Usa el aislamiento de lectura repetible

En esta página, se describe cómo usar el aislamiento de lectura repetible en Spanner.

La lectura repetible es un nivel de aislamiento que garantiza que todas las operaciones de lectura dentro de una transacción vean una instantánea coherente de la base de datos tal como existía al comienzo de la transacción. En Spanner, este nivel de aislamiento se implementa con una técnica que también se conoce comúnmente como aislamiento de instantánea. Este enfoque es beneficioso en situaciones de alta simultaneidad de lectura y escritura en las que numerosas transacciones leen datos que otras transacciones podrían modificar. Mediante el uso de una instantánea fija, la lectura repetible evita los impactos en el rendimiento del nivel de aislamiento serializable más riguroso. Con su simultaneidad optimista predeterminada, las lecturas se pueden ejecutar sin adquirir bloqueos y sin bloquear las escrituras simultáneas, lo que genera potencialmente menos transacciones anuladas que podrían necesitar reintentarse debido a conflictos de serialización. Con la simultaneidad pesimista, las operaciones de lectura usan instantáneas, pero los bloqueos exclusivos se aplican a los datos leídos de las consultas FOR UPDATE o las sugerencias lock_scanned_ranges=exclusive, y a los datos escritos con consultas DML. La simultaneidad pesimista también reduce la probabilidad de conflictos de escritura-escritura. Para obtener más información, consulta Descripción general del nivel de aislamiento y Control de simultaneidad.

Establece el nivel de aislamiento

Puedes establecer el nivel de aislamiento en las transacciones de lectura y escritura a nivel del cliente de la base de datos o de la transacción con los siguientes métodos:

Bibliotecas cliente

Go


import (
	"context"
	"fmt"
	"io"

	"cloud.google.com/go/spanner"
	pb "cloud.google.com/go/spanner/apiv1/spannerpb"
)

func writeWithTransactionUsingIsolationLevel(w io.Writer, db string) error {
	ctx := context.Background()

	// The isolation level specified at the client-level will be applied
	// to all RW transactions.
	cfg := spanner.ClientConfig{
		TransactionOptions: spanner.TransactionOptions{
			IsolationLevel: pb.TransactionOptions_SERIALIZABLE,
		},
	}
	client, err := spanner.NewClientWithConfig(ctx, db, cfg)
	if err != nil {
		return fmt.Errorf("failed to create client: %w", err)
	}
	defer client.Close()

	// The isolation level specified at the transaction-level takes
	// precedence over the isolation level configured at the client-level.
	// REPEATABLE_READ is used here to demonstrate overriding the client-level setting.
	txnOpts := spanner.TransactionOptions{
		IsolationLevel: pb.TransactionOptions_REPEATABLE_READ,
	}

	_, err = client.ReadWriteTransactionWithOptions(ctx, func(ctx context.Context, txn *spanner.ReadWriteTransaction) error {
		// Read the current album title
		key := spanner.Key{1, 1}
		row, err := txn.ReadRow(ctx, "Albums", key, []string{"AlbumTitle"})
		if err != nil {
			return fmt.Errorf("failed to read album: %v", err)
		}
		var title string
		if err := row.Column(0, &title); err != nil {
			return fmt.Errorf("failed to get album title: %v", err)
		}
		fmt.Fprintf(w, "Current album title: %s\n", title)

		// Update the album title
		stmt := spanner.Statement{
			SQL: `UPDATE Albums
				SET AlbumTitle = @AlbumTitle
				WHERE SingerId = @SingerId AND AlbumId = @AlbumId`,
			Params: map[string]interface{}{
				"SingerId":   1,
				"AlbumId":    1,
				"AlbumTitle": "New Album Title",
			},
		}
		count, err := txn.Update(ctx, stmt)
		if err != nil {
			return fmt.Errorf("failed to update album: %v", err)
		}
		fmt.Fprintf(w, "Updated %d record(s).\n", count)
		return nil
	}, txnOpts)

	if err != nil {
		return fmt.Errorf("transaction failed: %v", err)
	}
	return nil
}

