transaction.go

// Transaction update handler: This may get invoked multiple times due to retries.
withdraw100 := func(tn db.TransactionNode) (interface{}, error) {
  // Read the current state of the node.
  var acc Account
  if err := tn.Unmarshal(&acc); err != nil {
    return nil, err
  }
  // Mutate the state in memory.
  if acc.Balance < 100.0 {
    // Abort the transaction at anytime by returning an error.
    return nil, fmt.Errorf("insufficient funds: %.2f", acc.Balance)
  }
  acc.Balance -= 100.0

  // Return the new value which will be written back to the database.
  return acc, nil
}

ref := client.NewRef("accounts/alice")
if err := ref.Transaction(ctx, withdraw100); err != nil {
  log.Fatal(err)
}