Handling wsgi requests in my application - python

I've hit some conceptual road-block on something that should be simple.
I have a single-threaded, plain python application, which runs indefinitely.
I would like to be able to query information about the internal state of the application from the web, via an http request.
Most of the models I have seen for this, e.g. python's WSGIServer from flup.server.fcgi, require me to provide a callback function to handle a request. The callback then becomes the starting point of my program.
Instead of reworking my application's logic to fit this model of: request-->callback, I need to be able to hadnle requests within my own application's logic. For example, periodically throughout the execution of my application, I would check if there are any pending http/wsgi requests, handle them, and continue.
Do I have to open a sockets myself and deal with all the socket logic in order to achieve this? I don't necessarily mind, but I suspect I don't need to reinvent the wheel here.
Am I thinking about this incorrectly?

Related

Canceling a Flask backend process from a Web-based client

I have a Flask application running at backend, and delivering some data to a client Web application through some endpoints. E.g.
#app.route('/extract_entities_from_matching_docs', methods=['POST'])
def extract_entities_from_matching_docs():
data = request.form
entities = storage.get_entities_in_docs_by_keywords(data[“keywords”])
return jsonify(entities)
This is just an example, but the thing is sometimes, these kinds of methods take too much time to process, and the user may want to cancel the processing from the client (e.g. imagine you have a “Cancel” button on the client).
My question is: how can I cancel a running process at backend from the frontend? I thought that I may include a flag in the loops, so if the flag is set to true it continues looping and processing, otherwise, it just returns. But the problem with such a simple alternative is: what if the client just closes the browser’s tab? The process will continue running at the backend with no reason to do it. Maybe by posting to another flag from the client every x minutes, to make the backend know that there is a client waiting for the response. But maybe there is a more elegant solution that I'm ignoring.
I’m not really into the backend, but I need to solve this. So, can you suggest me any other alternative? Something to read? Or do you know if is there any good practice to face this?
What you want is not easily possible. Generally the solution is to not bother and keep the request running in the background and just live with it. In the end most requests will probably not be "cancelled" and thus the overhead for developing this kind of solution outweighs the minor benefits.
If your tasks are really long-running (and with that I mean minutes or even much more), then you should off-load them to something like Celery tasks. In that case, you can keep a reference to the task ID and revoke it (passing terminate=True should deal with an already-running task as well).

Performing a blocking request in django view

In one of the views in my django application, I need to perform a relatively lengthy network IO operation. The problem is other requests must wait for this request to be completed even though they have nothing to do with it.
I did some research and stumbled upon Celery but as I understand, it is used to perform background tasks independent of the request. (so I can not use the result of the task for the response to the request)
Is there a way to process views asynchronously in django so while the network request is pending other requests can be processed?
Edit: What I forgot to mention is that my application is a web service using django rest framework. So the result of a view is a json response not a page that I can later modify using AJAX.
The usual solution here is to offload the task to celery, and return a "please wait" response in your view. If you want, you can then use an Ajax call to periodically hit a view that will report whether the response is ready, and redirect when it is.
You want to maintain that HTTP connection for an extended period of time but still allow other requests to be managed, right? There's no simple solution to this problem. Also, any solution will be a level away from Django as it depends on how you process requests.
I don't know what you're currently using, so I can only tell you how I handled this in the past... I was using uwsgi to provide the WSGI interface between my python application and nginx. In uwsgi I used the asynchronous functions to suspend my long running connection when there was time to wait on the IO connections. The methods allow you to ask it to suspend things until there is something to read or write and then allow other connections to be serviced.
The above mentioned async calls use "green threads". It's much lighter weight then regular threads and you have control over when you move from thread to thread.
I am not saying that it is a good solution for your scenario[1], but the simple answer is using the following pattern:
async_result = some_task.delay(arg1)
result = async_result.get()
Check documentation for the get method. And instead of using the delay method you can use anything that returns an AsyncResult (like the apply_async method
[1] Why it may be a bad idea? Having an ongoing connection waiting a lot is bad for Django (it is not ready for long-lived connections), may conflict with the proxy configuration (if there is a reverse proxy somewhere) and may be identified as a timeout from the browser. So... it seems a Bad Idea[TM] to use this pattern for a Django Rest Framework view.

