An example of using NATS for pubsub messaging
An example of using NATS to do pubsub messaging.
This example starts a service that publishes CPU metrics every second to a metrics-pubsub topic. The topic is subscribed to by one-to-many clients each receiving the CPU metrics messages published by the service.
NATS publish subscribe is a one-to-many communication. A publisher sends a message on a subject. Any active subscriber listening on that subject receives the message.
The NATS pubsub model is “At-Most-Once-Delivery”. If a subscriber is not listening on the subject (no subject match), or is not active when the message is sent, the message is not received.
More information can be found in the NATS Documentation.
The examples require a local NATS server to be running. To start a NATS server as a Docker container run the following commands:
$ docker pull nats$ docker run -p 4222:4222 -p 6222:6222 -p 8222:8222 -d --name nats-main nats
You can start the PubSub Client using the following command:
$ ./gradlew :pubsub-client:run
Once the client and service is running you will see messages similar to the following in the terminal:
[main] INFO nats.example.pubsub.client.Main - Starting NATS Example PubSub Client[jnats-subscriptions] INFO nats.example.pubsub.client.Main - Received Message: "{\"cpuPercentage\":0.0,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":525.0}"[jnats-subscriptions] INFO nats.example.pubsub.client.Main - Received Message: "{\"cpuPercentage\":0.27,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":454.0}"[jnats-subscriptions] INFO nats.example.pubsub.client.Main - Received Message: "{\"cpuPercentage\":0.06,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":362.0}"
You can start multiple client instances to see them each receiving the same messages.
You can start the PubSub Service using the following command:
$ ./gradlew :pubsub-service:run
Once the service is running you will see messages similar to the following in the terminal:
[main] INFO nats.example.pubsub.service.Main - Starting NATS Example PubSub Service[Timer-0] INFO nats.example.pubsub.service.MetricsPublishTask - Publishing: {"cpuPercentage":0.0,"totalPhysicalMemory":16384.0,"freePhysicalMemory":552.0}[Timer-0] INFO nats.example.pubsub.service.MetricsPublishTask - Publishing: {"cpuPercentage":0.28,"totalPhysicalMemory":16384.0,"freePhysicalMemory":448.0}[Timer-0] INFO nats.example.pubsub.service.MetricsPublishTask - Publishing: {"cpuPercentage":0.04,"totalPhysicalMemory":16384.0,"freePhysicalMemory":564.0}
For bugs, questions and discussions please use the Github Issues.
MIT License
Copyright (c) 2018 Greg Whitaker
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