Subscribe to Google Pub/Sub

Databricks provides a built-in connector to subscribe to Google Pub/Sub in Databricks Runtime 13.3 LTS and above. This connector provides exactly-once processing semantics for records from the subscriber.

note

Pub/Sub might publish duplicate records, and records might arrive to the subscriber out of order. You should write Databricks code to handle duplicate and out-of-order records.

Syntax example

The following syntax example demonstrates configuring a Structured Streaming read from Pub/Sub using a service credential. For all authentication options, see Configure access to Pub/Sub.

Scala

val query = spark.readStream
  .format("pubsub")
  // we will create a Pubsub subscription if none exists with this id
  .option("subscriptionId", "mysub") // required
  .option("topicId", "mytopic") // required
  .option("projectId", "myproject") // required
  .option("serviceCredential", "service-credential-name") // required
  .load()

For more configuration options, see Configure options for Pub/Sub streaming read.

Configure access to Pub/Sub

The following table describes the roles required for the configured credentials:

Roles	Required or optional	How it is used
roles/pubsub.viewer or roles/viewer	Required	Check if subscription exists and get subscription
roles/pubsub.subscriber	Required	Fetch data from a subscription
roles/pubsub.editor or roles/editor	Optional	Enables creation of a subscription if one doesn’t exist and also enables use of the deleteSubscriptionOnStreamStop to delete subscriptions on stream termination

Databricks recommends configuring a service credential for Pub/Sub reads. Service credentials for Pub/Sub require Databricks Runtime 16.2 and above. See Manage access to external cloud services using service credentials.

If Databricks service credentials are not available, you can use a Google Service Account (GSA) directly.

If you configure compute to use a GSA, permissions for the GSA are available for all queries running on that cluster. See Google service account.

note

You cannot attach a GSA to compute configured with standard access mode.

You can configure the following options to pass the GSA directly to the stream:

• clientEmail
• clientId
• privateKey
• privateKeyId

Pub/Sub schema

The schema for the stream matches the records that are fetched from Pub/Sub, as described in the following table:

Field	Type
messageId	StringType
payload	ArrayType[ByteType]
attributes	StringType
publishTimestampInMillis	LongType

Configure options for Pub/Sub streaming read

The following table describes the options supported for Pub/Sub. All options are configured as part of a Structured Streaming read using .option("<optionName>", "<optionValue>") syntax.

note

Some Pub/Sub configuration options use the concept of fetches instead of micro-batches. This reflects internal implementation details, and options work similarly to corollaries in other Structured Streaming connectors, except that records are fetched and then processed.

Option	Default value	Description
numFetchPartitions	Set to one half of the number of executors present at stream initialization.	The number of parallel Spark tasks that fetch records from a subscription.
deleteSubscriptionOnStreamStop	false	If true, the subscription passed to the stream is deleted when the streaming job ends.
maxBytesPerTrigger	none	A soft limit for the batch size to be processed during each triggered micro-batch.
maxRecordsPerFetch	1000	The number of records to fetch per task before processing records.
maxFetchPeriod	10 seconds	The time duration for each task to fetch before processing records. Databricks recommends using the default value.

Incremental batch processing semantics for Pub/Sub

You can use Trigger.AvailableNow to consume available records from the Pub/Sub sources an an incremental batch.

Databricks records the timestamp when you begin a read with the Trigger.AvailableNow setting. Records processed by the batch include all previously fetched data and any newly published records with a timestamp less than the recorded stream start timestamp.

See Configuring incremental batch processing.

Monitoring streaming metrics

Structured Streaming progress metrics report the number of records fetched and ready to process, the size of the records fetched and ready to process, and the number of duplicates seen since stream start. The following is an example of these metrics:

JSON

"metrics" : {
  "numDuplicatesSinceStreamStart" : "1",
  "numRecordsReadyToProcess" : "1",
  "sizeOfRecordsReadyToProcess" : "8"
}

Limitations

Speculative execution (spark.speculation) is not supported with Pub/Sub.