Method: Google::Cloud::Datastore::Dataset#transaction

Defined in:
lib/google/cloud/datastore/dataset.rb

#transaction(deadline: nil, previous_transaction: nil) {|tx| ... } ⇒ Object

Creates a Datastore Transaction.

Transactions using the block syntax are committed upon block completion and are automatically retried when known errors are raised during commit. All other errors will be passed on.

All changes are accumulated in memory until the block completes. Transactions will be automatically retried when possible, until deadline is reached. This operation makes separate API requests to begin and commit the transaction.

Examples:

Runs the given block in a database transaction:

require "google/cloud/datastore"

datastore = Google::Cloud::Datastore.new

task = datastore.entity "Task", "sampleTask" do |t|
  t["type"] = "Personal"
  t["done"] = false
  t["priority"] = 4
  t["description"] = "Learn Cloud Datastore"
end

datastore.transaction do |tx|
  if tx.find(task.key).nil?
    tx.save task
  end
end

If no block is given, a Transaction object is returned:

require "google/cloud/datastore"

datastore = Google::Cloud::Datastore.new

task = datastore.entity "Task", "sampleTask" do |t|
  t["type"] = "Personal"
  t["done"] = false
  t["priority"] = 4
  t["description"] = "Learn Cloud Datastore"
end

tx = datastore.transaction
begin
  if tx.find(task.key).nil?
    tx.save task
  end
  tx.commit
rescue
  tx.rollback
end

Parameters:

  • deadline (Numeric) (defaults to: nil) —

    The total amount of time in seconds the transaction has to succeed. The default is 60.

  • previous_transaction (String) (defaults to: nil) —

    The transaction identifier of a transaction that is being retried. Read-write transactions may fail due to contention. A read-write transaction can be retried by specifying previous_transaction when creating the new transaction.

    Specifying previous_transaction provides information that can be used to improve throughput. In particular, if transactional operations A and B conflict, specifying the previous_transaction can help to prevent livelock. (See Transaction#id)

Yields:

  • (tx) —

    a block yielding a new transaction

Yield Parameters:

See Also:



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# File 'lib/google/cloud/datastore/dataset.rb', line 712

def transaction deadline: nil, previous_transaction: nil
  deadline = validate_deadline deadline
  backoff = 1.0
  start_time = Time.now

  tx = Transaction.new \
    service, previous_transaction: previous_transaction
  return tx unless block_given?

  begin
    yield tx
    tx.commit
  rescue Google::Cloud::UnavailableError => e
    # Re-raise if deadline has passed
    raise e if Time.now - start_time > deadline

    # Sleep with incremental backoff
    sleep backoff *= 1.3

    # Create new transaction and retry the block
    tx = Transaction.new service, previous_transaction: tx.id
    retry
  rescue StandardError
    begin
      tx.rollback
    rescue StandardError
      raise TransactionError,
            "Transaction failed to commit and rollback."
    end
    raise TransactionError, "Transaction failed to commit."
  end
end