[splix]

MongoDB has such feature (maybe its depends on driver, but at least JVM drivers have - http://api.mongodb.org/java/2.9.1/com/mongodb/WriteConcern.h...).

As for me, it's mostly the quesion of perfomance, and application architecture, most time you don't want to wait until it's replicated to slaves.

[mgummelt]

As far as I can tell, WriteConcerns don't protect from inconsistent reads in all cases. It looks like the most conservative setting is Majority, but even then there is no assurance that reads won't occur during the replication, nor that they won't occur to one of the minority of non-replicated servers.

[mrkurt]

You can set it to the total number of slaves you have and ensure data is on all of them. Normally that slows writes down enough that it's undesirable, though.