Sitecore uses MongoDB for tracking and analytics. If the production environment is split into several geographic locations, particularly in different continents, how should xDB be configured? If xDB can only have one writeable primary instance in any replica set, does this not force all front-end CD servers globally to write to the same node in one particular data centre? This doesn't seem ideal.
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You are correct, eventually all of the data comes down to a "one" Mongo, but that replica set itself can be geographically distributed as well. You can also shard MongoDB if you wish to. See the MongoDB manual on geographic redundancy for information on this type of scaling. From the manual:
While replica sets provide basic protection against single-instance failure, replica sets whose members are all located in a single facility are susceptible to errors in that facility. Power outages, network interruptions, and natural disasters are all issues that can affect replica sets whose members are colocated. To protect against these classes of failures, deploy a replica set with one or more members in a geographically distinct facility or data center to provide redundancy.
Also note that with xDB in a geograpically distributed environment, you'll need to have a session state server for each CD cluster. This gathers all the user's information during the session, prior to the session completion flush to the collection database. The Sitecore guide on 'clustered environments' has a diagram and some information on this geographic configuration. From the guide:
Each cluster could contain two or more content delivery instances, each with its own session state server. You could also group clusters together in the same place or spread them across different geographical locations.
I have asked platform developers exactly this question on Sitecore User Group London a week ago, they responded that all the data for xDB is internally kept in UTC format.
We also have had problem of servers in different time zones previously, but that time it affected Event Queue (they did not work when in different time zones), so keeping all your servers in the same time zone with time synchronized would do the job.
Here is official guidance for that from Sitecore: https://doc.sitecore.net/sitecore%20experience%20platform/utc/settings%20supporting%20utc%20implementation
