使用可重複讀取隔離

本頁說明如何在 Spanner 中使用可重複讀取隔離。

可重複讀取是一種隔離層級，可確保交易中的所有讀取作業，都會看到交易開始時資料庫的一致快照。在 Spanner 中，這項隔離等級是透過一種技術實作，這種技術也常稱為快照隔離。在讀寫並行程度高的情況下，許多交易會讀取其他交易可能修改的資料，因此這個方法很有幫助。使用固定快照時，可重複讀取作業可避免更嚴格的可序列化隔離層級對效能造成的影響。由於預設採用樂觀並行，讀取作業可以執行，不必取得鎖定，也不會封鎖並行寫入作業，因此因序列化衝突而可能需要重試的交易次數較少。使用悲觀並行時，讀取作業會使用快照，但專屬鎖定會套用至從 FOR UPDATE 查詢或 lock_scanned_ranges=exclusive 提示讀取的資料，以及使用 DML 查詢寫入的資料。悲觀並行控制也能降低寫入-寫入衝突的機率。詳情請參閱「隔離等級總覽」和「並行控制」。

設定隔離等級

您可以使用下列方法，在資料庫用戶端層級或交易層級，設定讀寫交易的隔離等級：

用戶端程式庫

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();
    }
}

REST

您可以使用 TransactionOptions.isolation_level REST API，在交易層級設定讀寫和唯讀交易的隔離等級。有效選項為 TransactionOptions.SERIALIZABLE 和 TransactionOptions.REPEATABLE_READ。根據預設，Spanner 會將隔離等級設為可序列化隔離。

您可以使用 Spanner 的驅動程式，在連線層級將隔離層級和讀取鎖定模式設為連線參數，或在交易層級設為 SET 陳述式選項。如要進一步瞭解各個驅動程式，請參閱「驅動程式總覽」。

您也可以為每個隔離等級設定鎖定並行。詳情請參閱「並行控制」。

不支援的用途

  • 您無法在分區 DML 交易中設定可重複讀取隔離等級。
  • 所有唯讀交易都已在固定快照中運作，且不需要鎖定，因此在這類交易中設定可重複讀取隔離等級，不會改變任何行為。
  • 您無法使用 Spanner 用戶端程式庫，在唯讀、單次使用和分割區作業中設定可重複讀取隔離層級。Spanner 用戶端程式庫不會提供選項，讓您在唯讀、單次使用和分割區查詢作業中設定可重複讀取隔離層級。

後續步驟