I've been experimenting with the consistency model of Cassandra, and I found
something that seems a bit unexpected. In my experiment, I have 2 processes, a
reader and a writer, each accessing a Cassandra cluster with a replication
factor greater than 1. In addition, sometimes I generate background traffic to
simulate a busy cluster by uploading a large data file to another table.
The writer executes a loop where it writes a single row that contains just an
sequentially increasing sequence number and a timestamp. In python this looks
something like:
while time.time() < start_time + duration:
target_server = random.sample(servers, 1)[0]
target_server = '%s:9160'%target_server
row = {'seqnum':str(seqnum), 'timestamp':str(time.time())}
seqnum += 1
# print 'uploading to server %s, %s'%(target_server, row)
pool = pycassa.connect('Keyspace1', [target_server])
cf = pycassa.ColumnFamily(pool, 'Standard1')
cf.insert('foo', row, write_consistency_level=consistency_level)
pool.dispose()
if sleeptime > 0.0:
time.sleep(sleeptime)
The reader simply executes a loop reading this row and reporting whenever a
sequence number is *less* than the previous sequence number. As expected, with
consistency_level=ConsistencyLevel.ONE there are many inconsistencies,
especially with a high replication factor.
What is unexpected is that I still detect inconsistencies when it is set at
ConsistencyLevel.QUORUM. This is unexpected because the documentation seems to
imply that QUORUM will give consistent results. With background traffic the
average difference in timestamps was 0.6s, and the maximum was >3.5s. This
means that a client sees a version of the row, and can subsequently see another
version of the row that is 3.5s older than the previous.
What I imagine is happening is this, but I'd like someone who knows that
they're talking about to tell me if it's actually the case:
I think Cassandra is not using an atomic commit protocol to commit to the
quorum of servers chosen when the write is made. This means that at some point
in the middle of the write, some subset of the quorum have seen the write,
while others have not. At this time, there is a quorum of servers that have
not seen the update, so depending on which quorum the client reads from, it may
or may not see the update.
Of course, I understand that the client is not *choosing* a bad quorum to read
from, it is just the first `q` servers to respond, but in this case it is
effectively random and sometimes an bad quorum is "chosen".
Does anyone have any other insight into what is going on here?
