Bringing the Mopidy music server to the browser

Note: This is a blog post draft that was originally written in January 2013, left unattended for 1689 days, and rediscovered and published unedited in August 2017.

During the five years since Mopidy entered the browser with its HTTP JSON-RPC API and the Mopidy.js JavaScript library, a number of successful Mopidy web clients have been built on top of this foundation. The APIs themselves have survived the test of time and have required minimal maintenance, just as I hoped when implementing the APIs back in November 2012.

Mopidy is a music server written in Python. It plays music from various sources, including local disk and Spotify. Mopidy can be remote controlled by, among others, MPD clients.

In Mopidy 0.10, released in the middle of December, we added an HTTP frontend. The HTTP frontend takes Mopidy’s full core API and makes it available from JavaScript in the browser. This means that you now can make your own web clients for Mopidy in JavaScript, and Wouter van Wijk have already started on his client (updated link).

I’d like to write a bit about how we made the HTTP client.

From the start, I was quite clear on having enough work to do on the server side of Mopidy. Thus, making my own client to test out REST APIs and to make sure the APIs we exposed were usable was mostly out of the question. Also, making a REST API would require us to spend huge amounts of time on figuring out how to best remap our procedural “core API” to REST resources. When the remapping would be complete, if ever, we would be stuck with the maintenance of yet another API on top of the core API, with all the associated mismatches and hacks required to match them up. We got enough of them in the MPD frontend.

Wouter had experimented a bit himself with making an HTTP frontend and suggested that we should go for an RPC model. I was reluctant at first. RPC reminds me of SOAP, the nineties, and other similar ideas which I don’t exactly regard as the state of the art.

After some thought, I figured that with an RPC API, I could probably make the entire API on the server side dynamically. In other words, I could get it done in a lot less time than a REST API, and it would maintain itself. At least, it would maintain itself to any degree such a thing is possible. If we added a new method to the core API, it would immediately be available through the HTTP RPC API. This would of course also mean that if we changed anything in the existing core API, we’d break any web client that use that part of the API. After some discussion we decided that we were OK with this drawback. After all, we intend the core API to become quite stable with time, where time approximately equals the release of v1.0. Also, the initial development work and future maintenance work associated with making an RPC API was within our reach, without distracting us for too long from work on the core code, the MPD and MPRIS frontends, and the local storage and Spotify backends. You see, we already got a healthy list of moving parts to keep oiled and working.

The part that really got me excited on an RPC API (of all things to get excited about) was the insight that if I added some introspection support to the API, I could make a JavaScript library to rebuild the entire API in the browser. So, instead of just providing a web service endpoint URL to potential client providers, we could offer them a complete API in the browser. Ready for development. Queue your first music track in a few minutes of development. That’s a good value proposition for aspiring client developers, if you ask me.

So, I made a new HTTP frontend. It started a CherryPy web server. Then it plugged ws4py into CherryPy, and we got a working WebSocket. Then I made my custom RPC API, using JSON as the transport format. I sent messages from Chrome’s console, and music started playing. It worked. It wasn’t tested, but I was happy so far.

Then Thomas, my main co-developer, wise as always, pasted the URL to the JSON-RPC 2.0 specification. Humpfh, I thought, not leaving it much chance. I read through the rather short spec, and concluded that it was really close to what I’d reinvented, minus support for calls with both positional and named arguments at the same time.

Cutting it short, I spent the next day or so implementing JSON-RPC 2.0, this time with tests. I plugged it into the HTTP frontend, and it worked.

Now you might say: Why didn’t you use one of the 25 or so existing JSON-RPC implementations on PyPI. Because there are 25 or so. How am I to review them in less time than it takes to implement the perfect one for my needs? Many of the alternatives provide examples of how to execute a Python file, and then magically a web server will be running. That’s not a selling point to me, since JSON-RPC got nothing to do with webservers, or the message transport for that mather. JSON-RPC is simply a mapping between a JSON format and method calls. It should be implemented by some function/object that accepts JSON, makes the required Python calls, and then returns the return values as JSON again. That’s it. No web server. Maybe some API introspection.

Digression aside, I’m considering extracting and releasing our JSON-RPC adapter. Then it’ll be N+1 competing standards, eh, I mean, implementations.

Next up was the JavaScript library. The main discussion here was actually where to place the code in our pure-Python repo. Bikeshedding of easily reversible decisions continues to be the easiest discussions to have. We ended on js/ in the root of the repo. How imaginative of us.

First thing in any new JavaScript project is of course to set up Buster.JS for testing. I also tried out the Grunt build tool for the first time. (My friend Pål recently wrote an introduction to Grunt featured at HN and in JavaScript Weekly.) Buster.JS in combination with Grunt and PhantomJS was a delight. If you simply run grunt watch and then modify a source file, your code is linted and tested in a headless browser in second or so. This makes JavaScript development for the browser feel like server side development. If this sounds interesting, check out our JavaScript project setup (updated link).


  • Mopidy.js usage examples
  • Invitation to develop clients

Note: To quickly address the above TODOs from January 2013: Usage examples can be found in the Mopidy docs, and an invitation to develop clients obviously wasn’t needed, as a dozen clients was made without it.