[Seaside] saving an image while serving

Igor Stasenko siguctua at gmail.com
Sat Apr 16 23:57:27 UTC 2011


This is really good to hear that.

I leaned something new. Usual way of design is to prevent failure(s).
But going that way we tend to forget that failures are inevitable, and
paying little attention towards what can be done
to restart/reload data from backups quickly and easily.
Indeed, embracing failure is the way we should design our systems.

On 17 April 2011 01:37, Nevin Pratt <nevin at bountifulbaby.com> wrote:
> Define "handful".  You'd be surprised.
>
> We are a large enough company with enough orders, to support more than a
> dozen employees, and a half a dozen contractors.  Our warehouse/office is
> now about 26,000 square feet.  And, we're growing, and profitable, with 2011
> looking to be our best year ever.
>
> The exact numbers are confidential, but I am confident that we are the
> largest company on the planet in our field (i.e., "Reborn Doll Supplies").
>
> Besides, the orders aren't exactly "hand reentered".  I just didn't give
> enough detail, because I didn't feel it was relevant.  We have an admin page
> that can easily slurp the data in rapidly, if needed.  It's not hard to do.
>
> And Squeak/Seaside has served us well.  And we just use the image for
> persistence, as previously mentioned.  And we are very happy with it.
>
> Nevin
>
>
> Of course that only works when you have a handful of orders to reenter,
> which isn't the case for most systems.
>
> Sent from my iPhone
> On 2011-04-16, at 12:41, "Sebastian Sastre" <sebastian at flowingconcept.com>
> wrote:
>
> So, 15 minutes times 4 makes a whole hour per year.
> Beautiful :)
> Doesn't even justify to implement automation of the manual rollback (that
> sounds feasible)
>
>
>
> On Apr 16, 2011, at 1:29 PM, Nevin Pratt wrote:
>
> Sebastion, that is *exactly* what I initially did at Bountiful Baby.
> However, it has been several *years* since I've had to do your #4, so things
> have changed slightly since.
>
> Bountiful Baby is an eCommerce site, and the critical "object graph" (your
> #2, below) information consists of inventory data (the website keeps track
> of our inventory), and gift certificate data (for gift certificates that
> have been issued).  Also, whenever either of those datums change, the
> website sends an email-- for example, emails are (obviously) sent for each
> order accepted, and, it sends emails when it issues a gift certificate.  So,
> it is easy to discover (via the emails) what data was lost since the last
> image crash.
>
> Consequently, currently this is what happens:
>
> 1. image is saved from time to time (usually daily), and copied to a
> separate "backup" machine.
> 2. if anything bad happens, the last image is grabbed, and the orders and/or
> gift certificates that were issued since the last image save are simply
> re-entered.
>
> And, #2 has been *very* rarely done-- maybe a two or three times a year, and
> then it turns out it is usually because I did something stupid.
>
> For us, it's a whole lot easier to do persistence this way than bothering
> with any persistence mechanism.  And, it turns out, it is *very* reliable,
> with exactly one easily fixable glitch:
>
> The glitch is: occasionally the production image UI will freeze, for no
> known reason.  It doesn't effect Seaside, though, so the website keeps going
> just fine.  And, if we run the "screenshot" app (in the configuration
> screen), there is a link at the top for "Suspend UI Process", and "Resume UI
> Process".  Just suspend and resume, and the UI becomes unstuck.
>
> We've been doing persistence this way for years now, and I've been
> *extremely* impressed with the reliability.
>
> Before that, we used PostgreSQL and GLORP for persistence.  But I yanked
> that code out years ago.  It wasn't worth the bother maintaining it.
>
> If you have a daily image save, then on average there will be 12 hours of
> lost data on a "random" crash.  Re-entering 12 hours of orders and/or gift
> certificates (discovered from the emails, as mentioned above) might take 10
> to 15 minutes.  Not a big deal at all for us.
>
> Ten years ago I wouldn't have even considered doing persistence this way,
> but I've changed.  Squeak has changed.  Seaside has changed.  And all for
> the better.
>
> Nevin
>
>
>
> Given that you don't need transactions, to make that style work, I suggest
> this:
> 1. save the image from time to time (as best suits your needs) like you're
> doing now
> 2. use a secondary way to dump the object graph
> 3. use the normal image for as long as things are good
> 4. when shit happens you "transplant" the object graph into a new
> "reincarnation" of your app in a fresh image
> 5. repeat
> For dumping the ODB you have options: image segments, SIXX comes to mind now
> If you make for yourself some "rescue" kit (script, tools, preloaded code in
> fresh image), you can make 4 quite painless (or, why not, monitored and
> automated)
> sebastian
> o/
>
>
> On Apr 16, 2011, at 12:23 PM, Michal wrote:
>
> hi -
>
> Despite the warnings, I am really interested in sticking to the
> simplest way of saving my seaside application data, ie periodically
> saving and backuping the image. The seaside book states that
>
>  "saving [the image] while processing http requests is a risk you
>  want to avoid."
>
> What is the status on that? Is that something we can fix? I have been
> running an image in this mode for a few weeks, with no ill effect so
> far, but I have had major problems with old image/vm combinations. So
> is this something that might be fixed already?
>
> Also, I recall that Avi had made a number of attempts at having an
> image saved in a forked background process, eg
>
> http://lists.squeakfoundation.org/pipermail/squeak-dev/2005-October/095547.html
>
> did anybody pick up on this, or did anything come out of it?
>
> thanks,
> Michal
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-- 
Best regards,
Igor Stasenko AKA sig.


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