alt Hacker News> When two transactions happen at nearly the same time on different nodes, the database must determine which happened first. If clocks are out of sync, the database might order them incorrectly, violating consistency guarantees.
This is only true if you use wall clock time as part of your database’s consistency algorithm. Generally I think this is a huge mistake. It’s almost always much easier to swap to a logical clock - which doesn’t care about wall time. And then you don’t have to worry about ntp.
The basic idea is this: event A happened before event B iff A (or something that happened after A) was observed by the node that generated B before B was generated. As a result, you end up with a dag of events - kind of like git. Some events aren’t ordered relative to one another. (We say, they happened concurrently). If you ever need a global order for all events, you can deterministically pick an arbitrary order for concurrent events by comparing ids or something. And this will give you a total order that will be the same on all peers.
If you make database events work like this, time is a little more complex. (It’s a graph traversal rather than simple numbers). But as a result the system clock doesn’t matter. No need to worry about atomic clocks, skew, drift, monotonicity, and all of that junk. It massively simplifies your system design.
Replies
I wouldn't say it's a mistake. Distributed algorithms that depend on wall clock time generally give better guarantees. Usually you want these guarantees. The downside is of course you need to keep accurate time. In the cases you don't need them (eg. for the case you described), sure, but as an engineer you don't always get to choose your constraints.
Unfortunately, some of us have to deal with things like billing, transaction timing to validate what a client's logs might have on their systems, and so on.
My take on this is that second timing is close enough for this. And that all my internal systems need agree on the time. So if I'm off by 200ms or some blather from the rest of the world, I'm not overly concerned. I am concerned, however, if a random internal system is not synced to my own ntp servers.
This doesn't mean I don't keep our servers synced, just that being off by some manner of ms doesn't bother me inordinately. And when it comes to timing of events, yes, auto-increment IDs or some such are easier to deal with.
Related in many ways: https://www.erlang.org/docs/22/apps/erts/time_correction
Also I still remember having fun with the "Determine the order of events by saving a tuple containing monotonic time and a strictly monotonically increasing integer as follows" part.