Fix Version/s: None
We believed we wanted to be rack aware because we want to ensure that loosing a rack only affects a single replica of any given row key.
When using rack awareness, the first problem you encounter immediately if you aren't careful is that you induce hotspots as a result of rack aware replica selection. Using the format rackname-nodename, consider a part of the ring that looks like this:
Due to the rack awareness, r2-n1 will be the second replica for all data whose primary replica is on r1-n1, r1-n2 and r1-n3 since they would all be forced to skip over any identical racks.
The way we end up allocating nodes in a cluster is to satisfy this criteria:
- Any node in rack r in a cluster of a replication factor of rf, must not have another node in r within rf-1 steps in the ring in either direction.
Any violation of this criteria implies the induction of hotspots due to rack awareness.
The realization however, that I had a few days ago, is that the rackawareness is not actually changing replica placement when using this ring topology. In other words, the way you have to use rack awareness is to construct the ring such that the rack awareness is a NOOP.
- Is there any non-hotspot inducing use-case where rack awareness can be used ("used" in the sense that it actually changes the placement relative to non-awareness) effectively without satisfying the criteria above?
- Is it misleading and counter-productive to teach people (via documentation for example) to rely on rack awareness in their rings instead of just giving them the rule above for ring topology?
- Would it be a better service to the user to provide an easy way to ensure that the ring topology adheres to this criteria (such as refusing to bootstrap a new node if rack awareness is requested, and taking it into consideration on automatic token selection (does anyone use that?)), than to "silently" generate hotspots by altering the replication strategy? (The "silence" problem is magnified by the fact that nodetool ring doesn't reflect this; so the user must take into account both the RF and the racks when interpreting nodetool ring output.)
FWIW, internally we just go with the criteria outlined above, and we have a separate tool which will print the actual ownership percentage of a node in the ring (based on the thrift describe_ring call). Any ring that has node selections that causes a violation of the criteria is effectively a bug/mis-configured ring, so only in the event of mistakes are we "using" the rack awareness (using the definition of "use" above).