JSON Serialization with Dancer and MooseX::Storage

Date: 27 April 2011

I’m hacking on a new tool in perl to manage my KVM cluster. Part of it is a RESTful interface using JSON. The objects I’m using are written using Moose and MooseX::Storable to simplify serialization. I can convert objects back and forth between perl objects and JSON all day. Unfortunately, there’s a fly in the ointment.

I’m using Dancer to provide the framework for the RESTful interface. The nice thing about Dancer is that it can automatically serialize perl data structures and it can do it via JSON. That’s all shiny except Dancer’s serializer doesn’t know about MooseX::Storable. In fact, Dancer chokes when I try to send an object back from one of the handlers.

All is not lost. There is a work around that appears to work in my tests. First of all, I needed to tell Dancer that it’s okay to try and convert my objects. This is done in config.yml.

engines:
  JSON:
    convert_blessed: '1'

That tells Dancer, via the JSON package, to try to convert objects using the method TO_JSON. But, wait! I don’t have one of those. Yet. Fortunately, MooseX::Storable has everything I need.

The MooseX::Storable API offers a couple of options. The first is to use freeze() to turn the object into a JSON string. That works if I’m only returning a single object but I’m often returning lists of things which complicates things.

sub TO_JSON { $_[0]->freeze }

Actually, it seems to work but Dancer has goes through the returned JSON string and encodes before sending it on. That’s actually a second round of encoding since MooseX::Storable did it once already. Here’s what a list containing an object looks like.

[
   "{\"__CLASS__\":\"YAVMM::Host-0.01\",\"cluster\":\"dev\",\"name\":\
"rbsmith-desktop\",\"ram\":3924056,\"num_cpus\":4}"
]

Ugly, huh? In fact, that might screw up anything non-perl that tries to use the RESTful interface. We can get around that by going down one level and using pack() instead.

pack() and the corresponding unpack() are required by anything implementing the MooseX::Storage API. Rather than going all the way to JSON, pack() spits back a serialized perl hashref. It turns out that that is exactly what Dancer needs to properly encode the object. So, I changed TO_JSON to look like this:

sub TO_JSON { $_[0]->pack }

The new output is below. Isn’t it pretty? That’s exactly what the JSON should look like.

[
   {
      "__CLASS__" : "YAVMM::Host-0.01",
      "cluster" : "dev",
      "name" : "rbsmith-desktop",
      "ram" : 3924056,
      "num_cpus" : 4
   }
]

On the client side, I still need to decode that mess. That’s actually very easy using the JSON module and feeding the JSON text into decode_json(). That spits back a perl hash or list reference that has the (mostly) deserialized data. I say mostly because my objects are still hashes and not objects. All that I need to do is feed those hashrefs to unpack() and I get my object back.

Here’s how I would translate that list above. Note: My object is called YAVMM::Host. You would, of course, use your own class name instead. Also, assume that $json_text has the JSON text. How it got there is beyond the scope of this document.

my @hosts = map { YAVMM::Host->unpack($_) } @{ decode_json($json_text) };

Now @hosts is a list with a bunch of YAVMM::Host objects that I can have all sorts of fun with.