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From Yang <teddyyyy...@gmail.com>
Subject Re: expiring + counter column?
Date Sun, 29 May 2011 18:41:03 GMT
yeah, then maybe we can make that a silent omission. less desirable, but
still better than unpredicted behavior. (this is not that bad: currently you
can't know whether a write result really reached a quorum, i.e. become
"effective", anyway)


regarding "we never look at SStables", I think right now counter adds do
require a read on SStables, although asynchronously:
StorageProxy:
private static void applyCounterMutation(final IMutation mutation, final
Multimap<InetAddress, InetAddress> hintedEndpoints, final
IWriteResponseHandler responseHandler, final String localDataCenter, final
ConsistencyLevel consistency_level, boolean executeOnMutationStage) {
......................

sendToHintedEndpoints(cm.makeReplicationMutation(), hintedEndpoints,
responseHandler, localDataCenter, false, consistency_level);
....
}


CounterMutation.java:
    public RowMutation makeReplicationMutation() throws IOException {

    ....
            Table table = Table.open(readCommand.table);
            Row row = readCommand.getRow(table);
................
}


I think the "getRow()" line above does what the .pdf design doc in the JIRA
described: replication to other replicas (non-leaders) replicate only the
**sum** that I own, not individual delta that I just received. actually I'm
not quite understanding why this approach was chosen, since it makes each
write into read---write (when getReplicateOnWrite() ) , which can be slow.
I'm still trying to understand that


Thanks
Yang

On Sun, May 29, 2011 at 3:45 AM, aaron morton <aaron@thelastpickle.com>wrote:

> Without commenting on the other parts of the design, this part is not
> possible "attempts to add to a dead counter throws an exception "
>
> All write operations are no look operations (write to the log, update
> memtables) we never look at the SSTables. It goes against the architecture
> of the write path to require a read from disk.
>
> Cheers
>
> -----------------
> Aaron Morton
> Freelance Cassandra Developer
> @aaronmorton
> http://www.thelastpickle.com
>
> On 29 May 2011, at 20:04, Yang wrote:
>
>
> sorry in the notation, instead of "ttl" I mean "timestamp"
>
>
> On Sun, May 29, 2011 at 12:24 AM, Yang <teddyyyy123@gmail.com> wrote:
>
>> sorry to beat on the dead horse.
>>
>> I looked at the link referred from #2103 :
>> https://issues.apache.org/jira/browse/CASSANDRA-2101
>> I agree with the reasoning in #2101 that the ultimate issue is that delete
>> and counter adds are not commutative. since by definition we can't achieve
>> predictable behavior with deletes + counter, can we redefine the behavior
>> of counter deletes, so that we can always guarantee the declared behavior?
>> --- specifically:
>>
>>
>> *we define that once a counter column is deleted, you can never add to it
>> again.*  attempts to add to a dead counter throws an exception  ---- all
>> future adds are just ignored.  i.e. a counter column has only one life,
>> until all tombstones are purged from system, after which it is possible for
>> the counter  to have a new incarnation.  basically instead of solving the
>> problem raised in #2103, we declare openly that it's unsolvable (which is
>> true), and make the code reflect this fact.
>>
>>
>>
>> I think this behavior would satisfy most use cases of counters. so instead
>> of relying on the advice to developers: "do not do updates for a period
>> after deletes, otherwise it probably wont' work", we enforce this into the
>> code.
>>
>>
>> the same logic can be carried over into expiring column, since they are
>> essentially automatically inserted deletes. that way #2103 could be "solved"
>>
>>
>> I'm attaching an example below, you can refer to them if needed.
>>
>> Thanks  a lot
>> Yang
>>
>>
>> example:
>> for simplicity we assume there is only one column family , one column, so
>> we omit column name and cf name in our notation, assume all counterColumns
>> have a delta value of 1, we only mark their ttl now. so c(123) means a
>> counter column of ttl=1, adding a delta of 1. d(456) means a tombstone with
>> ttl=456.
>>
>> then we can have the following operations
>>
>> operation                    result after operation
>> ----------------------------------------------------------------------
>> c(1)                            count=1
>> d(2)                            count = null ( counter not present )
>>
>> c(3)                            count = null ( add on dead counter
>> ignored)
>> ---------------------------------------------------
>>
>>
>> if the 2 adds arrive out of order ,  we would still guarantee eventual
>> consistency:
>>
>> operation                    result after operation
>>
>> --------------------------------------------------------------------------------
>> c(1)                            count=1
>> c(3)                            count=2   (we have 2 adds, each with
>> delta=1)
>> d(2)                            count=null (deleted)
>> --------------------------------------------------------------
>> at the end of both scenarios, the result is guaranteed to be null;
>> note that in the second scenario, line 2 shows a snapshot where we have a
>> state with count=2, which scenario 1 never sees this. this is fine, since
>> even regular columns can have this situation (just consider if the counter
>> columns were inserts/overwrites instead )
>>
>>
>>
>> On Fri, May 27, 2011 at 5:57 PM, Jonathan Ellis <jbellis@gmail.com>
>> wrote:
>> > No. See comments to
>> https://issues.apache.org/jira/browse/CASSANDRA-2103
>> >
>> > On Fri, May 27, 2011 at 7:29 PM, Yang <teddyyyy123@gmail.com> wrote:
>> >> is this combination feature available , or on track ?
>> >>
>> >> thanks
>> >> Yang
>> >>
>> >
>> >
>> >
>> > --
>> > Jonathan Ellis
>> > Project Chair, Apache Cassandra
>> > co-founder of DataStax, the source for professional Cassandra support
>> > http://www.datastax.com
>> >
>>
>>
>
>

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