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Choice of architecture for exposing CLIPS expert system as web application

I am relative new to developing web applications.
I would like your comments and suggestions of improvement for the following architectural considerations.
I have developed an expert system ES using CLIPS. Now I am planning to provide this to variety of users from our company as a web application. Before I start with going into greater details I am currently thinking about which technologies should be involved.
The goal is that the user of the web app is facing a chat-like animation which guides him to the final result while he or she provides more and more input to the ES.
After conducting some research on my own I came up with the following idea
In the backend I use PyCLIPS as an Interface between Python and CLIPS
Then I use DJANGO for integrating my python code into the web page dynamically altering the chat between user and ES.
There is one thing which is particularly still troubling me a lot: How shall I manage many concurrent users? Shall I use one ES with every user having an individual set of facts or shall every user have his or her own instance of the ES?
Do you have any other high level approaches for this problem which could be superior to this one?
I am looking forward to your experience and input regarding this matter.
Best

I'd suggest running the expert system in a stateless mode. Each time the user makes changes, you submit all data from the web page to the expert system and then retrieve the results to display on the web page. Doing it that way scales better if you have multiple users and makes it easier to implement undo logic if the user wants to change a response. There's an example showing how to do this with a CGI application at http://www.clipsrules.net/?q=Downloads/CLIPSCGI.

It is usually a good idea to split your Expert System into separate "shards".
It keeps the rule base simpler (as you don't need to distinguish to which user a fact is referring to) and allows you to scale horizontally when more users will be added.
If running one ES per user sounds overkill, you can decrease the granularity by sharding based on, for example, the first letter of the user surname or Id.
When designing similar solutions I tend to de-couple the frontend application with the ES by using a queueing system. This allows you to modify the cluster layout without the need to change the public APIs.
| Flask | ----> | RabbitMQ | ----> | ES Worker |
In case you want to change your sharding strategy, you can simply re-configure the broker queues layout without affecting the client/end-user.

Django - User Billing Platforms / Middleware, i.e., Tracking Expenses and Charges

I am writing a webapp and I would like to start charging my users. What are the recommended billing platforms for a python/Django webapp?
I would like something that keeps track of my users' purchase history, can elegantly handle subscription purchases, a la carte items, coupon codes, and refunds, makes it straightforward to generate invoices/receipts, and can easily integrate with most payment processors. Extra points if it comes with a fancy admin interface.
I found this django-billing project, are there any others? Also, do you rely on your payment processor to handle these tasks or do you do all of them yourself?
Note: I am not asking what payment processors to use, but rather what middleware/libraries one should run on their webapp itself.

The koalixcrm is perhaps something you could start with.
It offers some of your required functionality. Still it is in a prealpha stage but it already provides PDF export for Invoices and Quotes, there is already one included plugin for subscriptions.
also try the demo
As i am the developer of the koalixcrm im very interested to work with you - perhaps we can merge our projects.

It's not really clear why Django Community hasn't come up a with complete billing system or at least a generic one to start working on.
There's many packages that can be used for getting an idea how to implement such platform:
https://www.djangopackages.com/grids/g/payment-processing/

Simple API's to play around with for Python/Django?

Does anyone know of simple and well documented API's with plenty of hand holding examples that assumes very little or no prior knowledge of web development?
I've been messing around with Pyfacebook and Facebook-Python-SDK in trying to create a simple photo display app but I haven't been able to make much headway after spending the last few days on it. The main reason for this is simply because I just wasn't able to find a good tutorial that walks me through all the steps. So, I'm putting this mini project on pause and looking for lower hanging fruit.
In terms of skill level, I'm pretty ok on the basics of Python and Django.
Update
I've done the tutorials at http://www.djangoproject.com/ already. Really looking for ideas and suggestions on webapp projects that utilises an API. E.g, a twitter app that displays a user's most frequently used keywords in a tagcloud.
Update2
Side note: Having mess around with Twitter's API for a little bit, I would definitely recommend to start with Twitter first as opposed to Facebook. It's easier and better documented.

Best place to start is with tutorials on djangoproject.com.

