3 December 2015
When I was starting out on my writing career, I began with writing articles for technical magazines. Now, when I write article length pieces, they are all written for the web. Paper magazines still exist, but they are a shrinking minority, probably doomed to extinction. Yet despite the withering of paper magazines, many of the assumptions of paper magazines still exact a hold on writers and publishers. This has particularly risen up in some recent conversations with people working on articles I want to publish on my site.
Most web sites still follow the model of the Paper Age. These sites consist of articles that are grouped primarily due to when they were published. Such articles are usually written in one episode and published as a whole. Occasionally longer articles are split into parts, so they can be published in stages over time (if so they also may be written in parts). 
Yet these are constraints of a paper medium, where updating something already published is mostly impossible.  There's no reason to have an article split over distinct parts on the web, instead you can publish the first part and revise it by adding material later on. You can also substantially revise an existing article by changing the sections you've already published.
I do this whenever I feel the need on my site. Most of the longer-form articles that I've published on my site in the last couple of years were published in installments. For example, the popular article on Microservices was originally published over nine installments in March 2014. Yet it was written and conceived as a single article, and since that final installment, it's existed on the web as a single article.
Our first rationale for publishing in installments is the notion that people tend to prefer reading shorter snippets these days, so by releasing a 6000 word article in nine parts, we could keep each new slug to a size that people would prefer to read. A second reason is that multiple publications allows for more opportunities to grab people's attention, so makes it more likely that an article will find interested readers.
When I publish in installments, I add an item to my news feed and tweet for each installment. Since I'm describing an update, I link with a fragment URL to take readers to the new section (in future I may link to a temporary explanatory box to highlight what's in the new installment).
But whatever the way the article is released to the world, it is still a single conceptual item, so its best permanent form is a single article. Many people have read the microservices article since that March, and I suspect hardly any of them knew or cared that it was originally published in installments.
In that case we wrote the entire article before we started the installment publishing, but there's no reason against writing it in stages too. For my collection pipelines article, I wrote and published the original article over five installments in July 2014. As I was writing it, I was conscious that there were additional sections I could add. I decided to wait to see how the article was received before I put the effort in to write those sections. Since it was pretty popular, I made a number of revisions, for each one I announced it with a tweet and an item on my feed. 
Letting an article evolve like this is the kind of thing that's difficult in a print medium, but exactly the right thing to do on the web. A reader doesn't care that I revised the article to improve it, she just wants to read the best explanation of the topic at hand. 
I do like to provide some traces of such revisions. At the end of each article, I include a revision history which briefly summarizes the changes. For a couple of revisions, such as the 2006 revision of my article on Continuous Integration, I made the original article available on a different URL with a link from the revised article. I don't think the original article is useful to most readers, only really to those tracing the intellectual history of the idea, so shifting the original to a new URL makes sense.
The role of the feed is important in this. The traditional blog reinforces the Paper Age model by encouraging people to match an article with its feed entry. For longer articles, I prefer to consider them as different things, the feed is a notice of a new article or revision, which links to the article concerned. That way I generate feed entries each installment where the feed summarizes what's been added.
The point of all this is that we should consider web articles as information resources, resources that can and should be extended and revised as our understanding increases and as time and energy allow. We shouldn't let the Print Age notions of how articles should be constructed dictate the patterns of the Internet Age.
1: One of the infuriating things about these serial articles, is that there's often little thought to how the series should be read after publication. I often come across these articles where part 3 refers to parts 1 and 2, but part 1 doesn't get updated to refer to the later installments. This suggests to me that the publisher isn't thinking of the series as an article that's going to be valuable in the long-term.
2: There is a sort of an update mechanism, in that a series of articles might be republished as a single work. But that is relatively rare.
3: Another great example, written after I wrote this post, was Cade and Daniel's article on The Basics of Web Application Security. They wrote the first few installments in early 2016 and weren't sure what additional sections they would write (or even if they would have the energy to do them). They ended up writing a handful of further installments over the course of the next year.
4: Another form of revision is the wholesale revision of an article to incorporate what's been learned since the article was written. Jason Yip originally wrote his article on stand-up meetings in 2006, but did deep revisions in 2007, 2011, and 2016. (Sometimes it's worth the effort to make a serious revision to an article, and sometimes it's best to evolve it by just adding a couple of footnotes.)