by Example
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.
Code
require "nats"
require "json"
Use the JSON::Serializable mixin to be able to serialize and deserialize a
struct we define.
struct Payload
include JSON::Serializable
getter foo : String
getter bar : UInt8
def initialize(@foo, @bar)
end
end
Get the NATS_URL from the environment or fallback to the default. This can
be a comma-separated string. We convert it to an Array(URI) to pass to the
NATS client.
servers = ENV.fetch("NATS_URL", "nats://localhost:4222")
.split(',')
.map { |url| URI.parse(url) }
Create a client connection to an available NATS server.
nats = NATS::Client.new(servers)
When the program exits, we close the NATS client which waits for any pending messages (published or in a subscription) to be flushed.
at_exit { nats.close }
Create a subscription. We will receive both valid and invalid JSON here, so we
rescue the JSON::ParseException.
nats.subscribe "foo" do |msg|
json = String.new(msg.body)
payload = Payload.from_json(json)
puts "received valid JSON: #{payload}"
rescue JSON::ParseException
puts "received invalid JSON: #{json}"
end
Construct and serialize a Payload struct.
payload = Payload.new(foo: "bar", bar: 27).to_json
Publish the serialized payload
nats.publish "foo", payload
nats.publish "foo", "not json"Output
received valid JSON: Payload(@foo="bar", @bar=27) received invalid JSON: not json