Controlling a Twisted Server from Django

I'm trying to build a Twisted/Django mashup that will let me control various client connections managed by a Twisted server via Django's admin interface. Meaning, I want to be able to login to Django's admin and see what protocols are currently in use, any details specific to each connection (e.g. if the server is connected to freenode via IRC, it should list all the channels currently connected to), and allow me to disconnect or connect new clients by modifying or creating database records.
What would be the best way to do this? There are lots of posts out there about combining Django with Twisted, but I haven't found any prior art for doing quite what I've outlined. All the Twisted examples I've seen use hardcoded connection parameters, which makes it difficult for me to imagine how I would dynamically running reactor.connectTCP(...) or loseConnection(...) when signalled by a record in the database.
My strategy is to create a custom ClientFactory that solely polls the Django/managed database every N seconds for any commands, and to modify/create/delete connections as appropriate, reflecting the new status in the database when complete.
Does this seem feasible? Is there a better approach? Does anyone know of any existing projects that implement similar functionality?
Polling the database is lame, but unfortunately, databases rarely have good tools (and certainly there are no database-portable tools) for monitoring changes. So your approach might be okay.
However, if your app is in Django and you're not supporting random changes to the database from other (non-Django) clients, and your WSGI container is Twisted, then you can do this very simply by doing callFromThread(connectTCP, ...).
I've been working on yet another way of combing django and twisted. Fell free to give it a try: https://github.com/kowalski/featdjango.
The way it works, is slightly different that the others. It starts a twisted application and http site. The requests done to django are processed inside a special thread pool. What makes it special, is that that these threads can wait on Deferred, which makes it easy to combine synchronous django application code with asynchronous twisted code.
The reason I came up with structure like this, is that my application needs to perform a lot of http requests from inside the django views. Instead of performing them one by one I can delegate all of them at once to "the main application thread" which runs twisted and wait for them. The similarity to your problem is, that I also have an asynchronous component, which is a singleton and I access it from django views.
So this is, for example, this is how you would initiate the twisted component and later to get the reference from the view.
import threading
from django.conf import settings
_initiate_lock = threading.Lock()
def get_component():
global _initiate_lock
if not hasattr(settings, 'YOUR_CLIENT')
_initiate_lock.acquire()
try:
# other thread might have did our job while we
# were waiting for the lock
if not hasattr(settings, 'YOUR_CLIENT'):
client = YourComponent(**whatever)
threading.current_thread().wait_for_deferred(
client.initiate)
settings.YOUR_CLIENT = client
finally:
_initiate_lock.release()
return settings.YOUR_CLIENT
The code above, initiates my client and calls the initiate method on it. This method is asynchronous and returns a Deferred. I do all the necessary setup in there. The django thread will wait for it to finish before processing to next line.
This is how I do it, because I only access it from the request handler. You probably would want to initiate your component at startup, to call ListenTCP|SSL. Than your django request handlers could get the data about the connections just accessing some public methods on the your client. These methods could even return Deferred, in which case you should use .wait_for_defer() to call them.

How to create a polling script in Python?

I was trying to create a polling script in python that starts when another python script starts and then keeps supplying data back to this script.
I can obviously write an infinite loop but is that the right way to go about it? I might loose control over how the functions work and how many times a function should be called in an hour.
Edit:
What I am trying to accomplish is to poll the REST API of twitter and get new mentions and people who follow me. I obviously can't keep polling because I will run out of API requests per hour. Thus, the issue. This poller, will send the new mention and follower id/user to the main script that would be listening to any such update.
I highly suggest looking into Twisted, one of the most popular async frameworks using the reactor pattern.
The "infinite loop" you are looking for is really an application pattern that Twisted implements to respond to events asynchronously, and it almost never makes sense to roll your own.
Twisted is largely used for networking requirements, but the it has a LoopingCall interface to set up the kind of functionality you require. Using the core Twisted deferred as your request model allows you to set up a long-polling server that can perform the kind of conditional network test you need. It can intially be a little intimidating, but once you understand the core components (Factories, Reactors, Protocols etc) that you need to inherit it becomes much easier to visualize your problem.
This also might be a good tutorial to start looking at the basics of the "push" model:
http://carloscarrasco.com/simple-http-pubsub-server-with-twisted.html

Concurrency handling in python based webapp

I am developing web app on flask, python, sqlalchemy and postgresql.
My question is here regarding concurrency handling in this app.
How I wrote the app :
I take the example of adding user in database. I post the form and a view is called. I process all the form data and then call add_user(*arg) which uses sqlalchemy code to insert user in database and returns on successful execution and I return the response from the view.
What I assumed:
Ok now I assumed that my web server (which I have not decided yet) will either spawn a thread or a process if two users are trying to signup at the same time and will handle all the concurreny requirements.
Do i need to write threaded code here? By threaded code I mean that before writing I acquire a lock and after write release it.
I am pretty new to web development and multithreading/multiprocessing programing and would like some guidance on how write web app which can handle concurrency well.
Writing concurrency handling from start is right or this thought should come when a large number of concurrent users are using the webapp. Even If it should be done later I would like some pointers about it.
Basically I have no idea about concurrency part of webapp development. If you can point to resources from where I can learn more about it would be really helpful.
Flask will execute each request in a separate thread or even in separate processes. The number of threads and processes to spawn is determined by the WSGI server (for example, Apache with mod_wsgi).
If you use SQLAlchemy ScopedSessions, the session is perfectly thread-safe. You must not share ORM-controlled objects across threads (but in the large majority of cases, you won't let your objects live longer than a request anyway so this is usually not a concern).
In other words, as long as you don't intend to share state between requests other than through the database or cookies, you don't need to worry about concurrency issues. You don't need to create a lock for writing to the database.
If you create your own long-lived objects within your application, which you most likely don't need to do, and if those objects communicate or share state with request handling code, then you must take appropriate precautions to avoid synchronization issues (race conditions, deadlocks, use of libraries that are not thread-safe, etc.)

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