

Example Client/Server app

To run the example, call following.

npl "script/test/network/SimpleClientServer.lua" server="true"
npl "script/test/network/SimpleClientServer.lua" client="true"

The source code of this demo is also included in ParaCraftSDK/examples folder.

filename: script/test/network/SimpleClientServer.lua

Author: Li,Xizhi
Date: 2009-6-29
Desc: start one server, and at least one client. 
npl "script/test/network/SimpleClientServer.lua" server="true"
npl "script/test/network/SimpleClientServer.lua" client="true"
NPL.load("(gl)script/ide/commonlib.lua"); -- many sub dependency included

local nServerThreadCount = 2;
local initialized;
local isServerInstance = ParaEngine.GetAppCommandLineByParam("server","false") == "true";

-- expose these files. client/server usually share the same public files
local function AddPublicFiles()
    NPL.AddPublicFile("script/test/network/SimpleClientServer.lua", 1);

-- NPL simple server
local function InitServer()
    NPL.StartNetServer("", "60001");
    for i=1, nServerThreadCount do
        local rts_name = "worker"..i;
        local worker = NPL.CreateRuntimeState(rts_name, 0);
    LOG.std(nil, "info", "Server", "server is started with %d threads", nServerThreadCount);

-- NPL simple client
local function InitClient()

    -- since this is a pure client, no need to listen to any port. 
    NPL.StartNetServer("0", "0");
    -- add the server address
    NPL.AddNPLRuntimeAddress({host="", port="60001", nid="simpleserver"})
    LOG.std(nil, "info", "Client", "started");
    -- activate a remote neuron file on each thread on the server
    for i=1, nServerThreadCount do
        local rts_name = "worker"..i;
        while( NPL.activate(string.format("(%s)simpleserver:script/test/network/SimpleClientServer.lua", rts_name), 
            {TestCase = "TP", data="from client"}) ~=0 ) do
            -- if can not send message, try again.
            echo("failed to send message");

local function activate()
    if(not initialized) then
        initialized = true;
        if(isServerInstance) then
    elseif(msg and msg.TestCase) then
        LOG.std(nil, "info", "test", "%s got a message", isServerInstance and "server" or "client");

The above server is actually multi-threaded, please see MultiThreading for details.

The first time, NPL.activate calls a new remote server (with which we have not established TCP connection), the message is dropped and returned a non-zero value. NPLExtension.lua contains a number of helper functions to help you for sending a guaranteed message, such as NPL.activate_with_timeout.You need to include commonlib to use it.

Trusted Connections and NID

In the receiver’s activate function, it can assign any name or nid to incoming connection’s NPL runtime.

HelloWorld Example

Now, here comes a more complicated helloworld. It turns an ordinary helloworld.lua file into a neuron file, by associating an activate function with it. The file is then callable from any NPL thread or remote computer by its NPL address(url).

local function activate()
   if(msg) then
      print( or "");
   NPL.activate("(gl)helloworld.lua", {data="hello world!"})

Simple Web Server Example

NPL uses a HTTP-compatible protocol, so it is possible to handle standard HTTP request using the same NPL server. When NPL runtime receives a HTTP request message, it will send it to a publicly visible file with id -10. So we can create a simple HTTP web server with just a number of lines, like below:

filename: main.lua


local function StartWebServer()
    local host = "";
    local port = "8099";
    -- tell NPL runtime to route all HTTP message to the public neuron file `http_server.lua`
    NPL.AddPublicFile("source/SimpleWebServer/http_server.lua", -10);
    NPL.StartNetServer(host, port);
    LOG.std(nil, "system", "WebServer", "NPL Server started on ip:port %s %s", host, port);

local function activate()

filename: http_server.lua


local tostring = tostring;
local type = type;

local npl_http = commonlib.gettable("MyCompany.Samples.npl_http");

-- whether to dump all incoming stream;
npl_http.dump_stream = false;

-- keep statistics
local stats = {
    request_received = 0,

local default_msg = "HTTP/1.1 200 OK\r\nContent-Length: 31\r\nContent-Type: text/html\r\n\r\n<html><body>hello</body></html>";

local status_strings = {
    ok ="HTTP/1.1 200 OK\r\n",
    created ="HTTP/1.1 201 Created\r\n",
    accepted ="HTTP/1.1 202 Accepted\r\n",
    no_content = "HTTP/1.1 204 No Content\r\n",
    multiple_choices = "HTTP/1.1 300 Multiple Choices\r\n",
    moved_permanently = "HTTP/1.1 301 Moved Permanently\r\n",
    moved_temporarily = "HTTP/1.1 302 Moved Temporarily\r\n",
    not_modified = "HTTP/1.1 304 Not Modified\r\n",
    bad_request = "HTTP/1.1 400 Bad Request\r\n",
    unauthorized = "HTTP/1.1 401 Unauthorized\r\n",
    forbidden = "HTTP/1.1 403 Forbidden\r\n",
    not_found = "HTTP/1.1 404 Not Found\r\n",
    internal_server_error = "HTTP/1.1 500 Internal Server Error\r\n",
    not_implemented = "HTTP/1.1 501 Not Implemented\r\n",
    bad_gateway = "HTTP/1.1 502 Bad Gateway\r\n",
    service_unavailable = "HTTP/1.1 503 Service Unavailable\r\n",
npl_http.status_strings = status_strings;

-- make an HTML response
-- @param return_code: nil if default to "ok"(200)
function npl_http.make_html_response(nid, html, return_code, headers)
    if(type(html) == "table") then
        html = commonlib.Lua2XmlString(html);
    npl_http.make_response(nid, html, return_code, headers);

-- make a json response
-- @param return_code: nil if default to "ok"(200)
function npl_http.make_json_response(nid, json, return_code, headers)
    if(type(html) == "table") then
        json = commonlib.Json.Encode(json)
    npl_http.make_response(nid, json, return_code, headers);

-- make a string response
-- @param return_code: nil if default to "ok"(200)
-- @param body: must be string
-- @return true if send. 
function npl_http.make_response(nid, body, return_code, headers)
    if(type(body) == "string" and nid) then
        local out = {};
        out[#out+1] = status_strings[return_code or "ok"] or return_code["not_found"];
        if(body~="") then
            out[#out+1] = format("Content-Length: %d\r\n", #body);
        if(headers) then
            local name, value;
            for name, value in pairs(headers) do
                if(name ~= "Content-Length") then
                    out[#out+1] = format("%s: %s\r\n", name, value);
        out[#out+1] = "\r\n";
        out[#out+1] = body;

        -- if file name is "http",  the message body is raw http stream
        return NPL.activate(format("%s:http", nid), table.concat(out));

local function activate()
    stats.request_received = stats.request_received + 1;
    local msg=msg;
    local nid = msg.tid or msg.nid;
    if(npl_http.dump_stream) then
        log("HTTP:"); echo(msg);
    npl_http.make_response(nid, format("<html><body>hello world. req: %d. input is %s</body></html>", stats.request_received, commonlib.serialize_compact(msg)));

For a full-fledged build-in HTTP server framework in NPL, please see WebServer