examples/pubsub/pubsub.proto

// This file will be moved to a new location.

// Copyright 2015, Google Inc.
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
//     * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.


// Specification of the Pubsub API.

syntax = "proto2";

import "examples/pubsub/empty.proto";
import "examples/pubsub/label.proto";

package tech.pubsub;

// -----------------------------------------------------------------------------
// Overview of the Pubsub API
// -----------------------------------------------------------------------------

// This file describes an API for a Pubsub system.  This system provides a
// reliable many-to-many communication mechanism between independently written
// publishers and subscribers where the publisher publishes messages to "topics"
// and each subscriber creates a "subscription" and consumes messages from it.
//
// (a) The pubsub system maintains bindings between topics and subscriptions.
// (b) A publisher publishes messages into a topic.
// (c) The pubsub system delivers messages from topics into relevant
//     subscriptions.
// (d) A subscriber receives pending messages from its subscription and
//     acknowledges or nacks each one to the pubsub system.
// (e) The pubsub system removes acknowledged messages from that subscription.

// -----------------------------------------------------------------------------
// Data Model
// -----------------------------------------------------------------------------

// The data model consists of the following:
//
// * Topic: A topic is a resource to which messages are published by publishers.
//     Topics are named, and the name of the topic is unique within the pubsub
//     system.
//
// * Subscription: A subscription records the subscriber's interest in a topic.
//     It can optionally include a query to select a subset of interesting
//     messages.  The pubsub system maintains a logical cursor tracking the
//     matching messages which still need to be delivered and acked so that
//     they can retried as needed.  The set of messages that have not been
//     acknowledged is called the subscription backlog.
//
// * Message: A message is a unit of data that flows in the system.  It contains
//     opaque data from the publisher along with its labels.
//
// * Message Labels (optional): A set of opaque key, value pairs assigned
//     by the publisher which the subscriber can use for filtering out messages
//     in the topic.  For example, a label with key "foo.com/device_type" and
//     value "mobile" may be added for messages that are only relevant for a
//     mobile subscriber; a subscriber on a phone may decide to create a
//     subscription only for messages that have this label.

// -----------------------------------------------------------------------------
// Publisher Flow
// -----------------------------------------------------------------------------

// A publisher publishes messages to the topic using the Publish request:
//
//   PubsubMessage message;
//   message.set_data("....");
//   Label label;
//   label.set_key("foo.com/key1");
//   label.set_str_value("value1");
//   message.add_label(label);
//   PublishRequest request;
//   request.set_topic("topicName");
//   request.set_message(message);
//   PublisherService.Publish(request);

// -----------------------------------------------------------------------------
// Subscriber Flow
// -----------------------------------------------------------------------------

