// Copyright 2020 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and

/*
Package pubsublite provides an easy way to publish and receive messages using
Google Pub/Sub Lite.

Google Pub/Sub services are designed to provide reliable, many-to-many,
asynchronous messaging between applications. Publisher applications can send
messages to a topic and other applications can subscribe to that topic to
receive the messages. By decoupling senders and receivers, Google Pub/Sub allows
developers to communicate between independently written applications.

Compared to Cloud Pub/Sub, Pub/Sub Lite provides partitioned zonal data storage
with predefined throughput and storage capacity. Guidance on how to choose
between Cloud Pub/Sub and Pub/Sub Lite is available at
https://cloud.google.com/pubsub/docs/choosing-pubsub-or-lite.

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

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

Note: This library is in ALPHA. Backwards-incompatible changes may be made
before stable v1.0.0 is released.


Creating Topics

Messages are published to topics. Pub/Sub Lite topics may be created like so:

  const topicPath = "projects/my-project/locations/us-central1-c/topics/my-topic"
  topicConfig := pubsublite.TopicConfig{
    Name:                       topicPath,
    PartitionCount:             1,
    PublishCapacityMiBPerSec:   4,
    SubscribeCapacityMiBPerSec: 4,
    PerPartitionBytes:          30 * 1024 * 1024 * 1024,  // 30 GiB
    RetentionDuration:          pubsublite.InfiniteRetention,
  }
  adminClient, err := pubsublite.NewAdminClient(ctx, "us-central1")
  if err != nil {
    // TODO: Handle error.
  }
  topic, err = adminClient.CreateTopic(ctx, topicConfig)
  if err != nil {
    // TODO: Handle error.
  }

See https://cloud.google.com/pubsub/lite/docs/topics for more information about
how Pub/Sub Lite topics are configured.

See https://cloud.google.com/pubsub/lite/docs/locations for the list of regions
and zones where Pub/Sub Lite is available.


Publishing

The pubsublite/pscompat subpackage contains clients for publishing and receiving
messages, which have similar interfaces to their pubsub.Topic and
pubsub.Subscription counterparts in the Cloud Pub/Sub library:
https://pkg.go.dev/cloud.google.com/go/pubsub.

Pub/Sub Lite uses gRPC streams extensively for high throughput. For more
differences, see https://pkg.go.dev/cloud.google.com/go/pubsublite/pscompat.

To publish messages to a topic, first create a PublisherClient:

  publisher, err := pscompat.NewPublisherClient(ctx, topicPath)
  if err != nil {
    // TODO: Handle error.
  }

Then call Publish:

  result := publisher.Publish(ctx, &pubsub.Message{Data: []byte("payload")})

Publish queues the message for publishing and returns immediately. When enough
messages have accumulated, or enough time has elapsed, the batch of messages is
sent to the Pub/Sub Lite service. Thresholds for batching can be configured in
PublishSettings.

Publish returns a PublishResult, which behaves like a future; its Get method
blocks until the message has been sent (or has failed to be sent) to the
service:

  id, err := result.Get(ctx)
  if err != nil {
    // TODO: Handle error.
  }

Once you've finishing publishing, call Stop to flush all messages to the service
and close gRPC streams:

  publisher.Stop()

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


Creating Subscriptions

To receive messages published to a topic, create a subscription to the topic.
There may be more than one subscription per topic; each message that is
published to the topic will be delivered to all of its subscriptions.

Pub/Sub Lite subscriptions may be created like so:

  const subscriptionPath = "projects/my-project/locations/us-central1-c/subscriptions/my-subscription"
  subscriptionConfig := pubsublite.SubscriptionConfig{
    Name:                subscriptionPath,
    Topic:               topicPath,
    DeliveryRequirement: pubsublite.DeliverImmediately,
  }
  subscription, err = adminClient.CreateSubscription(ctx, subscriptionConfig)
  if err != nil {
    // TODO: Handle error.
  }

See https://cloud.google.com/pubsub/lite/docs/subscriptions for more information
about how subscriptions are configured.


Receiving

To receive messages for a subscription, first create a SubscriberClient:

  subscriber, err := pscompat.NewSubscriberClient(ctx, subscriptionPath)

Messages are then consumed from a subscription via callback.

  cctx, cancel := context.WithCancel(ctx)
  err = subscriber.Receive(cctx, func(ctx context.Context, m *pubsub.Message) {
    log.Printf("Got message: %s", m.Data)
    m.Ack()
  })
  if err != nil {
    // TODO: Handle error.
  }

The callback may be invoked concurrently by multiple goroutines (one per
partition that the subscriber client is connected to). To terminate a call to
Receive, cancel its context:

  cancel()

Clients must call pubsub.Message.Ack() or pubsub.Message.Nack() for every
message received. Pub/Sub Lite does not have ACK deadlines. Pub/Sub Lite also
does not actually have the concept of NACK. The default behavior terminates the
SubscriberClient. 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.

See https://cloud.google.com/pubsub/lite/docs/subscribing for more information
about receiving messages.
*/
package pubsublite // import "cloud.google.com/go/pubsublite"