Ryan Svihla created CASSANDRA-13315:
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Summary: Consistency is confusing for new users
Key: CASSANDRA-13315
URL: https://issues.apache.org/jira/browse/CASSANDRA-13315
Project: Cassandra
Issue Type: Improvement
Reporter: Ryan Svihla
New users really struggle with consistency level and fall into a large number
of tarpits trying to decide on the right one.
1. There are a LOT of consistency levels and it's up to the end user to reason
about what combinations are valid and what is really what they intend it to be.
Is there any reason why write at ALL and read at CL TWO is better than read at
CL ONE?
2. They require a good understanding of failure modes to do well. It's not
uncommon for people to use CL one and wonder why their data is missing.
3. The serial consistency level "bucket" is confusing to even write about and
easy to get wrong even for experienced users.
So I propose the following steps:
1. Remove the "serial consistency" level of consistency levels and just have
all consistency levels in one bucket at the protocol level.
2. To enable #1 just reject writes or updates done without a condition when
SERIAL/LOCAL_SERIAL is specified.
3. add 3 new consistency levels pointing to existing ones but that infer intent
much more cleanly:
* EVENTUALLY_CONSISTENT = LOCAL_ONE reads and writes
* HIGHLY_CONSISTENT = LOCAL_QUORUM reads and writes
* TRANSACTIONALLY_CONSISTENT = LOCAL_SERIAL reads and writes
for global levels of this I propose keeping the old ones around, they're rarely
used in the field except by accident or particularly opinionated and advanced
users.
Drivers should put the new consistency levels in a new package and docs should
be updated to suggest their use. Likewise setting default CL should only
provide those three settings and applying it for reads and writes at the same
time.
CQLSH I'm gonna suggest should default to HIGHLY_CONSISTENT. New sysadmins get
surprised by this frequently and I can think of a couple very major escalations
because people were confused what the default behavior was.
The benefit to all this change is we shrink the surface area that one has to
understand when learning Cassandra greatly, and we have far less bad initial
experiences and surprises. New users will more likely be able to wrap their
brains around those 3 ideas more readily then they can "what happens when I
have RF2, QUROUM writes and ONE reads". Advanced users get access to all the
way still, while new users don't have to learn all the ins and outs of
distributed theory just to write data and be able to read it back.
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