// The subscriber part of the API is richer than the publisher part and has a
// number of concepts w.r.t. subscription creation and monitoring:
//
// (1) A subscriber creates a subscription using the CreateSubscription call.
//     It may specify an optional "query" to indicate that it wants to receive
//     only messages with a certain set of labels using the label query syntax.
//     It may also specify an optional truncation policy to indicate when old
//     messages from the subcription can be removed.
//
// (2) A subscriber receives messages in one of two ways: via push or pull.
//
// (a) To receive messages via push, the PushConfig field must be specified in
//     the Subscription parameter when creating a subscription.  The PushConfig
//     specifies an endpoint at which the subscriber must expose the
//     PushEndpointService.  Messages are received via the HandlePubsubEvent
//     method.  The push subscriber responds to the HandlePubsubEvent method
//     with a result code that indicates one of three things: Ack (the message
//     has been successfully processed and the Pubsub system may delete it),
//     Nack (the message has been rejected, the Pubsub system should resend it
//     at a later time), or Push-Back (this is a Nack with the additional
//     semantics that the subscriber is overloaded and the pubsub system should
//     back off on the rate at which it is invoking HandlePubsubEvent).  The
//     endpoint may be a load balancer for better scalability.
//
// (b) To receive messages via pull a subscriber calls the Pull method on the
//     SubscriberService to get messages from the subscription.  For each
//     individual message, the subscriber may use the ack_id received in the
//     PullResponse to Ack the message, Nack the message, or modify the ack
//     deadline with ModifyAckDeadline.  See the
//     Subscription.ack_deadline_seconds field documentation for details on the
//     ack deadline behavior.
//
//     Note: Messages may be consumed in parallel by multiple subscribers making
//       Pull calls to the same subscription; this will result in the set of
//       messages from the subscription being shared and each subscriber
//       receiving a subset of the messages.
//
// (4) The subscriber can explicitly truncate the current subscription.
//
// (5) "Truncated" events are delivered when a subscription is
//     truncated, whether due to the subscription's truncation policy
//     or an explicit request from the subscriber.
//
// Subscription creation:
//
//   Subscription subscription;
//   subscription.set_topic("topicName");
//   subscription.set_name("subscriptionName");
//   subscription.push_config().set_push_endpoint("machinename:8888");
//   SubscriberService.CreateSubscription(subscription);
//
// Consuming messages via push:
//
//  The port 'machinename:8888' must be bound to a server that implements
//  the PushEndpointService with the following method:
//
//   int HandlePubsubEvent(PubsubEvent event) {
//     if (event.subscription().equals("subscriptionName")) {
//       if (event.has_message()) {
//         Process(event.message().data());
//       } else if (event.truncated()) {
//         ProcessTruncatedEvent();
//       }
//     }
//     return OK;  // This return code implies an acknowledgment
//   }
//
// Consuming messages via pull:
//
//  The subscription must be created without setting the push_config field.
//
//   PullRequest pull_request;
//   pull_request.set_subscription("subscriptionName");
//   pull_request.set_return_immediately(false);
//   while (true) {
//     PullResponse pull_response;
//     if (SubscriberService.Pull(pull_request, pull_response) == OK) {
//       PubsubEvent event = pull_response.pubsub_event();
//       if (event.has_message()) {
//         Process(event.message().data());
//       } else if (event.truncated()) {
//         ProcessTruncatedEvent();
//       }
//       AcknowledgeRequest ack_request;
//       ackRequest.set_subscription("subscriptionName");
//       ackRequest.set_ack_id(pull_response.ack_id());
//       SubscriberService.Acknowledge(ack_request);
//     }
//   }

// -----------------------------------------------------------------------------
// Reliability Semantics
// -----------------------------------------------------------------------------

// When a subscriber successfully creates a subscription using
// Subscriber.CreateSubscription, it establishes a "subscription point" with
// respect to that subscription - the subscriber is guaranteed to receive any
// message published after this subscription point that matches the
// subscription's query.  Note that messages published before the Subscription
// point may or may not be delivered.
//
// If the system truncates the subscription according to the specified
// truncation policy, the system delivers a subscription status event with the
// "truncated" field set to true.  We refer to such events as "truncation
// events".  A truncation event:
//
// * Informs the subscriber that part of the subscription messages have been
//   discarded.  The subscriber may want to recover from the message loss, e.g.,
//   by resyncing its state with its backend.
// * Establishes a new subscription point, i.e., the subscriber is guaranteed to
//   receive all changes published after the trunction event is received (or
//   until another truncation event is received).
//
// Note that messages are not delivered in any particular order by the pubsub
// system.  Furthermore, the system guarantees at-least-once delivery
// of each message or truncation events until acked.

// -----------------------------------------------------------------------------
// Deletion
// -----------------------------------------------------------------------------

// Both topics and subscriptions may be deleted.  Deletion of a topic implies
// deletion of all attached subscriptions.
//
// When a subscription is deleted directly by calling DeleteSubscription, all
// messages are immediately dropped.  If it is a pull subscriber, future pull
// requests will return NOT_FOUND.
//
// When a topic is deleted all corresponding subscriptions are immediately
// deleted, and subscribers experience the same behavior as directly deleting
// the subscription.

