List Subjects for a Specific Stream (Go)

List Subjects for a Specific Stream in JetStream

All clients have a way to get the list of subjects for any given stream, except it’s not completely obvious how to do this. These examples will show you how to get the list of subjects.

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$ nbe run jetstream/list-subjects/go

View the source code or learn how to run this example yourself

Code

package main


import (
	"context"
	"fmt"
	"os"
	"time"


	"github.com/nats-io/nats.go"
	"github.com/nats-io/nats.go/jetstream"
)


func main() {

Use the env variable if running in the container, otherwise use the default.

	url := os.Getenv("NATS_URL")
	if url == "" {
		url = nats.DefaultURL
	}

Create an unauthenticated connection to NATS.

	nc, _ := nats.Connect(url)

Drain is a safe way to ensure all buffered messages that were published are sent and all buffered messages received on a subscription are processed being closing the connection.

	defer nc.Drain()

Access JetStream which provides methods to create streams and consumers as well as convenience methods for publishing to streams and consuming messages from the streams.

	js, _ := jetstream.New(nc)

We will declare the initial stream configuration by specifying the name and subjects. Stream names are commonly uppercase to visually differentiate them from subjects, but this is not required. A stream can bind one or more subjects which almost always include wildcards. In addition, no two streams can have overlapping subjects otherwise the primary messages would be persisted twice. There are option to replicate messages in various ways, but that will be explained in later examples.

	cfg := jetstream.StreamConfig{
		Name:     "SUBJECTS",
		Subjects: []string{"plain", "greater.>", "star.*"},
		Storage:  jetstream.MemoryStorage,
	}

JetStream API uses context for timeouts and cancellation.

	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	defer cancel()

Delete the stream, so we always have a fresh start for the example don’t care if this, errors in this example, it will if the stream exists.

	_ = js.DeleteStream(ctx, "SUBJECTS")

Let’s add/create the stream with the default (no) limits.

	stream, _ := js.CreateStream(ctx, cfg)

Publish a message. In a real system we might ensure there is no error.

	js.Publish(ctx, "plain", []byte("plain-data"))

Stream Info contains State, which contains a map, Subjects, of subjects to their count but the server only collects and returns that information if WithSubjectFilter is set with a non-empty subject filter. To get all subjects, set the filter to >

	si, _ := stream.Info(ctx, jetstream.WithSubjectFilter(">"))
	fmt.Println("After publishing a message to a subject, it appears in state.")
	for k, v := range si.State.Subjects {
		fmt.Printf("  subject '%s' has %v message(s)\n", k, v)
	}

Publish some more messages, this time against wildcard subjects

	js.Publish(ctx, "greater.A", []byte("gtA-1"))
	js.Publish(ctx, "greater.A", []byte("gtA-2"))
	js.Publish(ctx, "greater.A.B", []byte("gtAB-1"))
	js.Publish(ctx, "greater.A.B", []byte("gtAB-2"))
	js.Publish(ctx, "greater.A.B.C", []byte("gtABC"))
	js.Publish(ctx, "greater.B.B.B", []byte("gtBBB"))
	js.Publish(ctx, "star.1", []byte("star1-1"))
	js.Publish(ctx, "star.1", []byte("star1-2"))
	js.Publish(ctx, "star.2", []byte("star2"))


	si, _ = stream.Info(ctx, jetstream.WithSubjectFilter(">"))
	fmt.Println("Wildcard subjects show the actual subject, not the template.")
	for k, v := range si.State.Subjects {
		fmt.Printf("  subject '%s' has %v message(s)\n", k, v)
	}

Subject Filtering

Instead of >, you can filter for a specific subject

	si, _ = stream.Info(ctx, jetstream.WithSubjectFilter("greater.>"))
	fmt.Println("Filtering the subject returns only matching entries ['greater.>']")
	for k, v := range si.State.Subjects {
		fmt.Printf("  subject '%s' has %v message(s)\n", k, v)
	}


	si, _ = stream.Info(ctx, jetstream.WithSubjectFilter("greater.A.>"))
	fmt.Println("Filtering the subject returns only matching entries ['greater.A.>']")
	for k, v := range si.State.Subjects {
		fmt.Printf("  subject '%s' has %v message(s)\n", k, v)
	}
}

Output

After publishing a message to a subject, it appears in state.
  subject 'plain' has 1 message(s)
Wildcard subjects show the actual subject, not the template.
  subject 'plain' has 1 message(s)
  subject 'star.1' has 2 message(s)
  subject 'star.2' has 1 message(s)
  subject 'greater.A' has 2 message(s)
  subject 'greater.A.B' has 2 message(s)
  subject 'greater.A.B.C' has 1 message(s)
  subject 'greater.B.B.B' has 1 message(s)
Filtering the subject returns only matching entries ['greater.>']
  subject 'greater.A' has 2 message(s)
  subject 'greater.A.B' has 2 message(s)
  subject 'greater.A.B.C' has 1 message(s)
  subject 'greater.B.B.B' has 1 message(s)
Filtering the subject returns only matching entries ['greater.A.>']
  subject 'greater.A.B' has 2 message(s)
  subject 'greater.A.B.C' has 1 message(s)

Recording

Note, playback is half speed to make it a bit easier to follow.