I've struggled for two days to understand how REST API Gateways should return GET requests to browsers when the backend service runs on AMQP (without using Web Sockets or polling).
Have successfully RPC'ed betweeen AMQP service (with RabbitMqs reply_to & correlation_id), but with Flask HTTP request waiting I'm still lost.
gateway.py - Response Handler Inside The HTTP Handler, Times out
def products_get():
def handler(ch=None, method=None, properties=None, body=None):
if body:
return body
return False
return_queue = 'products.get.return'
broker.channel.queue_declare(return_queue)
broker.channel.basic_consume(handler, return_queue)
broker.publish(exchange='', routing_key='products.get', body='Request data', properties=pika.BasicProperties(reply_to=return_queue))
now = time.time() # for timeout. Not having this returns 'no content' immediately
while time.time() < now + 1:
if handler():
return handler()
return 'Time out'
POST/PUT can simply send the AMQP message, return 200/201/201 immediately and the service work at its own pace. A separate REST interface just for GET requests seems implausible, but don't know the other options.
Regards
I think what you're asking is "how to perform asynchronous GET requests". and I reckon that the answer is - you can't. and should not. its bad practice or bad design. and it does not scale.
Why are you trying to get your GET response payload from AMQP?
If the paylaod (the content of the response) can be pulled from some DB, just pull it from there. that's called a synchronous request.
If the payload must be processed in some backend, send it away and don't have the requester wait for a response. You could assign some ID and have the requester ask again later (or collect some callback URL from the requester and have your backend POST the response once its ready - less common design).
EDIT:
so, given that you have to work with AMQP-backed backend, I would do something a little more elaborate: spawn a thread or a process in your front end that would constantly consume from AMQP and store the results locally or in some db. and serve GET results based on data that you stored locally. if the data isn't yet available, just return 404. ideally you'll need to re-shape your API: split it into "post" requests (that would trigger work at the backend) and "get" requests (that would return the results if they're available).
Related
I have a Flask endpoint that starts an automated email campaign to a list of contacts I have. The body of the POST request has a "emailAmount" key which tells the process how many emails to send out.
Essentially what I want it to do is start the process of sending these emails out, and after each email sent out, show live feedback on the frontend like a loading bar, or just a live count of how many emails have been sent since the process started, and then some sort of live feedback on the front end telling the user when the process has completed. The email process is abstracted to a class I built called "AutomateEmailClient".
from flask import Blueprint
from flask_login import login_required, current_user
from forms.startProcess import ProcessForm
from ..emails.client import AutomateEmailClient
email_router = Blueprint("email",__name__)
#email_router.route('/start', methods=['POST'])
#login__required()
def start_process():
form = ProcessForm() #Validate body of req with WTForms
if form.validate_on_submit():
#Interact with AutomateEmailClient object from different package a few levels up
return #? Return synchronously after email process has finished? or asynchronously and use websockets for live feedback to frontend?
However, after I return from the start_process() function, obviously the request/response cycle is dead, and if I just run the email process and have the return synchronously after, I would basically just have a hanging response for several minutes.
I'm assuming I need to use websockets, and possibly some sort of asynchronous functionality or multi processing, but I'm not sure what is the best approach or how to implement it.
I have a Flask app that generates video stream links. It connects to a server using login credentials and grabs a one time use link (that expires when a new link is generated using the same credentials). Using a list of credentials I am able to stream to as many devices as I like, so long as I have enough accounts.
The issue I am having is that one of the clients doesn't like the way the stream is returned.
#app.route("/play", methods=["GET"])
def play():
def streamData():
try:
useAccount(<credentials>)
with requests.get(link, stream=True) as r:
for chunk in r.iter_content(chunk_size=1024):
yield chunk
except:
pass
finally:
freeAccount(<credentials>)
...
# return redirect(link)
return Response(streamData())
If I return a redirect then there are no playback issues at all. The problem with a redirect is I don't have a way of marking the credentials as in use, then freeing them after.
The problem client is TVHeadend. I am able to get it to work by enabling the additional avlib inside of TVHeadend... But I shouldn't have to do that. I don't have to when I return a redirect.
What could be the cause of this?
Is it possible to make my app respond in the same way as the links server does?
My guess is that TVHeadend is very strict on if something complies to whatever standards... and I am guessing my app doesn't?
I have a couple different needs for asynchrony in my Python 3.6 Flask RESTful web service running under Gunicorn.
