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Subject: Re: About denormalization and keep consistent
From: J Chris Anderson <jchris@gmail.com>
In-Reply-To: <x2nf39928a61004082355u9ffd124fl1f983216d80619e9@mail.gmail.com>
Date: Fri, 9 Apr 2010 07:19:00 -0700
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To: user@couchdb.apache.org


On Apr 8, 2010, at 11:55 PM, faust 1111 wrote:

> Yes i understand that listen _changes is better to get round race =
conditions.
>=20
> Cannot get your suggesting about
> how i can track that all contents related to author was updated not 5
> of 50 but all.
>=20

I think your question is valid. The answer is also simple. There is no =
way to transactionally ensure that the author's name is updated =
everywhere it appears.

Your idea to do it in the app server, as the author changes their master =
record, is troubling because it can lead to race conditions. The changes =
method, where a name-update is an asynchronous process, is more robust, =
because you can know for sure that *eventually* the author's name will =
be changed everywhere it appears.

It is probably best to make this clear through the UI with a message =
like: "Your name has been changed in the master record. It could take a =
few minutes for the change to appear throughout the site."

In actuality, this is probably no different than in a relational =
database (as in a relational database, you'd probably have a caching =
layer that takes a few minutes to expire anyway.)

> Thats ok.
> I don't understand if listen feed _chenges, feed give me info only
> about id & rev of changed doc, how i can get that author name is
> changed?
>=20

My method is to have a view of docs by author, and then query that view =
for the old author's name, updating any docs that appear. This way if =
new writes come in with the old name (due to there being out of date =
replicas of the master record lingering, for instance) they will be =
eventually updated as well. You could have a time-to-live of something =
like 5 minutes (or longer if your system is giant) for the process which =
is running the query for docs-that-say-Joe-but-should-say-Joseph and =
updating them.

_changes is just a convenient way to trigger that view query (so that =
you aren't polling the view when nothing has happened in the database.) =
With filtered changes, you can even be sure that you are only polling =
the view when there will be something relevant to see. However, all this =
_changes stuff is really just an optimization over brute force polling =
the view once every N seconds, so you can add it later, when your app is =
big enough that load starts to matter.

Chris

>=20
> 2010/4/9 Nicholas Orr <nicholas.orr@zxgen.net>:
>> i don't think you are getting what the above people are suggesting...
>>=20
>> Go read up on the _changes API :)
>>=20
>> The basics are, every single change in the database is pushed into =
this
>> feed. All race conditions that are caused by your ruby way (via the =
filter)
>> are averted :)
>>=20
>> Nick
>>=20
>> On Fri, Apr 9, 2010 at 4:34 AM, faust 1111 <faust451@gmail.com> =
wrote:
>>=20
>>> i means
>>> when i do
>>> Content.by_author(self).each {|content|
>>>          content.author_name =3D self.name;
>>>          content.save(bulk=3Dtrue)
>>>       }
>>>=20
>>> i don't sure that all contents will updated may be only 5 and then
>>> process crushed.
>>>=20
>>> 2010/4/8 Andrew Melo <andrew.melo@gmail.com>:
>>>> On Thu, Apr 8, 2010 at 12:53 PM, faust 1111 <faust451@gmail.com> =
wrote:
>>>>> What difference?
>>>>> if do
>>>>> Author
>>>>>  after_save
>>>>>     if name_changed?
>>>>>        Content.by_author(self).each {|content|
>>>>>           content.author_name =3D self.name;
>>>>>           content.save(bulk=3Dtrue)
>>>>>       }
>>>>>=20
>>>>> or i start backend process to track Author _changes.
>>>>>=20
>>>>> This code not guarantee that all contents will updated.
>>>>=20
>>>> I don't get your question. You asked how to make sure that you =
could
>>>> change a number of documents consistently, we suggested that you =
watch
>>>> _changes to catch any silly race conditions. Then, you told us you
>>>> didn't need to use _changes, but you were worried that things would =
be
>>>> inconsistent.
>>>>=20
>>>> Even with your code above, you get a race condition (if I =
understand
>>>> your ruby right, I don't know ruby much at all). Something could
>>>> happen between when you check to see if a document needs to be =
changed
>>>> and the actual change occurs. Then, you're gonna get a conflict and
>>>> have to write up the logic to handle that intelligently.
>>>>=20
>>>> best,
>>>> Andrew
>>>>=20
>>>>=20
>>>>>=20
>>>>> 2010/4/8 Andrew Melo <andrew.melo@gmail.com>:
>>>>>> On Thu, Apr 8, 2010 at 12:29 PM, faust 1111 <faust451@gmail.com>
>>> wrote:
>>>>>>> I can catch changes in my app before save author, may be backend
>>>>>>> process is surplus in my case.
>>>>>>> i need consistent, when i update author name i must know that =
all
>>>>>>> contents with author was updated success.
>>>>>>=20
>>>>>> Then their suggestion of watching _changes works for you. Start
>>>>>> watching _changes. Make all your changes to the documents' =
authors.
>>>>>> Any changes that come through on _changes, make sure they have =
the
>>>>>> proper author. Keep watching _changes until you're sure that =
nobody
>>>>>> has stale data they're waiting submit.
>>>>>>=20
>>>>>>=20
>>>>>>>=20
>>>>>>>=20
>>>>>>> 2010/4/8 Zachary Zolton <zachary.zolton@gmail.com>:
>>>>>>>> I suggest you check out the _changes API:
>>>>>>>> http://books.couchdb.org/relax/reference/change-notifications
>>>>>>>>=20
>>>>>>>> Basically, if you have doc types A & B, where B maintains a =
denormed
>>>>>>>> bit of A, then you can watch the _changes feed in a backend =
process.
>>>>>>>> When an A gets updated, hit a view of all B's related to that
>>>>>>>> particular A, and update the dernomed data.
>>>>>>>>=20
>>>>>>>> On Thu, Apr 8, 2010 at 10:20 AM, faust 1111 =
<faust451@gmail.com>
>>> wrote:
>>>>>>>>> Hi guy's
>>>>>>>>> I return back to my problem with denormalization.
>>>>>>>>>=20
>>>>>>>>> is it possible to keep consistent when apply denormalization?
>>>>>>>>> For example
>>>>>>>>> Content
>>>>>>>>>   have author (we store author name and id in Content)
>>>>>>>>>=20
>>>>>>>>> When author name changed(that's happens not frequently)
>>>>>>>>> i need find all content belong to this author and update =
author name
>>>>>>>>> but what if this operation not finished (not all docs was =
updated)
>>>>>>>>>=20
>>>>>>>>> What i can do in this case?
>>>>>>>>> Thanks.
>>>>>>>>>=20
>>>>>>>>=20
>>>>>>>=20
>>>>>>=20
>>>>>>=20
>>>>>>=20
>>>>>> --
>>>>>> --
>>>>>> Andrew Melo
>>>>>>=20
>>>>>=20
>>>>=20
>>>>=20
>>>>=20
>>>> --
>>>> --
>>>> Andrew Melo
>>>>=20
>>>=20
>>=20