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From Jason Smith <jason.h.sm...@gmail.com>
Subject Re: Purging documents and view invalidation
Date Mon, 08 Jul 2013 07:17:18 GMT
Paul, I believe you are correct on both counts: It would not re-replicate
but IMO it is a fragile thing to depend on.

The database has a purge_seq value which tracks the number of purges. I do
not recall if that is factored into the replication ID. It should be. If
the target database has undergone a purge since you last met you have no
idea what its state is. Note, the database name is relevant to the
replication id, so simply copying foo.couch to bar.couch would trigger
re-replicating the purged documents.

To me, purging is as if a document had never existed. A replication should
recreate it (unless you change your filter policy or validate_doc_update).
This seems to be what Steven is doing so I think he is using it correctly.
CouchDB purge is like Git rebase: sure it is dangerous, but that's because
it is powerful; and sometimes power users need power tools.


On Mon, Jul 8, 2013 at 2:04 PM, Paul Hirst <paul.hirst@sophos.com> wrote:

> Wouldn't the _local document which tracks replication prevent that?
> Provided the source and destinations databases don't change URL it should
> pick up where it left off every time, and therefore never consider the
> documents due for consideration unless they change. Are you suggesting that
> it's rather fragile to rely on that?
>
> -----Original Message-----
> From: Jason Smith [mailto:jason.h.smith@gmail.com]
> Sent: 05 July 2013 16:23
> To: user@couchdb.apache.org
> Subject: Re: Purging documents and view invalidation
>
> If you do that, and you re-run replication (or potentially if you use
> continuous replication) then those documents will be re-replicated back to
> the remote site. Purging is as if the document was never created at all. So
> when replication runs, the couches will want to copy it from the "master"
> source.
>
>
> On Fri, Jul 5, 2013 at 8:12 PM, Steven Barlow <stemail23@gmail.com> wrote:
>
> > Purged at the remote site. The master always contains the complete
> > data set, the remote sites replicate partial data sets for their
> > immediate needs, and then clean themselves up once the tasks are
> > complete.
> >
> > On 05/07/2013, at 9:57 PM, Jason Smith <jason.h.smith@gmail.com> wrote:
> >
> > > On which database will you perform the purging?
> > >
> > >
> > > On Fri, Jul 5, 2013 at 6:52 PM, Steven Barlow <stemail23@gmail.com>
> > wrote:
> > >
> > >> Sorry if this is a tangent, but I wanted to pick up on the "rarely
> > >> used in the wild" thread: I personally intend to use purge, because
> > >> I have temporary partial (filtered) replications of a "master"
> > >> database at remote sites. When the data has been consumed by the
> > >> remote site, I figured I could purge it (to save space). Is this
> > >> not a valid, or common use case for purging?
> > >>
> > >> On 05/07/2013, at 7:21 PM, Jason Smith <jason.h.smith@gmail.com>
> wrote:
> > >>
> > >>> I slightly disagree with Bob, but he is right that all purge buys
> > >>> you
> > >> (vs.
> > >>> filtered replication and then swapping DBs) is a little bit of
> uptime.
> > >>> Purge is not "untested" but it is rarely used in the wild, so the
> > >>> cost/benefit for your uptime is something between "risky" and
> > "unknown."
> > >>>
> > >>> (For me, personally, I would purge.)
> > >>>
> > >>>
> > >>> On Fri, Jul 5, 2013 at 3:31 PM, Robert Newson <rnewson@apache.org>
> > >> wrote:
> > >>>
> > >>>> Paul,
> > >>>>
> > >>>> If you replicate this database to another database and use a
> > >>>> filter that blocks deleted documents, the target will not contain
> > >>>> a trace of your 100 million deletes (that is, you can build a new
> > >>>> database without cruft without messing with your existing
> > >>>> database). During the replication, you can query the view on the
> > >>>> target to build it incrementally, or wait till the end, query it
> > >>>> once and wait for completion. At the end, flip your app to look
> > >>>> at the new database instead.
> > >>>>
> > >>>> The _purge feature is really only for the case where you
> > >>>> accidentally write your root password down in a document id or
> > >>>> something (since compaction will sweep away old document
> > >>>> contents). I advise against using it for any other reason.
> > >>>>
> > >>>> B.
> > >>>>
> > >>>>
> > >>>>
> > >>>>
> > >>>> On 5 July 2013 09:17, Jason Smith <jhs@apache.org> wrote:
> > >>>>> Hi, Paul. I wrote up some thoughts on purging here:
> > >>>>> https://github.com/iriscouch/cqs#purging-couchdb
> > >>>>>
> > >>>>> Note, that procedure is untested. It works as a thought
> > >>>>> experiment
> > >> only.
> > >>>>>
> > >>>>> The procedure looks complicated, but all you will need is the
> > >>>>> core
> > >> purge,
> > >>>>> view, purge, view, etc. cadence as described in Damien's email
I
> > linked
> > >>>> to.
> > >>>>> As long as you never purge twice before hitting the view, you
> > >>>>> are
> > fine.
> > >>>>> Again, to my knowledge, the purge code is less well tested
than
> > >>>>> other
> > >>>> parts
> > >>>>> of CouchDB, so perhaps copy your .couch file and try with that
> > >>>>> until
> > >> you
> > >>>>> are confident.
> > >>>>>
> > >>>>>
> > >>>>> On Fri, Jul 5, 2013 at 2:37 PM, Paul Hirst
> > >>>>> <paul.hirst@sophos.com>
> > >>>> wrote:
> > >>>>>
> > >>>>>> I would like to purge a few (~100 million) documents from
my
> > database.
> > >>>>>> I've been going through deleting them all, and that'll
be
> > >>>>>> complete
> > in
> > >>>> the
> > >>>>>> next few days but I would like to free up some extra space
by
> > purging
> > >>>> them
> > >>>>>> also.
> > >>>>>>
> > >>>>>> My concern is around a comment on the wiki page here
> > >>>>>> http://wiki.apache.org/couchdb/Purge_Documents
> > >>>>>>
> > >>>>>> 'If you have purged more than one document between querying
> > >>>>>> your
> > >> views,
> > >>>>>> you will find that they will rebuild from scratch.'
> > >>>>>>
> > >>>>>> Since I have already deleted the documents I know they
aren't
> > showing
> > >> up
> > >>>>>> in the view any longer. Is there any way I can avoid this
view
> > >>>>>> invalidation? (My views take about 10 days to build from
> > >>>>>> scratch so
> > I
> > >>>> can't
> > >>>>>> afford the hit).
> > >>>>>>
> > >>>>>> I have a replica of the database. I could do the purge
on the
> > replica,
> > >>>>>> wait for the view to rebuild, switch over, purge on the
> > >>>>>> original db,
> > >>>> wait
> > >>>>>> for the view, switch back, unless there are any obvious
> > >>>>>> problems
> > with
> > >>>> this
> > >>>>>> approach?
> > >>>>>>
> > >>>>>> Cheers,
> > >>>>>> Paul
> > >>>>>>
> > >>>>>> ________________________________
> > >>>>>>
> > >>>>>> Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon,
> > >>>>>> OX14
> > >> 3YP,
> > >>>>>> United Kingdom.
> > >>>>>> Company Reg No 2096520. VAT Reg No GB 991 2418 08.
> > >>
> >
>
> ________________________________
>
> Sophos Limited, The Pentagon, Abingdon Science Park, Abingdon, OX14 3YP,
> United Kingdom.
> Company Reg No 2096520. VAT Reg No GB 991 2418 08.
>

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