1) I'd like for one of my service's routes to be able to send an HTTP request to another HTTP service and, without waiting for the response, send a response back to the client that called my service.
Some example code:
#route
def fire_and_forget():
# Send request to other server without waiting
# for it to send a response.
# Return my own response.
2) I'd like for another one of my service's routes to be able to send 2 or more asynchronous HTTP requests to other HTTP services and wait for them all to reply before my service sends a response.
Some example code:
#route
def combine_results():
# Send request to service A
# Send request to service B
# Wait for both to return.
# Do something with both responses
# Return my own response.
Thanks in advance.
EDIT: I am trying to avoid the additional complexity of using a queue (e.g. celery).
You can use eventlets for the the second use case. It's pretty easy to do:
import eventlet
providers = [EventfulPump(), MeetupPump()]
try:
pool = eventlet.GreenPool()
pile = eventlet.GreenPile(pool)
for each in providers:
pile.spawn(each.get, [], 5, loc) # call the interface method
except (PumpFailure, PumpOverride):
return abort(503)
results = []
for res in pile:
results += res
You can wrap each of your api endpoints in a class that implements a "common interface" (in the above it is the get method) and you can make the calls in parallel. I just place them all in a list.
Your other use case is harder to accomplish in straight python. At least a few years ago you would be forced to introduce some sort of worker process like celery to get something like that done. This question seems to cover all the issues:
Making an asynchronous task in Flask
Perhaps things have changed in flask land?
I'm working on my first Flask app (version 0.10.1), and also my first Python (version 3.5) app. One of its pieces needs to work like this:
Submit a form
Run a Celery task (which makes some third-party API calls)
When the Celery task's API calls complete, send a JSON post to another URL in the app
Get that JSON data and update a database record with it
Here's the relevant part of the Celery task:
if not response['errors']: # response comes from the Salesforce API call
# do something to notify that the task was finished successfully
message = {'flask_id' : flask_id, 'sf_id' : response['id']}
message = json.dumps(message)
print('call endpoint now and update it')
res = requests.post('http://0.0.0.0:5000/transaction_result/', json=message)
And here's the endpoint it calls:
#app.route('/transaction_result/', methods=['POST'])
def transaction_result():
result = jsonify(request.get_json(force=True))
print(result.flask_id)
return result.flask_id
So far I'm just trying to get the data and print the ID, and I'll worry about the database after that.
The error I get though is this: requests.exceptions.ConnectionError: None: Max retries exceeded with url: /transaction_result/ (Caused by None)
My reading indicates that my data might not be coming over as JSON, hence the Force=True on the result, but even this doesn't seem to work. I've also tried doing the same request in CocoaRestClient, with a Content-Type header of application/json, and I get the same result.
Because both of these attempts break, I can't tell if my issue is in the request or in the attempt to parse the response.
First of all request.get_json(force=True) returns an object (or None if silent=True). jsonify converts objects to JSON strings. You're trying to access str_val.flask_id. It's impossible. However, even after removing redundant jsonify call, you'll have to change result.flask_id to result['flask_id'].
So, eventually the code should look like this:
#app.route('/transaction_result/', methods=['POST'])
def transaction_result():
result = request.get_json()
return result['flask_id']
And you are absolutely right when you're using REST client to test the route. It crucially simplifies testing process by reducing involved parts. One well-known problem during sending requests from a flask app to the same app is running this app under development server with only one thread. In such case a request will always be blocked by an internal request because the current thread is serving the outermost request and cannot handle the internal one. However, since you are sending a request from the Celery task, it's not likely your scenario.
UPD: Finally, the last one reason was an IP address 0.0.0.0. Changing it to the real one solved the problem.
I'm using Flask to develop a web server in a python app. I'm achieving this scenario: the client (it won't be a browser) sends a request, the server does some long task in background and on completion sends the response back to the client asynchronously. Is it possible to do that?
What you ask cannot be done with the HTTP protocol. Each request receives a response synchronously. The closest thing to achieve what you want would be this:
The client sends the request and the server responds with a job id immediately, while it also starts a background task for this long calculation.
The client can then poll the server for status by sending the job id in a new request. The response is again immediate and contains a job status, such as "in progress", "completed", "failed", etc. The server can also return a progress percentage, which the client can use to render a progress bar.
You could also implement web sockets, but that will require socket enabled server and client.