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From "Ryan Svihla (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-13315) Consistency is confusing for new users
Date Thu, 09 Mar 2017 16:36:38 GMT
Ryan Svihla created CASSANDRA-13315:
---------------------------------------

             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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