// -----------------------------------------------------------------------------
// The Publisher service and its protos.
// -----------------------------------------------------------------------------

// The service that an application uses to manipulate topics, and to send
// messages to a topic.
service PublisherService {

  // Creates the given topic with the given name.
  rpc CreateTopic(Topic) returns (Topic) {
  }

  // Adds a message to the topic.  Returns NOT_FOUND if the topic does not
  // exist.
  rpc Publish(PublishRequest) returns (proto2.Empty) {
  }

  // Adds one or more messages to the topic. Returns NOT_FOUND if the topic does
  // not exist.
  rpc PublishBatch(PublishBatchRequest) returns (PublishBatchResponse) {
  }

  // Gets the configuration of a topic. Since the topic only has the name
  // attribute, this method is only useful to check the existence of a topic.
  // If other attributes are added in the future, they will be returned here.
  rpc GetTopic(GetTopicRequest) returns (Topic) {
  }

  // Lists matching topics.
  rpc ListTopics(ListTopicsRequest) returns (ListTopicsResponse) {
  }

  // Deletes the topic with the given name.  All subscriptions to this topic
  // are also deleted. Returns NOT_FOUND if the topic does not exist.
  // After a topic is deleted, a new topic may be created with the same name.
  rpc DeleteTopic(DeleteTopicRequest) returns (proto2.Empty)  {
  }
}

// A topic resource.
message Topic {
  // Name of the topic.
  optional string name = 1;
}

// A message data and its labels.
message PubsubMessage {
  // The message payload.
  optional bytes data = 1;

  // Optional list of labels for this message. Keys in this collection must
  // be unique.
  repeated tech.label.Label label = 2;

  // ID of this message assigned by the server at publication time. Guaranteed
  // to be unique within the topic. This value may be read by a subscriber
  // that receives a PubsubMessage via a Pull call or a push delivery. It must
  // not be populated by a publisher in a Publish call.
  optional string message_id = 3;
}

// Request for the GetTopic method.
message GetTopicRequest {
  // The name of the topic to get.
  optional string topic = 1;
}

// Request for the Publish method.
message PublishRequest {
  // The message in the request will be published on this topic.
  optional string topic = 1;

  // The message to publish.
  optional PubsubMessage message = 2;
}

// Request for the PublishBatch method.
message PublishBatchRequest {
  // The messages in the request will be published on this topic.
  optional string topic = 1;

  // The messages to publish.
  repeated PubsubMessage messages = 2;
}

// Response for the PublishBatch method.
message PublishBatchResponse {
  // The server-assigned ID of each published message, in the same order as
  // the messages in the request. IDs are guaranteed to be unique within
  // the topic.
  repeated string message_ids = 1;
}

// Request for the ListTopics method.
message ListTopicsRequest {
  // A valid label query expression.
  // (-- Which labels are required or supported is implementation-specific. --)
  optional string query = 1;

  // Maximum number of topics to return.
  // (-- If not specified or <= 0, the implementation will select a reasonable
  // value. --)
  optional int32 max_results = 2;

  // The value obtained in the last <code>ListTopicsResponse</code>
  // for continuation.
  optional string page_token = 3;

}

// Response for the ListTopics method.
message ListTopicsResponse {
  // The resulting topics.
  repeated Topic topic = 1;

  // If not empty, indicates that there are more topics that match the request,
  // and this value should be passed to the next <code>ListTopicsRequest</code>
  // to continue.
  optional string next_page_token = 2;
}

// Request for the Delete method.
message DeleteTopicRequest {
  // Name of the topic to delete.
  optional string topic = 1;
}

// -----------------------------------------------------------------------------
// The Subscriber service and its protos.
// -----------------------------------------------------------------------------

// The service that an application uses to manipulate subscriptions and to
// consume messages from a subscription via the pull method.
service SubscriberService {

  // Creates a subscription on a given topic for a given subscriber.
  // If the subscription already exists, returns ALREADY_EXISTS.
  // If the corresponding topic doesn't exist, returns NOT_FOUND.
  //
  // If the name is not provided in the request, the server will assign a random
  // name for this subscription on the same project as the topic.
  rpc CreateSubscription(Subscription) returns (Subscription) {
  }

  // Gets the configuration details of a subscription.
  rpc GetSubscription(GetSubscriptionRequest) returns (Subscription) {
  }

