This document gives an overview of how subscriptions work in Google Cloud Pub/Sub. For details on pull and push delivery subscriptions, see the Pull Subscriber Guide and the Push Subscriber Guide.
To receive messages published to a topic, you must create a subscription to that topic. Only messages published to the topic after the subscription is created are available to subscriber applications. The subscription connects the topic to a subscriber application that receives and processes messages published to the topic. A topic can have multiple subscriptions, but a given subscription belongs to a single topic.
To learn about creating and updating subscriptions, see Managing Topics and Subscriptions.
At-Least-Once Delivery
Google Cloud Pub/Sub delivers each published message at least once for every subscription. There are some exceptions to this at-least-once behavior: each subscription has a configurable maximum retention time for a message. A message that could not be delivered within that time is deleted and is no longer accessible. This typically happens when subscribers do not keep up with the flow of messages. A message published before a given subscription was created will usually not be delivered. Thus, a message published to a topic with no subscription cannot be retrieved.
Once a message is sent to a subscriber, the subscriber must either acknowledge
or drop the message. A message is considered outstanding once it has been sent
out for delivery and before a subscriber acknowledges it. Google Cloud Pub/Sub will
repeatedly attempt to deliver any message that has not been acknowledged or that
is not outstanding. A subscriber has a configurable, limited amount of time, or
ackDeadline, to acknowledge the message. Once the deadline has passed,
an outstanding message becomes unacknowledged.
Typically, Pub/Sub delivers each message once and in the order in which it was published.
However, messages may sometimes be delivered out of order or more than once.
In general, accommodating more-than-once delivery requires your subscriber to be
idempotent when
processing messages. You can achieve exactly once processing of Google Cloud Pub/Sub message streams using
Cloud Dataflow PubsubIO.
PubsubIO de-duplicates messages on custom message identifiers or those assigned by
Google Cloud Pub/Sub. You can also achieve ordered processing with Cloud Dataflow by using the standard
sorting APIs of the service. Alternatively, to achieve ordering, the publisher of the topic to which
you subscribe can include a sequence token in the message. See
Message Ordering for more information.
Push and Pull Delivery
A subscription can use either the push or pull mechanism for message delivery. You can change or configure the mechanism at any time. In push delivery, Pub/Sub initiates requests to your subscriber application to deliver messages. In pull delivery, your subscriber application initiates requests to the Pub/Sub server to retrieve messages.
For a push subscription, the Pub/Sub server sends each message as an HTTPs request to the subscriber application at a pre-configured endpoint. The endpoint acknowledges the message by returning an HTTP success status code. A non-success response indicates that the message should be resent. Pub/Sub dynamically adjusts the rate of push requests based on the rate at which it receives success responses.
In a pull subscription, the subscribing application explicitly calls the
pull method, which requests messages for delivery. The Pub/Sub server
responds with the message (or an error if the queue is
empty), and an ack ID. The subscriber then explicitly calls the
acknowledge method, using the returned ack ID, to acknowledge
receipt.
The following table offers some guidance in choosing the appropriate delivery mechanism for your application:
| Pull | Push |
|---|---|
|
|
The following table compares pull and push delivery:
| Pull | Push | |
|---|---|---|
| Endpoints | Any device on the internet that has authorized credentials is able to call the Google Cloud Pub/Sub API. | An HTTPS server with non-self-signed certificate accessible on the public web. The receiving endpoint may be decoupled from the Pub/Sub subscription, so that messages from multiple subscriptions may be sent to a single endpoint. |
| Load balancing | Multiple subscribers can make pull calls to the same "shared" subscription. Each subscriber will receive a subset of the messages. | The push endpoint can be a load balancer. |
| Configuration | No configuration is necessary. | No configuration is necessary for App Engine apps in the same project as
the subscriber. Configuration (and verification) of push endpoints is required in the Google Cloud Platform Console for all other endpoints. Endpoints must be reachable via DNS names and have SSL certificates installed. |
| Flow control | The subscriber client controls the rate of delivery. The subscriber can dynamically modify the ack deadline, allowing message processing to be arbitrarily long. | The Pub/Sub server automatically implements flow control. There is no need to handle message flow at the client side, although it is possible to indicate that the client cannot handle the current message load by passing back an HTTP error. |
| Efficiency and throughput | Achieves high throughput at low CPU and bandwidth by allowing batched delivery and acknowledgments as well as massively parallel consumption. May be inefficient if aggressive polling is used to minimize message delivery time. | Delivers one message per request and limits maximum number of outstanding messages. |
Configuring Subscriptions
A subscription is created for a single topic. It has several properties that can be set at creation time or updated later, including:
- Delivery method: By default, Pub/Sub subscriptions use the pull method. You can switch to push delivery by specifying a non-empty, valid HTTPs push endpoint URL. You can switch back to pull delivery by specifying an empty URL.
- An acknowledgment deadline: If your code doesn't acknowledge the message before the deadline, the message is sent again. The default is 10 seconds. The maximum custom deadline you can specify is 600 seconds (10 minutes).
- Maximum message retention duration: The maximum amount of time for which Google Cloud Pub/Sub will retain and attempt to deliver a message. Default and maximum values are seven days. Ten minutes is the minimum.
Lifecycle of a Subscription
Subscriptions with no activity (push successes or pull requests) for 31 days may be deleted automatically. You can also delete a subscription manually. While you can create new subscriptions with the same name as a deleted one, note that the new subscription has no relation to the old one, even though they have the same name. Therefore, the new subscription has no backlog at the time it is created (no messages waiting for delivery), even if the deleted subscription had a large number of unacknowledged messages.
