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Data Streaming with Zef

Zef offers a built-in streaming capability as a core feature. This allows users to build reactive systems without requiring any additional libraries. In fact, a significant part of the ZefDB is built on top of streams inherently. Users can leverage this streaming capability to react to various events in the database such as the creation, assignment, or removal of entities. The resulting reactive system can be used for a range of applications such as push notifications or real-time data processing.

A data stream consists of three stages: a data producer (source), a processing engine, and a data sink.

In this tutorial, we will explore three different types of data sources for streaming that zef can produce, and learn how to write simple code to react to them.

Reacting to Database Events

Let's say we want to build a reactive system that sends push notifications every time a new movie is created in the database. We can subscribe to the events emitted every time an ET.Movie is created using the on operator:

db | on[Instantiated[ET.Movie]] | subscribe[print]  

In Zef, a stream is considered lazy until at least one data sink (or consumer) subscribes to it. The subscribe operator essentially activates the stream and triggers it to start producing and emitting data.

Whenever a new movie is created, Zef will emit an Instantiated event. Let's create a new movie:

new_movie = ET.Movie(title="Hunger Games")  
new_movie | db | run

You should see the following output:

Instantiated({'target': <ZefRef #97 ET.Movie slice=2>})  

If we want to send a push notification to users, we can do

def send_push_notification(event):  
# do something with the event emitted
message = f"{ | F.title | collect} is created!"

effect = FX.HTTP.Request(
"message" : message
return effect

db | on[Instantiated[ET.Movie]] # Data Source
| map[send_push_notification] # Stream Processing
| subscribe[run] # Data Sink

There are two other graph events that we can subscribe to, Assigned and Terminated. For example:

db | on[Assigned[AET]] | subscribe[print]  
db | on[Terminated[ET.Movie]] | subscribe[print]

Assigned event is emitted usually when a value is assigned to a node, and usually it is an Attribute Entitiy (AET). Going back to the same example above, when new_movie is created, the emitted Assigned event looks like this.

Assigned({'target': <ZefRef #127 AET.String slice=2>, 'prev': None, 'current': 'Hunger Games'})  

When a movie is removed from the database, the Terminated event is emitted:

Terminated({'target': <ZefRef #302 ET.Movie slice=5>})  

Pushable Streams

In addition to graph events, Zef also allows us to create pushable streams where we can push messages or data and react to them. It works similarly to creating a "Topic" in Kafka.

To create a pushable stream, we can run the following command:

stream =  FX.Stream.CreatePushableStream() | run  

After that, we can create a subscription to the stream like this:

def do_something(data):  
return f"Doing something with {data}"

stream # Data Stream
| map[do_something] # Stream Processing
| subscribe[print] # Data Sink

Now, if we have other systems pushing data to this stream, such as data from a HTTP request or a websocket, we can push data to the stream like this:

"new data" | push[stream] | run  

Note that pushing an event into a stream is a side-effectful operation itself (the stream is potentially observable and not part of the pure functional core).

The output console should show the following:

Doing something with new data  

Web Requests as Data Stream

One can think of web requests as an stream of data. Any incoming requests from a HTTP Server or a Websocket Server can be treated as an event that trigger a series of downstream computation.

handle = FX.HTTP.StartServer(  
) | run

The return value of executing the HTTP.StartServer side effect is a dictionary that consist of:

  • server_uuid - unique identifier of the server
  • stream - A pushable stream where all incoming requests will flow into

Similar to the last example, we can subscribe to the request stream by doing the following:

def handle_request(req: Dict) -> FX:  
# construct and return an effect (data)
return FX.HTTP.SendResponse(
response="Welcome to ZefFX!"

handle['stream'] # Data Stream
| map[handle_request] # Stream Processing
| subscribe[run] # Data Sink

The HTTP server converts external requests to a stream of events and expects a response in form of an effect object (action) to be returned to the runtime within the timeout limit.
subscribe is an operator which takes an impure function and executes this using the incoming event as an argument, as the events arrive in the stream.


Zef is continuously improving its reactive component and adding more features, such as more advanced stream processing operators and joins. Additionally, the complexity of building distributed streaming process systems will be absorbed by ZefHub. The goal is to make it easy for users to build streaming and reactive systems.