I think the general assumption is that DC failover happens at the client app level rather than the Cassandra level due to the potentially very significant difference in request latency if you move from a app-local DC to a remote DC. The preferred pattern for most people is that the app fails in a failed DC and some load balancer above the app redirects traffic to a different DC.
The other factor is that the fail-back scenario from a failed DC and LOCAL_* consistencies is potentially complex. Do you want to immediately start using the new DC when it becomes available (with missing data) or wait until it catches up on writes (and how do you know when that has happened)? Note also QUORUM is a clear majority of replicas across both DCs. Some people run 3 DCs with RF 3 in each and QUORUM to maintain strong consistency across DCs even with DC failure. Cheers Ben On Tue, 21 Mar 2017 at 10:00 Shannon Carey <sca...@expedia.com> wrote: Specifically, this puts us in an awkward position because LOCAL_QUORUM is desirable so that we don't have unnecessary cross-DC traffic from the client by default, but we can't use it because it will cause complete failure if the local DC goes down. And we can't use QUORUM because it would fail if there's not a quorum in either DC (as would happen if one DC goes down). So it seems like we are forced to use a lesser consistency such as ONE or TWO. -Shannon From: Shannon Carey <sca...@expedia.com> Date: Monday, March 20, 2017 at 5:25 PM To: "user@cassandra.apache.org" <user@cassandra.apache.org> Subject: Consistency Level vs. Retry Policy when no local nodes are available I am running DSE 5.0, and I have a Java client using the Datastax 3.0.0 client library. The client is configured to use a DCAwareRoundRobinPolicy wrapped in a TokenAwarePolicy. Nothing special. When I run my query, I set a custom retry policy. I am testing cross-DC failover. I have disabled connectivity to the "local" DC (relative to my client) in order to perform the test. When I run a query with the first consistency level set to LOCAL_ONE (or local anything), my retry policy is never called and I always get this exception: "com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (no host was tried)" getErrors() on the exception is empty. This is contrary to my expectation that the first attempt would fail and would allow my RetryPolicy to attempt a different (non-LOCAL) consistency level. I have no choice but to avoid using any kind of LOCAL consistency level throughout my applications. Is this expected? Or is there anything I can do about it? Thanks! It certainly seems like a bug to me or at least something that should be improved. -Shannon -- *Ben Slater* *Chief Product Officer <https://www.instaclustr.com/>* <https://www.facebook.com/instaclustr> <https://twitter.com/instaclustr> <https://www.linkedin.com/company/instaclustr> Read our latest technical blog posts here <https://www.instaclustr.com/blog/>. This email has been sent on behalf of Instaclustr Pty. Limited (Australia) and Instaclustr Inc (USA). This email and any attachments may contain confidential and legally privileged information. If you are not the intended recipient, do not copy or disclose its content, but please reply to this email immediately and highlight the error to the sender and then immediately delete the message.