Java

static void isolationLevelSetting(DatabaseId db) {
  // The isolation level specified at the client-level will be applied to all
  // RW transactions.
  DefaultReadWriteTransactionOptions transactionOptions =
      DefaultReadWriteTransactionOptions.newBuilder()
          .setIsolationLevel(IsolationLevel.SERIALIZABLE)
          .build();
  SpannerOptions options =
      SpannerOptions.newBuilder()
          .setDefaultTransactionOptions(transactionOptions)
          .build();
  Spanner spanner = options.getService();
  DatabaseClient dbClient = spanner.getDatabaseClient(db);
  dbClient
      // The isolation level specified at the transaction-level takes precedence
      // over the isolation level configured at the client-level.
      .readWriteTransaction(Options.isolationLevel(IsolationLevel.REPEATABLE_READ))
      .run(transaction -> {
        // Read an AlbumTitle.
        String selectSql =
            "SELECT AlbumTitle from Albums WHERE SingerId = 1 and AlbumId = 1";
        String title = null;
        try (ResultSet resultSet = transaction.executeQuery(Statement.of(selectSql))) {
          if (resultSet.next()) {
            title = resultSet.getString("AlbumTitle");
          }
        }
        System.out.printf("Current album title: %s\n", title);

        // Update the title.
        String updateSql =
            "UPDATE Albums "
                + "SET AlbumTitle = 'New Album Title' "
                + "WHERE SingerId = 1 and AlbumId = 1";
        long rowCount = transaction.executeUpdate(Statement.of(updateSql));
        System.out.printf("%d record updated.\n", rowCount);
        return null;
      });
}

Node.js

// Imports the Google Cloud Spanner client library
const {Spanner, protos} = require('@google-cloud/spanner');
// The isolation level specified at the client-level will be applied
// to all RW transactions.
const defaultTransactionOptions = {
  isolationLevel:
    protos.google.spanner.v1.TransactionOptions.IsolationLevel.SERIALIZABLE,
};

// Instantiates a client with defaultTransactionOptions
const spanner = new Spanner({
  projectId: projectId,
  defaultTransactionOptions,
});

function runTransactionWithIsolationLevel() {
  // Gets a reference to a Cloud Spanner instance and database
  const instance = spanner.instance(instanceId);
  const database = instance.database(databaseId);
  // The isolation level specified at the request level takes precedence over the isolation level configured at the client level.
  const isolationOptionsForTransaction = {
    isolationLevel:
      protos.google.spanner.v1.TransactionOptions.IsolationLevel
        .REPEATABLE_READ,
  };

  database.runTransaction(
    isolationOptionsForTransaction,
    async (err, transaction) => {
      if (err) {
        console.error(err);
        return;
      }
      try {
        const query =
          'SELECT AlbumTitle FROM Albums WHERE SingerId = 1 AND AlbumId = 1';
        const results = await transaction.run(query);
        // Gets first album's title
        const rows = results[0].map(row => row.toJSON());
        const albumTitle = rows[0].AlbumTitle;
        console.log(`previous album title ${albumTitle}`);

        const update =
          "UPDATE Albums SET AlbumTitle = 'New Album Title' WHERE SingerId = 1 AND AlbumId = 1";
        const [rowCount] = await transaction.runUpdate(update);
        console.log(
          `Successfully updated ${rowCount} record in Albums table.`,
        );
        await transaction.commit();
        console.log(
          'Successfully executed read-write transaction with isolationLevel option.',
        );
      } catch (err) {
        console.error('ERROR:', err);
        transaction.end();
      } finally {
        // Close the database when finished.
        await database.close();
      }
    },
  );
}
runTransactionWithIsolationLevel();

Python

# instance_id = "your-spanner-instance"
# database_id = "your-spanner-db-id"
from google.cloud.spanner_v1 import TransactionOptions, DefaultTransactionOptions

# The isolation level specified at the client-level will be applied to all RW transactions.
isolation_options_for_client = TransactionOptions.IsolationLevel.SERIALIZABLE

spanner_client = spanner.Client(
    default_transaction_options=DefaultTransactionOptions(
        isolation_level=isolation_options_for_client
    )
)
instance = spanner_client.instance(instance_id)
database = instance.database(database_id)

# The isolation level specified at the request level takes precedence over the isolation level configured at the client level.
isolation_options_for_transaction = (
    TransactionOptions.IsolationLevel.REPEATABLE_READ
)

def update_albums_with_isolation(transaction):
    # Read an AlbumTitle.
    results = transaction.execute_sql(
        "SELECT AlbumTitle from Albums WHERE SingerId = 1 and AlbumId = 1"
    )
    for result in results:
        print("Current Album Title: {}".format(*result))

