How can I load Jinja2 templates dynamically from Google Cloud Storage in App Engine?

I have an application on App Engine and want to deploy my Jinja2 templates in a way that lets me dynamically update them without needing to redeploy my entire application.

Ideally they would be stored in Google Cloud Storage, which would allow me to just replace the template files in a bucket and have them be immediately used by the live application. However, Flask seems to require the templates to be local to the application.

Is this at all possible?

2 Answers
2

This would be possible by loading each template directly from Google Cloud Storage for every request, and using the render_template_string function from Flask.

render_template_string

For example, if your template file hello.html looks like this:

hello.html

<h1>Hello name !</h1>


Add the Google Cloud Storage resource to your application, create a new bucket (let's call it your-bucket), and upload this file to the bucket.

your-bucket

In your requirements.txt:

requirements.txt

flask
google-cloud-storage


In your main.py:

main.py

from flask import Flask, render_template_string
from google.cloud import storage

app = Flask(__name__)

# Initialize the bucket you created containing the templates
bucket = storage.Client().bucket('your-bucket')

@app.route('/')
def hello():
# Load the template string from Cloud Storage
template_string = bucket.blob('hello.html').download_as_string().decode('ascii')

# Now use render_template_string the same way you'd use render_template
return render_template_string(template_string, name='World')


Note that since this application would re-download the template for every request, for high-traffic applications it would possibly incur significant additional request time, as well as cost.

Due to this, it would be ideal to "cache" the template string in some way (for example, via Cloud Memorystore), and then use an Object Change Notification (possibly via a Google Cloud Function trigger) to determine when the file has changed in the bucket, and update the cache.

Here's a nice wrapper of the Flask render_template() call to achieve this that either

You can then copy your /templates file to your Google CDN and reference the public URL

-

from flask import Flask, render_template, render_template_string


def renderTemplateLocalOrRemote(file, **kwargs):
if REMOTE_LOADING_ENABLED is defined: # Load the template file remotely
r = requests.get('https://'+YOUR_BASE_URL+"/templates/"+file)
template_string = r.content.decode('utf-8')
return render_template_string(template_string, **kwargs)
else: # Load the template file from local, pass on to standard method
return render_template(file, **kwargs)


You can then use this wrapper everywhere you were using flask.render_template()

@app.route('/')
def hello():
return renderTemplateLocalOrRemote('hello.html', name='lalala', another_param='lilili')

@app.route('/another_route')
def hello2():
return renderTemplateLocalOrRemote('hello2.html', different_param='lilili')


and toggle it on/off using

REMOTE_LOADING_ENABLED


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