Have you tried the Django tutorial? It is pretty basic, but touches on all important points required to develop your own basic app.

django-basic-apps contains a collection of apps you might enjoy reading.
Edit: Check out this good list of web services I found. :)

As far as I know you can't write facebook apps with Django. Facebook uses their own API and stuff. They are completely different.
And for the twitter API thingy I have an idea.
Develop a django app which can used to
scrap and backup tweets.
The scenario is during any FOSS
conference, they are using a #hastag
to identify tweets related to that
conf. But after sometime these tweets
don't show up even on search. For
example we used #inpycon2010 tag for
Pycon conf in India. But now when I
search for this tag, nothing shows up.
So what you can do allow users to
register a hastag and set a time
interval. Within that time interval
your app should scrap all the tweets
and backup them. The user should be
able to retreive from that later.
If you start this a foss project, I'm ready to jump in :)

I'm searching for a messaging platform (like XMPP) that allows tight integration with a web application

At the company I work for, we are building a cluster of web applications for collaboration. Things like accounting, billing, CRM etc.
We are using a RESTfull technique:
For database we use CouchDB
Different applications communicate with one another and with the database via http.
Besides, we have a single sign on solution, so that when you login in one application, you are automatically logged to the other.
For all apps we use Python (Pylons).
Now we need to add instant messaging to the stack.
We need to support both web and desktop clients. But just being able to chat is not enough.
We need to be able to achieve all of the following (and more similar things).
When somebody gets assigned to a task, they must receive a message. I guess this is possible with some system daemon.
There must be an option to automatically group people in groups by lots of different properties. For example, there must be groups divided both by geographical location, by company division, by job type (all the programers from different cities and different company divisions must form a group), so that one can send mass messages to a group of choice.
Rooms should be automatically created and destroyed. For example when several people visit the same invoice, a room for them must be automatically created (and they must auto-join). And when all leave the invoice, the room must be destroyed.
Authentication and authorization from our applications.
I can implement this using custom solutions like hookbox http://hookbox.org/docs/intro.html
but then I'll have lots of problems in supporting desktop clients.
I have no former experience with instant messaging. I've been reading about this lately. I've been looking mostly at things like ejabberd. But it has been a hard time and I can't find whether what I want is possible at all.
So I'd be happy if people with experience in this field could help me with some advice, articles, tales of what is possible etc.

Like frx suggested above, the StropheJS folks have an excellent book about web+xmpp coding but since you mentioned you have no experience in this type of coding I would suggest talking to some folks who have :) It will save you time in the long run - not that I'm saying don't try to implement what frx outlines, it could be a fun project :)
I know of one group who has implemented something similar and chatting with them would help solidify what you have in mind: http://andyet.net/ (I'm not affiliated with them at all except for the fact that the XMPP dev community is small and we tend to know each other :)

All goals could be achieved with ejabberd, strophe and little server side scripting
When someone gets assigned to task, server side script could easily authenticate to xmpp server and send message stanza to assigned JID. That its trivial task.
To group different people in groups, it is easily can be done from web chat app if those user properties are stored somewhere. Just join them in particular multi user chat room after authentication.
Ejabberd has option to automatically create and destroy rooms.
Ejabberd has various authorization methods including database and script auth
You could take look at StropheJS library, they have great book (paperback) released. Really recommend to read this book http://professionalxmpp.com/

Does Django scale? [closed]

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I'm building a web application with Django. The reasons I chose Django were:
I wanted to work with free/open-source tools.
I like Python and feel it's a long-term language, whereas regarding Ruby I wasn't sure, and PHP seemed like a huge hassle to learn.
I'm building a prototype for an idea and wasn't thinking too much about the future. Development speed was the main factor, and I already knew Python.
I knew the migration to Google App Engine would be easier should I choose to do so in the future.
I heard Django was "nice".
Now that I'm getting closer to thinking about publishing my work, I start being concerned about scale. The only information I found about the scaling capabilities of Django is provided by the Django team (I'm not saying anything to disregard them, but this is clearly not objective information...).
My questions:
What's the "largest" site that's built on Django today? (I measure size mostly by user traffic)
Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?
Could a site like Stack Overflow run on Django?

