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A variant of Msgpack-RPC that has faster decoding.
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Framed Msgpack RPC

Framed Msgpack RPC (FMPRPC) is an RPC system for node.js. It allows clients to call remote procedures on servers. An RPC consists of: (1) a simple string name; (2) an argument that is a single JSON object; (3) a reply that is also a single JSON object. Of course, those objects can be arrays, or dictionaries, so arguments and return values can be complex and interesting.

FMPRPC is a variant of the Msgpack-RPC protocol specification for node.js. Msgpack-RPC communicates binary JSON objects that are efficiently encoded and decoded with the MessagePack serialization format.

"Framed" Msgpack-RPC differs from standard Msgpack-RPC in a small way: the encoding of the length of the packet is prepended to each packet. This way, receivers can efficiently buffer data until a full packet is available to decode. In an event-based context like node.js, framing simplifies implementation, and yields a faster decoder, especially for very large messages.

By convention, RPCs are grouped into programs, which can have one or more versions. Each (prog,vers) pair then has a collection of procedures, meaning an RPC is identified unabmiguously by a (prog,vers,proc) triple. In practice, these three strings are joined with "." characters, and the dotted triple is the RPC name.

Due to framing, this protocol is not compatible with existing Msgpack-RPC systems. This implementation supports TCP transports only at the current time.


The simplest way to write a server is with the Server class as below:

var rpc = require('framed-msgpack-rpc');
var srv= new rpc.Server ({
    programs : {
        "myprog.1" : {
            add : function(arg, response) {
                response.result(arg.a + arg.b);
    port : 8000 
srv.listen(function (err) {
    if (err) {
        console.log("Error binding: " + err);
    } else {

a corresponding client might look like:

var x = rpc.createTransport({ host: '', port : 8000 });
x.connect(function (err) {
    if (err) {
        console.log("error connecting");
    } else {
        var c = new rpc.Client(x, "myprog.1");
        c.invoke('add', { a : 5, b : 4}, function(err, response) {
            if (err) {
                console.log("error in RPC: " + err);
            } else { 
                assert.equal(9, response);

Or, equivalently, in beautiful IcedCoffeeScript:

x = rpc.createTransport { host: '', port : 8000 }
await x.connect defer err
if err
    console.log "error connecting"
    c = new rpc.Client x, "myprog.1"
    await c.invoke 'add', { a : 5, b : 4}, defer err, response
    if err? then console.log "error in RPC: #{err}"
    else assert.equal 9, response


It should work to just install with npm:

npm install -g framed-msgpack-rpc

If you install by hand, you will need to install the one dependency, which is the the Purepack Msgpack library, available as purepack on npm:

npm install -g purepack

Full API Documentation

If you are building real applications, it's good to look deeper than the simple API introduced above. The full library is based on an abstraction called an FMPRPC Transport. This class represents a stream of FMPRPC packets. Clients and servers are built on top of these streams, but not in one-to-one correspondence. That is, several clients and several servers can share the same Transport object. Thus, FMPRPC supports multiplexing of many logically separated application-level streams over the same underlying TCP stream.


The transport mechanics are available via the submodule transport:

var rpc = require('framed-msgpack-rpc');
var transport = rpc.transport;

Transports are auto-allocated in the case of servers (as part of the listen and connect process), but for clients, you'll find yourself allocating and connecting them explicitly.

All transports are stream transports and for now are built atop TCP streams. Eventually we'll roll out support for Unix domain sockets, but there is no plan for UDP support right now.


var x = new transport.Transport(opts);

Make a new TCP transport, where opts are:

  • port - the port to connect to
  • host - the host to connect to, or localhost if none was given
  • path - the path to connect to, if using Unix domain sockets
  • tcp_opts - TCP options to pass to node's net.connect method, which is {} by default
  • log_obj - An object to use to log info, warnings, and errors on this transport. By default, the default logging to console.log will be used. See Logging below.
  • do_tcp_delay - By default, the Transport will setNoDelay on TCP streams, but if you specify this flag as true, that behavior will be suppressed.
  • hooks - Hooks to be called on connection error and EOF. Especially useful for RobustTransports (see below). The known hooks are
    • hooks.connected - Called when a transport is connected
    • hooks.eof - Called when a transport hits EOF.
  • dbgr - A debugging object. If set, it will turn on RPC tracing via the given debugging object. See Debugging below. I would have liked to call this a debugger, but that's a reserved keyword in node.

