pscompat

package
v1.8.2 Latest Latest
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 Go to latest Published: Jun 5, 2024 License: Apache-2.0 Imports: 18 Imported by: 10

Documentation

Overview

Package pscompat contains clients for publishing and subscribing using the Pub/Sub Lite service.

This package is designed to compatible with the Cloud Pub/Sub library: https://pkg.go.dev/cloud.google.com/go/pubsub. If interfaces are defined by the client application, PublisherClient and SubscriberClient can be used as substitutions for pubsub.Topic.Publish() and pubsub.Subscription.Receive(), respectively, from the pubsub package. See the following examples: https://pkg.go.dev/cloud.google.com/go/pubsublite/pscompat#example-NewPublisherClient-Interface and https://pkg.go.dev/cloud.google.com/go/pubsublite/pscompat#example-NewSubscriberClient-Interface.

The Cloud Pub/Sub and Pub/Sub Lite services have some differences:

  • Pub/Sub Lite does not support NACK for messages. By default, this will terminate the SubscriberClient. A custom function can be provided for ReceiveSettings.NackHandler to handle NACKed messages.
  • Pub/Sub Lite has no concept of ACK deadlines. Subscribers must ACK or NACK every message received and can take as much time as they need to process the message.
  • Pub/Sub Lite PublisherClients and SubscriberClients can fail permanently when an unretryable error occurs.
  • Publishers and subscribers will be throttled if Pub/Sub Lite publish or subscribe throughput limits are exceeded. Thus publishing can be more sensitive to buffer overflow than Cloud Pub/Sub.
  • Pub/Sub Lite utilizes bidirectional gRPC streams extensively to maximize publish and subscribe throughput.

More information about Pub/Sub Lite is available at https://cloud.google.com/pubsub/lite.

Information about choosing between Cloud Pub/Sub vs Pub/Sub Lite is available at https://cloud.google.com/pubsub/docs/choosing-pubsub-or-lite.

Complete sample programs can be found at https://github.com/GoogleCloudPlatform/golang-samples/tree/master/pubsublite.

See https://pkg.go.dev/cloud.google.com/go for authentication, timeouts, connection pooling and similar aspects of this package.

Index

Examples

Constants

View Source 
const (
	// MaxPublishRequestCount is the maximum number of messages that can be
	// batched in a single publish request.
	MaxPublishRequestCount = wire.MaxPublishRequestCount

	// MaxPublishRequestBytes is the maximum allowed serialized size of a single
	// publish request (containing a batch of messages) in bytes.
	MaxPublishRequestBytes = wire.MaxPublishRequestBytes
)
View Source 
const EventTimeAttributeKey = "x-goog-pubsublite-event-time-timestamp-proto"

EventTimeAttributeKey is the key of the attribute whose value is an encoded Timestamp proto. The value will be used to set the event time property of a Pub/Sub Lite message.

Variables

View Source 
var (
	// ErrOverflow is set for a PublishResult when publish buffers overflow. This
	// can occur when backends are unavailable or the actual publish throughput
	// of clients exceeds the allocated publish throughput for the Pub/Sub Lite
	// topic. Use errors.Is for comparing errors.
	ErrOverflow = wire.ErrOverflow

	// ErrOversizedMessage is set for a PublishResult when a published message
	// exceeds MaxPublishRequestBytes. Publishing this message will never succeed.
	// Use errors.Is for comparing errors.
	ErrOversizedMessage = wire.ErrOversizedMessage

	// ErrPublisherStopped is set for a PublishResult when a message cannot be
	// published because the publisher client has stopped or is in the process of
	// stopping. It may be stopping due to a fatal error. PublisherClient.Error()
	// returns the error that caused the publisher client to terminate (if any).
	// Use errors.Is for comparing errors.
	ErrPublisherStopped = wire.ErrServiceStopped

	// ErrBackendUnavailable indicates that the backend service has been
	// unavailable for a period of time. The timeout can be configured using
	// PublishSettings.Timeout or ReceiveSettings.Timeout. Use errors.Is for
	// comparing errors.
	ErrBackendUnavailable = wire.ErrBackendUnavailable
)
View Source 
var DefaultPublishSettings = PublishSettings{
	DelayThreshold:    10 * time.Millisecond,
	CountThreshold:    100,
	ByteThreshold:     1e6,
	Timeout:           7 * 24 * time.Hour,
	BufferedByteLimit: 1e10,
	EnableIdempotence: true,
}

DefaultPublishSettings holds the default values for PublishSettings.

