NATS Logo by Example

Pull Consumer - Applying Limits in JetStream

When configuring a pull consumer, there are a few limits that can be set that are enforced server-side. There are several options including:

  • the maximum number of non-acked in-flight messages
  • the maximum number of in-flight pull requests (across subscribers)
  • the maximum pull batch size for any given request
  • the maximum timeout on a given pull request
  • the maximum total size (in bytes) of a pulled batch

In addition, when a pull request (fetch) is made, both the batch size and the timeout can be specified (and changed) per request and across different subscribers if desired.

CLI Go Python Deno Node Rust C# Java Ruby Elixir C
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$ nbe run jetstream/pull-consumer-limits/go
View the source code or learn how to run this example yourself

Code

package main


import (
	"fmt"
	"os"
	"sync"
	"time"


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


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)
	defer nc.Drain()

Access the JetStreamContext for managing streams and consumers as well as for publishing and subscription convenience methods.

	js, _ := nc.JetStream()

Declare a simple limits-based stream.

	streamName := "EVENTS"


	js.AddStream(&nats.StreamConfig{
		Name:     streamName,
		Subjects: []string{"events.>"},
	})

Define a basic pull consumer without any limits and a short ack wait time for the purpose of this example. These default options will be reused when we update the consumer to show-case various limits. If you haven’t seen the first pull consumer example yet, check that out first!

	consumerName := "processor"
	ackWait := 10 * time.Second
	ackPolicy := nats.AckExplicitPolicy

One quick note. This example show cases how consumer configuration can be changed on-demand. This one exception is MaxWaiting which cannot be updated on a consumer as of now. This must be set up front when the consumer is created.

	js.AddConsumer(streamName, &nats.ConsumerConfig{
		Durable:    consumerName,
		AckPolicy:  ackPolicy,
		AckWait:    ackWait,
		MaxWaiting: 1,
	})

Bind a subscription to the consumer.

	sub, _ := js.PullSubscribe("", consumerName, nats.Bind(streamName, consumerName))

Max in-flight messages

The first limit to explore is the max in-flight messages. This will limit how many un-acked in-flight messages there are across all subscriptions bound to this consumer. We can update the consumer config on-the-fly with the MaxAckPending setting.

	fmt.Println("--- max in-flight messages (n=1) ---")


	js.UpdateConsumer(streamName, &nats.ConsumerConfig{
		Durable:       consumerName,
		AckPolicy:     ackPolicy,
		AckWait:       ackWait,
		MaxAckPending: 1,
	})

Let’s publish a couple events for this section.

	js.Publish("events.1", nil)
	js.Publish("events.2", nil)

We can request a larger batch size, but we will only get one back since only one can be un-acked at any given time. This essentially forces serial processing messages for a pull consumer.

	msgs, _ := sub.Fetch(3)
	fmt.Printf("requested 3, got %d\n", len(msgs))

This limit becomes more apparent with the second fetch which would timeout since we haven’t acked the previous one yet.

	_, err := sub.Fetch(1, nats.MaxWait(time.Second))
	fmt.Printf("%s\n", err)

Let’s ack it and then try another fetch.

	msgs[0].Ack()

It works this time!

	msgs, _ = sub.Fetch(1)
	fmt.Printf("requested 1, got %d\n", len(msgs))
	msgs[0].Ack()

Max fetch batch size

This one limits the max batch size any one fetch can receive. This can be used to keep the fetches to a reasonable size.

	fmt.Println("\n--- max fetch batch size (n=2) ---")


	js.UpdateConsumer(streamName, &nats.ConsumerConfig{
		Durable:         consumerName,
		AckPolicy:       ackPolicy,
		AckWait:         ackWait,
		MaxRequestBatch: 2,
	})

Publish a couple events for this section…

	js.Publish("events.1", nil)
	js.Publish("events.2", nil)

If a batch size is larger than the limit, it is considered an error.

	_, err = sub.Fetch(10)
	fmt.Printf("%s\n", err)

Using the max batch size (or less) will, of course, work.

	msgs, _ = sub.Fetch(2)
	fmt.Printf("requested 2, got %d\n", len(msgs))


	msgs[0].Ack()
	msgs[1].Ack()

Max waiting requests

The next limit defines the maximum number of fetch requests that are all waiting in parallel to receive messages. This prevents building up too many requests that the server will have to distribute to for a given consumer.

	fmt.Println("\n--- max waiting requests (n=1) ---")

Since MaxWaiting was already set to 1 when the consumer was created, this is a no-op.

	js.UpdateConsumer(streamName, &nats.ConsumerConfig{
		Durable:    consumerName,
		AckPolicy:  ackPolicy,
		AckWait:    ackWait,
		MaxWaiting: 1,
	})


	fmt.Printf("is valid? %v\n", sub.IsValid())

Since Fetch is blocking, we will put these in a few goroutines to simulate the behavior. There are no more messages in the stream so we will expect two timeouts and one with the max waiting error since we are allowing at most two requests to be waiting.

	wg := &sync.WaitGroup{}
	wg.Add(3)


	go func() {
		_, err := sub.Fetch(1, nats.MaxWait(time.Second))
		fmt.Printf("fetch 1: %s\n", err)
		wg.Done()
	}()


	go func() {
		_, err := sub.Fetch(1, nats.MaxWait(time.Second))
		fmt.Printf("fetch 2: %s\n", err)
		wg.Done()
	}()


	go func() {
		_, err := sub.Fetch(1, nats.MaxWait(time.Second))
		fmt.Printf("fetch 3: %s\n", err)
		wg.Done()
	}()


	wg.Wait()

Max fetch timeout

Normally each fetch call can specify it’s own max wait timeout, i.e. how long the client wants to wait to receive at least one message. It may be desirable to limit defined on the consumer to prevent requests waiting too long for messages.

	fmt.Println("\n--- max fetch timeout (d=1s) ---")


	js.UpdateConsumer(streamName, &nats.ConsumerConfig{
		Durable:           consumerName,
		AckPolicy:         ackPolicy,
		AckWait:           ackWait,
		MaxRequestExpires: time.Second,
	})

Using a max wait equal or less than MaxRequestExpires will result in the expected timeout since there are no messages currently.

	t0 := time.Now()
	_, err = sub.Fetch(1, nats.MaxWait(time.Second))
	fmt.Printf("timeout occured? %v in %s\n", err == nats.ErrTimeout, time.Since(t0))

However, trying to use a longer timeout will return immediately with an error.

	t0 = time.Now()
	_, err = sub.Fetch(1, nats.MaxWait(5*time.Second))
	fmt.Printf("%s in %s\n", err, time.Since(t0))

Max total bytes per fetch

This is not yet implememented in the Go client..

}

Output

Network cb225c3a_default  Creating
Network cb225c3a_default  Created
Container cb225c3a-nats-1  Creating
Container cb225c3a-nats-1  Created
Container cb225c3a-nats-1  Starting
Container cb225c3a-nats-1  Started
--- max in-flight messages (n=1) ---
requested 3, got 1
nats: timeout
requested 1, got 1

--- max fetch batch size (n=2) ---
nats: Exceeded MaxRequestBatch of 2
requested 2, got 2

--- max waiting requests (n=1) ---
is valid? true
fetch 1: nats: Exceeded MaxWaiting
fetch 3: nats: Exceeded MaxWaiting
fetch 2: nats: timeout

--- max fetch timeout (d=1s) ---
timeout occured? true in 1.000437048s
nats: Exceeded MaxRequestExpires of 1s in 339.805┬Ás

Recording

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