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
*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.