View Source 
var DefaultReceiveSettings = ReceiveSettings{
	MaxOutstandingMessages: 1000,
	MaxOutstandingBytes:    1e9,
	Timeout:                7 * 24 * time.Hour,
}

DefaultReceiveSettings holds the default values for ReceiveSettings.

Functions

func DecodeEventTimeAttribute added in v1.8.0 

func DecodeEventTimeAttribute(value string) (*tspb.Timestamp, error)

DecodeEventTimeAttribute decodes a timestamp that was encoded with EncodeEventTimeAttribute.

Example 
package main

import (
	"context"
	"fmt"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	subscriber, err := pscompat.NewSubscriberClient(ctx, subscription)
	if err != nil {
		// TODO: Handle error.
	}
	cctx, cancel := context.WithCancel(ctx)
	err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
		m.Ack()
		if v, ok := m.Attributes[pscompat.EventTimeAttributeKey]; ok {
			eventTime, err := pscompat.DecodeEventTimeAttribute(v)
			if err != nil {
				// TODO: Handle error.
			}
			fmt.Printf("Received message with event time: %v\n", eventTime)
		}
	})
	if err != nil {
		// TODO: Handle error.
	}

	// Call cancel from the receiver callback or another goroutine to stop
	// receiving.
	cancel()
}

Output:

func EncodeEventTimeAttribute added in v1.8.0 

func EncodeEventTimeAttribute(eventTime *tspb.Timestamp) (string, error)

EncodeEventTimeAttribute encodes a timestamp in a way that it will be interpreted as an event time if published on a message with an attribute named EventTimeAttributeKey.

Example 
package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
	"google.golang.org/protobuf/types/known/timestamppb"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	publisher, err := pscompat.NewPublisherClient(ctx, topic)
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	v, err := pscompat.EncodeEventTimeAttribute(&timestamppb.Timestamp{
		Seconds: 1672531200,
		Nanos:   500000000,
	})
	if err != nil {
		// TODO: Handle error.
	}

	r := publisher.Publish(ctx, &pubsub.Message{
		Data: []byte("hello world"),
		Attributes: map[string]string{
			pscompat.EventTimeAttributeKey: v,
		},
	})
	_, err = r.Get(ctx)
	if err != nil {
		// TODO: Handle error.
	}
}

Output:

Types

type KeyExtractorFunc 

type KeyExtractorFunc func(*pubsub.Message) []byte

KeyExtractorFunc is a function that extracts an ordering key from a Message.

type MessageMetadata added in v0.7.0 

type MessageMetadata struct {
	// The topic partition the message was published to.
	Partition int

	// The offset the message was assigned.
	//
	// If this MessageMetadata was returned for a publish result and publish
	// idempotence was enabled, the offset may be -1 when the message was
	// identified as a duplicate of an already successfully published message,
	// but the server did not have sufficient information to return the message's
	// offset at publish time. Messages received by subscribers will always have
	// the correct offset.
	Offset int64
}

MessageMetadata holds properties of a message published to the Pub/Sub Lite service.

func ParseMessageMetadata added in v0.7.0 

func ParseMessageMetadata(id string) (*MessageMetadata, error)

ParseMessageMetadata creates MessageMetadata from the ID string of a pubsub.PublishResult returned by PublisherClient or pubsub.Message.ID received from SubscriberClient.

Example (Publisher) 
package main

import (
	"context"
	"fmt"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	publisher, err := pscompat.NewPublisherClient(ctx, topic)
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	result := publisher.Publish(ctx, &pubsub.Message{Data: []byte("payload")})
	id, err := result.Get(ctx)
	if err != nil {
		// TODO: Handle error.
	}
	metadata, err := pscompat.ParseMessageMetadata(id)
	if err != nil {
		// TODO: Handle error.
	}
	fmt.Printf("Published message to partition %d with offset %d\n", metadata.Partition, metadata.Offset)
}

