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186 lines
4.4 KiB
Markdown
186 lines
4.4 KiB
Markdown
# Explore NATS Pub/Sub
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NATS is a publish subscribe messaging system. Subscribers listening on a subject name receive messages on that subject. If the subscriber is not actively listening on the subject, the message is not received. Subscribers can use the wildcard subjects `*` to match a single token to match the tail of a subject.
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<div class="graphviz"><code data-viz="dot">
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digraph nats_pub_sub {
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graph [splines=ortho, nodesep=1];
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sub1 [shape="box", label="SUB\ncom.msg.one"];
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pub1 [shape="box", label="PUB\ncom.msg.one"];
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non_active [shape="box", label="Non-Active\nSubscriber"];
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{
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rank=same
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pub1 sub1 non_active
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}
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gnatsd [shape="box", label="NATS", width=8];
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sub2 [shape="box", label="SUB\ncom.msg.one"];
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sub3 [shape="box", label="SUB\ncom.msg.two"];
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sub4 [shape="box", label="SUB\ncom.msg.*"];
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{
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rank=same
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sub2 sub3 sub4
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}
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pub1 -> gnatsd [penwidth=2];
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gnatsd -> sub1 [penwidth=2];
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gnatsd -> non_active [style=dashed color=red arrowhead="none"];
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gnatsd -> sub2 [penwidth=2];
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gnatsd -> sub3 [style=dashed color=red arrowhead="none"];
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gnatsd -> sub4 [penwidth=2];
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}
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</code></div>
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## Prerequisites
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Go and the NATS server should be installed.
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## 1. Start the NATS server
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```sh
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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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```sh
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[1] 2015/08/12 15:18:22.301550 [INF] Starting gnatsd version 0.6.4
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[1] 2015/08/12 15:18:22.301762 [INF] Listening for client connections on 0.0.0.0:4222
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[1] 2015/08/12 15:18:22.301769 [INF] gnatsd 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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```sh
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% cd $GOPATH/src/github.com/nats-io/nats/examples
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```
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## 4. Run the client subscriber program
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```sh
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% go run nats-sub.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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```sh
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% go run nats-sub.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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```sh
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% cd $GOPATH/src/github.com/nats-io/nats/examples
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```
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## 7. Publish a NATS message
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```sh
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% go run nats-pub.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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```sh
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% go run nats-pub.go msg.test hello
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```
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or
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```sh
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% go run nats-pub.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: *Published [msg.test] : 'NATS MESSAGE'*
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And that the subscriber receives the message: *[#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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```sh
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% go run nats-pub.go msg.test "NATS MESSAGE 2"
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```
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You should see that the subscriber receive 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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```sh
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% cd $GOPATH/src/github.com/nats-io/nats/examples
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```
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## 12. Subscribe to the message
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```sh
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% go run nats-sub.go msg.test
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```
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## 13. Publish another message using the publisher client
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```sh
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% go run nats-pub.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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```sh
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% cd $GOPATH/src/github.com/nats-io/nats/examples
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```
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## 16. Subscribe to a different message
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```sh
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% go run nats-sub.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. (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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```sh
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% go run nats-sub.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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