"What are the largest sites built on Django today?"
There isn't any single place that collects information about traffic on Django built sites, so I'll have to take a stab at it using data from various locations. First, we have a list of Django sites on the front page of the main Django project page and then a list of Django built sites at djangosites.org. Going through the lists and picking some that I know have decent traffic we see:
Instagram: What Powers Instagram: Hundreds of Instances, Dozens of Technologies.
Pinterest: Alexa rank 37 (21.4.2015) and 70 Million users in 2013
Bitbucket: 200TB of Code and 2.500.000 Users
Disqus: Serving 400 million people with Python.
curse.com: 600k daily visits.
tabblo.com: 44k daily visits, see Ned Batchelder's posts Infrastructure for modern web sites.
chesspark.com: Alexa rank about 179k.
pownce.com (no longer active): alexa rank about 65k.
Mike Malone of Pownce, in his EuroDjangoCon presentation on Scaling Django Web Apps says "hundreds of hits per second". This is a very good presentation on how to scale Django, and makes some good points including (current) shortcomings in Django scalability.
HP had a site built with Django 1.5: ePrint center. However, as for novemer/2015 the entire website was migrated and this link is just a redirect. This website was a world-wide service attending subscription to Instant Ink and related services HP offered (*).
"Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?"
Yes, see above.
"Could a site like Stack Overflow run on Django?"
My gut feeling is yes but, as others answered and Mike Malone mentions in his presentation, database design is critical. Strong proof might also be found at www.cnprog.com if we can find any reliable traffic stats. Anyway, it's not just something that will happen by throwing together a bunch of Django models :)
There are, of course, many more sites and bloggers of interest, but I have got to stop somewhere!
Blog post about Using Django to build high-traffic site michaelmoore.com described as a top 10,000 website. Quantcast stats and compete.com stats.
(*) The author of the edit, including such reference, used to work as outsourced developer in that project.

We're doing load testing now. We think we can support 240 concurrent requests (a sustained rate of 120 hits per second 24x7) without any significant degradation in the server performance. That would be 432,000 hits per hour. Response times aren't small (our transactions are large) but there's no degradation from our baseline performance as the load increases.
We're using Apache front-ending Django and MySQL. The OS is Red Hat Enterprise Linux (RHEL). 64-bit. We use mod_wsgi in daemon mode for Django. We've done no cache or database optimization other than to accept the defaults.
We're all in one VM on a 64-bit Dell with (I think) 32Gb RAM.
Since performance is almost the same for 20 or 200 concurrent users, we don't need to spend huge amounts of time "tweaking". Instead we simply need to keep our base performance up through ordinary SSL performance improvements, ordinary database design and implementation (indexing, etc.), ordinary firewall performance improvements, etc.
What we do measure is our load test laptops struggling under the insane workload of 15 processes running 16 concurrent threads of requests.

Not sure about the number of daily visits but here are a few examples of large Django sites:
disqus.com (talk from djangocon)
bitbucket.org (write up)
lanyrd.com (source)
support.mozilla.com (source code)
addons.mozilla.org (source code) (talk from djangocon)
theonion.com (write up)
The guardian.co.uk comment system uses Django (source)
instagram
pinterest
rdio
Here is a link to list of high traffic Django sites on Quora.

What's the "largest" site that's built on Django today? (I measure size mostly by user traffic)
In the US, it was Mahalo. I'm told they handle roughly 10 million uniques a month. Now, in 2019, Mahalo is powered by Ruby on Rails.
Abroad, the Globo network (a network of news, sports, and entertainment sites in Brazil); Alexa ranks them in to top 100 globally (around 80th currently).
Other notable Django users include PBS, National Geographic, Discovery, NASA (actually a number of different divisions within NASA), and the Library of Congress.
Can Django deal with 100k users daily, each visiting the site for a couple of hours?
Yes -- but only if you've written your application right, and if you've got enough hardware. Django's not a magic bullet.
Could a site like StackOverflow run on Django?
Yes (but see above).
Technology-wise, easily: see soclone for one attempt. Traffic-wise, compete pegs StackOverflow at under 1 million uniques per month. I can name at least dozen Django sites with more traffic than SO.

Scaling Web apps is not about web frameworks or languages, is about your architecture.
It's about how you handle you browser cache, your database cache, how you use non-standard persistence providers (like CouchDB), how tuned is your database and a lot of other stuff...