Output:
Example (Subscriber) 
package main

import (
	"context"
	"fmt"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	subscriber, err := pscompat.NewSubscriberClient(ctx, subscription)
	if err != nil {
		// TODO: Handle error.
	}
	err = subscriber.Receive(ctx, func(ctx context.Context, m *pubsub.Message) {
		// TODO: Handle message.
		m.Ack()
		metadata, err := pscompat.ParseMessageMetadata(m.ID)
		if err != nil {
			// TODO: Handle error.
		}
		fmt.Printf("Received message from partition %d with offset %d\n", metadata.Partition, metadata.Offset)
	})
	if err != nil {
		// TODO: Handle error.
	}
}

Output:

func (*MessageMetadata) String added in v0.7.0 

func (m *MessageMetadata) String() string

type NackHandler 

type NackHandler func(*pubsub.Message) error

NackHandler is invoked when pubsub.Message.Nack() is called. Pub/Sub Lite does not have a concept of 'nack'. If the nack handler implementation returns nil, the message is acknowledged. If an error is returned, the SubscriberClient will consider this a fatal error and terminate.

In Pub/Sub Lite, only a single subscriber for a given subscription is connected to any partition at a time, and there is no other client that may be able to handle messages.

type PublishMessageTransformerFunc 

type PublishMessageTransformerFunc func(*pubsub.Message, *pb.PubSubMessage) error

PublishMessageTransformerFunc transforms a pubsub.Message to a Pub/Sub Lite PubSubMessage API proto. If this returns an error, the pubsub.PublishResult will be errored and the PublisherClient will consider this a fatal error and terminate.

type PublishSettings 

type PublishSettings struct {
	// Publish a non-empty batch after this delay has passed. If DelayThreshold is
	// 0, it will be treated as DefaultPublishSettings.DelayThreshold. Otherwise
	// must be > 0.
	DelayThreshold time.Duration

	// Publish a batch when it has this many messages. The maximum is
	// MaxPublishRequestCount. If CountThreshold is 0, it will be treated as
	// DefaultPublishSettings.CountThreshold. Otherwise must be > 0.
	CountThreshold int

	// Publish a batch when its size in bytes reaches this value. The maximum is
	// MaxPublishRequestBytes. If ByteThreshold is 0, it will be treated as
	// DefaultPublishSettings.ByteThreshold. Otherwise must be > 0.
	ByteThreshold int

	// The maximum time that the client will attempt to open a publish stream
	// to the server. If Timeout is 0, it will be treated as
	// DefaultPublishSettings.Timeout, otherwise will be clamped to 2 minutes. In
	// the future, setting Timeout to less than 2 minutes will result in an error.
	//
	// If your application has a low tolerance to backend unavailability, set
	// Timeout to a lower duration to detect and handle. When the timeout is
	// exceeded, the PublisherClient will terminate with ErrBackendUnavailable and
	// details of the last error that occurred while trying to reconnect to
	// backends. Note that if the timeout duration is long, ErrOverflow may occur
	// first.
	//
	// If no failover operations need to be performed by the application, it is
	// recommended to just use the default timeout value to avoid the
	// PublisherClient terminating during short periods of backend unavailability.
	Timeout time.Duration

	// The maximum number of bytes that the publisher will keep in memory before
	// returning ErrOverflow. If BufferedByteLimit is 0, it will be treated as
	// DefaultPublishSettings.BufferedByteLimit. Otherwise must be > 0.
	//
	// Note that this setting applies per partition. If BufferedByteLimit is being
	// used to bound memory usage, keep in mind the number of partitions in the
	// topic.
	//
	// Note that Pub/Sub Lite topics are provisioned a publishing throughput
	// capacity, per partition, shared by all publisher clients. Setting a large
	// buffer size can mitigate transient publish spikes. However, consistently
	// attempting to publish messages at a much higher rate than the publishing
	// throughput capacity can cause the buffers to overflow. For more
	// information, see https://cloud.google.com/pubsub/lite/docs/topics.
	BufferedByteLimit int

	// Whether idempotence is enabled, where the server will ensure that unique
	// messages within a single publisher session are stored only once. Default
	// true.
	EnableIdempotence optional.Bool

	// Optional custom function that extracts an ordering key from a Message. The
	// default implementation extracts the key from Message.OrderingKey.
	KeyExtractor KeyExtractorFunc

	// Optional custom function that transforms a pubsub.Message to a
	// PubSubMessage API proto.
	MessageTransformer PublishMessageTransformerFunc
	// contains filtered or unexported fields
}

PublishSettings configure the PublisherClient. Batching settings (DelayThreshold, CountThreshold, ByteThreshold, BufferedByteLimit) apply per partition.