    # Update the AlbumTitle.
    row_ct = transaction.execute_update(
        "UPDATE Albums SET AlbumTitle = 'A New Title' WHERE SingerId = 1 and AlbumId = 1"
    )

    print("{} record(s) updated.".format(row_ct))

database.run_in_transaction(
    update_albums_with_isolation, isolation_level=isolation_options_for_transaction
)

C++

void IsolationLevelSetting(std::string const& project_id,
                           std::string const& instance_id,
                           std::string const& database_id) {
  namespace spanner = ::google::cloud::spanner;
  using ::google::cloud::Options;
  using ::google::cloud::StatusOr;

  auto db = spanner::Database(project_id, instance_id, database_id);

  // The isolation level specified at the client-level will be applied
  // to all RW transactions.
  auto options = Options{}.set<spanner::TransactionIsolationLevelOption>(
      spanner::Transaction::IsolationLevel::kSerializable);
  auto client = spanner::Client(spanner::MakeConnection(db, options));

  auto commit = client.Commit(
      [&client](
          spanner::Transaction const& txn) -> StatusOr<spanner::Mutations> {
        // Read an AlbumTitle.
        auto sql = spanner::SqlStatement(
            "SELECT AlbumTitle from Albums WHERE SingerId = @SingerId and "
            "AlbumId = @AlbumId",
            {{"SingerId", spanner::Value(1)}, {"AlbumId", spanner::Value(1)}});
        auto rows = client.ExecuteQuery(txn, std::move(sql));
        for (auto const& row :
             spanner::StreamOf<std::tuple<std::string>>(rows)) {
          if (!row) return row.status();
          std::cout << "Current album title: " << std::get<0>(*row) << "\n";
        }

        // Update the title.
        auto update_sql = spanner::SqlStatement(
            "UPDATE Albums "
            "SET AlbumTitle = @AlbumTitle "
            "WHERE SingerId = @SingerId and AlbumId = @AlbumId",
            {{"AlbumTitle", spanner::Value("New Album Title")},
             {"SingerId", spanner::Value(1)},
             {"AlbumId", spanner::Value(1)}});
        auto result = client.ExecuteDml(txn, std::move(update_sql));
        if (!result) return result.status();
        std::cout << result->RowsModified() << " record updated.\n";

        return spanner::Mutations{};
      },
      // The isolation level specified at the transaction-level takes
      // precedence over the isolation level configured at the client-level.
      // kRepeatableRead is used here to demonstrate overriding the client-level
      // setting.
      Options{}.set<spanner::TransactionIsolationLevelOption>(
          spanner::Transaction::IsolationLevel::kRepeatableRead));

  if (!commit) throw std::move(commit).status();
  std::cout << "Update was successful [spanner_isolation_level_setting]\n";
}

C#


using Google.Cloud.Spanner.Data;
using System;
using System.Threading;
using System.Threading.Tasks;
using IsolationLevel = System.Data.IsolationLevel;

public class IsolationLevelAsyncSample
{
    public async Task IsolationLevelAsync(string projectId, string instanceId, string databaseId)
    {
        // Create client with IsolationLevel=Serializable.
        string connectionString = $"Data Source=projects/{projectId}/instances/{instanceId}/databases/{databaseId};IsolationLevel=Serializable";

        using var connection = new SpannerConnection(connectionString);
        await connection.OpenAsync();

        // Create transaction options overriding IsolationLevel to RepeatableRead.
        var transactionOptions = SpannerTransactionCreationOptions.ReadWrite
            .WithIsolationLevel(IsolationLevel.RepeatableRead);

        using var transaction = await connection.BeginTransactionAsync(transactionOptions, null, CancellationToken.None);

        using var cmd = connection.CreateSelectCommand("SELECT AlbumTitle FROM Albums WHERE SingerId = 1 AND AlbumId = 1");
        cmd.Transaction = transaction;
        using (var reader = await cmd.ExecuteReaderAsync())
        {
            while (await reader.ReadAsync())
            {
                Console.WriteLine($"AlbumTitle: {reader.GetFieldValue<string>("AlbumTitle")}");
            }
        }

        using var updateCmd = connection.CreateDmlCommand("UPDATE Albums SET AlbumTitle = 'A New Title' WHERE SingerId = 1 AND AlbumId = 1");
        updateCmd.Transaction = transaction;
        var rowCount = await updateCmd.ExecuteNonQueryAsync();
        Console.WriteLine($"{rowCount} records updated.");

        await transaction.CommitAsync();
    }
}

PHP

use Google\Cloud\Spanner\SpannerClient;
use Google\Cloud\Spanner\Transaction;
use Google\Cloud\Spanner\V1\TransactionOptions\IsolationLevel;