Playing devil's advocate a little bit:
You should check the DjangoCon 2008 Keynote, delivered by Cal Henderson, titled "Why I hate Django" where he pretty much goes over everything Django is missing that you might want to do in a high traffic website. At the end of the day you have to take this all with an open mind because it is perfectly possible to write Django apps that scale, but I thought it was a good presentation and relevant to your question.

The largest django site I know of is the Washington Post, which would certainly indicate that it can scale well.
Good design decisions probably have a bigger performance impact than anything else. Twitter is often cited as a site which embodies the performance issues with another dynamic interpreted language based web framework, Ruby on Rails - yet Twitter engineers have stated that the framework isn't as much an issue as some of the database design choices they made early on.
Django works very nicely with memcached and provides some classes for managing the cache, which is where you would resolve the majority of your performance issues. What you deliver on the wire is almost more important than your backend in reality - using a tool like yslow is critical for a high performance web application. You can always throw more hardware at your backend, but you can't change your users bandwidth.

I was at the EuroDjangoCon conference the other week, and this was the subject of a couple of talks - including from the founders of what was the largest Django-based site, Pownce (slides from one talk here). The main message is that it's not Django you have to worry about, but things like proper caching, load balancing, database optimisation, etc.
Django actually has hooks for most of those things - caching, in particular, is made very easy.

I'm sure you're looking for a more solid answer, but the most obvious objective validation I can think of is that Google pushes Django for use with its App Engine framework. If anybody knows about and deals with scalability on a regular basis, it's Google. From what I've read, the most limiting factor seems to be the database back-end, which is why Google uses their own...

As stated in High Performance Django Book
and Go through this Cal Henderson
See further details as mentioned below:
It’s not uncommon to hear people say “Django doesn’t scale”. Depending on how you look at it, the statement is either completely true or patently false. Django, on its own, doesn’t scale.
The same can be said of Ruby on Rails, Flask, PHP, or any other language used by a database-driven dynamic website.
The good news, however, is that Django interacts beautifully with a suite of caching and
load balancing tools that will allow it to scale to as much traffic as you can throw at it.
Contrary to what you may have read online,
it can do so without replacing core components often labeled as “too slow” such as the database ORM or the template layer.
Disqus serves over 8 billion page views per month. Those are some huge numbers.
These teams have proven Django most certainly does scale.
Our experience here at Lincoln Loop backs it up.
We’ve built big Django sites capable of spending the day on the Reddit homepage without breaking a sweat.
Django’s scaling success stories are almost too numerous to list at this point.
It backs Disqus, Instagram, and Pinterest. Want some more proof? Instagram was able to sustain over 30 million users on Django with only 3 engineers (2 of which had no back-end development

Today we use many web apps and sites for our needs. Most of them are highly useful. I will show you some of them used by python or django.
Washington Post
The Washington Post’s website is a hugely popular online news source to accompany their daily paper. Its’ huge amount of views and traffic can be easily handled by the Django web framework.
Washington Post - 52.2 million unique visitors (March, 2015)
NASA
The National Aeronautics and Space Administration’s official website is the place to find news, pictures, and videos about their ongoing space exploration. This Django website can easily handle huge amounts of views and traffic.
2 million visitors monthly
The Guardian
The Guardian is a British news and media website owned by the Guardian Media Group. It contains nearly all of the content of the newspapers The Guardian and The Observer. This huge data is handled by Django.
The Guardian (commenting system) - 41,6 million unique visitors (October, 2014)
YouTube
We all know YouTube as the place to upload cat videos and fails. As one of the most popular websites in existence, it provides us with endless hours of video entertainment. The Python programming language powers it and the features we love.
DropBox
DropBox started the online document storing revolution that has become part of daily life. We now store almost everything in the cloud. Dropbox allows us to store, sync, and share almost anything using the power of Python.
Survey Monkey
Survey Monkey is the largest online survey company. They can handle over one million responses every day on their rewritten Python website.
Quora
Quora is the number one place online to ask a question and receive answers from a community of individuals. On their Python website relevant results are answered, edited, and organized by these community members.
Bitly
A majority of the code for Bitly URL shortening services and analytics are all built with Python. Their service can handle hundreds of millions of events per day.
Reddit
Reddit is known as the front page of the internet. It is the place online to find information or entertainment based on thousands of different categories. Posts and links are user generated and are promoted to the top through votes. Many of Reddit’s capabilities rely on Python for their functionality.
Hipmunk
Hipmunk is an online consumer travel site that compares the top travel sites to find you the best deals. This Python website’s tools allow you to find the cheapest hotels and flights for your destination.
Click here for more:
25-of-the-most-popular-python-and-django-websites,
What-are-some-well-known-sites-running-on-Django

I think we might as well add Apple's App of the year for 2011, Instagram, to the list which uses django intensively.

