Flask-Mercure-SSE

Provide push capabilities using server-sent events to your Flask apps. Based on the Mercure protocol.

• Built-in hub for development
• Use any external Mercure hub
• Use external hub like the Mercure.rocks hub for production and scaling

Installation

pip install flask-mercure-sse

Getting started

Enable the MercureSSE extension:

from flask import Flask
from flask_mercure_sse import MercureSSE

app = Flask(__name__)
mercure = MercureSSE(app)

Publish messages from anywhere in your app:

mercure.publish("topic", "message")

Generate subscription urls in your templates:

<script>
const es = new EventSource("{{ mercure_hub_url('topic') }})");
// ...
</script>

About the built-in hub

The built-in hub is for development only as it is not scalable at all.

It implements the all the required part of the Mercure specification including authorization. subscriptions are not implemented.

Configuration

Key	Description	Default
MERCURE_HUB_URL	External hub url	None
MERCURE_PUBLISHER_JWT	The authorization JWT to publish on external hubs	Required when hub url is provided
MERCURE_AUTHZ_COOKIE_NAME	Authorization cookie name	mercureAuthorization
MERCURE_HUB_ALLOW_PUBLISH	Whether to allow publishing via HTTP with the built-in hub	False
MERCURE_HUB_ALLOW_ANONYMOUS	Whether to allow anonymous subscribers to connect	True
MERCURE_SUBSCRIBER_SECRET_KEY	Secret key to generate subscriber JWTs	app.config["SECRET_KEY"]
MERCURE_PUBLISHER_SECRET_KEY	Secret key to generate publisher JWTs	app.config["SECRET_KEY"]

Authorization

Publish privately using private=True in publish().

Using external hubs

Provide the authorization JWT to the frontend:

• Use mercure_hub_url(topics, "SUBSCRIBER_JWT") to generate subscription urls with the authorization parameter.
• Use MercureSSE.set_authz_cookie(response, jwt="SUBSCRIBER_JWT") to define the mercureAuthorization cookie.

Use mercure_subscriber_jwt(topics) in templates to generate a JWT.

Using the built-in hub

First, ensure that a secret key is defined in your app config.

By default, publishing is not possible via the HTTP api for security reason. You will only need to publish internally using MercureSSE.publish().

Create authorization JWT for subscribers using MercureSSE.create_subscription_jwt(topics).

To authorize subscribers:

• Pass the JWT to mercure_hub_url() in templates like external hubs
• Use MercureSSE.set_authz_cookie(response, topics) to define the mercureAuthorization cookie.
• Use MercureSSE.set_authz_session(topics) to define the authorization via the session

When publishing via HTTP is allowed app.extensions["mercure"].publisher_jwt as the authorization JWT.

CLI

Some CLI commands are available.

Start with flask mercure --help.

Going to production

It is recommended to use the official Mercure.rocks hub in production environments.