Utilizzare l'isolamento di lettura ripetibile

Questa pagina descrive come utilizzare l'isolamento di lettura ripetibile in Spanner.

La lettura ripetibile è un livello di isolamento che garantisce che tutte le operazioni di lettura all'interno di una transazione vedano uno snapshot coerente del database così com'era all'inizio della transazione. In Spanner, questo livello di isolamento viene implementato utilizzando una tecnica comunemente chiamata anche isolamento dello snapshot. Questo approccio è vantaggioso in scenari di concorrenza di lettura/scrittura elevata in cui numerose transazioni leggono dati che altre transazioni potrebbero modificare. Utilizzando uno snapshot fisso, la lettura ripetibile evita gli impatti sul rendimento del livello di isolamento serializzabile più rigoroso. Con la sua concorrenza ottimistica predefinita, le letture possono essere eseguite senza acquisire blocchi e senza bloccare le scritture simultanee, il che comporta potenzialmente un numero inferiore di transazioni interrotte che potrebbero dover essere ritentate a causa di conflitti di serializzazione. Con la concorrenza pessimistica, le operazioni di lettura utilizzano gli snapshot, ma i blocchi esclusivi si applicano ai dati letti dalle query FOR UPDATE o dagli hint lock_scanned_ranges=exclusive e ai dati scritti con query DML. La concorrenza pessimistica riduce anche la probabilità di conflitti di scrittura-scrittura. Per ulteriori informazioni, consulta Panoramica del livello di isolamento e Controllo della concorrenza.

Imposta il livello di isolamento

Puoi impostare il livello di isolamento per le transazioni di lettura/scrittura a livello di client di database o di transazione utilizzando i seguenti metodi:

Librerie client

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

Puoi utilizzare l'API REST TransactionOptions.isolation_level per impostare il livello di isolamento delle transazioni di lettura/scrittura e di sola lettura a livello di transazione. Le opzioni valide sono TransactionOptions.SERIALIZABLE e TransactionOptions.REPEATABLE_READ. Per impostazione predefinita, Spanner imposta il livello di isolamento su isolamento serializzabile.

Puoi utilizzare i driver di Spanner per impostare il livello di isolamento e la modalità di blocco di lettura come parametro di connessione a livello di connessione o come opzione di istruzione a livello di transazione.SET Per saperne di più su ciascun driver, consulta la panoramica dei driver.

Puoi anche configurare la concorrenza di blocco per ogni livello di isolamento. Per maggiori informazioni, consulta Controllo della concorrenza.

Casi d'uso non supportati

  • Non puoi impostare l'isolamento in lettura continua nelle transazioni DML partizionate.
  • Tutte le transazioni di sola lettura operano già a uno snapshot fisso e non richiedono blocchi, quindi l'impostazione dell'isolamento di lettura ripetibile in questo tipo di transazione non modifica alcun comportamento.
  • Non puoi impostare l'isolamento di lettura ripetibile su operazioni di sola lettura, monouso e di partizione utilizzando le librerie client Spanner Le librerie client Spanner non avranno l'opzione per impostare l'isolamento di lettura ripetibile su operazioni di query di sola lettura, monouso e di partizione.

Passaggi successivi