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Comment by orthecreedence | Hacker News Reader

Riak does. You say, when writing, "please don't return until this data is replicated on 2 servers." And when reading, "please only return a successful read if this data is read from 2 servers."

So you have R = 2, W = 2, R+W = 4, and if your replication (N) val is 3, you're fine (you're always going to get consistency if R+W > N).

Riak is cool.

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Riak is cool and what you have described is correct however under failure conditions[1] you may not get this desired behavior.

[1]http://docs.basho.com/riak/latest/references/appendices/conc...

I believe Cassandra does as well, not 100% sure though.

Cassandra does, you can write with a write consistency of W and read with a read consistency of R, and as long as their sum is greater than the replication factor (number of copies to store across the cluster) you have consistent reads. W + R > N.

http://wiki.apache.org/cassandra/ArchitectureOverview#line-1...

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.

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.

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.

Does Riak support distributed transactions? If not, I don't see how they handle the possibility of a read occuring during the replicated write.

No, Riak is nontransactional. But neither is mongo if it's important here. Riak is apparently getting some kind of strong consistency though. Calvin looks interesting for a nosql with distributed transactions.

This is also something we desperately needed at my last company, but we couldn't find FOSS that supported it. mysql offers XA, but I've heard mixed reviews.

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