Trello is a collaboration tool that organizes your projects into boards. In one glance, Trello tells you what's being worked on, who's working on what, and where something is in a process.


  • When the data request returns, Backbone.js gets busy. The idea with Backbone is that we render each Model that comes down from the server with a View, and then Backbone provides an easy way to:

    1) Watch for DOM events within the HTML generated by the View and tie those to methods on the corresponding Model, which re-syncs with the server

    2) Watch the model for changes, and re-render the model’s HTML block to reflect them

    Neat! Using that general approach, we get a fairly regular, comprehensible, and maintainable client. We custom-built a client-side Model cache to handle updates and simplify client-side Model reuse.


  • We use Mustache, a logic-less templating language, to represent our models as HTML. While ‘harnessing the full power of [INSERT YOUR FAVORITE LANGUAGE HERE] in your templates’ sounds like a good idea, it seems that in practice it requires a lot of developer discipline to maintain comprehensible code. We’ve been very happy with the ‘less is more’ approach of Mustache, which allows us to re-use template code without encouraging us to mingle it with our client logic and make a mess of things.


  • Where we have browser support (recent Chrome, Firefox, and Safari), we make a WebSocket connection so that the server can push changes made by other people down to browsers listening on the appropriate channels. We use a modified version* of the Socket.io client and server libraries that allows us to keep many thousands of open WebSockets on each of our servers at very little cost in terms of CPU or memory usage. So when anything happens to a board you’re watching, that action is published to our server processes and propagated to your watching browser with very minimal latency, usually well under a second.


  • The server side of Trello is built in Node.js. We knew we wanted instant propagation of updates, which meant that we needed to be able to hold a lot of open connections, so an event-driven, non-blocking server seemed like a good choice. Node also turned out to be an amazing prototyping tool for a single-page app. The prototype version of the Trello server was really just a library of functions that operated on arrays of Models in the memory of a single Node.js process, and the client simply invoked those functions through a very thin wrapper over a WebSocket. This was a very fast way for us to get started trying things out with Trello and making sure that the design was headed in the right direction. We used the prototype version to manage the development of Trello and other internal projects at Fog Creek.


  • We use HAProxy to load balance between our webservers. It balances TCP between the machines round robin and leaves everything else to Node.js, leaving the connections open with a reasonably long time to live to support WebSockets and re-use of a TCP connection for AJAX polling.


  • Trello uses Redis for ephemeral data that needs to be shared between server processes but not persisted to disk. Things like the activity level of a session or a temporary OpenID key are stored in Redis, and the application is built to recover gracefully if any of these (or all of them) are lost. We run with allkeys-lru enabled and about five times as much space as its actual working set needs, so Redis automatically discards data that hasn’t been accessed lately, and reconstructs it when necessary.


  • MongoDB fills our more traditional database needs. We knew we wanted Trello to be blisteringly fast. One of the coolest and most performance-obsessed teams we know is our next-door neighbor and sister company StackExchange. Talking to their dev lead David at lunch one day, I learned that even though they use SQL Server for data storage, they actually primarily store a lot of their data in a denormalized format for performance, and normalize only when they need to.


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