/**
 * Shows how to run a Read Write transaction with isolation level options.
 *
 * Example:
 * ```
 * isolation_level($instanceId, $databaseId);
 * ```
 *
 * @param string $instanceId The Spanner instance ID.
 * @param string $databaseId The Spanner database ID.
 */
function isolation_level(string $instanceId, string $databaseId): void
{
    // The isolation level specified at the client-level will be applied to all
    // RW transactions.
    $spanner = new SpannerClient([
        'isolationLevel' => IsolationLevel::SERIALIZABLE
    ]);
    $instance = $spanner->instance($instanceId);
    $database = $instance->database($databaseId);

    // The isolation level specified at the request level takes precedence over
    // the isolation level configured at the client level.
    $database->runTransaction(function (Transaction $t) {
        // Read an AlbumTitle.
        $results = $t->execute('SELECT AlbumTitle from Albums WHERE SingerId = 1 and AlbumId = 1');
        foreach ($results as $row) {
            printf('Current Album Title: %s' . PHP_EOL, $row['AlbumTitle']);
        }

        // Update the AlbumTitle.
        $rowCount = $t->executeUpdate('UPDATE Albums SET AlbumTitle = \'A New Title\' WHERE SingerId = 1 and AlbumId = 1');

        // Commit the transaction!
        $t->commit();

        printf('%d record(s) updated.' . PHP_EOL, $rowCount);
    }, [
        'transactionOptions' => [
            'isolationLevel' => IsolationLevel::REPEATABLE_READ
        ]
    ]);
}

Ruby

require "google/cloud/spanner"

def spanner_isolation_level project_id:, instance_id:, database_id:
  # Instantiates a client with isolation_level: :SERIALIZABLE
  spanner = Google::Cloud::Spanner.new project: project_id
  client = spanner.client instance_id, database_id, isolation_level: :SERIALIZABLE

  # Overrides isolation_level to :REPEATABLE_READ at transaction level
  client.transaction isolation_level: :REPEATABLE_READ do |tx|
    results = tx.execute_query "SELECT AlbumTitle FROM Albums WHERE SingerId = 1 AND AlbumId = 1"

    results.rows.each do |row|
      puts "AlbumTitle: #{row[:AlbumTitle]}"
    end

    row_count = tx.execute_update "UPDATE Albums SET AlbumTitle = 'A New Title' WHERE SingerId = 1 AND AlbumId = 1"

    puts "#{row_count} records updated."
  end
end

REST

Puedes usar la TransactionOptions.isolation_level API de REST para establecer el nivel de aislamiento en las transacciones de lectura y escritura y de solo lectura a nivel de la transacción. Las opciones válidas son TransactionOptions.SERIALIZABLE y TransactionOptions.REPEATABLE_READ. De forma predeterminada, Spanner establece el nivel de aislamiento en el aislamiento serializable.

Puedes usar los controladores de Spanner para establecer el nivel de aislamiento y el modo de bloqueo de lectura como un parámetro de conexión a nivel de la conexión o como una opción de instrucción SET a nivel de la transacción. Para obtener más información sobre cada controlador, consulta Descripción general de los controladores.

También puedes configurar la simultaneidad de bloqueo para cada nivel de aislamiento. Para obtener más información, consulta Control de simultaneidad.

Casos de uso no compatibles

  • No puedes establecer el aislamiento de lectura repetible en las transacciones DML particionadas.
  • Todas las transacciones de solo lectura ya operan en una instantánea fija y no requieren bloqueos, por lo que establecer el aislamiento de lectura repetible en este tipo de transacción no cambia ningún comportamiento.
  • No puedes establecer el aislamiento de lectura repetible en las operaciones de solo lectura, de un solo uso y de partición con las bibliotecas cliente de Spanner. Las bibliotecas cliente de Spanner no tendrán la opción de establecer el aislamiento de lectura repetible en las operaciones de consulta de solo lectura, de un solo uso y de partición.

¿Qué sigue?