The following two options are used internally by Server and Listener classes, and should not be accessed directly:

  • tcp_stream - Wrap an existing TCP stream
  • parent - A parent listener object


var x = new transport.RobustTransport(opts, ropts);

A subclass of the above; with some more features:

  • If disconnected, will attempt to reconnect until successful.
  • Will queue calls issued in between a disconnect and a reconnect.
  • Will warn of RPCs that are outstanding for more than the given threshholds.

The opts dictionary is as in Transport, but there are additional options that can be specified via ropts:

  • reconnect_delay - a float - the number of seconds to wait between connection attempts.
  • queue_max - the maximum number of RPCs to queue while reconnecting
  • warn_threshhold - RPCs that take more than this number of seconds are warned about via the logging object.
  • error_threshhold - RPCs that take more than this number of seconds are errored about via the logging object. Also, a timer will be set up to warn after this many seconds if the RPC isn't completed in time, while the RPC is still outstanding.


x.connect(function (err) { if (!err) { console.log("connected!") } });

Connect a transport if it's not already connected. Takes a single callback, which takes one parameter --- an error that's null in the case of a success, and non-null otherwise. In the case of a RobustTransport, the callback will be fired after the initial connection attempt, but will continue to reconnect in the background. Additional error and warnings are issued via the logger object, and an info is issued when a connection succeeds. Also, if a hooks.connected was passed, it will be called on a successful connection, both the first time, and after any subsequent reconnect.


var b = x.is_connected();

Returns a bool, which is true if the transport is currently connected, and false otherwise.



Call to actively close the given connection. It will trigger all of the regular hooks and warnings that an implicit close would. In the case of a RobustTransport, the transport will not attempt a reconnection.


var ip = x.remote_address();

Get the IP address of the remote side of the connection. Note that this can change for a RobustTransport, if the DNS resolution for the given hostname was updated and the connection was reestablished. Will return a string in dotted-quad notation.


var g = x.get_generation()

Get the generation number of this stream connection. In the case of a regular Transport, it's always going to be 1. In the case of a RobustTransport, this number is incremented every time the connection is reestablished.


var l = x.get_logger()

If you want to grab to the logger on the given transport, use this method. For instance, you can change the verbosity level with x.get_logger().set_level(2) if you are using the standard logging object.


x.set_logger(new logger.Logger({prefix : ">", level : logger.WARN}));

Set the logger object on this Transport to be the passed logger. You can pass a subclass of the given Logger class if you need custom behavior to fit in with your logging system.



Set a debugging object on a transport. After this is done, the core will report that an RPC call was made or answered, either on the server or client. See Debugging below for more details.



Call set_debugger as above but with an object that will be allocated. The object is of type debug.Debugger, and is initialized with flags given by flags. All debug traces are set to transport's logger object at the log.levels.DEBUG level.

These flags can either be in numerical form (e.g., 0xfff ) or string literal form (e.g., "a1m"). If in the latter form, the flags will be converted into the numerical form via sflags_to_flags.

transport.createTransport or rpc.createTransport

var x = rpc.createTransport(opts)

Create either a new Transport or RobustTransport with just one call. The opts array is as above, but with a few differences. First, the opts here is the merge of the opts and ropts above for the case of RobustTransports; and second, an option of robust : true will enable the robust variety of the transport.

Note that by default, I like function to use underscores rather than camel case, but there's a lot of functions like createConnection in the standard library, so this particular function is in camel case. Sorry for the inconsistency.


