[jira] [Created] (CASSANDRA-13315) Consistency is confusing for new users

["Ryan Svihla (JIRA)" <ji...@apache.org>]

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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