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
