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155 lines
4.0 KiB
Markdown
155 lines
4.0 KiB
Markdown
# Explore NATS Pub/Sub
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NATS is a [publish subscribe](../../nats-concepts/pubsub.md) messaging system [based on subjects](../../nats-concepts/subjects.md). Subscribers listening on a subject receive messages published on that subject. If the subscriber is not actively listening on the subject, the message is not received. Subscribers can use the wildcard tokens such as `*` and `>` to match a single token or to match the tail of a subject.
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## Prerequisites
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Go and the NATS server should be installed. Optionally you can use the demo server located at `nats://demo.nats.io`
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### 1. Start the NATS server
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```bash
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% nats-server
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```
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When the server starts successfully, you will see the following messages:
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```bash
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[1] 2019/31/05 15:18:22.301550 [INF] Starting nats-server version 2.0.0
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[1] 2019/31/05 15:18:22.301762 [INF] Listening for client connections on 0.0.0.0:4222
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[1] 2019/31/05 15:18:22.301769 [INF] nats-server is ready
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```
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The NATS server listens for client connections on TCP Port 4222.
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### 2. Start a shell or command prompt session
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You will use this session to run an example NATS client subscriber program.
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### 3. CD to the Go client examples directory
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```bash
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% cd $GOPATH/src/github.com/nats-io/nats.go/examples
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```
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### 4. Run the client subscriber program
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```bash
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% go run nats-sub/main.go <subject>
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```
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Where `<subject>` is a subject to listen on. A valid subject is a string that is unique in the system.
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For example:
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```bash
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% go run nats-sub/main.go msg.test
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```
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You should see the message: _Listening on \[msg.test\]_
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### 5. Start another shell or command prompt session
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You will use this session to run a NATS publisher client.
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### 6. CD to the examples directory
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```bash
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% cd $GOPATH/src/github.com/nats-io/nats.go/examples
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```
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### 7. Publish a NATS message
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```bash
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% go run nats-pub/main.go <subject> <message>
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```
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Where `<subject>` is the subject name and `<message>` is the text to publish.
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For example:
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```bash
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% go run nats-pub/main.go msg.test hello
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```
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or
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```bash
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% go run nats-pub/main.go msg.test "NATS MESSAGE"
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```
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### 8. Verify message publication and receipt
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You should see that the publisher sends the message and prints: _Published \[msg.test\] : 'NATS MESSAGE'_
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And that the subscriber receives the message and prints: _\[\#1\] Received on \[msg.test\]: 'NATS MESSAGE'_
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Note that if the receiver does not get the message, check that you are using the same subject name for the publisher and the subscriber.
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### 9. Publish another message
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```bash
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% go run nats-pub/main.go msg.test "NATS MESSAGE 2"
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```
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You should see that the subscriber receives message 2. Note that the message count is incremented each time your subscribing client receives a message on that subject:
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### 10. Start another shell or command prompt session
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You will use this session to run a second NATS subscriber.
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### 11. CD to the examples directory
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```bash
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% cd $GOPATH/src/github.com/nats-io/nats.go/examples
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```
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### 12. Start a second client subscriber program
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```bash
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% go run nats-sub/main.go msg.test
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```
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### 13. Publish another message using the publisher client
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```bash
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% go run nats-pub/main.go msg.test "NATS MESSAGE 3"
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```
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Verify that both subscribing clients receive the message.
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### 14. Start another shell or command prompt session
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You will use this session to run a third NATS subscriber.
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### 15. CD to the examples directory
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```bash
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% cd $GOPATH/src/github.com/nats-io/nats.go/examples
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```
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### 16. Subscribe to a different subject
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```bash
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% go run nats-sub/main.go msg.test.new
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```
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All the but last subscriber receives the message. Why? Because that subscriber is not listening on the message subject used by the publisher.
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### 17. Update the last subscriber to use a wildcard
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NATS supports the use of wildcard characters for message subscribers only. You cannot publish a message using a wildcard subject.
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Change the last subscriber the listen on msg.\* and run it:
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```bash
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% go run nats-sub/main.go msg.*
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```
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### 18. Publish another message
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This time, all three subscribing clients should receive the message.
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