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From Jeffrey Kesselman <jef...@gmail.com>
Subject Re: What does a write lock ?
Date Fri, 08 Jul 2011 02:34:44 GMT
Its not really isolation, btw, because we arent talking about anyone seeing
an update mid-update.    Rather, we are talking about when updates are
allowed to occur.

Atomicity means that all the updates happen together or they don't happen at
all.
Isolation means that no results of the update are visible until the entire
update operation is complete.

This really lies somewhere in the middle of the two concepts.   Its part of
the results of the combined effects of ACID


On Thu, Jul 7, 2011 at 10:27 PM, Jonathan Ellis <jbellis@gmail.com> wrote:

> Sounds to me like you're confusing atomicity with isolation.
>
> On Thu, Jul 7, 2011 at 2:54 PM, Jeffrey Kesselman <jeffpk@gmail.com>
> wrote:
> > Yup, im even more confused.    Lets talk about the model, not the
> > implementation.
> > AIUI updates to a row are atomic across all columns in that row at once,
> > true?
> > If true then the next question is, does the validation happen inside or
> > outside of that guarantee, and is the row guaranteed not to change
> between
> > validation and update?
> > If that is *not* the case then it makes a whole class of solutions to
> > synchronization problems fail and puts my larger project
> > in serious question.
> >
> > On Thu, Jul 7, 2011 at 3:43 PM, Yang <teddyyyy123@gmail.com> wrote:
> >>
> >> no , the memtable is a concurrentskiplistmap
> >>
> >> insertion can happen in parallel
> >>
> >> On Jul 7, 2011 9:24 AM, "Jeffrey Kesselman" <jeffpk@gmail.com> wrote:
> >> > This has me more confused.
> >> >
> >> > Does this mean that ALL rows on a given node are only updated
> >> > sequentially,
> >> > never in parallel?
> >> >
> >> > On Thu, Jul 7, 2011 at 3:21 PM, Yang <teddyyyy123@gmail.com> wrote:
> >> >
> >> >> just to add onto what jonathan said
> >> >>
> >> >> the columns are immutable . if u overwrite/ reconcile a new obj is
> >> >> created and shoved into the memtable
> >> >>
> >> >> there is a shared lock for all writes though which guard against an
> >> >> exclusive lock on memtable switching/flushing
> >> >> On Jul 7, 2011 7:51 AM, "A J" <s5alye@gmail.com> wrote:
> >> >> > Does a write lock:
> >> >> > 1. Just the columns in question for the specific row in question
?
> >> >> > 2. The full row in question ?
> >> >> > 3. The full CF ?
> >> >> >
> >> >> > I doubt read does any locks.
> >> >> >
> >> >> > Thanks.
> >> >>
> >> >
> >> >
> >> >
> >> > --
> >> > It's always darkest just before you are eaten by a grue.
> >
> >
> >
> > --
> > It's always darkest just before you are eaten by a grue.
> >
>
>
>
> --
> Jonathan Ellis
> Project Chair, Apache Cassandra
> co-founder of DataStax, the source for professional Cassandra support
> http://www.datastax.com
>



-- 
It's always darkest just before you are eaten by a grue.

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