Safety and liveness: Eventual consistency is not safe

<i>[1] Eventual convergence is likely the strongest convergence property we can guarantee given unbounded partition durations.</i><p>I don't think this is true. Consider the property I call "eventually known consistency", wherein the system can be asked "are all operations performed before time T visible everywhere?", with a "yes"/"maybe" response, where "yes" is guaranteed to eventually be returned after some bounded period of non-partition.<p>Eventually known consistency can be used to get AP (just ask for the data), CP (let T be the current time; spin until ConsistentUpTo(T) returns true; then perform the read), or CA (in the sense that if as long as a partition does not occur, the algorithm for CP provides a response within a bounded time), and is thus strictly stronger than other properties.

FWIW, the original Dynamo concept supported safety by returning not just the latest version, but all conflicting versions: the client then had the opportunity to make a merged version of the data that was newer than either of the inputs, and store that as a replacement.<p>It should therefore be remembered that many/most implementations of "eventual consistency" have these issues, it is not a requirement of the mechanism, and some implementations realize this and either have merge implementations or have plans to provide them.<p>(I am not certain where Cassandra is on this axis, but last I paid attention they were actively trying to decide whether to modify the client protocol to match Dynamo, or provide server-assisted merge operators more similar to their existing server-assisted comparison operators.)

tl;dr. "Eventual consistency" is a b.s. marketing term that actually has nearly no meaning.<p>The easiest way to see just how empty the definition is, is by negating it and seing what undesirable property a system would have to have in order to fail to be eventually consistent. Take the definition from wikipedia:<p>"Given a sufficiently long period of time over which no changes are sent, all updates can be expected to propagate eventually through the system and all the replicas will be consistent."<p>Negation:<p>"No matter how long you wait without sending changes, updates may not propagate through the system, and disagreement may continue to exist between replicas indefinitely."<p>So basically, saying that something is "eventually consistent" just means "we won't completely ignore you forever". Great.