Scaling Instances in Elastic Beanstalk breaks dynamic image link of python flask app
Question
The following code works brilliantly in local machine as well as in deployment at AWS Elastic BeanStalk - serves filxed image as well as dynamically created image. But, as soon as I scale the deployment to multiple EC2 instances, the dynamically created image becomes unavailable, even though I can find (by ssh) that dynamic image is present in the EC2 instance!
I suspect, it has something to do with how I am referencing to the static images in html or the way the load balancer handles static images. But, have no clue where to begin the investigation. Help please! :(
<code>
from flask import Flask, render_template, request, url_for
import os, datetime
import thread
import time
from copy import deepcopy
import pyqrcode # generating image of QR code from given string
application = Flask(__name__)
class QR(object):
def __init__(self):
self.input_string = None
self.output_string = None
self.input_image_filename = None
def encode(self, input_string, QR_image_filename, QR_image_scale):
"""Return the image of QR code"""
if not input_string:
raise RuntimeError('Input string is empty!') # checking that input_string is not empty
else:
self.input_string = input_string
try:
qr = pyqrcode.create(self.input_string)
qr.png(QR_image_filename, scale=QR_image_scale)
return True
except Exception as e:
print "printing exception : " + str(e)
raise RuntimeError('QR creation failed!')
def background_deletion_task(path):
print "wait starts"
# gevent.sleep(5)
time.sleep(5)
print "wait done"
if os.path.exists(path):
os.remove(path)
print "deletion done"
@application.route('/')
def hello():
gen_qr = QR()
gen_qr.qr_time = datetime.datetime.utcnow().isoformat()
gen_qr.image_file_name = str(gen_qr.qr_time[-6:]) + ".png"
gen_qr.full_path = os.path.join(application.root_path, 'static', gen_qr.image_file_name)
gen_qr.encode(str(gen_qr.image_file_name), gen_qr.full_path, 4)
# ---------------deletion by thread------------
#thread.start_new_thread(background_deletion_task, (gen_qr.full_path,))
return render_template('hello.html', gen_qr = gen_qr)
if __name__ == "__main__":
#application.debug=True
application.run()
</code>
<html> <body> Fixed Image <img src="/static/EYN_Logo.png" style="max-width: 150px;"> Dynamically Created Image <img src="/static/{{gen_qr.image_file_name}}"> </body> </html>
2 answers
solution1
2 2018-02-23 13:20:16
It seems like you are creating the instance on a single server, but in a load-balanced environment you probably have more than one server, and those servers can come & go at any time.
So basically, never store any state information on an EC2 instance.
To get around this, you have a couple of options. The best option would be to store the images in S3 - if you still want them to be referenced under /static/ then you could use Cloudfront to map that path to an S3 bucket.
The other option would be to use EFS to mount a shared file system on all of your EC2 instance.
Keep in mind that EFS is about 10x the cost of S3 storage.
solution2
0 2018-02-23 15:31:32
I found a workaround which works, but not completely convinced that's the right approach. This is what works for me now.
First, I added a ANY_NAME.config under .exbextension with the following option_settings: aws:elasticbeanstalk:container:python:staticfiles: "/static/": "static/" This is to explicitly say to EBS where to find the static files.
Second, I have invoked the html by <img src="{{ url_for('static', filename=fname ) }}"> instead of <img src="/{{fname}}">
where return render_template('hello.html', fname = fname)
After this, eb scale 2 serves well the dynamic files.
As chris mentioned, although it works, perhaps the better approach is to serve the images ( whether dynamically created or static) from S3 bucket.
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