Introduction
Server-sent events (SSE) is a way to send data to the browser without reloading the page. This allows you to use streaming data and build real-time applications that can be used in a variety of scenarios.
FastAPI is a Python framework that makes it easy to build APIs.
In this tutorial, we will use FastAPI to create a simple SSE server that will send a message every second.
Prerequisites
In order to follow this tutorial, you will need to have a Python and pip installed on your machine:
https://www.python.org/downloads/
Installing FastAPI
To install FastAPI and all of its dependencies, you can use the following command:
pip install "fastapi[all]"
This will also include the uvicorn
server, which is used to run the server.
Installing sse-starlette
Once you've installed FastAPI, you can install the sse-starlette
extension to add support for SSE to your FastAPI project:
pip install sse-starlette
Let's also add asyncio
to our project:
pip install asyncio
Creating a simple hello world endpoint
Once you've installed FastAPI, you can create a simple hello world endpoint to get started.
Create a new file called main.py
and add the following code:
import asyncio
import uvicorn
from fastapi import FastAPI, Request
app = FastAPI()
@app.get("/")
async def root():
return {"message": "Hello World"}
Running the uvicorn
server
To run the server, you can use the following command:
uvicorn main:app --reload
This will run the server on port 8000
. The --reload
flag will automatically reload the server when you make changes to the code so you don't have to restart the server every time you make a change.
Visit the server in your browser and you should see the following output:
{
"message": "Hello World"
}
FastAPI will automatically generate a /docs
endpoint that will show you the API documentation. If you were to visit /docs
, you would see the following:
Adding SSE support to your FastAPI project
Next, let's extend the main.py
file to add SSE support. To do so you can add SSE support to your project by adding the following line to your main.py
file:
from sse_starlette.sse import EventSourceResponse
Then you can use the EventSourceResponse
class to create a response that will send SSE events. Let's create a new endpoint that will send an event every second:
STREAM_DELAY = 1 # second
RETRY_TIMEOUT = 15000 # milisecond
@app.get('/stream')
async def message_stream(request: Request):
def new_messages():
# Add logic here to check for new messages
yield 'Hello World'
async def event_generator():
while True:
# If client closes connection, stop sending events
if await request.is_disconnected():
break
# Checks for new messages and return them to client if any
if new_messages():
yield {
"event": "new_message",
"id": "message_id",
"retry": RETRY_TIMEOUT,
"data": "message_content"
}
await asyncio.sleep(STREAM_DELAY)
return EventSourceResponse(event_generator())
Now if you visit the /stream
endpoint in your browser, you would see an event sent every second without you having to reload the page.
FastAPI with streaming data and Materialize
To learn more about streaming data, you can check out this tutorial here on how to use FastAPI with Materialize:
How to use FastAPI with Materialize for real-time data processing
The tutorial also includes a demo project that you could run to get a feel on how it all works.
Here is a quick diagram of the project:
What is Materialize?
Materialize is a streaming database that takes data coming from different sources like Kafka, PostgreSQL, S3 buckets, and more and allows users to write views that aggregate/materialize that data and let you query those views using pure SQL with very low latency.
Streaming data with Materialize
For the demo project, we are using the [TAIL](https://materialize.com/docs/sql/tail/#conceptual-framework)
statement. TAIL
streams updates from a source, table, or view as they occur which allows you to query the data as it is being updated and is a perfect fit for the SSE example.
Here is the code for the /stream
endpoint that uses TAIL
to stream data:
@app.get('/stream')
async def message_stream(request: Request):
def new_messages():
# Check if data in table
results = engine.execute('SELECT count(*) FROM sensors_view_1s')
if results.fetchone()[0] == 0:
return None
else:
return True
async def event_generator():
while True:
# If client was closed the connection
if await request.is_disconnected():
break
# Checks for new messages and return them to client if any
if new_messages():
connection = engine.raw_connection()
with connection.cursor() as cur:
cur.execute("DECLARE c CURSOR FOR TAIL sensors_view_1s")
cur.execute("FETCH ALL c")
for row in cur:
yield row
await asyncio.sleep(MESSAGE_STREAM_DELAY)
return EventSourceResponse(event_generator())
As you can see, we have just extended the new_message
function to check if there are any new messages in the sensors_view_1s
view. If there are no new messages, we will return None
and the EventSourceResponse
will not send any events. If there are new messages, we will return True
and the EventSourceResponse
will send the new messages.
Then in the event_generator
async function, we are using TAIL
with the FETCH ALL
statement to get all the messages in the sensors_view_1s
view. We are using the DECLARE CURSOR
statement to create a cursor that will stream the data as it is being updated.
Conclusion
To learn more about FastAPI, check out the FastAPI documentation.
For more information on how to use FastAPI with Materialize, check out this tutorial.
To learn more about Materialize, check out the Materialize documentation.
Top comments (2)
Thanks dude, I have the perfect place for this in my app
Nice! That is super cool!