JSON for Message Payloads (Python)

JSON for Message Payloads in Messaging

The basic structure of a NATS message modeled in the client libraries includes the subject the message was published to, the application-defined payload, and an optional set of headers (for requests, there is also a reply-to subject). The payload is a sequence of bytes, so it is up to the application to define how to serialize and deserialize the payload.

JSON is ubiquitous and simple data-interchange format that is supported in virtually all programming languages. This example demonstrates how to serialize and deserialize a message payload using a JSON library.

CLI Go Python JavaScript Rust C# C#2 Java Ruby Elixir Crystal C

Jump to the output or the recording

$ nbe run messaging/json/python

View the source code or learn how to run this example yourself

Code

import asyncio
import json
import os
from dataclasses import asdict, dataclass


import nats




@dataclass
class Payload:
    foo: str
    bar: int




async def main():

Use the NATS_URL env variable if defined, otherwise fallback to the default.

    nats_url = os.getenv("NATS_URL", "nats://localhost:4222")


    client = await nats.connect(nats_url)

Create a subscription that receives two messages. One message will contain a valid serialized payload and the other will not.

    subscriber = await client.subscribe("foo", max_msgs=2)

Construct a Payload value and serialize it.

    payload = Payload(foo="bar", bar=27)
    bytes_ = json.dumps(asdict(payload)).encode()

Publish the serialized payload.

    await client.publish("foo", bytes_)
    await client.publish("foo", b"not json")

Loop through the expected messages and attempt to deserialize the payload into a Payload value. If deserialization into this type fails, alternate handling can be performed, either discarding or attempting to derialize in a more general type (such as an untyped dict).

    async for message in subscriber.messages:
        try:
            payload = Payload(**json.loads(message.data))
            print(f"received valid JSON payload: {payload.foo=} {payload.bar=}")
        except json.decoder.JSONDecodeError:
            print(f"received invalid JSON payload: {message.data=}")




if __name__ == "__main__":
    asyncio.run(main())

Output

received valid JSON payload: payload.foo='bar' payload.bar=27
received invalid JSON payload: message.data=b'not json'

Recording

Note, playback is half speed to make it a bit easier to follow.