Pagination over a constantly changing list is hard. Really hard. In fact it should be one of those computer science problems up there with naming and cache invalidation.

The issue especially surfaces in news sites, like Hacker News. The hierarchy of posts is constantly changing. By the time you click ‘next page’, page two has an entirely different set of posts. So how do you ensure that the next page contains what the user expects to see?

Paginating on offset is out, since the valid offset is constantly changing. Paginating on a post’s date is also out, since HN’s sort order is not determined by date, but rather by a dynamic score.

Well one one method is to store every possible sort order in memory. In fact, this is exactly what Hacker News does. When you click ‘next page’, HN knows what you’re expecting to see, because it has the particular sort order you’re looking at stored in a closure.

However this is a lot of data. And it only grows. This means that every so often the garbage collector comes through and expires old links. And old can mean a couple of minutes - far too quick for practical pagination in my opinion.

@fat came up with a neat solution for Medium, which he shared with me recently. In fact the solution is so simple that I nearly kicked myself.

Send the server the IDs of the posts you already have when paginating. Sort posts by score, limit to your pagination offset, and omit those supplied IDs from the returned collection.

In other words I send an array containing all the previously fetched post IDs as a parameter whenever I’m fetching more paginated posts. Then I make sure that any posts fetched from the DB aren’t referenced in those IDs.

That’s a pretty pragmatic solution, that in my opinion draws the right compromises. Since I’m paginating via infinite scrolling, rather than links, users won’t notice ugly post IDs in the URLs. And since I’m keeping the list current using a stream of realtime updates from the server, posts are unlikely to be out of order. All in all, I’m pretty happy with the approach.

The Nokia Lumia 1020 is pricey, but its whopping 41-megapixel camera takes impressive photos.

It’s amazing how much cell-phone cameras have changed over the years. A decade ago, I excitedly purchased my first phone with a built-in camera, the Sony Ericsson T610, whose 0.1-megapixel camera took tiny, grainy color photos that seemed awesome at the time. A day ago, I started playing around with the Nokia Lumia 1020, which includes a staggering 41-megapixel sensor capable of snapping sharp, bright, intensely detailed images.

I got to be a guest on Tim Smith's The East Wing podcast. I talk about some of the stuff that's happened in my life since the last time I was on the show over a year ago, like the Kickstarter and CodePen. We also get into some random topics like freelancing and frameworks.

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The East Wing, Part 2 is a post from CSS-Tricks

The Maryland Department of Business and Economic Development (DBED) and the Economic Alliance of Greater Baltimore (EAGB) announced today that five Maryland companies have been selected out of 21 applicants to participate in Advance Maryland, a business program designed to support second-stage companies that are growth-oriented and have moved beyond the startup phase. Accepted companies include Amethyst Technologies (Baltimore County); Aquafin (Elkton); MRE Technology Solutions (Annapolis); Unleashed Technologies (Columbia); and Parking Panda (Baltimore).