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:

Go

  1. nc, err := nats.Connect("demo.nats.io",
  2.     nats.ErrorHandler(func(nc *nats.Conn, s *nats.Subscription, err error) {
  3.         if s != nil {
  4.         log.Printf("Async error in %q/%q: %v", s.Subject, s.Queue, err)
  5.         } else {
  6.         log.Printf("Async error outside subscription: %v", err)
  7.         }
  8.     }))
  9. if err != nil {
  10.     log.Fatal(err)
  11. }
  12. defer nc.Close()
  13. ec, err := nats.NewEncodedConn(nc, nats.JSON_ENCODER)
  14. if err != nil {
  15.     log.Fatal(err)
  16. }
  17. defer ec.Close()
  19. // Define the object
  20. type stock struct {
  21.     Symbol string
  22.     Price  int
  23. }
  25. wg := sync.WaitGroup{}
  26. wg.Add(1)
  28. // Subscribe
  29. // Decoding errors will be passed to the function supplied via
  30. // nats.ErrorHandler above, and the callback supplied here will
  31. // not be invoked.
  32. if _, err := ec.Subscribe("updates", func(s *stock) {
  33.     log.Printf("Stock: %s - Price: %v", s.Symbol, s.Price)
  34.     wg.Done()
  35. }); err != nil {
  36.     log.Fatal(err)
  37. }
  39. // Wait for a message to come in
  40. wg.Wait()

Java

  1. class StockForJsonSub {
  2.     public String symbol;
  3.     public float price;
  5.     public String toString() {
  6.         return symbol + " is at " + price;
  7.     }
  8. }
  10. public class SubscribeJSON {
  11.     public static void main(String[] args) {
  13.         try {
  14.             Connection nc = Nats.connect("nats://demo.nats.io:4222");
  16.             // Use a latch to wait for 10 messages to arrive
  17.             CountDownLatch latch = new CountDownLatch(10);
  19.             // Create a dispatcher and inline message handler
  20.             Dispatcher d = nc.createDispatcher((msg) -> {
  21.                 Gson gson = new Gson();
  23.                 String json = new String(msg.getData(), StandardCharsets.UTF_8);
  24.                 StockForJsonSub stk = gson.fromJson(json, StockForJsonSub.class);
  26.                 // Use the object
  27.                 System.out.println(stk);
  29.                 latch.countDown();
  30.             });
  32.             // Subscribe
  33.             d.subscribe("updates");
  35.             // Wait for a message to come in
  36.             latch.await(); 
  38.             // Close the connection
  39.             nc.close();
  40.         } catch (Exception e) {
  41.             e.printStackTrace();
  42.         }
  43.     }
  44. }

JavaScript

  1. const jc = JSONCodec();
  2. const sub = nc.subscribe(subj, {
  3.   callback: (_err, msg) => {
  4.     t.log(`${jc.decode(msg.data)}`);
  5.   },
  6.   max: 1,
  7. });

Python

  1. import asyncio
  2. import json
  3. from nats.aio.client import Client as NATS
  4. from nats.aio.errors import ErrTimeout
  6. async def run(loop):
  7.     nc = NATS()
  9.     await nc.connect(servers=["nats://127.0.0.1:4222"], loop=loop)
  11.     async def message_handler(msg):
  12.         data = json.loads(msg.data.decode())
  13.         print(data)
  15.     sid = await nc.subscribe("updates", cb=message_handler)
  16.     await nc.flush()
  18.     await nc.auto_unsubscribe(sid, 2)
  19.     await nc.publish("updates", json.dumps({"symbol": "GOOG", "price": 1200 }).encode())
  20.     await asyncio.sleep(1, loop=loop)
  21.     await nc.close()

Ruby

  1. require 'nats/client'
  2. require 'json'
  4. NATS.start(servers:["nats://127.0.0.1:4222"]) do |nc|
  5.   nc.subscribe("updates") do |msg|
  6.     m = JSON.parse(msg)
  8.     # {"symbol"=>"GOOG", "price"=>12}
  9.     p m
  10.   end
  11. end

C

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

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