nats-queue-example

An example of using NATS for queue messaging.

This example starts a service that publishes CPU metrics every second to a metrics-queue subject and queue group queuegroup1. The queue is subscribed to by one-to-many clients and each message is received by only a single client.

Background

NATS supports message queueing using point-to-point (one-to-one) communication.

To create a message queue, subscribers register a queue name. All subscribers with the same queue name form the queue group. As messages on the registered subject are published, one member of the group is chosen randomly to receive the message. Although queue groups have multiple subscribers, each message is consumed by only one.

Queue subscribers can be asynchronous, in which case the message handler callback function processes the delivered message. Synchronous queue subscribers must build in logic to processes the message.

More information can be found in the NATS Documentation.

Prerequisites

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

Running the Example

Start the Queue Client

You can start the Queue Client using the following command:

$ ./gradlew :queue-client:run

Once the client and service is running you will see messages similar to the following in the terminal:

[main] INFO nats.example.queue.client.Main - Starting NATS Example Queue Client
[jnats-subscriptions] INFO nats.example.queue.client.Main - Received Message: "{\"id\":\"d2a47686-38de-4f34-9ed8-f2088ea190a6\",\"cpuPercentage\":0.06,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":244.0}"
[jnats-subscriptions] INFO nats.example.queue.client.Main - Received Message: "{\"id\":\"aab40392-e716-4546-9cd9-472d150a74d8\",\"cpuPercentage\":0.06,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":275.0}"
[jnats-subscriptions] INFO nats.example.queue.client.Main - Received Message: "{\"id\":\"9fa01ff2-0e82-45f0-aa77-53321a2a6635\",\"cpuPercentage\":0.06,\"totalPhysicalMemory\":16384.0,\"freePhysicalMemory\":221.0}"

You can start multiple clients to verify that each message produced by the service is only being received by a single client.

Start the Queue Service

You can start the Queue Service using the following command:

$ ./gradlew :queue-service:run

Once the service is running you will see messages similar to the following in the terminal:

[main] INFO nats.example.queue.service.Main - Starting NATS Example Queue Service
[Timer-0] INFO nats.example.queue.service.MetricsPublishTask - Publishing: {"id":"e215288f-c739-4823-ad0b-f807798fc07f","cpuPercentage":0.0,"totalPhysicalMemory":16384.0,"freePhysicalMemory":373.0}
[Timer-0] INFO nats.example.queue.service.MetricsPublishTask - Publishing: {"id":"536f3274-5fca-4041-9fe0-dc3f68190eaf","cpuPercentage":0.24,"totalPhysicalMemory":16384.0,"freePhysicalMemory":273.0}
[Timer-0] INFO nats.example.queue.service.MetricsPublishTask - Publishing: {"id":"be451f72-4224-412d-81e0-d4784183e6f4","cpuPercentage":0.04,"totalPhysicalMemory":16384.0,"freePhysicalMemory":349.0}

Bugs and Feedback

For bugs, questions and discussions please use the Github Issues.

License

MIT License

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of this software and associated documentation files (the “Software”), to deal
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The above copyright notice and this permission notice shall be included in all
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