Start multiple threads. They write some data from them. One of them crashes. Is it safe to just restart that thread and continue?<p>Not always, because that thread might have written to so some shared data structure and left it in an inconsistent state. Restarting that one thread might seem to work but because there is no guarantee, the safest way is to restart the OS process.<p>It might not even be your code. Maybe your RPC library or some other package did it internally. You might restart and 99% of time it will work, but it will not be a guaranteed thing.<p>Now, of course the equivalent counterpart to this is not threads but you can do this with OS processes. And I&#x27;ve done it in Python. You can use pipes, file system writes, sockets to create a reliable system were some processes can crash without bringing down the whole service. But it is cumbersome.<p>But imagine the sever handles a million connections. Certainly doable in Erlang. Each one handled by one lightweight process. Connection does something unusual, maybe uses a new feature which was added last night. It crashes, that&#x27;s fine. Other 999999 are ok. If this was a C++ program, it might have segfault-ed and caused the other 999999 connections to be dropped.<p>I&#x27;ve seen systems which had a periodically crashing subsystem that was being restarted behind the scenes for a while most of the system stayed up without noticeable issues. In business terms that means a smaller ops team, it means not having to wake up at 4am to answer pages (if subsystem heals itself, can just fix it in the morning) and so on.<p>Moreover, live code hot-patching is often touted as a gimmick. But I&#x27;ve used on a large production clusters while it was serving tens of thousands of requests per second without stopping. Sure it was scary and by that time something has already gone wrong (to need that fix), and it was nice not having to to shut down everything just to add an extra trace or log statement.