Subscription

Subscription

A Subscription object will give you access to your Cloud Pub/Sub subscription.

Subscriptions are sometimes retrieved when using various methods:

  • PubSub#getSubscriptions
  • Topic#getSubscriptions

Subscription objects may be created directly with:

  • PubSub#createSubscription
  • Topic#createSubscription

All Subscription objects are instances of an EventEmitter. The subscription will pull for messages automatically as long as there is at least one listener assigned for the message event. Available events:

Upon receipt of a message: on(event: 'message', listener: (message: Message) => void): this;

Upon receipt of an error: on(event: 'error', listener: (error: Error) => void): this;

Upon receipt of a (non-fatal) debug warning: on(event: 'debug', listener: (msg: DebugMessage) => void): this;

Upon the closing of the subscriber: on(event: 'close', listener: Function): this;

By default Subscription objects allow you to process 1000 messages at the same time. You can fine tune this value by adjusting the options.flowControl.maxMessages option.

If your subscription is seeing more re-deliveries than preferable, you might try increasing your options.ackDeadline value or decreasing the options.streamingOptions.maxStreams value.

Subscription objects handle ack management, by automatically extending the ack deadline while the message is being processed, to then issue the ack or nack of such message when the processing is done. Note: message redelivery is still possible.

By default each PubSub instance can handle 100 open streams, with default options this translates to less than 20 Subscriptions per PubSub instance. If you wish to create more Subscriptions than that, you can either create multiple PubSub instances or lower the options.streamingOptions.maxStreams value on each Subscription object.

Constructor

new Subscription(pubsub, name, optionsopt)

Parameters:
Name Type Attributes Description
pubsub PubSub

PubSub object.

name string

The name of the subscription.

options SubscriberOptions <optional>

Options for handling messages.

Examples
From PubSub#getSubscriptions
```
const {PubSub} = require('@google-cloud/pubsub');
const pubsub = new PubSub();

pubsub.getSubscriptions((err, subscriptions) => {
  // `subscriptions` is an array of Subscription objects.
});

```
From Topic#getSubscriptions
```
const topic = pubsub.topic('my-topic');
topic.getSubscriptions((err, subscriptions) => {
  // `subscriptions` is an array of Subscription objects.
});

```
Topic#createSubscription
```
const topic = pubsub.topic('my-topic');
topic.createSubscription('new-subscription', (err, subscription) => {
  // `subscription` is a Subscription object.
});

```
Topic#subscription
```
const topic = pubsub.topic('my-topic');
const subscription = topic.subscription('my-subscription');
// `subscription` is a Subscription object.

```
Once you have obtained a subscription object, you may begin to register listeners. This will automatically trigger pulling for messages.
```
// Register an error handler.
subscription.on('error', (err) => {});

// Register a debug handler, to catch non-fatal errors and other messages.
subscription.on('debug', msg => { console.log(msg.message); });

// Register a close handler in case the subscriber closes unexpectedly
subscription.on('close', () => {});

// Register a listener for `message` events.
function onMessage(message) {
  // Called every time a message is received.

  // message.id = ID of the message.
  // message.ackId = ID used to acknowledge the message receival.
  // message.data = Contents of the message.
  // message.attributes = Attributes of the message.
  // message.publishTime = Date when Pub/Sub received the message.

  // Ack the message:
  // message.ack();

  // This doesn't ack the message, but allows more messages to be retrieved
  // if your limit was hit or if you don't want to ack the message.
  // message.nack();
}
subscription.on('message', onMessage);

// Remove the listener from receiving `message` events.
subscription.removeListener('message', onMessage);

```
To apply a fine level of flow control, consider the following configuration
```
const subscription = topic.subscription('my-sub', {
  flowControl: {
    maxMessages: 1,
    // this tells the client to manage and lock any excess messages
    allowExcessMessages: false
  }
});
```

Members

iam

IAM (Identity and Access Management) allows you to set permissions on individual resources and offers a wider range of roles: editor, owner, publisher, subscriber, and viewer. This gives you greater flexibility and allows you to set more fine-grained access control.

The IAM access control features described in this document are Beta, including the API methods to get and set IAM policies, and to test IAM permissions. Cloud Pub/Sub's use of IAM features is not covered by any SLA or deprecation policy, and may be subject to backward-incompatible changes.

Mixes In:
See:
Example
```
//-
// Get the IAM policy for your subscription.
//-
subscription.iam.getPolicy((err, policy) => {
  console.log(policy);
});

//-
// If the callback is omitted, we'll return a Promise.
//-
subscription.iam.getPolicy().then((data) => {
  const policy = data[0];
  const apiResponse = data[1];
});
```

isOpen

Indicates if the Subscription is open and receiving messages.

projectId

Methods

open()

Opens the Subscription to receive messages. In general this method shouldn't need to be called, unless you wish to receive messages after calling Subscription#close. Alternatively one could just assign a new message event listener which will also re-open the Subscription.

Example
```
subscription.on('message', message => message.ack());

// Close the subscription.
subscription.close(err => {
  if (err) {
    // Error handling omitted.
  }

  The subscription has been closed and messages will no longer be received.
});

// Resume receiving messages.
subscription.open();
```

setOptions(options)

Sets the Subscription options.

Parameters:
Name Type Description
options SubscriberOptions

The options.

snapshot(name) → {Snapshot}

Create a Snapshot object. See Subscription#createSnapshot to create a snapshot.

Parameters:
Name Type Description
name string

The name of the snapshot.

Returns:
Type Description
Snapshot
Throws:

If a name is not provided.

Type
Error
Example
```
const snapshot = subscription.snapshot('my-snapshot');
```