#django

haven't used it, but seems like an option

you just gotta compare prices/features really and get what works for your project

or roll your own if you want to deal with the headache

but cloudinary seems a bit an overkill. it allows for effects, face detection, etc.

I imagine you can disable the stuff you don't need

but they start at $90/m

that probably includes the r&d costs

for face detection!

i probably just roll my own with boto

is it a good idea to use django for the backend server of a mobile app?

but django provides so much front end stuff, like templates, etc. is it designed to be used as a backend server?

You can use whatever frontends you want

mun: the only downside maybe is django needs a third party lib to do json-y api-y stuff better

pokmo: You could have a look at libpixel.com, they do resizing and some basic filtering

pete___ thanks for the pointer

pete___ looks like libpixel taps into your image store

so image storage needs to be done separately

pokmo: yeap, we use django-storages / boto to get the stuff into s3, then make calls via a template tag to get django to call the url of the image we need

pete___ they're definitely more economical though :)

pokmo: yea, the service is pretty good as well, we don't use them for much more than responsive images of products but they do a pretty decent job of that

since we use heroku we use s3 from media content anyways so it was a bit of a no-brainer

Hey, I have a critical section in my code that could be lead to a problem if two people do something "in parallel". While that probably won't be the case for my web application (it's low traffic), I want to do it the right way nonetheless.

However, first of all I'd like to test this before and get the error, then fix, and run the test again

pete___ have you tried thumbor? https://github.com/thumbor/thumbor

Is there something to say to Django "please run THIS code in parallel"

ovalseven8: No. But it all depends what are you trying to do.

pokmo: I think I found that after using libpixel for a while

but you'll have to host it yourself

I can't find the way to render the app root in template, I've declared this: path('', TemplateView.as_view(template_name="gda/dashboard.html")),

pokmo: honestly for the time involved I didn't think it was worth the switch

jtiai, I have an event where people can register for. But when someone register AT THE SAME TIME, it could lead that they register 4 tickets in total although only 2 available (both register 2)

but I can't use {% url '' %}

ovalseven8: select for update would be useful for you.

jtiai, Mh but I cannot reproduce this potential failure?

ovalseven8: Testing parallelism is quite difficult.

pete___ but libpixel isn't cheap either. probably my use case is a bit different

jtiai, Would not this be enough for me? https://docs.djangoproject.com/en/2.0/topics/db...

Or what's the difference to select for update

ovalseven8: Nope. It just ties transaction to sequence.

my app is like a time lapse camera, capturing images over intervals

...sequence? where I got that. I meant request. :)

so most image requests are going to be unique :(

ovalseven8: select_for_update locks select row from editing.

(called also as pessimistic locking)

jtiai, Does the transaction not do locking by default?

pokmo: maybe, like I said, we just use it for product images that don't change all that often. I guess you gotta consider the costs of running a server (including maintenance etc) vs the scaling up costs of something like libpixel

ovalseven8: Nope. It doesn't have anything to do with locking.

jtiai, But in normal DBMS when I start a transaction and write to a tupel, it sets a lock normally. So that's different to Django here?

ovalseven8: No it doesn't do that.

pokmo: one alternative that we looked into was imgix (https://www.imgix.com/pricing) which is a similar system. The only issue was that they wanted $50 a month for SSL certificates and since we can use a few domains this was a deal breaker for us

jtiai, You mean DBMS or Django? :D

dbms

Django doesn't change how transactions in databases do work.

pete___ oh i've looked them up too. $3/1000 image requests should be pretty good for you otherwise

pokmo: actually, I just dug out the email from them. $100 Annual + $50 a month for a regular cert, $600 annual + $50 a month for a wildcard!

ovalseven8: transactions doesn't protect from race conditions. To handle it you need to do it yourself.

Is there a way to retrieve one row per day using Django ? I tried DISTINCT but Im using SQLite

And for that django provides you way to lock rows from editing.

converge: one row per day?

they don't mention any of this up front and have the cheek to say it's costs passed on from their CDN (which is just cloudfront!)

jtiai, Does not it depend on the isolation level? E.g. serializable

pete___ gosh

jtiai, yes, I have (id, followers, last_update) -> (1, 100, '2018-05-30') (2, 200, '2018-05-30') (3, 300, '2018-05-31') (4, 350, '2018-05-31') , I want to retrieve the first or last record of the day

ovalseven8: A bit. but read committed is usual isolation level.

pete___ maybe that's how users make up for the cheap request rates

pete___ does libpixel charge for SSL certs?

pokmo: nope, I don't think so

jtiai, Mh, in that case I do not have a problem? https://stackoverflow.com/a/1005218

cool

ovalseven8: Well sqlite locks the whole database. But sqlite is not a "real" rdbms engine. Real ones like postgres do much more sophisticated locking.

Having trouble binding data to a filefield when validation fails. The input type file widget just shows “no file selected”. I make sure to pass in request.FILES into the form’s data in the view

jtiai, could I do that using custom manager ?

converge: There isn't actally single query that would fetch the data.

jtiai, right, could I add some logic to a custom manager method to do it ?

I mean, is it a good idea ?

That's one possibilty. Not sure is that a good idea.

jtiai, So when I understand correctly, I have to use select_for_update() WITHIN an atomic function? So @transaction.atomic

ovalseven8: No you don't.

ovalseven8: select_for_update() tells database to lock selected rows exclusively. That way two (even separate) processes can't modify same rows. Now if two concurrent access happens - latter one needs to wait for lock to be released.

..or you can define that exception is raised when rows are locked to avoid waiting for long periods.

select_for_update() doesn't work outside of a transaction

jtiai, Yeah, but in the docs it says "Returns a queryset that will lock rows until the end of the transaction"

table/row locks are always held until the end of the transaction, they don't work in autocommit mode

don't they?

knbk, But Django does autocommit by default

Well I never run my code in autocommit mode.

That's why I asked if I have to use @transaction.atomic

For that view which may modify that data you want to have an exlusive lock.

Mh, in Django a transcation only begins when I do queries like "Model.objects.get(pk=1)". But when I have select_for_update() there is no transaction yet

unless you disable autocommit (django enables it by default), you need to use transaction.atomic to start a transaction

knbk, Yeah, that what I wrote. :)

knbk, Do you know if there's a way to simulate parallel changes? I have a view that could lead to probelsm when executed in parallel. I want to fix it with select_for_update() but test it before and after.

To see if my code change really does what I except it to do :D

testing locks/race conditions is hard, there's no clean way to test it since you can't control the order of operations

knbk, So when I understand correctly THE FIRST example is completely without effect?

And the second solution is okay?

which examples?

knbk, Oh, sorry: https://dpaste.de/CPAh

you don't evaluate the queryset in either case, so they both don't lock the rows

but if you evaluate it in the first one, you should get a TransactionManagementError

knbk, I do not get an error :/

do you run with ATOMIC_REQUESTS or with autocommit disabled?

or inside a TestCase?

knbk, Completely default normal function view in Django.

with "Registration.objects.select_for_update().filter(event=event)" in the view

so did you evaluate the queryset?

knbk, What do you mean exactly, sorry

read this: https://docs.djangoproject.com/en/2.0/ref/model...

querysets are lazy, what you're doing doesn't actually execute a query

knbk, Works: https://dpaste.de/e00u

which database are you using?