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From "Jeremiah Jordan" <JEREMIAH.JOR...@morningstar.com>
Subject RE: Cassandra ACID
Date Thu, 30 Jun 2011 19:57:31 GMT
For your Consistency case, it is actually an ALL read that is needed,
not an ALL write.  ALL read, with what ever consistency level of write
that you need (to support machines dyeing) is the only way to get
consistent results in the face of a failed write which was at > ONE that
went to one node, but not the others.

________________________________

From: AJ [mailto:aj@dude.podzone.net] 
Sent: Friday, June 24, 2011 11:28 PM
To: user@cassandra.apache.org
Subject: Re: Cassandra ACID


Ok, here it is reworked; consider it a summary of the thread.  If I left
out an important point that you think is 100% correct even if you
already mentioned it, then make some noise about it and provide some
evidence so it's captured sufficiently.  And, if you're in a debate,
please try and get to a resolution; all will appreciate it.

It will be evident below that Consistency is not the only thing that is
"tunable", at least indirectly.  Unfortunately, you still can't
tunafish.  Ar ar ar.

Atomicity
All individual writes are atomic at the row level.  So, a batch mutate
for one specific key will apply updates to all the columns for that one
specific row atomically.  If part of the single-key batch update fails,
then all of the updates will be reverted since they all pertained to one
key/row.  Notice, I said 'reverted' not 'rolled back'.  Note: atomicity
and isolation are related to the topic of transactions but one does not
imply the other.  Even though row updates are atomic, they are not
isolated from other users' updates or reads.    
Refs: http://wiki.apache.org/cassandra/FAQ#batch_mutate_atomic

Consistency
Cassandra does not provide the same scope of Consistency as defined in
the ACID standard.  Consistency in C* does not include referential
integrity since C* is not a relational database.  Any referential
integrity required would have to be handled by the client.  Also, even
though the official docs say that QUORUM writes/reads is the minimal
consistency_level setting to guarantee full consistency, this assumes
that the write preceding the read does not fail (see comments below).
Therefore, an ALL write would be necessary prior to a QUORUM read of the
same data.  For a multi-dc scenario use an ALL write followed by a
EACH_QUORUM read.
Refs: http://wiki.apache.org/cassandra/ArchitectureOverview

Isolation
NOTHING is isolated; because there is no transaction support in the
first place.  This means that two or more clients can update the same
row at the same time.  Their updates of the same or different columns
may be interleaved and leave the row in a state that may not make sense
depending on your application.  Note: this doesn't mean to say that two
updates of the same column will be corrupted, obviously; columns are the
smallest atomic unit ('atomic' in the more general thread-safe context).
Refs: None that directly address this explicitly and clearly and in one
place.

Durability
Updates are made highly durable at the level comparable to a DBMS by the
use of the commit log.  However, this requires "commitlog_sync: batch"
in cassandra.yaml.  For "some" performance improvement with "some" cost
in durability you can specify "commitlog_sync: periodic".  See
discussion below for more details.
Refs: Plenty + this thread.



On 6/24/2011 1:46 PM, Jim Newsham wrote: 

	On 6/23/2011 8:55 PM, AJ wrote: 

		Can any Cassandra contributors/guru's confirm my
understanding of Cassandra's degree of support for the ACID properties?
		
		I provide official references when known.  Please let me
know if I missed some good official documentation.
		
		Atomicity
		All individual writes are atomic at the row level.  So,
a batch mutate for one specific key will apply updates to all the
columns for that one specific row atomically.  If part of the single-key
batch update fails, then all of the updates will be reverted since they
all pertained to one key/row.  Notice, I said 'reverted' not 'rolled
back'.  Note: atomicity and isolation are related to the topic of
transactions but one does not imply the other.  Even though row updates
are atomic, they are not isolated from other users' updates or reads.

		Refs:
http://wiki.apache.org/cassandra/FAQ#batch_mutate_atomic
		
		Consistency
		If you want 100% consistency, use consistency level
QUORUM for both reads and writes and EACH_QUORUM in a multi-dc scenario.

		Refs:
http://wiki.apache.org/cassandra/ArchitectureOverview
		
		


	This is a pretty narrow interpretation of consistency.  In a
traditional database, consistency prevents you from getting into a
logically inconsistent state, where records in one table do not agree
with records in another table.  This includes referential integrity,
cascading deletes, etc.  It seems to me Cassandra has no support for
this concept whatsoever.
	
	

		Isolation
		NOTHING is isolated; because there is no transaction
support in the first place.  This means that two or more clients can
update the same row at the same time.  Their updates of the same or
different columns may be interleaved and leave the row in a state that
may not make sense depending on your application.  Note: this doesn't
mean to say that two updates of the same column will be corrupted,
obviously; columns are the smallest atomic unit ('atomic' in the more
general thread-safe context).
		Refs: None that directly address this explicitly and
clearly and in one place.
		
		Durability
		Updates are made durable by the use of the commit log.
No worries here.
		Refs: Plenty.
		


	Jim
	



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