A zero PublishSettings will result in values equivalent to DefaultPublishSettings.

type PublisherClient 

type PublisherClient struct {
	// contains filtered or unexported fields
}

PublisherClient is a Pub/Sub Lite client to publish messages to a given topic. A PublisherClient is safe to use from multiple goroutines.

PublisherClients are expected to be long-lived and used for the duration of the application, rather than for publishing small batches of messages. Stop must be called to release resources when a PublisherClient is no longer required.

See https://cloud.google.com/pubsub/lite/docs/publishing for more information about publishing.

func NewPublisherClient 

func NewPublisherClient(ctx context.Context, topic string, opts ...option.ClientOption) (*PublisherClient, error)

NewPublisherClient creates a new Pub/Sub Lite publisher client to publish messages to a given topic, using DefaultPublishSettings. A valid topic path has the format: "projects/PROJECT_ID/locations/LOCATION/topics/TOPIC_ID".

Stop must be called to release resources when a PublisherClient is no longer required.

Example (EarlyTokenRefresh)

This example illustrates how to configure OAuth tokens to be refreshed 5 minutes before they expire, in order to mitigate delays which may occur during refresh. 

package main

import (
	"context"
	"fmt"
	"log"
	"time"

	"cloud.google.com/go/pubsub"
	api "cloud.google.com/go/pubsublite/apiv1"
	"cloud.google.com/go/pubsublite/pscompat"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/option"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	params := google.CredentialsParams{
		Scopes:            api.DefaultAuthScopes(),
		EarlyTokenRefresh: 5 * time.Minute,
	}
	creds, err := google.FindDefaultCredentialsWithParams(ctx, params)
	if err != nil {
		log.Fatalf("No 'Application Default Credentials' found: %v.", err)
	}
	publisher, err := pscompat.NewPublisherClient(ctx, topic, option.WithTokenSource(creds.TokenSource))
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	var results []*pubsub.PublishResult
	r := publisher.Publish(ctx, &pubsub.Message{
		Data: []byte("hello world"),
	})
	results = append(results, r)
	// Publish more messages ...

	var publishFailed bool
	for _, r := range results {
		id, err := r.Get(ctx)
		if err != nil {
			// TODO: Handle error.
			publishFailed = true
			continue
		}
		fmt.Printf("Published a message with a message ID: %s\n", id)
	}

	// NOTE: A failed PublishResult indicates that the publisher client
	// encountered a fatal error and has permanently terminated. After the fatal
	// error has been resolved, a new publisher client instance must be created to
	// republish failed messages.
	if publishFailed {
		fmt.Printf("Publisher client terminated due to error: %v\n", publisher.Error())
	}
}

Output:
Example (Interface)

This example illustrates how to declare a common interface for publisher clients from Cloud Pub/Sub (cloud.google.com/go/pubsub) and Pub/Sub Lite (cloud.google.com/go/pubsublite/pscompat). 

package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	// publisherInterface is implemented by both pscompat.PublisherClient and
	// pubsub.Topic.
	type publisherInterface interface {
		Publish(context.Context, *pubsub.Message) *pubsub.PublishResult
		Stop()
	}

	publish := func(publisher publisherInterface) {
		defer publisher.Stop()
		// TODO: Publish messages.
	}

	// Create a Pub/Sub Lite publisher client.
	ctx := context.Background()
	publisher, err := pscompat.NewPublisherClient(ctx, "projects/my-project/locations/region-or-zone/topics/my-topic")
	if err != nil {
		// TODO: Handle error.
	}
	publish(publisher)

	// Create a Cloud Pub/Sub topic to publish.
	client, err := pubsub.NewClient(ctx, "my-project")
	if err != nil {
		// TODO: Handle error.
	}
	topic := client.Topic("my-topic")
	publish(topic)
}

Output:

func NewPublisherClientWithSettings 

func NewPublisherClientWithSettings(ctx context.Context, topic string, settings PublishSettings, opts ...option.ClientOption) (*PublisherClient, error)

NewPublisherClientWithSettings creates a new Pub/Sub Lite publisher client to publish messages to a given topic, using the specified PublishSettings. A valid topic path has the format: "projects/PROJECT_ID/locations/LOCATION/topics/TOPIC_ID".

