structure.md

Structured Data

Client libraries may provide tools to help receive structured data, like JSON. The core traffic to the NATS server will always be opaque byte arrays. The server does not process message payloads in any form. For libraries that don't provide helpers, you can always encode and decode data before sending the associated bytes to the NATS client.

For example, to receive JSON you could do:

{% tabs %} {% tab title="Go" %}

nc, err := nats.Connect("demo.nats.io",
    nats.ErrorHandler(func(nc *nats.Conn, s *nats.Subscription, err error) {
        if s != nil {
        log.Printf("Async error in %q/%q: %v", s.Subject, s.Queue, err)
        } else {
        log.Printf("Async error outside subscription: %v", err)
        }
    }))
if err != nil {
    log.Fatal(err)
}
defer nc.Close()
ec, err := nats.NewEncodedConn(nc, nats.JSON_ENCODER)
if err != nil {
    log.Fatal(err)
}
defer ec.Close()

// Define the object
type stock struct {
    Symbol string
    Price  int
}

wg := sync.WaitGroup{}
wg.Add(1)

// Subscribe
// Decoding errors will be passed to the function supplied via
// nats.ErrorHandler above, and the callback supplied here will
// not be invoked.
if _, err := ec.Subscribe("updates", func(s *stock) {
    log.Printf("Stock: %s - Price: %v", s.Symbol, s.Price)
    wg.Done()
}); err != nil {
    log.Fatal(err)
}

// Wait for a message to come in
wg.Wait()

{% endtab %}

{% tab title="Java" %}

class StockForJsonSub {
    public String symbol;
    public float price;

    public String toString() {
        return symbol + " is at " + price;
    }
}

public class SubscribeJSON {
    public static void main(String[] args) {

        try {
            Connection nc = Nats.connect("nats://demo.nats.io:4222");

            // Use a latch to wait for 10 messages to arrive
            CountDownLatch latch = new CountDownLatch(10);

            // Create a dispatcher and inline message handler
            Dispatcher d = nc.createDispatcher((msg) -> {
                Gson gson = new Gson();

                String json = new String(msg.getData(), StandardCharsets.UTF_8);
                StockForJsonSub stk = gson.fromJson(json, StockForJsonSub.class);

                // Use the object
                System.out.println(stk);

                latch.countDown();
            });

            // Subscribe
            d.subscribe("updates");

            // Wait for a message to come in
            latch.await(); 

            // Close the connection
            nc.close();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

{% endtab %}

{% tab title="JavaScript" %}

const jc = JSONCodec();
const sub = nc.subscribe(subj, {
  callback: (_err, msg) => {
    t.log(`${jc.decode(msg.data)}`);
  },
  max: 1,
});

{% endtab %}

{% tab title="Python" %}

import asyncio
import json
from nats.aio.client import Client as NATS
from nats.aio.errors import ErrTimeout

async def run(loop):
    nc = NATS()

    await nc.connect(servers=["nats://127.0.0.1:4222"], loop=loop)

    async def message_handler(msg):
        data = json.loads(msg.data.decode())
        print(data)

    sid = await nc.subscribe("updates", cb=message_handler)
    await nc.flush()

    await nc.auto_unsubscribe(sid, 2)
    await nc.publish("updates", json.dumps({"symbol": "GOOG", "price": 1200 }).encode())
    await asyncio.sleep(1, loop=loop)
    await nc.close()

{% endtab %}

{% tab title="Ruby" %}

require 'nats/client'
require 'json'

NATS.start(servers:["nats://127.0.0.1:4222"]) do |nc|
  nc.subscribe("updates") do |msg|
    m = JSON.parse(msg)

    # {"symbol"=>"GOOG", "price"=>12}
    p m
  end
end

{% endtab %}

{% tab title="C" %}

// Structured data is not configurable in C NATS Client.

{% endtab %} {% endtabs %}