Sean Corfield: Clojure and CFML Sitting in a Tree

Last night I attended the Adobe User Group here in Sydney. That might strike some of you as a big surprise given my relationship with Adobe is pretty much limited to fiddling with Photoshop/Lightroom to get my photos looking nice.

However the reason for which I attended the meetup is that Sean Corfield - a prolific member of the Clojure community - gave a presentation on how he introduced and migrated most of his backend code at World Singles from CFML to Clojure - hence my interest.

What follows is a mix of my own notes and what I could remember from the slides:

Going faster

The original platform was built in CFML between 2001 and 2008. It was essentially monolithic procedural code. It was rewritten in 2009 - when they introduced OO and a new version of CFML

Brief Stats

  • 3 million members
  • 1 million emails sent every day
  • 2k concurrent users 24x7, on average

Clojure and WorldSingles

  • Tried .NET - ran into all sorts of trouble. Didn’t work well in production.
  • Tried Scala - Memory leaks in the built-in actor library were a deal breaker. The type system also wasn’t a good cultural fit.
  • Evaluated Clojure in 2010 - seemed like a good language to squeeze performance out of lower level components
  • Clojure version released in production in 2011
    • Rewrote remaining Scala Code
    • 6.3k LOC - 1.5k Tests LOC
    • equivalent to 4x the CFML LOC
    • Clojure is essentially used in the model (as in MVC)

Why add Clojure?

  • It’s fast - compiles down to JVM bytecode (and it’s faster than CFML)
  • Immutability (automatic thread safety)
    • they found several thread safety bugs in the third party ColdFusion libraries being used
  • Built-in concurrency / parallelism support - future, pmap, pvalues etc.
  • Lazy sequences - being able to work with potentially ininite collections without bringing your server down.
  • All high quality, production ready Java libraries easily accessible via java interop.

What do they use it for?

  • Email
    • html generation & sending
    • tracking & log file analysis
  • Geolocation (rest/json)
  • i18n
  • Reporting (parallel queries) - he showed a bit of this code. Heavy use of futures + deref
  • Search engine integration (json/xml)
    • This breaks down to two Clojure components:
    • One feeds the search engine based on changes to the users profiles
    • The other then runs against the search engine 24x7 trying to find new matches for users
  • (lightweight?) Persistence layer
    • no ORM - thin framework over sql instead
    • Same interface for both Mysql & MongoDB
    • MongoDB being used after hitting performance issues in Mysql

Clojure ecosystem/community

Having an active Community is crucial for language adoption and the clojure community got this right:

  • Community
    • 6700 developers on the mailing list
    • ~300 developers on IRC 24x7
  • Active library development
    • #23 language on Github
    • Over 7k projects on github
    • Nearly 2k libs on Clojars

The Future

  • Looking into Cascalog for big data processing
  • All new back-end/model code written in Clojure
  • Views/Controllers will remain in CFML
    • This ties up with Sean’s answer to ‘What would you not use Clojure for?’
    • He mentioned HTML rendering as the major reason. You can’t hand hiccup code to a designer.
    • Similarly, while designers can work on Enlive templates, updating the dynamic bits in Clojure can be cumbersome.


It was great to hear the many good things Sean had to say about Clojure. WorldSingles seem pretty happy with the decision of migration their heavy-lifting code to it and hopefully these slide notes will give you some insight into the sort of things Clojure is good at.

Sean is likely to put his slides up somewhere so I’ll link to it as soon as I have it. Here they are.

If you want to know more about Clojure and be involved in the community, you should come to the next clj-syd meetup.