Stop must be called to release resources when a PublisherClient is no longer required.

Example (BatchingSettings)

This example illustrates how to set batching settings for publishing. Note that batching settings apply per partition. If BufferedByteLimit is being used to bound memory usage, keep in mind the number of partitions in the topic. 

package main

import (
	"context"
	"fmt"
	"time"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	settings := pscompat.PublishSettings{
		DelayThreshold:    50 * time.Millisecond,
		CountThreshold:    200,
		BufferedByteLimit: 5e8,
	}
	publisher, err := pscompat.NewPublisherClientWithSettings(ctx, topic, settings)
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	var results []*pubsub.PublishResult
	r := publisher.Publish(ctx, &pubsub.Message{
		Data: []byte("hello world"),
	})
	results = append(results, r)
	// Publish more messages ...

	var publishFailed bool
	for _, r := range results {
		id, err := r.Get(ctx)
		if err != nil {
			// TODO: Handle error.
			publishFailed = true
			continue
		}
		fmt.Printf("Published a message with a message ID: %s\n", id)
	}

	// NOTE: A failed PublishResult indicates that the publisher client
	// encountered a fatal error and has permanently terminated. After the fatal
	// error has been resolved, a new publisher client instance must be created to
	// republish failed messages.
	if publishFailed {
		fmt.Printf("Publisher client terminated due to error: %v\n", publisher.Error())
	}
}

Output:

func (*PublisherClient) Error 

func (p *PublisherClient) Error() error

Error returns the error that caused the publisher client to terminate. The error returned here may contain more context than PublishResult errors. The return value may be nil if Stop() was called.

func (*PublisherClient) Publish 

func (p *PublisherClient) Publish(ctx context.Context, msg *pubsub.Message) *pubsub.PublishResult

Publish publishes `msg` to the topic asynchronously. Messages are batched and sent according to the client's PublishSettings. Publish never blocks.

Publish returns a non-nil PublishResult which will be ready when the message has been sent (or has failed to be sent) to the server. Retryable errors are automatically handled. If a PublishResult returns an error, this indicates that the publisher client encountered a fatal error and can no longer be used. Fatal errors should be manually inspected and the cause resolved. A new publisher client instance must be created to republish failed messages.

Once Stop() has been called or the publisher client has failed permanently due to an error, future calls to Publish will immediately return a PublishResult with error ErrPublisherStopped.

Error() returns the error that caused the publisher client to terminate and may contain more context than the error returned by PublishResult.

Example 
package main

import (
	"context"
	"fmt"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	publisher, err := pscompat.NewPublisherClient(ctx, topic)
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	var results []*pubsub.PublishResult
	r := publisher.Publish(ctx, &pubsub.Message{
		Data: []byte("hello world"),
	})
	results = append(results, r)
	// Publish more messages ...

	var publishFailed bool
	for _, r := range results {
		id, err := r.Get(ctx)
		if err != nil {
			// TODO: Handle error.
			publishFailed = true
			continue
		}
		fmt.Printf("Published a message with a message ID: %s\n", id)
	}

	// NOTE: A failed PublishResult indicates that the publisher client
	// encountered a fatal error and has permanently terminated. After the fatal
	// error has been resolved, a new publisher client instance must be created to
	// republish failed messages.
	if publishFailed {
		fmt.Printf("Publisher client terminated due to error: %v\n", publisher.Error())
	}
}

Output:
Example (ErrorHandling)

This example illustrates how to handle various publishing errors. Some errors can be automatically handled (e.g. backend unavailable and buffer overflow), while others are fatal errors that should be inspected. If the application has a low tolerance to backend unavailability, set a lower PublishSettings.Timeout value to detect and alert. 

package main

import (
	"context"
	"errors"
	"fmt"
	"sync"
	"time"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
	"golang.org/x/sync/errgroup"
)

func main() {
	ctx := context.Background()
	const topic = "projects/my-project/locations/region-or-zone/topics/my-topic"
	settings := pscompat.PublishSettings{
		// The PublisherClient will terminate when it cannot connect to backends for
		// more than 10 minutes.
		Timeout: 10 * time.Minute,
		// Sets a conservative publish buffer byte limit, per partition.
		BufferedByteLimit: 1e8,
	}
	publisher, err := pscompat.NewPublisherClientWithSettings(ctx, topic, settings)
	if err != nil {
		// TODO: Handle error.
	}
	defer publisher.Stop()

