How to Implement Real-Time Comment Updates in Jinja2 Templates Using WebSockets and FastAPI?

Getting Updated List of Items in Jinja2 Template Using FastAPI

In this tutorial, we demonstrate how to implement a real-time chat application using FastAPI and Jinja2 for dynamic updates of comment lists. The application leverages WebSockets to establish a bidirectional communication channel between the server and multiple clients.

Problem:

Imagine building a blog comment system where new comments need to be dynamically added to the frontend without requiring a page refresh. Jinja2 templates are used to render existing comments on the page, but a mechanism is needed to retrieve and display newly added comments in real-time.

Solution:

To achieve this functionality, we introduce WebSockets, a technology that allows for real-time, bidirectional communication over a single TCP connection. By implementing WebSockets with FastAPI and Jinja2, we can establish a persistent connection between the server and the client, enabling the server to broadcast new comments to all connected clients in real-time.

Implementing WebSockets:

To set up WebSockets in FastAPI, we create a WebSocket endpoint that handles incoming connections and establishes a dedicated connection for each client. The following code shows our WebSocket endpoint:

<code class="python">@app.websocket("/ws")
async def websocket_endpoint(websocket: WebSocket):
    await manager.connect(websocket)
    try:
        while True:
            data = await websocket.receive_json()
            comments.append(Comment(data['author'], data['content']))
            await manager.broadcast(data)
    except (WebSocketDisconnect, ConnectionClosed):
        manager.disconnect(websocket)</code>

In this code:

• manager is an instance of ConnectionManager, which handles the list of active connections and manages message broadcasting.
• When a client connects to the /ws endpoint, the connect method is called, adding the client to the list of active connections.
• The while loop handles incoming messages from the client. When a new message is received, a new comment is created and added to the comments list. The updated list is then broadcast to all connected clients using the broadcast method.
• If a client closes the connection or an exception occurs, the disconnect method is called, removing the client from the list of active connections.

Updating the Jinja2 Template:

To display the updated comments in the Jinja2 template, we use JavaScript and WebSockets to listen for incoming messages from the server and dynamically add new comments to the page. In our index.html template, we include the following JavaScript code:

<code class="html"><script>
  var ws = new WebSocket("ws://localhost:8000/ws");
  ws.onmessage = function(event) {
    var comments = document.getElementById('comments');
    var comment = document.createElement('li');
    var jsonObj = JSON.parse(event.data);
    var authorNode = document.createElement('h3');
    authorNode.innerHTML = jsonObj.author;
    var contentNode = document.createElement('p');
    contentNode.innerHTML = jsonObj.content;
    comment.appendChild(authorNode);
    comment.appendChild(contentNode);
    comments.appendChild(comment);
  };
</script></code>

This JavaScript code:

• Establishes a WebSocket connection to the server using the provided WebSocket URL.
• Defines an event listener that listens for incoming messages from the server.
• When a message is received, it parses the JSON data from the message and creates a new HTML comment element with the received author and content.
• Finally, the new comment element is appended to the comments container in the HTML, dynamically updating the page with the new comment.

Conclusion

By integrating WebSockets with FastAPI and Jinja2, we have implemented a dynamic comment system that allows for real-time updates. This technique can be applied to various scenarios where you need to send and receive messages or data between clients and a server in real-time, providing a highly interactive and responsive user experience.

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