[redbad]

It seems that antirez believes he's allowing Redis to approach AP and/or CP semantics by providing primitives that users can compose in ad-hoc ways according to their requirements.

Unfortunately, that's not a valid methodology to achieve availability or (especially) consistency.

[antirez]

In Redis Cluster there is nothing you can do to tune availability or consistency. The only thing you can tune is write "safety" using WAIT, but actually most users should never use it since the system is mostly designed to be low-latency.

Redis Cluster is not AP nor CP. It resembles more AP since it has no strong consistency, however it is not able to reach "A" of AP that is very strict. Like AP systems have some form of consistency, but not "C" of CAP. Redis does this for "A" as well, it has some form of availability, but not as strict as "A" of CAP.

Why this? You'll notice that there are not AP database systems with the powerful commands of Redis, and that there are not CP systems with the latency of Redis. So it is a very opinionated trade off to get both sacrificing other stuff (especially "A").

[leobelle]

If it's not AP or CP, then what is the value? Cassandra is eventually consistent. Consistency is pretty important. It seems like Redis Cluster doesn't have consistency, nor availability, nor partition tolerance. Maybe I misunderstand it though.

[antirez]

> Cassandra is eventually consistent. Consistency is pretty important.

Your above statement unfortunately has no match in distributed systems theory or practice. Basically eventually consistent means that the system is not consistent from the point of view of CAP, so it does not feature strong consistency.

As an alternative eventual consistency means: our contract with the user is, that, at least, when there are no partitions, the system will no show split-brain conditions, so a given information will converge to a given value for all the nodes.

This alone is a contract with the user, but does not tell a lot, it just tells you:

1) No strong consistency, sorry. 2) No split-brain conditions when the partitions heal.

So "It features eventual consistency, consistency is important" statement, I'm afraid, means that you don't have the tradeoffs about distributed systems clear.

Now, if EC alone does not mean a lot, how to classify an EC system? It depends on the exact implementation of what happens when there is a partition, and what happens when the partition heals.

There are EC systems that are configured by default to accept writes in the minority side (even in partitions when there is a single DB node if we want to honor "A" of CAP), and later will merge depending on "wall clock" timestamps.

This is a form of consistency (and is weak), but is not Consistency with capital-C of CAP.

Other systems will use vector clocks instead of timestamps, which is stronger but slower. Other systems will instead merge values together doing a "Set union" operation. Here we still have no strong consistency, but the safety of the system is improved.

Now that we understand what means strong consistency, and what means eventual consistency, we can talk about Redis Cluster from this point of view. In Redis Cluster, which is eventual consistent, when the partition heals all the nodes will agree by selecting an history between the histories available for a given "hash slot" (a set of keys). However we need to consider what happens during partitions to have a better picture of what happens. In the case of Redis Cluster the minority side of the partition will stop accepting writes after some time. This gives up "A" of CAP in order to put a bound to how much two histories can diverge.

Ok, now let's explore "A". CAP Availability is the ability to reply with a positive reply (so, basically, to be able to accept writes is required) to requests even in a minority partition with a single node.

This is a very strong idea of availability, but this is not the only possibility to be fault tolerant. For example CP systems don't feature "A", but they are able to work correctly even if N/2-1 nodes will fail. This is a pretty strong failure tolerance! So they are reliable systems for a lot of works even if they don't have "A".

As a counter example, Cassandra or Riak would still be very useful tools even without "A" availability, but just with N/2-1 nodes allowed to fail.

Ok, back to Redis Cluster. It does not provides "A" since minority partitions are not available. It does not provide "C" since it is eventually consistent. But it still provides a form of consistency (its contract with the user about what happens during partitions) and provides some degree of resilience to failures.

AP or CP are just the theoretical limits you can get if you want to exploit the max in this regard, however it is not like a system is useless if it is not AP or CP.

In the case of Redis Cluster to renounce to "C" was obvious since it is asynchronously replicated and is not a good fit for high latency interactions. To renounce to the strong availability of "A" was a tradeoff to feature Redis data structures that are limited in size (number of elements) only by available memory.

Suggested reading about EC: http://www.bailis.org/blog/safety-and-liveness-eventual-cons...

[leobelle]

Thank you for your replies to my questions! For what it's worth I love the redis interface and appreciate your indepth information.