Yes it can. It could be Django with Python or Ruby on Rails. It will still scale.
There are few different techniques. First, caching is not scaling. You could have several application servers balanced with nginx as the front in addition to hardware balancer(s).
To scale on the database side you can go pretty far with read slave in MySQL / PostgreSQL if you go the RDBMS way.
Some good examples of heavy traffic websites in Django could be:
Pownce when they were still there.
Discus (generic shared comments manager)
All the newspaper related websites: Washington Post and others.
You can feel safe.

Here's a list of some relatively high-profile things built in Django:
The Guardian's "Investigate your MP's expenses" app
Politifact.com (here's a Blog post talking about the (positive) experience. Site won a Pulitzer.
NY Times' Represent app
EveryBlock
Peter Harkins, one of the programmers over at WaPo, lists all the stuff they’ve built with Django on his blog
It's a little old, but someone from the LA Times gave a basic overview of why they went with Django.
The Onion's AV Club was recently moved from (I think Drupal) to Django.
I imagine a number of these these sites probably gets well over 100k+ hits per day. Django can certainly do 100k hits/day and more. But YMMV in getting your particular site there depending on what you're building.
There are caching options at the Django level (for example caching querysets and views in memcached can work wonders) and beyond (upstream caches like Squid). Database Server specifications will also be a factor (and usually the place to splurge), as is how well you've tuned it. Don't assume, for example, that Django's going set up indexes properly. Don't assume that the default PostgreSQL or MySQL configuration is the right one.
Furthermore, you always have the option of having multiple application servers running Django if that is the slow point, with a software or hardware load balancer in front.
Finally, are you serving static content on the same server as Django? Are you using Apache or something like nginx or lighttpd? Can you afford to use a CDN for static content? These are things to think about, but it's all very speculative. 100k hits/day isn't the only variable: how much do you want to spend? How much expertise do you have managing all these components? How much time do you have to pull it all together?

The developer advocate for YouTube gave a talk about scaling Python at PyCon 2012, which is also relevant to scaling Django.
YouTube has more than a billion users, and YouTube is built on Python.

I have been using Django for over a year now, and am very impressed with how it manages to combine modularity, scalability and speed of development. Like with any technology, it comes with a learning curve. However, this learning curve is made a lot less steep by the excellent documentation from the Django community. Django has been able to handle everything I have thrown at it really well. It looks like it will be able to scale well into the future.
BidRodeo Penny Auctions is a moderately sized Django powered website. It is a very dynamic website and does handle a good number of page views a day.

Note that if you're expecting 100K users per day, that are active for hours at a time (meaning max of 20K+ concurrent users), you're going to need A LOT of servers. SO has ~15,000 registered users, and most of them are probably not active daily. While the bulk of traffic comes from unregistered users, I'm guessing that very few of them stay on the site more than a couple minutes (i.e. they follow google search results then leave).
For that volume, expect at least 30 servers ... which is still a rather heavy 1,000 concurrent users per server.

My experience with Django is minimal but I do remember in The Django Book they have a chapter where they interview people running some of the larger Django applications. Here is a link. I guess it could provide some insights.
It says curse.com is one of the largest Django applications with around 60-90 million page views in a month.

