nice lightweight SOA implementation

some good API practices

4 min, 688 words

I've evangelized service-oriented architecture (SOA) before.

To review, briefly and roughly: SOA promotes decoupled services. For example, a Fahrenheit-to-Celsius converter would likely be implemented as a web-service, instead of as a function/method embedded/tied into some bigger program. The benefits of this are multiple: 1) The service can be written in any programming language, and accessed by other services written in different languages. 2) SOA makes the idealized promise of code-reuse a reality.

I have a programmer friend who works for a large corporation who is familiar with implementing SOA using industrial-scale best-practices; I'm familiar with implementing it in a lightweight, seat-of-the-pants fashion.

Over the past year+ I've created well over a dozen or so SOA web-services for different projects. But I recently implemented one I put some best-practice effort into that'll be a model for my future SOA work. Some links:

[2023 update -- the march of progress has resulted in the retirement of these services, so no active links. Sorry!]

What I like about this one...

  • The api urls offer 'discovery' via embedding, in the built-in returned data, contact and documentation information. Having just one of these pieces of info would be great; having both is particularly nice because web urls and staff change over time. Why is this useful? If someone is looking at the code that calls this service 5 years from now, and if I'm not around, the documentation will provide info on some extra features of the service that otherwise wouldn't be apparent if, say, the web-service just returned the word 'English'

  • The api urls are 'hackable', another way of enhancing discovery. One can intuitively try entering a code other than 'enk' to see what comes up (like 'tlh'). Also, reasonably appropriate things happen if one lops off increasing sections of the url (in this case, redirects to documentation pages).

  • The api urls are versioned. Key:value pairs can be added to this api -- but the existing key:value pairs must never be changed. The reason is that post-release, I don't know who's using it for what, thus I have to assume any changes could break someone's app. So if I want to change the label 'response' to 'language', and deliver it in xml, I can leave the existing one as is, and label the new one 'api_v2'.

  • All these urls utilize server-caching. This is an implementation rather than a design feature, but worth mentioning. Django offers a flexible and easy-to-use caching feature; I have it set so that the list and api urls only have to hit the database once a day, no matter how many times the urls are hit. Further, django's caching is intelligent: its response includes 'Cache-Control', 'Etag', and 'Expires' http-headers so that a browser or well-designed code doesn't even have to call the web-service again to redisplay the data. Nice. This would be particularly important and useful for something like RSS feeds.

Good info...

  • A terrific, hands-on review-resource on http-headers: The web-services chapter of Mark Pilgrims 'Dive Into Python' website & book.

  • Many of the features of this language_translator web-service were informed by the book 'RESTful Web Services', by Richardson & Ruby. Some parts are a bit dense, but it's chock-full of terrific detailed info and food for thought. I came across it after having written a half-dozen or so SOA web-services, each one a little different and better, and it directly addressed many issues I had begun to think about or saw referenced via web-research.

[Acknowledgements to Peter Murray's article and Richard Akerman's Access_2006 presentation that first inspired my SOA thinking.]