  // Lists matching subscriptions.
  rpc ListSubscriptions(ListSubscriptionsRequest)
      returns (ListSubscriptionsResponse) {
  }

  // Deletes an existing subscription. All pending messages in the subscription
  // are immediately dropped. Calls to Pull after deletion will return
  // NOT_FOUND.
  rpc DeleteSubscription(DeleteSubscriptionRequest) returns (proto2.Empty) {
  }

  // Removes all the pending messages in the subscription and releases the
  // storage associated with them. Results in a truncation event to be sent to
  // the subscriber. Messages added after this call returns are stored in the
  // subscription as before.
  rpc TruncateSubscription(TruncateSubscriptionRequest) returns (proto2.Empty) {
  }

  //
  // Push subscriber calls.
  //

  // Modifies the <code>PushConfig</code> for a specified subscription.
  // This method can be used to suspend the flow of messages to an endpoint
  // by clearing the <code>PushConfig</code> field in the request. Messages
  // will be accumulated for delivery even if no push configuration is
  // defined or while the configuration is modified.
  rpc ModifyPushConfig(ModifyPushConfigRequest) returns (proto2.Empty) {
  }

  //
  // Pull Subscriber calls
  //

  // Pulls a single message from the server.
  // If return_immediately is true, and no messages are available in the
  // subscription, this method returns FAILED_PRECONDITION. The system is free
  // to return an UNAVAILABLE error if no messages are available in a
  // reasonable amount of time (to reduce system load).
  rpc Pull(PullRequest) returns (PullResponse) {
  }

  // Pulls messages from the server. Returns an empty list if there are no
  // messages available in the backlog. The system is free to return UNAVAILABLE
  // if there are too many pull requests outstanding for the given subscription.
  rpc PullBatch(PullBatchRequest) returns (PullBatchResponse) {
  }

  // Modifies the Ack deadline for a message received from a pull request.
  rpc ModifyAckDeadline(ModifyAckDeadlineRequest) returns (proto2.Empty) {
  }

  // Acknowledges a particular received message: the Pub/Sub system can remove
  // the given message from the subscription. Acknowledging a message whose
  // Ack deadline has expired may succeed, but the message could have been
  // already redelivered. Acknowledging a message more than once will not
  // result in an error. This is only used for messages received via pull.
  rpc Acknowledge(AcknowledgeRequest) returns (proto2.Empty) {
  }

  // Refuses processing a particular received message. The system will
  // redeliver this message to some consumer of the subscription at some
  // future time. This is only used for messages received via pull.
  rpc Nack(NackRequest) returns (proto2.Empty) {
  }
}

// A subscription resource.
message Subscription {
  // Name of the subscription.
  optional string name = 1;

  // The name of the topic from which this subscription is receiving messages.
  optional string topic = 2;

  // If <code>query</code> is non-empty, only messages on the subscriber's
  // topic whose labels match the query will be returned. Otherwise all
  // messages on the topic will be returned.
  // (-- The query syntax is described in label_query.proto --)
  optional string query = 3;

  // The subscriber may specify requirements for truncating unacknowledged
  // subscription entries. The system will honor the
  // <code>CreateSubscription</code> request only if it can meet these
  // requirements. If this field is not specified, messages are never truncated
  // by the system.
  optional TruncationPolicy truncation_policy = 4;

  // Specifies which messages can be truncated by the system.
  message TruncationPolicy {
    oneof policy {
      // If <code>max_bytes</code> is specified, the system is allowed to drop
      // old messages to keep the combined size of stored messages under
      // <code>max_bytes</code>. This is a hint; the system may keep more than
      // this many bytes, but will make a best effort to keep the size from
      // growing much beyond this parameter.
      int64 max_bytes = 1;

      // If <code>max_age_seconds</code> is specified, the system is allowed to
      // drop messages that have been stored for at least this many seconds.
      // This is a hint; the system may keep these messages, but will make a
      // best effort to remove them when their maximum age is reached.
      int64 max_age_seconds = 2;
    }
  }

