Dear Hiro, Thank you! Yes, this is exactly my understanding. Is this explicitly written down anywhere or can any expert confirm? I suppose I should read the code but its a little intimidating.
Best wishes, Craig On Fri, May 24, 2019 at 11:11 AM Hiroyuki Yamada <mogwa...@gmail.com> wrote: > Hi Craig, > > Now I probably understand what the python doc is saying. > > As long as `serial_consistency_level` is set to SERIAL for paxos phase > and `consistency_level` is set to SERIAL for the later read, > conflicts in paxos table can be properly detected, so > `consistency_level` for commit phase can be anything (can be ANY as > the doc says). > Unfinished record write (commit) will be repaired in the read if any. > But, if `consistency_level` is set to others (like ALL) for the later > read, it won't be able to detect conflicts in the paxos table, so it > does not work as expected. > > I'm not sure if it answers your question, but makes sense ? > Craig. > > Is this understanding correct ? > C* professionals. > > Thanks, > Hiro > > On Fri, May 24, 2019 at 10:49 AM Craig Pastro <siyo...@gmail.com> wrote: > > > > Dear Hiro, > > > > Thank you for your response! > > > > Hmm, my understanding is slightly different I think. Please let me try > to explain one situation and let me know what you think. > > > > 1. Do a LWT write with serial_consistency = SERIAL (default) and > consistency = ONE. > > 2. LWT starts its Paxos phase and has communicated with a quorum of nodes > > 3. At this point a read of that data is initiated with consistency = > SERIAL. > > > > Now, here is where I am confused. What I think happens is that a SERIAL > read will read from a quorum of nodes and detect that the Paxos phase is > underway and... maybe wait until it is over before responding with the > latest data? The Paxos phase happens between a quorum so basically even > though the consistency level is ONE (or indeed ANY as the Python docs > state), doing a read with SERIAL implies that the write actually took place > at a consistency level equivalent to QUORUM. > > > > Here also what I think is that a read initiated when the Paxos phase is > underway with a consistency level of QUORUM or ALL will not detect that a > Paxos phase is underway and return the old current data. > > > > Is this correct? > > > > Thank you for any help! > > > > Best wishes, > > Craig > > > > > > > > > > > > > > On Fri, May 24, 2019 at 9:58 AM Hiroyuki Yamada <mogwa...@gmail.com> > wrote: > >> > >> Hi Craig, > >> > >> I'm not 100 % sure about some corner cases, > >> but I'm sure that LWT should be used with the following consistency > >> levels usually. > >> > >> LWT write: > >> serial_consistency_level: SERIAL > >> consistency_level: QUORUM > >> > >> LWT read: > >> consistency_level: SERIAL > >> (It's a bit weird and mis-leading as a design that you can set SERIAL > >> to consistency_level in read where as you can't for write.) > >> > >> BTW, I doubt the python doc is correct in especially the following part. > >> "But if the regular consistency_level of that write is ANY, then only > >> a read with a consistency_level of SERIAL is guaranteed to see it > >> (even a read with consistency ALL is not guaranteed to be enough)." > >> It is really true? > >> It doesn't really make sense to me because SERIAL read mostly returns > >> by seeing quorum of replications, > >> and write with ANY returns by writing mostly one replication, so they > >> don't overlap in that case. > >> It would be great if anyone can clarify this. > >> > >> Thanks, > >> Hiro > >> > >> > >> On Thu, May 23, 2019 at 3:53 PM Craig Pastro <siyo...@gmail.com> wrote: > >> > > >> > Hello! > >> > > >> > I am trying to understand the consistency level (not serial > consistency) required for LWTs. Basically what I am trying to understand is > that if a consistency level of ONE is enough for a LWT write operation if I > do my read with a consistency level of SERIAL? > >> > > >> > It would seem so based on what is written for the datastax python > driver: > >> > > >> > > http://datastax.github.io/python-driver/api/cassandra/query.html#cassandra.query.Statement.serial_consistency_level > >> > > >> > However, that is the only place that I can find this information so I > am a little hesitant to believe it 100%. > >> > > >> > By the way, I did find basically the same question ( > https://www.mail-archive.com/user@cassandra.apache.org/msg45453.html) but > I am unsure if the answer there really answers my question. > >> > > >> > Thank you in advance for any help! > >> > > >> > Best regards, > >> > Craig > >> > > >> > > >> > >> --------------------------------------------------------------------- > >> To unsubscribe, e-mail: user-unsubscr...@cassandra.apache.org > >> For additional commands, e-mail: user-h...@cassandra.apache.org > >> > > --------------------------------------------------------------------- > To unsubscribe, e-mail: user-unsubscr...@cassandra.apache.org > For additional commands, e-mail: user-h...@cassandra.apache.org > >
