It's a glorified IM app running at global scale. Once you run at global scale like Twitter does, maintaining even a TODO app introduces horrifying new problems

It's a glorified IM app running at global scale. Once you run at global scale like Twitter does, maintaining even a TODO app introduces horrifying new problems you hadn't considered before. Couple this with a drive to move quickly, and the solutions implemented frequently aren't, well, _good._ At a minimum, they're not robust, and require specialized knowledge to keep running. So while the problem being solved isn't terribly technically difficult, from like a 100k foot view, the scale of the solution required plus the decade's worth of hacks and permanent "temporary" solutions ends up being incredibly complex. EDIT: Not sure why the post is flagged, it doesn't seem to break any guidelines, but sure okay. Maybe too much Twitter news recently. Anyway, also replying here, since parent edited their comment to reply. I'm not arguing that Twitter is special or unique or anything, or that their problems cannot be easily solved in a vacuum. The problem is that Twitter is not in a vacuum, they've got a decade of systems that grew in fits and spurts. They don't have common, easy-to-administer stuff across their whole stack, they've got weird, custom stuff in critical paths. And more businesses are like this than you think. Businesses don't grow cleanly, they don't adopt technology cleanly. They grow with business needs, and they grow usually with less time allotted than you'd want to do things right. As a consequence, while something _like_ Twitter would be simple to build in concept, the _actual_ systems running at Twitter are going to be very specialized and require special knowledge to run efficiently.