Interest-based Stream (JavaScript)

Interest-based Stream in JetStream

As the name suggests, the interest retention policy for a stream retains messages for as long as there are consumers which have interest in a particular message.

The base case is where there are no consumers for the streams and messages are being appended. What happens to those messages? By definition, they are immediately deleted from the stream since there are no consumers.

An interest-based stream provides a middle ground between an at-most-once quality of service (QoS) that core NATS provides, requiring all subscribers to be connected to receive a message, and a pure limits-based stream. As long as there are consumers for the stream whose subject filter overlaps with a message appended to a stream, the message won’t be deleted until a subscription bound to each consumer has successfully acks the message, terminates it, or the max redelivery has been reached.

Note that this retention policy is additive to any limits set on the stream. As a contrived example, if max-msgs is set to one with old messages being discarded, every new message that is received by the stream will result in the prior message being deleted regardless if any of the consumer subscriptions were available to process the message.

In this example, we will walk through the interest-based retention behaviors in code. If you are new to streams, it is recommended to read the limits-based stream example prior to reading this one. Alternatively, if you are in need of a stream behaving as a queue, check out the work-queue stream.

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$ nbe run jetstream/interest-stream/deno

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

Code

import the library - in node.js import {connect, etc} from "nats"; or if not doing a module, const {connect, etc} = require("nats");

import {
  AckPolicy,
  connect,
  RetentionPolicy,
} from "https://deno.land/x/nats@v1.16.0/src/mod.ts";

Get the passed NATS_URL or fallback to the default. This can be a comma-separated string.

const servers = Deno.env.get("NATS_URL") || "nats://localhost:4222";

Create a client connection to an available NATS server.

const nc = await connect({
  servers: servers.split(","),
});

access JetStream

const js = nc.jetstream();

CRUD operations in jetstream are performed by the JetStreamManager:

const jsm = await js.jetstreamManager();

Creating the stream

Define the stream configuration, specifying RetentionPolicy.Interest for retention, and create the stream.

await jsm.streams.add({
  name: "EVENTS",
  retention: RetentionPolicy.Interest,
  subjects: ["events.>"],
});
console.log("created the stream");

To demonstrate the base case behavior of the stream without any consumers, we will publish a few messages to the stream.

await js.publish("events.page_loaded");
await js.publish("events.mouse_clicked");
let ack = await js.publish("events.input_focused");
console.log("published 3 messages");

We confirm that all three messages were published and the last message sequence is 3.

console.log("last message seq: ", ack.seq);

Checking out the stream info, notice how zero messages are present in the stream, but the last_seq is 3 which matches the last ack’ed publish sequence above. Also notice that the first_seq is one greater which behaves as a sentinel value indicating the stream is empty. This sequence has not been assigned to a message yet, but can be interpreted as no messages available in this context.

console.log("# Stream info without any consumers");
console.log((await jsm.streams.info("EVENTS")).state);

Adding a consumer

Now let’s add a consumer and publish a few more messages

await jsm.consumers.add("EVENTS", {
  durable_name: "processor-1",
  ack_policy: AckPolicy.Explicit,
});


await js.publish("events.mouse_clicked");
await js.publish("events.input_focused");

If we inspect the stream info again, we will notice a few differences. It shows two messages (which we expect) and the first and last sequences corresponding to the two messages we just published. We also see that the consumer_count is now one.

console.log("# Stream info with one consumer");
console.log((await jsm.streams.info("EVENTS")).state);

Now that the consumer is there and showing interest in the messages, we know they will remain until we process the messages. Let’s create a Consumer and process the messages - note we are accessing JetStream, not JetStreamManager here

const c = await js.consumers.get("EVENTS", "processor-1");
let iter = await c.fetch({ max_messages: 2, expires: 1000 });
await (async () => {
  for await (const m of iter) {
    m.ack();
  }
})();

What do we expect in the stream? No messages and the first_seq has been set to the next sequence number like in the base case.

console.log("# Stream info with one consumer and acked messages");
console.log((await jsm.streams.info("EVENTS")).state);

Two or more consumers

Since each consumer represents a separate view over a stream, we would expect that if messages were processed by one consumer, but not the other, the messages would be retained. This is indeed the case.