  // If push delivery is used with this subscription, this field is
  // used to configure it.
  optional PushConfig push_config = 5;

  // For either push or pull delivery, the value is the maximum time after a
  // subscriber receives a message before the subscriber should acknowledge or
  // Nack the message. If the Ack deadline for a message passes without an
  // Ack or a Nack, the Pub/Sub system will eventually redeliver the message.
  // If a subscriber acknowledges after the deadline, the Pub/Sub system may
  // accept the Ack, but it is possible that the message has been already
  // delivered again. Multiple Acks to the message are allowed and will
  // succeed.
  //
  // For push delivery, this value is used to set the request timeout for
  // the call to the push endpoint.
  //
  // For pull delivery, this value is used as the initial value for the Ack
  // deadline. It may be overridden for a specific pull request (message) with
  // <code>ModifyAckDeadline</code>.
  // While a message is outstanding (i.e. it has been delivered to a pull
  // subscriber and the subscriber has not yet Acked or Nacked), the Pub/Sub
  // system will not deliver that message to another pull subscriber
  // (on a best-effort basis).
  optional int32 ack_deadline_seconds = 6;

  // If this parameter is set to n, the system is allowed to (but not required
  // to) delete the subscription when at least n seconds have elapsed since the
  // client presence was detected. (Presence is detected through any
  // interaction using the subscription ID, including Pull(), Get(), or
  // acknowledging a message.)
  //
  // If this parameter is not set, the subscription will stay live until
  // explicitly deleted.
  //
  // Clients can detect such garbage collection when a Get call or a Pull call
  // (for pull subscribers only) returns NOT_FOUND.
  optional int64 garbage_collect_seconds = 7;
}

// Configuration for a push delivery endpoint.
message PushConfig {
  // A URL locating the endpoint to which messages should be pushed.
  // For example, a Webhook endpoint might use "https://example.com/push".
  // (-- An Android application might use "gcm:<REGID>", where <REGID> is a
  // GCM registration id allocated for pushing messages to the application. --)
  optional string push_endpoint = 1;
}

// An event indicating a received message or truncation event.
message PubsubEvent {
  // The subscription that received the event.
  optional string subscription = 1;

  oneof type {
    // A received message.
    PubsubMessage message = 2;

    // Indicates that this subscription has been truncated.
    bool truncated = 3;

    // Indicates that this subscription has been deleted. (Note that pull
    // subscribers will always receive NOT_FOUND in response in their pull
    // request on the subscription, rather than seeing this boolean.)
    bool deleted = 4;
  }
}

// Request for the GetSubscription method.
message GetSubscriptionRequest {
  // The name of the subscription to get.
  optional string subscription = 1;
}

// Request for the ListSubscriptions method.
message ListSubscriptionsRequest {
  // A valid label query expression.
  // (-- Which labels are required or supported is implementation-specific.
  // TODO(eschapira): This method must support to query by topic. We must
  // define the key URI for the "topic" label. --)
  optional string query = 1;

  // Maximum number of subscriptions to return.
  // (-- If not specified or <= 0, the implementation will select a reasonable
  // value. --)
  optional int32 max_results = 3;

  // The value obtained in the last <code>ListSubscriptionsResponse</code>
  // for continuation.
  optional string page_token = 4;
}

// Response for the ListSubscriptions method.
message ListSubscriptionsResponse {
  // The subscriptions that match the request.
  repeated Subscription subscription = 1;

  // If not empty, indicates that there are more subscriptions that match the
  // request and this value should be passed to the next
  // <code>ListSubscriptionsRequest</code> to continue.
  optional string next_page_token = 2;
}

// Request for the TruncateSubscription method.
message TruncateSubscriptionRequest {
  // The subscription that is being truncated.
  optional string subscription = 1;
}

// Request for the DeleteSubscription method.
message DeleteSubscriptionRequest {
  // The subscription to delete.
  optional string subscription = 1;
}

// Request for the ModifyPushConfig method.
message ModifyPushConfigRequest {
  // The name of the subscription.
  optional string subscription = 1;

  // An empty <code>push_config</code> indicates that the Pub/Sub system should
  // pause pushing messages from the given subscription.
  optional PushConfig push_config = 2;
}

