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From Jeffrey Zhong <jzh...@hortonworks.com>
Subject Re: Timestamp resolution
Date Thu, 12 Jun 2014 22:01:20 GMT

In situations such as client "timestamp" is used to support transactions,
it's not bad to disable server side TTL because TTL clean up doesn't care
about transaction semantics. Therefore, it likely breaks data integrity
across multiple tables or regions.

It's still a good use case to support TTL on "client provided timestamp".
I think we can provide a plug-in so that during compaction we allow user
to interpret their own timestamp to decide if to GC old data.


On 6/11/14 10:39 PM, "lars hofhansl" <larsh@apache.org> wrote:

>The issues you cite are all orthogonal. We have client/RS time now, we
>have clock skew now, that is completely independent from the time
>I explained the need I saw for this before. Lemme include:
>On Fri, May 23, 2014 at 06:16PM, lars hofhansl wrote:
>> The specific discussion here was a transaction engine doing snapshot
>> isolation using the HBase timestamps, but still be close to wall clock
>> as much as possible.
>> In that scenario, with ms resolution you can only do 1000
>> and so you need to turn the timestamp into something that is not wall
>> time as HBase understands it (and hence TTL, etc, will no longer work,
>> well as any other tools you've written that use the HBase timestamp).
>> 1m transactions/sec are good enough (for now, I envision in a few years
>> we'll be sitting here wondering how we could ever think that 1m
>> transaction/sec are sufficient) :)
>The point is: Even if you had timestamp oracle (that can resolve ms and
>fill inside ms resolution with a counter), there'd be no way to use this
>as the HBase timestamp while being close to wall clock (so that TTL, etc,
>still works).
>So specifically I was not advocating an automatic higher time resolution
>(as far as I know that cannot be done reliably in Java across
>multiple cores). I was advocating allowing clients with access to a
>(perhaps, but not necessarily single threaded) timestamp oracle to store
>those timestamps and still make use of all HBase optimization (filtering
>HFiles, TTL, etc).
>-- Lars
> From: Michael Segel <michael_segel@hotmail.com>
>To: dev@hbase.apache.org
>Cc: lars hofhansl <larsh@apache.org>
>Sent: Wednesday, June 11, 2014 2:03 PM
>Subject: Re: Timestamp resolution
>Weirdly enough I find that I have to agree with Andrew.
>First, how do you get time in units smaller than a ms?
>Second clock skew becomes an issue.
>Third, which clock are you using? The client machine? The RS? And then
>how do you synchronize each of the RS to be within a ms of each other?
>Correct me if I¹m wrong but NTP doesn¹t give that close of a sync.
>Sorry, but really, not a good idea.
>If you want thisŠ you can store the temporal data as a column.
>Time really is relative.
>On May 25, 2014, at 12:53 AM, Stack <stack@duboce.net> wrote:
>> On Fri, May 23, 2014 at 5:27 PM, lars hofhansl <larsh@apache.org> wrote:
>>> We have discussed this in the past. It just came up again during an
>>> internal discussion.
>>> Currently we simply store a Java timestamp (millisec since epoch),
>>>i.e. we
>>> have ms resolution.
>>> We do have 8 bytes for the TS, though. Not enough to store nanosecs
>>> would only cover 2^63/10^9/3600/24/365.24 = 292.279 years), but enough
>>> microseconds (292279 years).
>>> Should we just store he TS is microseconds? We could do that right now
>>> (and just keep the ms resolution for now - i.e. the us part would
>>>always be
>>> 0 for now).
>>> Existing data must be in ms of course, so we'd grandfather that in, but
>>> new tables could store by default in us.
>>> We'd need to make this configurable both the column family level and
>>> client level, so clients could still opt to see data in ms.
>>> Comments? Too much to bite off?
>>> -- Lars
>> I'm a fan.  As Enis cites, HBASE-8927 has good discussion.  No
>> configuration I'd say.  Just move to the new regime (though I suppose we
>> should let you turn it off).
>> I think it was Liu Shaohui (IIRC) who made a suggestion that had us put
>> together ms and nanos under a synchronized block stamping the ts on
>> (left-shift the currentTimeMillis and fill in the bottom bytes with as
>> of the nanos as fits; i.e. your micros).  Rather than nanos/micros, we
>> could use a counter instead if a Cell arrives in the same ms.  Would be
>> costly having all ops go via one code block to get 'time' across cores
>> handlers.
>> St.Ack

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