await jsm.consumers.add("EVENTS", {
  durable_name: "processor-2",
  ack_policy: AckPolicy.Explicit,
});


await js.publish("events.input_focused");
await js.publish("events.mouse_clicked");

Here we get the second consumer processor-2, followed by a fetch and ack. There are two observations to make here. First the fetched messages are the latest two messages that were published just above and not any prior messages since these were already deleted from the stream. This should be apparent now, but this reinforces that a late consumer cannot retroactively show interest. The second point is that the stream info shows that the latest two messages are still present in the stream. This is also expected since the first consumer had not yet processed them.

const c2 = await js.consumers.get("EVENTS", "processor-2");
iter = await c2.fetch({ max_messages: 2, expires: 1000 });
await (async () => {
  for await (const m of iter) {
    console.log(`msg stream seq: ${m.info.streamSequence}`);
    m.ack();
  }
})();


console.log(
  "# Stream info with two consumers, but only one set of acked messages",
);
console.log((await jsm.streams.info("EVENTS")).state);

Fetching and ack’ing from the first consumer subscription will result in the messages being deleted.

iter = await c.fetch({ max_messages: 2, expires: 1000 });
await (async () => {
  for await (const m of iter) {
    m.ack();
  }
})();


console.log("# Stream info with two consumers having both acked");
console.log((await jsm.streams.info("EVENTS")).state);

A final callout is that interest respects the FilterSubject on a consumer. For example, if a consumer defines a filter only for events.mouse_clicked events then it won’t be considered interested in events such as events.input_focused.

await jsm.consumers.add("EVENTS", {
  durable_name: "processor-3",
  ack_policy: AckPolicy.Explicit,
  filter_subject: "events.mouse_clicked",
});


await js.publish("events.input_focused");

Retrieve and term (also works) and ack from the first consumers that do have interest.

let m = await c.next({ expires: 1000 });
m.term();


m = await c2.next({ expires: 1000 });
m.ack();


console.log("# Stream info with three consumers with interest from two");
console.log((await jsm.streams.info("EVENTS")).state);


await nc.drain();

Output

[1A[1B[0G[?25l[+] Building 0.0s (0/0)                                                                                                                                                                                                             
[?25h[1A[1B[0G[?25l[+] Building 0.0s (0/0)                                                                                                                                                                                                             
[?25hcreated the stream
published 3 messages
last message seq:  3
# Stream info without any consumers
{
  messages: 0,
  bytes: 0,
  first_seq: 4,
  first_ts: "2023-08-24T22:02:32.622546225Z",
  last_seq: 3,
  last_ts: "2023-08-24T22:02:32.622546225Z",
  consumer_count: 0
}
# Stream info with one consumer
{
  messages: 2,
  bytes: 100,
  first_seq: 4,
  first_ts: "2023-08-24T22:02:32.626834616Z",
  last_seq: 5,
  last_ts: "2023-08-24T22:02:32.627634468Z",
  num_subjects: 2,
  consumer_count: 1
}
# Stream info with one consumer and acked messages
{
  messages: 1,
  bytes: 50,
  first_seq: 5,
  first_ts: "2023-08-24T22:02:32.627634468Z",
  last_seq: 5,
  last_ts: "2023-08-24T22:02:32.627634468Z",
  num_subjects: 1,
  consumer_count: 1
}
msg stream seq: 6
msg stream seq: 7
# Stream info with two consumers, but only one set of acked messages
{
  messages: 2,
  bytes: 100,
  first_seq: 6,
  first_ts: "2023-08-24T22:02:32.677448443Z",
  last_seq: 7,
  last_ts: "2023-08-24T22:02:32.677936731Z",
  num_subjects: 2,
  consumer_count: 2
}
# Stream info with two consumers having both acked
{
  messages: 1,
  bytes: 50,
  first_seq: 7,
  first_ts: "2023-08-24T22:02:32.677936731Z",
  last_seq: 7,
  last_ts: "2023-08-24T22:02:32.677936731Z",
  num_subjects: 1,
  consumer_count: 2
}
# Stream info with three consumers with interest from two
{
  messages: 1,
  bytes: 50,
  first_seq: 8,
  first_ts: "2023-08-24T22:02:32.765346449Z",
  last_seq: 8,
  last_ts: "2023-08-24T22:02:32.765346449Z",
  num_subjects: 1,
  consumer_count: 3
}

Recording

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