You are viewing a plain text version of this content. The canonical link for it is here.
Posted to user@couchdb.apache.org by Ido Ran <id...@gmail.com> on 2010/09/12 14:43:59 UTC
Revsion Explained
Hi,
I've read that the _rev fiel in each document is composed of two parts -
integer which increment by 1 on each change and MD5 hash of the JSON of the
document.
What part of the JSON is included in that hash?
I've created a new document with _id of "foo" and single field name "start"
with value "99". I've save the document and it got _rev of
"1-b563730fcffb6583ff48980c1533e439".
I didn't change anything, just hit save again in Futon and not the _rev is
"2-75a41bb3bc74cb4d768466efa2a0c1ca".
If I hit save again I get another hash.
Why is that?
Thank you,
Ido
Re: Revsion Explained
Posted by Sebastian Cohnen <se...@googlemail.com>.
Correct, you cannot use the _rev field (nor parts of it) to test for equality of two documents. you can however use the _rev token to verify that the given documents and their history are equal, since that is what conflict detection actually does.
On 12.09.2010, at 15:24, Ido Ran wrote:
> Thank you for the answer.
>
> I though it is possible to compare two _rev fields of two documents and
> based on the hash part (not the integer) to tell if they have the same
> content. I guess it is not possible.
>
> Ido
>
> On Sun, Sep 12, 2010 at 3:14 PM, Sebastian Cohnen <
> sebastiancohnen@googlemail.com> wrote:
>
>> Hi Ido,
>>
>> consider the _rev token as an opaque value necessary for the mvcc (I'd call
>> it mvcc-token, since "revision" is often misunderstood). The hash algorithm
>> may even change someday. AFAIK it contains the document itself and also its
>> history (at least parts of it). The algorithm ensures that the same document
>> with the same history result in the same _rev token even on two different
>> nodes. This behavior is required for conflict detection/resolution.
>>
>> Best
>>
>> Sebastian
>>
>>
>> On 12.09.2010, at 14:43, Ido Ran wrote:
>>
>>> Hi,
>>> I've read that the _rev fiel in each document is composed of two parts -
>>> integer which increment by 1 on each change and MD5 hash of the JSON of
>> the
>>> document.
>>> What part of the JSON is included in that hash?
>>>
>>> I've created a new document with _id of "foo" and single field name
>> "start"
>>> with value "99". I've save the document and it got _rev of
>>> "1-b563730fcffb6583ff48980c1533e439".
>>> I didn't change anything, just hit save again in Futon and not the _rev
>> is
>>> "2-75a41bb3bc74cb4d768466efa2a0c1ca".
>>> If I hit save again I get another hash.
>>>
>>> Why is that?
>>>
>>> Thank you,
>>> Ido
>>
>>
Re: Revsion Explained
Posted by Ido Ran <id...@gmail.com>.
Thank you for the answer.
I though it is possible to compare two _rev fields of two documents and
based on the hash part (not the integer) to tell if they have the same
content. I guess it is not possible.
Ido
On Sun, Sep 12, 2010 at 3:14 PM, Sebastian Cohnen <
sebastiancohnen@googlemail.com> wrote:
> Hi Ido,
>
> consider the _rev token as an opaque value necessary for the mvcc (I'd call
> it mvcc-token, since "revision" is often misunderstood). The hash algorithm
> may even change someday. AFAIK it contains the document itself and also its
> history (at least parts of it). The algorithm ensures that the same document
> with the same history result in the same _rev token even on two different
> nodes. This behavior is required for conflict detection/resolution.
>
> Best
>
> Sebastian
>
>
> On 12.09.2010, at 14:43, Ido Ran wrote:
>
> > Hi,
> > I've read that the _rev fiel in each document is composed of two parts -
> > integer which increment by 1 on each change and MD5 hash of the JSON of
> the
> > document.
> > What part of the JSON is included in that hash?
> >
> > I've created a new document with _id of "foo" and single field name
> "start"
> > with value "99". I've save the document and it got _rev of
> > "1-b563730fcffb6583ff48980c1533e439".
> > I didn't change anything, just hit save again in Futon and not the _rev
> is
> > "2-75a41bb3bc74cb4d768466efa2a0c1ca".
> > If I hit save again I get another hash.
> >
> > Why is that?
> >
> > Thank you,
> > Ido
>
>
Re: Revsion Explained
Posted by Sebastian Cohnen <se...@googlemail.com>.
Hi Ido,
consider the _rev token as an opaque value necessary for the mvcc (I'd call it mvcc-token, since "revision" is often misunderstood). The hash algorithm may even change someday. AFAIK it contains the document itself and also its history (at least parts of it). The algorithm ensures that the same document with the same history result in the same _rev token even on two different nodes. This behavior is required for conflict detection/resolution.
Best
Sebastian
On 12.09.2010, at 14:43, Ido Ran wrote:
> Hi,
> I've read that the _rev fiel in each document is composed of two parts -
> integer which increment by 1 on each change and MD5 hash of the JSON of the
> document.
> What part of the JSON is included in that hash?
>
> I've created a new document with _id of "foo" and single field name "start"
> with value "99". I've save the document and it got _rev of
> "1-b563730fcffb6583ff48980c1533e439".
> I didn't change anything, just hit save again in Futon and not the _rev is
> "2-75a41bb3bc74cb4d768466efa2a0c1ca".
> If I hit save again I get another hash.
>
> Why is that?
>
> Thank you,
> Ido