	var toRepublish []*pubsub.Message
	var mu sync.Mutex
	g := new(errgroup.Group)

	for i := 0; i < 10; i++ {
		msg := &pubsub.Message{
			Data: []byte(fmt.Sprintf("message-%d", i)),
		}
		result := publisher.Publish(ctx, msg)

		g.Go(func() error {
			id, err := result.Get(ctx)
			if err != nil {
				// NOTE: A failed PublishResult indicates that the publisher client has
				// permanently terminated. A new publisher client instance must be
				// created to republish failed messages.
				fmt.Printf("Publish error: %v\n", err)
				// Oversized messages cannot be published.
				if !errors.Is(err, pscompat.ErrOversizedMessage) {
					mu.Lock()
					toRepublish = append(toRepublish, msg)
					mu.Unlock()
				}
				return err
			}
			fmt.Printf("Published a message with a message ID: %s\n", id)
			return nil
		})
	}
	if err := g.Wait(); err != nil {
		fmt.Printf("Publisher client terminated due to error: %v\n", publisher.Error())
		switch {
		case errors.Is(publisher.Error(), pscompat.ErrBackendUnavailable):
			// TODO: Create a new publisher client to republish failed messages.
		case errors.Is(publisher.Error(), pscompat.ErrOverflow):
			// TODO: Create a new publisher client to republish failed messages.
			// Throttle publishing. Note that backend unavailability can also cause
			// buffer overflow before the ErrBackendUnavailable error.
		default:
			// TODO: Inspect and handle fatal error.
		}
	}
}

Output:

func (*PublisherClient) Stop 

func (p *PublisherClient) Stop()

Stop sends all remaining published messages and closes publish streams. Returns once all outstanding messages have been sent or have failed to be sent. Stop should be called when the client is no longer required.

type ReassignmentHandlerFunc added in v1.2.0 

type ReassignmentHandlerFunc func(previousPartitions, nextPartitions []int) error

ReassignmentHandlerFunc is called any time a new partition assignment is received from the server. It will be called with both the previous and new partition numbers as decided by the server. Both slices of partition numbers are sorted in ascending order.

When this handler is called, partitions that are being assigned away are stopping and new partitions are starting. Acks and nacks for messages from partitions that are being assigned away will have no effect, but message deliveries may still be in flight.

The client library will not acknowledge the assignment until this handler returns. The server will not assign any of the partitions in `previousPartitions` to another client unless the assignment is acknowledged, or a client takes too long to acknowledge (currently 30 seconds from the time the assignment is sent from server's point of view).

Because of the above, as long as reassignment handling is processed quickly, it can be used to abort outstanding operations on partitions which are being assigned away from this client.

If this handler returns an error, the SubscriberClient will consider this a fatal error and terminate.

type ReceiveMessageTransformerFunc 

type ReceiveMessageTransformerFunc func(*pb.SequencedMessage, *pubsub.Message) error

ReceiveMessageTransformerFunc transforms a Pub/Sub Lite SequencedMessage API proto to a pubsub.Message. The implementation must not set pubsub.Message.ID.

If this returns an error, the SubscriberClient will consider this a fatal error and terminate.

type ReceiveSettings 

type ReceiveSettings struct {
	// MaxOutstandingMessages is the maximum number of unacknowledged messages.
	// If MaxOutstandingMessages is 0, it will be treated as
	// DefaultReceiveSettings.MaxOutstandingMessages. Otherwise must be > 0.
	MaxOutstandingMessages int

	// MaxOutstandingBytes is the maximum size (in quota bytes) of unacknowledged
	// messages. If MaxOutstandingBytes is 0, it will be treated as
	// DefaultReceiveSettings.MaxOutstandingBytes. Otherwise must be > 0.
	//
	// Note that this setting applies per partition. If MaxOutstandingBytes is
	// being used to bound memory usage, keep in mind the number of partitions in
	// the associated topic.
	MaxOutstandingBytes int

