Tuesday, 29 August 2017

Don't Fetch What You Don't Need

I've been using GraphQL a bit at work recently - it's an interesting approach that seems in many ways to be the next evolution of RESTful APIs, where the client gets to choose exactly what they'd like the server to return to them.

GraphQL is a work-in-progress. The data type primitives are very limiting (how can I represent a UNIX/JavaScript timestamp with just an Int?), and always POSTing to the server seems like a backward step to the bad-old-days of SOAP. But as with all things in the JavaScript world, it's improving at a truly breakneck pace.

Something that I immediately saw as valuable was being able to save bandwidth by not including fields in the desired response - it also felt familiar, and yesterday I realised why - in MongoDB it's trivially easy to do this whenever you write a query. This excellent feature was sadly not exposed in my Mondrian library for Scala; something I've now rectified in the 0.6.x release.

Some quick tests involving documents that had large arrays of heavyweight fields showed that dropping them using a projection typically saved 50ms of latency, even on very small collections of documents. This has worked out very well for the use case of my current top-secret side project, where upon arrival at the front page, we need to quickly fetch a "summary" version of the most-recent 10 documents. The page visitor can then browse these and we can paginate for more summaries, or, if they are interested in a particular summary, we perform a findById and get the full "heavy" object.

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