Protobuf for node

Protobuf for Node adds idiomatic  protocol buffer  handling to  node.JS .
It is actually two things in one: firstly, you can marshal protocol messages to and from Node byte buffers and send them over the wire in pure JS.
Secondly, you can use protocol messages as a native interface from JS to C++ add-ons in the same process. It takes away the details of the V8 and the eio threadpool APIs and makes native extensions to Node much easier to implement.
How to use
To read/write protocol buffers, protobuf for node needs the parsed representation of your protocol buffer schema (which is in protobuf format itself). The protobuf compiler spits out this format:

$ vi feeds.proto
package feeds;
message Feed {
  optional string title = 1;
  message Entry {
    optional string title = 1;
  }
  repeated Entry entry = 2;
}
:wq
$ $PROTO/bin/protoc --descriptor_set_out=feeds.desc --include_imports feeds.proto

Using this file (read into a Buffer) you can create a 'Schema' object:

 var fs = require('fs');
 var Schema = require('protobuf_for_node').Schema;
 var schema = new Schema(fs.readFileSync('feeds.desc'));

A Schema object maps fully qualified protocol buffer names to type objects that know how to marshal JS objects to and from Buffers:

 var Feed = schema['feeds.Feed'];
 var aFeed = Feed.parse(aBuffer);
 var serialized = Feed.serialize(aFeed);

When marshalling, the serializer accepts any JavaScript object (but only picks the properties defined in the schema):

 var serialized = Feed.serialize({ title: 'Title', ignored: 42 });
 var aFeed = Feed.parse(serialized);  => { title: 'Title' }

Note that property names are the camel-cased version of the field names in the protocol buffer description, like for the Java version ( http://code.google.com/apis/protocolbuffers/docs/reference/java-generated.html#fields ). E.g. "optional string title_string = 1"will translate into a "titleString"JS property.
Note how we're using uppercase for the type objects like for constructor functions. That's because they are: when parsing, objects are constructed using the respective constructor for the message type. This means that you are able to attach methods:

  Feed.prototype.numEntries = function() {
    return this.entry.length;
  };
  var aFeed = Feed.parse(Feed.serialize({ entry: [{}, {}] }));
  aFeed.numEntries()  =>  2

Native Interface
Protocol buffers aren't only great for data interchange between processes - you can also use them to send data between code written in JS and C++ within the same process. Protobuf for node makes it simple to implement a native add-on without having to touch the V8 api at all: you implement a  protobuf service  instead. It's three easy steps:
1. Define the add-on service interface:

// An example service to query pwd(3).
package pwd;

// Empty (=no arg) request message.
message EntriesRequest {
}

message Entry {
  optional string name = 1;
  optional int32 uid = 2;
  optional int32 gid = 3;
  optional string home = 4;
  optional string shell = 5;
}
message EntriesResponse {
  repeated Entry entry = 1;
}

service Pwd {
  rpc GetEntries(EntriesRequest) returns (EntriesResponse);
}

... and generate the C++ code for it:

proto/bin/protoc --cpp_out=. service.proto

2. Implement and export the service in an add-on:

extern "C" void init(v8::Handle<v8::Object> target) {
  // Look Ma - no V8 api required!

  // Simple synchronous implementation.
  protobuf_for_node::ExportService(
      target, "pwd",
      new (class : public pwd::Pwd {
        virtual void GetEntries(google::protobuf::RpcController*,
                                const pwd::EntriesRequest* request,
                                pwd::EntriesResponse* response,
                                google::protobuf::Closure* done) {
          struct passwd* pwd;
          while ((pwd = getpwent())) {
            pwd::Entry* e = response->add_entry();
            e->set_name(pwd->pw_name);
            e->set_uid(pwd->pw_uid);
            e->set_gid(pwd->pw_gid);
            e->set_home(pwd->pw_dir);
            e->set_shell(pwd->pw_shell);
          }
          setpwent();
          done->Run();
        }
      }));

3. Use it from JS:

// prints the user database
puts(JSON.stringify(require('pwd').pwd.GetEntries({}), null, 2));

If your service is CPU intensive, you should call it with an extra callback argument: the invocation is automatically placed on the eio thread pool and does not block node:

// prints the user database
require('pwd').pwd.GetEntries({}, function(response) {
  puts(JSON.stringify(response), null, 2);
});

Your service is free to finish and call the "done"closure asynchronously.
Note that marshalling between JS and the request and response protos necessarily happens on the JS thread. Passing tons of data will block just as much using asynchronous invocation.
Speed
The Protobuf for Node add-on relies on the protobuf C++ runtime but it does not require any generation, compilation or linkage of generated C++ code. It works reflectively and is thus able to deal with arbitrary schemas. This means however, that it is not as fast as with generated code. Simple measurements show that unmarshalling is about between 20% and 50% faster than V8's native JSON support.
Calling C++ services is faster since it avoids marshalling to bytes but transfers data directly between the JS objects and the protocol messages. Also, in this case you compile the generated service interface into your code, so reflection is faster.
Building and running
You need to have node.js 0.4 and protobuf 2.4.0a installed. protobuf_for_node depends on the respective versions of v8 and eio shipped by node. Use branch node2 for node0.2/protobuf 2.3.0. If you want to use a later protobuf version, you may need to re-generate the example protoservice code: cd example; protoc --cpp_out=. protoservice.proto.
To build protobuf for node, do:

PROTOBUF=<protobuf prefix> /path/to/node/bin/node-waf configure clean build

This will build a) the protobuf_for_node library (build/default/libprotobuf_for_node_lib.so). It is linked to by your service-exporting add-ons and by the b) protobuf_for_node add-on (build/default/protobuf_for_node.node).
You need a) and the protobuf library in your DYLD_LIBRARY_PATH and b) in your NODE_PATH if you require('protobuf_for_node').
Running the examples
The examples should be run from within the protobuf-for-node directory, e.g. "node example/feeds.js".
Limitations
There are mismatches between the protocol buffer and javascript type systems. For example, the JS Number datatype does not offer enough precision to represent proto's uint64 datatypes.
There is no specification for the type conversions performed during JS->proto conversion. There may be surprises when you marshal, say, a JS object into an int64 proto field. That being said, the behavior is more or less in line with the ECMA conversions like ToNumber et al.
The marshalling code does not preserve unknown fields nor extensions.