What's the "largest" site that's built on Django today? (I measure size mostly by user traffic)
Pinterest
disqus.com
More here: https://www.shuup.com/en/blog/25-of-the-most-popular-python-and-django-websites/
Can Django deal with 100,000 users daily, each visiting the site for a couple of hours?
Yes but use proper architecture, database design, use of cache, use load balances and multiple servers or nodes
Could a site like Stack Overflow run on Django?
Yes just need to follow the answer mentioned in the 2nd question

I don't think the issue is really about Django scaling.
I really suggest you look into your architecture that's what will help you with your scaling needs.If you get that wrong there is no point on how well Django performs. Performance != Scale. You can have a system that has amazing performance but does not scale and vice versa.
Is your application database bound? If it is then your scale issues lay there as well. How are you planning on interacting with the database from Django? What happens when you database cannot process requests as fast as Django accepts them? What happens when your data outgrows one physical machine. You need to account for how you plan on dealing with those circumstances.
Moreover, What happens when your traffic outgrows one app server? how you handle sessions in this case can be tricky, more often than not you would probably require a shared nothing architecture. Again that depends on your application.
In short languages is not what determines scale, a language is responsible for performance(again depending on your applications, different languages perform differently). It is your design and architecture that makes scaling a reality.
I hope it helps, would be glad to help further if you have questions.

Another example is rasp.yandex.ru, Russian transport timetable service. Its attendance satisfies your requirements.

If you have a site with some static content, then putting a Varnish server in front will dramatically increase your performance. Even a single box can then easily spit out 100 Mbit/s of traffic.
Note that with dynamic content, using something like Varnish becomes a lot more tricky.

I develop high traffic sites using Django for the national broadcaster in Ireland. It works well for us. Developing a high performance site is more than about just choosing a framework. A framework will only be one part of a system that is as strong as it's weakest link. Using the latest framework 'X' won't solve your performance issues if the problem is slow database queries or a badly configured server or network.

The problem is not to know if django can scale or not.
The right way is to understand and know which are the network design patterns and tools to put under your django/symfony/rails project to scale well.
Some ideas can be :
Multiplexing.
Inversed proxy. Ex : Nginx, Varnish
Memcache Session. Ex : Redis
Clusterization on your project and db for load balancing and fault tolerance : Ex : Docker
Use third party to store assets. Ex : Amazon S3
Hope it help a bit. This is my tiny rock to the mountain.

Even-though there have been a lot of great answers here, I just feel like pointing out, that nobody have put emphasis on..
It depends on the application
If you application is light on writes, as in you are reading a lot more data from the DB than you are writing. Then scaling django should be fairly trivial, heck, it comes with some fairly decent output/view caching straight out of the box. Make use of that, and say, redis as a cache provider, put a load balancer in front of it, spin up n-instances and you should be able to deal with a VERY large amount of traffic.
Now, if you have to do thousands of complex writes a second? Different story. Is Django going to be a bad choice? Well, not necessarily, depends on how you architect your solution really, and also, what your requirements are.
Just my two cents :-)

If you want to use Open source then there are many options for you. But python is best among them as it has many libraries and a super awesome community.
These are a few reasons which might change your mind:
Python is very good but it is a interpreted language which makes it slow. But many accelerator and caching services are there which partly solve this problem.
If you are thinking about rapid development then Ruby on Rails is best among all. The main motto of this(ROR) framework is to give a comfortable experience to the developers. If you compare Ruby and Python both have nearly the same syntax.
Google App Engine is very good service but it will bind you in its scope, you don't get chance to experiment new things. Instead of it you can use Digital Ocean cloud which will only take $5/Month charge for its simplest droplet. Heroku is another free service where you can deploy your product.
Yes! Yes! What you heard is totally correct but here are some examples which are using other technologies
Rails: Github, Twitter(previously), Shopify, Airbnb, Slideshare, Heroku etc.
PHP: Facebook, Wikipedia, Flickr, Yahoo, Tumbler, Mailchimp etc.
Conclusion is a framework or language won't do everything for you. A better architecture, designing and strategy will give you a scalable website. Instagram is the biggest example, this small team is managing such huge data. Here is one blog about its architecture must read it.

You can definitely run a high-traffic site in Django. Check out this pre-Django 1.0 but still relevant post here: http://menendez.com/blog/launching-high-performance-django-site/

Check out this micro news aggregator called EveryBlock.
It's entirely written in Django. In fact they are the people who developed the Django framework itself.

Spreading the tasks evenly, in short optimizing each and every aspect including DBs, Files, Images, CSS etc. and balancing the load with several other resources is necessary once your site/application starts growing. OR you make some more space for it to grow. Implementation of latest technologies like CDN, Cloud are must with huge sites. Just developing and tweaking an application won't give your the cent percent satisfation, other components also play an important role.