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From AJ ...@dude.podzone.net>
Subject Re: Cassandra ACID
Date Sat, 25 Jun 2011 04:27:34 GMT
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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