Clients are thin wrappers around Transports, allowing RPC client calls. Several clients can share the same Transport. Import the client libraries as a submodule:

var client = require('framed-msgpack-rpc').client;

The API is as follows:


Make a new RPC client:

var c = new client.Client(x, prog);

Where x is a transport.Transport and prog is the name of an RPC program. Examples for prog are of the form myprog.1, meaning the program is called myprog and the version is 1.

Given a client, you can now make RPC calls over the specified connection:


Use a Client to invoke an RPC as follows:

c.invoke(proc, arg, function(err, res) {});

The parameters are:

  • proc - The name of the RPC procedure. It is joined with the RPC program.version specified when the client was allocated, yielding a dotted triple that's sent over the wire.
  • arg - A JSON object that's the argument to the RPC.
  • cb - A callback that's fired once there is a reply to the RPC. err is null in the success case, and non-null otherwise. The res object is optionally returned in a success case, giving the reply to the RPC. If the server supplied a null result, then res can still be null in the case of success.


As above, but don't wait for a reply:

c.notify(proc, arg);

Here, there is no callback, and no way to check if the sever received the message (or got an error). Notifying seems weird to me, but it was in the original MsgpackRpc system, so it's reproduced here.


To write a server, the programmer must specify a series of hooks that handle individual RPCs. There are a few ways to achieve these ends with this library. The big difference is what is the this object for the hook. In the case of the server.Server and server.SimpleServer classes, the this object is the server itself. In the server.ContextualServer class, the this object is a per-connection context object. The first two are good for most cases.

You can get the server library through the submodule server:

var server = require('framed-msgpack-rpc').server;

But most of the classes are also rexported from the top-level module.


Create a new server object; specify a port to bind to, a host IP address to bind to, and also a set of RPC handlers.

var s = new server.Server(opts);

For opts, the fields are:

  • port - A port to bind to
  • host - A host IP to bind to
  • path - A socket path to bind to, if being run on as Unix domain socket.
  • TransportClass - A transport class to use when allocating a new Transport for an incoming connection. By default, it's transport.Transport
  • log_obj - A log object to log errors, and also to assign to (via make_child) to child connections. Use the default log class (which logs to console.log) if unspecified.
  • programs - Programs to support, following this JSON schema:
    prog_1 : {
        proc_1 : function (arg, res, x) { /* ... */ },
        proc_2 : function (arg, res, x) { /* ... */ },
        /* etc ... */
    prog_2 : {
        proc_1 : function (arg, res, x) { /* ... */ }

Each hook in the object is called once per RPC. The arg argument is the argument specified by the remote client. The res argument is what the hook should call to send its reply to the client (by calling res.result(some_object)). A server can also reject the RPC via res.error(some_error_string)). The final argument, x, is the transport over which the RPC came in to the server. For instance, the server can call x.remote_address() to figure out who the remote client is.


A SimpleServer behaves like a Server but is simplified in some ways. First off, it only handles one program, which is typically set on object construction. Second off, it depends on inheritance; I've used CoffeeScript here, but you can use hand-rolled JavaScript style inheritance too. Finally, it infers your method hooks: on construction, it iterates over all methods in the current object, and infers that a hook of the form h_foo handles the RPC foo.

Here's an example:

class MyServer extends server.SimpleServer

  constructor : (d) ->
    super d 
    @set_program_name "myprog.1"

  h_reflect : (arg, res, x) -> res.result arg
  h_null    : (arg, res, x) -> res.result null
  h_add     : (arg, res, x) -> res.result { sum : arg.x + arg.y }

Most methods below are good for both SimpleServer and Server. The former has a few extra; see the code in server.iced.