// -----------------------------------------------------------------------------
// The protos used by a pull subscriber.
// -----------------------------------------------------------------------------

// Request for the Pull method.
message PullRequest {
  // The subscription from which a message should be pulled.
  optional string subscription = 1;

  // If this is specified as true the system will respond immediately even if
  // it is not able to return a message in the Pull response. Otherwise the
  // system is allowed to wait until at least one message is available rather
  // than returning FAILED_PRECONDITION. The client may cancel the request if
  // it does not wish to wait any longer for the response.
  optional bool return_immediately = 2;
}

// Either a <code>PubsubMessage</code> or a truncation event. One of these two
// must be populated.
message PullResponse {
  // This ID must be used to acknowledge the received event or message.
  optional string ack_id = 1;

  // A pubsub message or truncation event.
  optional PubsubEvent pubsub_event = 2;
}

// Request for the PullBatch method.
message PullBatchRequest {
  // The subscription from which messages should be pulled.
  optional string subscription = 1;

  // If this is specified as true the system will respond immediately even if
  // it is not able to return a message in the Pull response. Otherwise the
  // system is allowed to wait until at least one message is available rather
  // than returning no messages. The client may cancel the request if it does
  // not wish to wait any longer for the response.
  optional bool return_immediately = 2;

  // The maximum number of PubsubEvents returned for this request. The Pub/Sub
  // system may return fewer than the number of events specified.
  optional int32 max_events = 3;
}

// Response for the PullBatch method.
message PullBatchResponse {

  // Received Pub/Sub messages or status events. The Pub/Sub system will return
  // zero messages if there are no more messages available in the backlog. The
  // Pub/Sub system may return fewer than the max_events requested even if
  // there are more messages available in the backlog.
  repeated PullResponse pull_responses = 2;
}

// Request for the ModifyAckDeadline method.
message ModifyAckDeadlineRequest {
  // The name of the subscription from which messages are being pulled.
  optional string subscription = 1;

  // The acknowledgment ID.
  optional string ack_id = 2;

  // The new Ack deadline. Must be >= 0.
  optional int32 ack_deadline_seconds = 3;
}

// Request for the Acknowledge method.
message AcknowledgeRequest {
  // The subscription whose message is being acknowledged.
  optional string subscription = 1;

  // The acknowledgment ID for the message being acknowledged. This was
  // returned by the Pub/Sub system in the Pull response.
  repeated string ack_id = 2;
}

// Request for the Nack method.
message NackRequest {
  // The subscription whose message is being Nacked.
  optional string subscription = 1;

  // The acknowledgment ID for the message being refused. This was returned by
  // the Pub/Sub system in the Pull response.
  repeated string ack_id = 2;
}

// -----------------------------------------------------------------------------
// The service and protos used by a push subscriber.
// -----------------------------------------------------------------------------

// The service that a subscriber uses to handle messages sent via push
// delivery.
// This service is not currently exported for HTTP clients.
// TODO(eschapira): Explain HTTP subscribers.
service PushEndpointService {
  // Sends a <code>PubsubMessage</code> or a subscription status event to a
  // push endpoint.
  // The push endpoint responds with an empty message and a code from
  // util/task/codes.proto. The following codes have a particular meaning to the
  // Pub/Sub system:
  // OK          - This is interpreted by Pub/Sub as Ack.
  // ABORTED     - This is intepreted by Pub/Sub as a Nack, without implying
  //               pushback for congestion control.  The Pub/Sub system will
  //               retry this message at a later time.
  // UNAVAILABLE - This is intepreted by Pub/Sub as a Nack, with the additional
  //               semantics of push-back.  The Pub/Sub system will use an AIMD
  //               congestion control algorithm to backoff the rate of sending
  //               messages from this subscription.
  // Any other code, or a failure to respond, will be interpreted in the same
  // way as ABORTED; i.e. the system will retry the message at a later time to
  // ensure reliable delivery.
  rpc HandlePubsubEvent(PubsubEvent) returns (proto2.Empty);
}