	// The maximum time that the client will attempt to open a subscribe stream
	// to the server. If Timeout is 0, it will be treated as
	// DefaultReceiveSettings.Timeout, otherwise will be clamped to 2 minutes. In
	// the future, setting Timeout to less than 2 minutes will result in an error.
	//
	// If your application has a low tolerance to backend unavailability, set
	// Timeout to a lower duration to detect and handle. When the timeout is
	// exceeded, the SubscriberClient will terminate with ErrBackendUnavailable
	// and details of the last error that occurred while trying to reconnect to
	// backends.
	//
	// If no failover operations need to be performed by the application, it is
	// recommended to just use the default timeout value to avoid the
	// SubscriberClient terminating during short periods of backend
	// unavailability.
	Timeout time.Duration

	// The topic partition numbers (zero-indexed) to receive messages from.
	// Values must be less than the number of partitions for the topic. If not
	// specified, the SubscriberClient will use the partition assignment service
	// to determine which partitions it should connect to.
	Partitions []int

	// Optional custom function to handle pubsub.Message.Nack() calls. If not set,
	// the default behavior is to terminate the SubscriberClient.
	NackHandler NackHandler

	// Optional custom function that transforms a SequencedMessage API proto to a
	// pubsub.Message.
	MessageTransformer ReceiveMessageTransformerFunc

	// Optional custom function that is called when a new partition assignment has
	// been delivered to the client.
	ReassignmentHandler ReassignmentHandlerFunc
}

ReceiveSettings configure the SubscriberClient. Flow control settings (MaxOutstandingMessages, MaxOutstandingBytes) apply per partition.

A zero ReceiveSettings will result in values equivalent to DefaultReceiveSettings.

type SubscriberClient 

type SubscriberClient struct {
	// contains filtered or unexported fields
}

SubscriberClient is a Pub/Sub Lite client to receive messages for a given subscription.

See https://cloud.google.com/pubsub/lite/docs/subscribing for more information about receiving messages.

func NewSubscriberClient 

func NewSubscriberClient(ctx context.Context, subscription string, opts ...option.ClientOption) (*SubscriberClient, error)

NewSubscriberClient creates a new Pub/Sub Lite client to receive messages for a given subscription, using DefaultReceiveSettings. A valid subscription path has the format: "projects/PROJECT_ID/locations/LOCATION/subscriptions/SUBSCRIPTION_ID".

Example (Interface)

This example illustrates how to declare a common interface for subscriber clients from Cloud Pub/Sub (cloud.google.com/go/pubsub) and Pub/Sub Lite (cloud.google.com/go/pubsublite/pscompat). 

package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	// subscriberInterface is implemented by both pscompat.SubscriberClient and
	// pubsub.Subscription.
	type subscriberInterface interface {
		Receive(context.Context, func(context.Context, *pubsub.Message)) error
	}

	receive := func(subscriber subscriberInterface) {
		// TODO: Receive messages.
	}

	// Create a Pub/Sub Lite subscriber client.
	ctx := context.Background()
	subscriber, err := pscompat.NewSubscriberClient(ctx, "projects/my-project/locations/region-or-zone/subscriptions/my-subscription")
	if err != nil {
		// TODO: Handle error.
	}
	receive(subscriber)

	// Create a Cloud Pub/Sub subscription to receive.
	client, err := pubsub.NewClient(ctx, "my-project")
	if err != nil {
		// TODO: Handle error.
	}
	subscription := client.Subscription("my-subscription")
	receive(subscription)
}

Output:

func NewSubscriberClientWithSettings 

func NewSubscriberClientWithSettings(ctx context.Context, subscription string, settings ReceiveSettings, opts ...option.ClientOption) (*SubscriberClient, error)

NewSubscriberClientWithSettings creates a new Pub/Sub Lite client to receive messages for a given subscription, using the specified ReceiveSettings. A valid subscription path has the format: "projects/PROJECT_ID/locations/LOCATION/subscriptions/SUBSCRIPTION_ID".

Example (ManualPartitionAssignment)

This example shows how to manually assign which topic partitions a SubscriberClient should connect to. If not specified, the SubscriberClient will use Pub/Sub Lite's partition assignment service to automatically determine which partitions it should connect to. 

package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	settings := pscompat.ReceiveSettings{
		// NOTE: The corresponding topic must have 2 or more partitions.
		Partitions: []int{0, 1},
	}
	subscriber, err := pscompat.NewSubscriberClientWithSettings(ctx, subscription, settings)
	if err != nil {
		// TODO: Handle error.
	}
	cctx, cancel := context.WithCancel(ctx)
	err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
		// TODO: Handle message.
		// NOTE: May be called concurrently; synchronize access to shared memory.
		m.Ack()
	})
	if err != nil {
		// TODO: Handle error.
	}

