Some things simply don't scale. You can't do some things in real-time. You have to do it in batch-jobs or avoid them. If you know about the O-notation, you will

Some things simply don't scale. You can't do some things in real-time. You have to do it in batch-jobs or avoid them. If you know about the O-notation, you will probably know certain algos don't scale. Friendster insisted to calculate the friends-of-friends-of-friends relationship in real-time and was not able to scale it. MySpace and Facebook did the same calc in the early days too, but both scrapped the feature as they were not able to scale it. Other examples are big data (e.g. log analysis), if you choose the wrong software stack (database) or wrong indexes you won't be able to scale it (especially if it's real time. You will bleed money in no time for cloud servers with TBs of RAM, when you could do it with a couple of servers with a normal sharded SQL database and one hand tuned index.