Time synchronization done right
Time synchronization between computers is important for many applications. For some applications (e.g. Apache Cassandra databases), it is even critical for data consistency.
Still, there are quite a few common misconceptions about how time synchronization in a network should be done correctly. Unfortunately, those misconceptions can easily lead to synchronization schemes that are by far less than optimal.
Accidentally, when looking for something else, I came a cross a series of two articles (part one, part two) that excellently describe the problems of the synchronization schemes that are commonly used and explain how to setup a scheme that actually provides precise synchronization.
In short, the only proper way for getting clock synchronization with the properties that most people want is setting up an internal pool of NTP servers that synchronizes against external references and have all other computers in the network synchronize against this internal pool. This is also the scheme that I have been successfully using for years.
However, you do not have worry: As long as you only need rough synchronization (so that the clock will show about the right time and not drift away more and more), the common scheme of synchronizing each individual computer against an external pool is typically okay, too. You just should be aware that there will allways be some clock skew between the computers and that you the computers will drift apart significantly if the connection to the external pool is interrupted for an extended period of time.
Addendum:
Shortly after writing this article, I found two more articles that are loosely connected to this topic. The first one basically tells us that we cannot rely on synchronized clocks because there are just too many ways how things can go wrong. The second one gives a practical example of how quickly things can go wrong when it comes to time synchronization.