	// Call cancel from the receiver callback or another goroutine to stop
	// receiving.
	cancel()
}

Output:
Example (MaxOutstanding)

This example shows how to throttle SubscriberClient.Receive, which aims for high throughput by default. By limiting the number of messages and/or bytes being processed at once, you can bound your program's resource consumption. Note that ReceiveSettings apply per partition, so keep in mind the number of partitions in the associated topic. 

package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	settings := pscompat.ReceiveSettings{
		MaxOutstandingMessages: 5,
		MaxOutstandingBytes:    10e6,
	}
	subscriber, err := pscompat.NewSubscriberClientWithSettings(ctx, subscription, settings)
	if err != nil {
		// TODO: Handle error.
	}
	cctx, cancel := context.WithCancel(ctx)
	err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
		// TODO: Handle message.
		// NOTE: May be called concurrently; synchronize access to shared memory.
		m.Ack()
	})
	if err != nil {
		// TODO: Handle error.
	}

	// Call cancel from the receiver callback or another goroutine to stop
	// receiving.
	cancel()
}

Output:

func (*SubscriberClient) Receive 

func (s *SubscriberClient) Receive(ctx context.Context, f func(context.Context, *pubsub.Message)) error

Receive calls f with the messages from the subscription. It blocks until ctx is done, or the service returns a non-retryable error.

The standard way to terminate a Receive is to cancel its context: 

cctx, cancel := context.WithCancel(ctx)
err := sub.Receive(cctx, callback)
// Call cancel from callback, or another goroutine.

If there is a fatal service error, Receive returns that error after all of the outstanding calls to f have returned. If ctx is done, Receive returns nil after all of the outstanding calls to f have returned and all messages have been acknowledged. The context passed to f will be canceled when ctx is Done or there is a fatal service error.

Receive calls f concurrently from multiple goroutines if the SubscriberClient is connected to multiple partitions. Only one call from any connected partition will be outstanding at a time, and blocking in the receiver callback f will block the delivery of subsequent messages for the partition.

All messages received by f must be ACKed or NACKed. Failure to do so can prevent Receive from returning. Messages may be processed by the client concurrently and ACKed asynchronously to increase throughput.

Each SubscriberClient may have only one invocation of Receive active at a time.

Example 
package main

import (
	"context"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	subscriber, err := pscompat.NewSubscriberClient(ctx, subscription)
	if err != nil {
		// TODO: Handle error.
	}
	cctx, cancel := context.WithCancel(ctx)
	err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
		// TODO: Handle message.
		// NOTE: May be called concurrently; synchronize access to shared memory.
		m.Ack()
	})
	if err != nil {
		// TODO: Handle error.
	}

	// Call cancel from the receiver callback or another goroutine to stop
	// receiving.
	cancel()
}

Output:
Example (ErrorHandling)

If the application has a low tolerance to backend unavailability, set a lower ReceiveSettings.Timeout value to detect and alert. 

package main

import (
	"context"
	"errors"
	"fmt"
	"time"

	"cloud.google.com/go/pubsub"
	"cloud.google.com/go/pubsublite/pscompat"
)

func main() {
	ctx := context.Background()
	const subscription = "projects/my-project/locations/region-or-zone/subscriptions/my-subscription"
	settings := pscompat.ReceiveSettings{
		// The SubscriberClient will terminate when it cannot connect to backends
		// for more than 5 minutes.
		Timeout: 5 * time.Minute,
	}
	subscriber, err := pscompat.NewSubscriberClientWithSettings(ctx, subscription, settings)
	if err != nil {
		// TODO: Handle error.
	}

	for {
		cctx, cancel := context.WithCancel(ctx)
		err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
			// TODO: Handle message.
			// NOTE: May be called concurrently; synchronize access to shared memory.
			m.Ack()
		})
		if err != nil {
			cancel()
			fmt.Printf("Subscriber client stopped receiving due to error: %v\n", err)
			if errors.Is(err, pscompat.ErrBackendUnavailable) {
				// TODO: Alert if necessary. Receive can be retried.
			} else {
				// TODO: Handle fatal error.
				break
			}
		}

		// Call cancel from the receiver callback or another goroutine to stop
		// receiving.
		cancel()
	}
}

Output:

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