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From Amit Patel <amit.pa...@cloudera.com>
Subject Re: [DISCUSS] Merging Hybrid Logical Clocks with Master
Date Fri, 25 Aug 2017 17:54:43 GMT
Just as an update with regard to performance: getting the time from or
updating the hybrid logical clock is quite expensive (see clocks
microbenchmark
<https://issues.apache.org/jira/browse/HBASE-18542?focusedCommentId=16141932&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-16141932>)
to the extent that it would be a blocker for this work. There is however
certainly room for improvement given that the current implementation uses
synchronized blocks for both Clock#now and Clock#update.

On Wed, Aug 23, 2017 at 6:18 PM, Enis Söztutar <enis@apache.org> wrote:

> Thanks Amit for continuing on the work.
>
> I am biased, but of course would like to see this merged back to master
> (and possibly make it to 2.0) once the issues are ironed out. We can
> eliminate a very large class of problems with a mathematically sound
> strategy.
>
> Enis
>
> On Tue, Aug 22, 2017 at 1:28 PM, Stack <stack@duboce.net> wrote:
>
> > Look at the pretty HLC timestamps in meta after a run:
> >
> >  hbase:meta
> >           column=table:state, timestamp=1576463246879096833,
> value=\x08\x00
> >  hbase:namespace
> >            column=table:state, timestamp=1576460369910562817,
> > value=\x08\x00
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:regioninfo, timestamp=1576462593304821761,
> > value={ENCODED => 6d365281d5ca8ee0bc0fa12cdf33ecb5, NAME =>
> > 'hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.',
> > STARTKEY => '', ENDKEY => ''}
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:seqnumDuringOpen, timestamp=1576462593304821761,
> > value=\x00\x00\x00\x00\x00\x00\x00\x15
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:server, timestamp=1576462593304821761, value=
> > ve0536.halxg.cloudera.com:16020
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:serverstartcode, timestamp=1576462593304821761,
> > value=1503431013602
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:sn, timestamp=1576462593050017795, value=
> > ve0536.halxg.cloudera.com,16020,1503431013602
> >  hbase:namespace,,1503429763610.6d365281d5ca8ee0bc0fa12cdf33ecb5.
> >           column=info:state, timestamp=1576462593304821761, value=OPEN
> >
> > St.Ack
> >
> >
> >
> > On Tue, Aug 22, 2017 at 1:26 PM, Stack <stack@duboce.net> wrote:
> >
> > > Nice work Amit. Idea of merging sounds good to me. Our metadata system
> > > will run w/ a new rigor as HLC allows us leave behind a whole class of
> > > issues.
> > >
> > > I ran the HLC branch on small cluster for a while w/ chaos. The ITBLL
> job
> > > passed so basically works I'd say. +1 from me on merge.
> > >
> > > St.Ack
> > >
> > > On Mon, Aug 21, 2017 at 1:29 PM, Amit Patel <amit.patel@cloudera.com>
> > > wrote:
> > >
> > >> Hi everyone,
> > >>
> > >> I’d like to begin the discussion of merging the current HLC work with
> > >> master. Note that the current work is not ready to merge yet since
> there
> > >> are a few remaining issues (see below). For reference, HBASE-14070
> > >> <https://issues.apache.org/jira/browse/HBASE-14070> proposes using
> > hybrid
> > >> logical clocks, which combine both logical and physical clocks to
> > capture
> > >> not only causality relationship between events, but also physical time
> > at
> > >> which events occur. HLC can be enabled on a per-table basis and at the
> > >> moment just the meta table has HLC enabled by default (the other
> tables
> > >> default to use the system clock). An initial design document created
> by
> > >> Enis Soztutar can be found here
> > >> <https://docs.google.com/document/d/1LL2GAodiYi0waBz5ODGL4LD
> > >> T4e_bXy8P9h6kWC05Bhw/edit#>.
> > >> The more recent status document created by Sai Teja Ranuva can be
> found
> > >> here
> > >> <https://docs.google.com/document/d/1n32DUKoL3LSoKQ1_NuF8TbQ
> > >> kZiZCwFhbHZ7JXRjg3oA/edit#heading=h.881hwl2sdbvx>.
> > >>
> > >>
> > >> Changes
> > >>
> > >> The new classes that would be introduced include: a Clock interface
> with
> > >> three clock implementations mentioned above and enums for the clock
> and
> > >> timestamp type.  The three clock implementations are hybrid logical,
> > >> system
> > >> monotonic, and system clocks and each generate 64-bit timestamps.
> > >> Timestamps from the system monotonic and system clock represent
> physical
> > >> time and are directly backwards compatible.
> > >>
> > >> Users can set the clock type of a table during table creation. A
> table’s
> > >> clock type cannot be changed after the table has been created. For
> > tables
> > >> whose clock type is HLC, users would not be permitted to set their own
> > >> timestamps for mutations. Certain events such as region assignment,
> > >> unassignment, and recovery update the clocks upon receiving timestamps
> > >> externally.
> > >>
> > >> Benefits
> > >>
> > >>
> > >>    -
> > >>
> > >>    Hybrid logical clocks (and even system monotonic clocks) can solve
> a
> > >>    variety of long standing issues related to time highlighted here
> > >>    <https://docs.google.com/document/d/1LL2GAodiYi0waBz5ODGL4L
> > >> DT4e_bXy8P9h6kWC05Bhw/edit#heading=h.msabldfs8oly>.
> > >>
> > >>    -
> > >>
> > >>    Future work with hybrid logical clocks would include enabling them
> > with
> > >>    user tables. Ideally we would want HLC to be used everywhere.
> Further
> > >> work
> > >>    such as global point-in-time snapshots can leverage HLC
> > >>
> > >>
> > >> Remaining issues
> > >>
> > >> There are still a few remaining issues to close out before actually
> > >> merging, but I wanted to at least start the discussion. Currently I am
> > >> working on fixing remaining tests that are either failing or timing
> out
> > in
> > >> the public branch <https://builds.apache.org/job/HBASE-14070.HLC/>
as
> > >> well
> > >> as doing local performance tests, and cluster tests. Some of the
> > remaining
> > >> issues we are tracking are:
> > >>
> > >>
> > >>    -
> > >>
> > >>    HBASE-18542 (Perf numbers)
> > >>    <https://issues.apache.org/jira/browse/HBASE-18542>
> > >>    -
> > >>
> > >>    HBASE-18508 (Fixing unit tests)
> > >>    <https://issues.apache.org/jira/browse/HBASE-18508>
> > >>    -
> > >>
> > >>    HBASE-18432 (Clock skew)
> > >>    <https://issues.apache.org/jira/browse/HBASE-18432>
> > >>
> > >> Some brainstorming/discussions we also are tracking:
> > >>
> > >>    -
> > >>
> > >>    (HBASE-18643) Adding transaction id to Result
> > >>    <https://issues.apache.org/jira/browse/HBASE-18643>
> > >>    -
> > >>
> > >>    (HBASE-18642) Deprecate setting of timestamp in client for HLC
> > >>    <https://issues.apache.org/jira/browse/HBASE-18642>
> > >>
> > >
> > >
> >
>

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