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:
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 objecttype stock struct {Symbol stringPrice 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 inwg.Wait()
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 arriveCountDownLatch latch = new CountDownLatch(10);​// Create a dispatcher and inline message handlerDispatcher 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 objectSystem.out.println(stk);​latch.countDown();});​// Subscribed.subscribe("updates");​// Wait for a message to come inlatch.await();​// Close the connectionnc.close();} catch (Exception e) {e.printStackTrace();}}}
let nc = NATS.connect({url: "nats://demo.nats.io:4222",json: true});​nc.subscribe('updates', (msg) => {if(msg && msg.ticker === 'TSLA') {t.log('got message:', msg);}});
import asyncioimport jsonfrom nats.aio.client import Client as NATSfrom 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()
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 mendend
let nc = await connect({url: "nats://demo.nats.io:4222",payload: Payload.JSON});​nc.subscribe('updates', (err, msg) => {t.log('got message:', msg.data ? msg.data : "no payload");});
// Structured data is not configurable in C NATS Client.