Here's an example:

class Prog1 extends server.Handler
  h_foo : (arg, res) -> 
    console.log "RPC to foo() from #{@transport.remote_address()}"
    res.result { y : arg.i + 2 }
  h_bar : (arg, res) -> res.result { y : arg.j * arg.k }

class Prog2 extends server.Handler
  h_foo : (arg, res) -> res.error "not yet implemented"
  h_bar : (arg, res) -> res.error "not yet implemented"

s = new server.ContextualServer 
   port : 8881 
   classes : 
     "prog.1" : Prog1
     "prog.2" : Prog2

await s.listen defer err
console.log "Error: #{err}" if err?

This code constructs a server.ContextualServer with a classes object that maps program names to classes. Whenever a new connection is established in the above example, a new Prog1 object and a new Prog2 object is created. The former will handle all RPCs to prog.1 on that connection; the latter will handle all RPCs to prog.2. Note that the this object here is per-connection, not per-server. This allows you to store all sorts of interesting per-connection state. For more info, please see server.iced.


Bind to a port, and listen for incoming connections

s.listen(function(err) {});

On success, the callback is fired with null, and otherwise, an error object is passed.


As above, but keep retrying if binding failed:

s.listen_retry(delay, function(err) {});

The retry happens every delay seconds. The given function is called back with null once the reconnection happens, or with the actual error if it was other than err.code = 'EADDRINUSE'.


Close a server, and give back its port to the OS.


Before calling listen, you can use this method to set the port that the Server is going to bind to.


Walk the list of children, calling the specified function on each child connection in the list:

s.walk_children (function(ch) {});

Logging Hooks

As you could imagine, an RPC can generate a lot of errors, warnings, and informational messages. Examples include: unmarshalling failures, unexpected EOFs, connection breaking, unhandled RPCs, etc.

This package has an extensible logging system to fit in with your application, and a default logging system that should work for a lot of cases too.

The basic classes can be found in the log submodule, accessible as:

var log = require('framed-msgpack-rpc').log;

When a new Listener or Transport class is instantiated, it will need a new logger object (note that Listener is the base class for the various Server classes). It will try the following steps to pick a log.Logger object:

  1. Access the opt.log_obj passed to the Transport or Listener constructor. This is often times an object of a custom subclass of log.Logger.
  2. If that is was not specified, allocate a new log.Logger object:
    1. If log.set_default_logger_class was previous called, allocate one of those objects.
    2. Otherwise, allocate a log.Logger object.

Once this log.Logger object is allocated, the Transport or Listener class will call set_remote on it, so that subsequent log lines will show which client or server generated the message.

Logging is via the following methods, in ascending order of severity:


They all, by default, write the message msg to console.log while prepending the remote_address supplied above. The default log level is set to log.levels.INFO, but you can set it to log.levels.WARN, log.levels.ERRORS, etc. Warnings at lower levels will be silently swallowed. For the default logger object, the method log.Logger.set_level can be used to set the logging level as desired.

To make a custom logger class, you can subclass log.Logger, or use duck-typing: just make sure your class implements set_remote and the five leveled logging methods above.

See VLogger in test/all.iced for one example of a different logger --- it's used to make the regression tests look pretty.

See log.iced for more details.

Debugging and Tracing

An debugger is a JavaScript object that is passed into the FMPRPC core, and if available, is used to dump RPC debug traces. These debuggers can be installed when a Transport is allocated, by specifying the opts.dbgr option, or by calling set_debugger on most FMPRPC objects.

If a debugging object is active, it is called with debug.Message object when an RPC comes in or goes out. The debug.Message object contains a bunch of fields:

F =
  METHOD : 0x1
  REMOTE : 0x2
  SEQID : 0x4
  ERR : 0x10  
  ARG : 0x20
  RES : 0x40
  TYPE : 0x80
  DIR : 0x100          # which direction -- incoming or outgoing?

Debugging objects can choose to spam some or all of these fields, depending on how bad the bug is. For most purposes, the supplied debug.Debugger makes a nice debugger object that you can easily tune to print only the fields of your choosing (via the flags parameter).

See debug.iced for more details.



To come. See packetizer.iced for details.


To come. See dispatch.iced for details.

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