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From William Oberman <ober...@civicscience.com>
Subject Re: What does a write lock ?
Date Fri, 08 Jul 2011 12:30:26 GMT
Questions like this seem to come up a lot:
http://stackoverflow.com/questions/6033888/cassandra-atomicity-isolation-of-column-updates-on-a-single-row-on-on-single-no
http://stackoverflow.com/questions/2055037/cassandra-atomic-reads-writes-within-a-single-columnfamily
http://www.mail-archive.com/user@cassandra.apache.org/msg14701.html

Lets say you read state A (from one key in one CF), you change the data to
A' in your client, and you write A'.  Are you worried that someone else
might have changed A to B during this process (making the "new" state a race
between A' and B)?  It doesn't sound to me like you are...  It sounds to me
like you're worried about a set of columns for the key being in a consistent
state before, during, and after a process.  And A -> A' and A -> B will each
be atomic for the key (based on my understanding).  But, if A' and B are
changes to a different set of columns, I believe that would interleave,
which itself could be "inconsistent" from your application's point of view.


will

On Thu, Jul 7, 2011 at 11:41 PM, Jeffrey Kesselman <jeffpk@gmail.com> wrote:

> Really, as i lay in the bath thinking nabout it, I concluded what I am
> looking for is a very limited form of Consistency.
>
> Its consistency over a single row on a single node just for the period of
> update.
>
>
> On Thu, Jul 7, 2011 at 10:34 PM, Jeffrey Kesselman <jeffpk@gmail.com>wrote:
>
>> 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.
>>
>
>
>
> --
> It's always darkest just before you are eaten by a grue.
>

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