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From "Edward Capriolo (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4734) Move CQL3 consistency to protocol
Date Tue, 16 Apr 2013 19:19:16 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4734?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13633224#comment-13633224
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Edward Capriolo commented on CASSANDRA-4734:
--------------------------------------------

Anecdotal, I feel few actually understand CL, (heck maybe I do not even understand it). What
I find, is that users come to realize that Cassandra is not atomic in the "atomic isolated"
sense. Once that mental hurdle is crossed users fall into a thinking similar to CL.ONE is
the most likely to succeed, and the fastest. Naturally since ONE is fastest and most likely
to not cause problems it becomes the application default. 

IMHO there is a logic to setting CL at the column family level, however that implies that
all operations on a column family would require the same CL which is not true, even inside
a row key some columns might require a different CL based on application requirements. Personally,
I do not see how anything but a per operation CL could be correct. If only 1/3 natural endpoints
is up I would not want my client to deny me the ability to write because the column family,
keyspace,...whatever dictates the CL it should be written at.
                
> Move CQL3 consistency to protocol
> ---------------------------------
>
>                 Key: CASSANDRA-4734
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4734
>             Project: Cassandra
>          Issue Type: Task
>          Components: API
>            Reporter: Jonathan Ellis
>            Assignee: Sylvain Lebresne
>             Fix For: 1.2.0 beta 2
>
>         Attachments: 0001-Move-consistency-level-to-the-protocol-level-2.txt, 0001-Move-consistency-level-to-the-protocol-level-3.txt,
0001-Move-consistency-level-to-the-protocol-level.txt, 0002-Remove-remains-of-4448-3.txt,
0002-Remove-remains-of-4448.txt, 0002-Thrift-generated-file-diffs-2.txt, 0003-Thrift-generated-file-diffs-3.txt,
0003-Thrift-generated-file-diffs.txt
>
>
> Currently, in CQL3, you set the consistency level of an operation in
> the language, eg 'SELECT * FROM foo USING CONSISTENCY QUORUM'.  It now
> looks like this was a mistake, and that consistency should be set at
> the protocol level, i.e. as a separate parameter along with the query.
> The reasoning is that the CL applies to the guarantee provided by the
> operation being successful, not to the query itself.  Specifically,
> having the CL being part of the language means that CL is opaque to
> low level client libraries without themselves parsing the CQL, which
> we want to avoid.  Thus,
> - Those libraries can't implement automatic retries policy, where a query would be retried
with a smaller CL.  (I'm aware that this is often a Bad Idea, but it does have legitimate
uses and not having that available is seen as a regression from the Thrift api.)
> - We had to introduce CASSANDRA-4448 to allow the client to configure some  form of default
CL since the library can't handle that anymore, which is  hackish.
> - Executing prepared statements with different CL requires preparing multiple statements.
> - CL only makes sense for BATCH operations as a whole, not the sub-statements within
the batch. Currently CQL3 "fixes" that by validating the given CLs match, but it would be
much more clear if the CL was on the protocol side.

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