[Seaside] saving an image while serving

Nevin Pratt nevin at bountifulbaby.com
Sat Apr 16 16:29:52 UTC 2011

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.


> 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 <http://twitter.com/#%21/sebastianconcpt>
> 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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