reconsider rack awareness
-------------------------
Key: CASSANDRA-3810
URL: https://issues.apache.org/jira/browse/CASSANDRA-3810
Project: Cassandra
Issue Type: Task
Reporter: Peter Schuller
Assignee: Peter Schuller
Priority: Minor
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:
{code}
...
r1-n1
r1-n2
r1-n3
r2-n1
r3-n1
r4-n1
...
{code}
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*.
So, questions:
* 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).
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
