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From Andrew Melo <andrew.m...@gmail.com>
Subject Re: About denormalization and keep consistent
Date Fri, 09 Apr 2010 18:47:26 GMT
On Fri, Apr 9, 2010 at 1:44 PM, faust 1111 <faust451@gmail.com> wrote:
> Thanks Chris.
>
>> 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.
>>
>
> You told about more robust method, you mean run backend process listen
> _changes feed for Authors,
> and when changes come try update all contents related to author?
> If i get you right, i don't understand you
>>because you can know for sure that *eventually* the author's name will be changed
everywhere it appears.
> What you mean?
>
>
>> 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name (due to there being out of date replicas of the master record lingering, for instance)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.
>>
>
> Probably i don't get you:
> I track _changes feed, when change come you suggest query that view
> for the old author's name, but how i know old name, author doc all
> ready with newest name.

When you spawn the background process, you'll have to pass it that
information somehow.



>
> Sorry for getting your time, to answer for stupid questions.
>
>
> 2010/4/9 J Chris Anderson <jchris@gmail.com>:
>>
>> On Apr 8, 2010, at 11:55 PM, faust 1111 wrote:
>>
>>> Yes i understand that listen _changes is better to get round race conditions.
>>>
>>> Cannot get your suggesting about
>>> how i can track that all contents related to author was updated not 5
>>> of 50 but all.
>>>
>>
>> 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?
>>>
>>
>> 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
>>
>>>
>>> 2010/4/9 Nicholas Orr <nicholas.orr@zxgen.net>:
>>>> i don't think you are getting what the above people are suggesting...
>>>>
>>>> Go read up on the _changes API :)
>>>>
>>>> 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 :)
>>>>
>>>> Nick
>>>>
>>>> On Fri, Apr 9, 2010 at 4:34 AM, faust 1111 <faust451@gmail.com> wrote:
>>>>
>>>>> i means
>>>>> when i do
>>>>> Content.by_author(self).each {|content|
>>>>>          content.author_name = self.name;
>>>>>          content.save(bulk=true)
>>>>>       }
>>>>>
>>>>> i don't sure that all contents will updated may be only 5 and then
>>>>> process crushed.
>>>>>
>>>>> 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 = self.name;
>>>>>>>           content.save(bulk=true)
>>>>>>>       }
>>>>>>>
>>>>>>> or i start backend process to track Author _changes.
>>>>>>>
>>>>>>> This code not guarantee that all contents will updated.
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>> 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.
>>>>>>
>>>>>> best,
>>>>>> Andrew
>>>>>>
>>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>>>>
>>>>>>>> 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.
>>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> 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
>>>>>>>>>>
>>>>>>>>>> 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.
>>>>>>>>>>
>>>>>>>>>> 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.
>>>>>>>>>>>
>>>>>>>>>>> is it possible to keep consistent when apply
denormalization?
>>>>>>>>>>> For example
>>>>>>>>>>> Content
>>>>>>>>>>>   have author (we store author name and id in
Content)
>>>>>>>>>>>
>>>>>>>>>>> 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)
>>>>>>>>>>>
>>>>>>>>>>> What i can do in this case?
>>>>>>>>>>> Thanks.
>>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> --
>>>>>>>> --
>>>>>>>> Andrew Melo
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> --
>>>>>> Andrew Melo
>>>>>>
>>>>>
>>>>
>>
>>
>



-- 
--
Andrew Melo

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