Schreibgeschützte Transaktion

Verwenden Sie eine schreibgeschützte Transaktion.

Weitere Informationen

Eine ausführliche Dokumentation, die dieses Codebeispiel enthält, finden Sie hier:

Codebeispiel

C++

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

void ReadOnlyTransaction(google::cloud::spanner::Client client) {
  namespace spanner = ::google::cloud::spanner;
  auto read_only = spanner::MakeReadOnlyTransaction();

  spanner::SqlStatement select(
      "SELECT SingerId, AlbumId, AlbumTitle FROM Albums");
  using RowType = std::tuple<std::int64_t, std::int64_t, std::string>;

  // Read#1.
  auto rows1 = client.ExecuteQuery(read_only, select);
  std::cout << "Read 1 results\n";
  for (auto& row : spanner::StreamOf<RowType>(rows1)) {
    if (!row) throw std::move(row).status();
    std::cout << "SingerId: " << std::get<0>(*row)
              << " AlbumId: " << std::get<1>(*row)
              << " AlbumTitle: " << std::get<2>(*row) << "\n";
  }
  // Read#2. Even if changes occur in-between the reads the transaction ensures
  // that Read #1 and Read #2 return the same data.
  auto rows2 = client.ExecuteQuery(read_only, select);
  std::cout << "Read 2 results\n";
  for (auto& row : spanner::StreamOf<RowType>(rows2)) {
    if (!row) throw std::move(row).status();
    std::cout << "SingerId: " << std::get<0>(*row)
              << " AlbumId: " << std::get<1>(*row)
              << " AlbumTitle: " << std::get<2>(*row) << "\n";
  }
}

C#

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

public static async Task ReadOnlyTransaction(string connectionString)
{
    await using var connection = new SpannerConnection(connectionString);
    await connection.OpenAsync();

    // Start a read-only transaction on this connection.
    await using var transaction = await connection.BeginReadOnlyTransactionAsync();

    await using var command = connection.CreateCommand();
    command.Transaction = transaction;
    command.CommandText = "SELECT SingerId, AlbumId, AlbumTitle " +
                          "FROM Albums " +
                          "ORDER BY SingerId, AlbumId";
    await using (var reader = await command.ExecuteReaderAsync())
    {
        while (await reader.ReadAsync())
        {
            Console.WriteLine(
                $"{reader["SingerId"]} {reader["AlbumId"]} {reader["AlbumTitle"]}");
        }
    }

    // Execute another query using the same read-only transaction.
    command.CommandText = "SELECT SingerId, AlbumId, AlbumTitle " +
                          "FROM Albums " +
                          "ORDER BY AlbumTitle";
    await using (var reader = await command.ExecuteReaderAsync())
    {
        while (await reader.ReadAsync())
        {
            Console.WriteLine(
                $"{reader["SingerId"]} {reader["AlbumId"]} {reader["AlbumTitle"]}");
        }
    }

    // End the read-only transaction by calling Commit.
    await transaction.CommitAsync();
}

Go

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

import (
	"context"
	"fmt"

	"github.com/jackc/pgx/v5"
)

func ReadOnlyTransaction(host string, port int, database string) error {
	ctx := context.Background()
	connString := fmt.Sprintf(
		"postgres://uid:pwd@%s:%d/%s?sslmode=disable",
		host, port, database)
	conn, err := pgx.Connect(ctx, connString)
	if err != nil {
		return err
	}
	defer conn.Close(ctx)

	// Start a read-only transaction by supplying additional transaction options.
	tx, err := conn.BeginTx(ctx, pgx.TxOptions{AccessMode: pgx.ReadOnly})

	albumsOrderedById, err := tx.Query(ctx, "SELECT singer_id, album_id, album_title FROM albums ORDER BY singer_id, album_id")
	defer albumsOrderedById.Close()
	if err != nil {
		return err
	}
	for albumsOrderedById.Next() {
		var singerId, albumId int64
		var title string
		err = albumsOrderedById.Scan(&singerId, &albumId, &title)
		if err != nil {
			return err
		}
		fmt.Printf("%v %v %v\n", singerId, albumId, title)
	}

	albumsOrderedTitle, err := tx.Query(ctx, "SELECT singer_id, album_id, album_title FROM albums ORDER BY album_title")
	defer albumsOrderedTitle.Close()
	if err != nil {
		return err
	}
	for albumsOrderedTitle.Next() {
		var singerId, albumId int64
		var title string
		err = albumsOrderedTitle.Scan(&singerId, &albumId, &title)
		if err != nil {
			return err
		}
		fmt.Printf("%v %v %v\n", singerId, albumId, title)
	}

	// End the read-only transaction by calling Commit().
	return tx.Commit(ctx)
}

Java

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

class ReadOnlyTransaction {
  static void readOnlyTransaction(String host, int port, String database) throws SQLException {
    String connectionUrl = String.format("jdbc:postgresql://%s:%d/%s", host, port, database);
    try (Connection connection = DriverManager.getConnection(connectionUrl)) {
      // Set AutoCommit=false to enable transactions.
      connection.setAutoCommit(false);
      // This SQL statement instructs the JDBC driver to use
      // a read-only transaction.
      connection.createStatement().execute("set transaction read only");

      try (ResultSet resultSet =
          connection
              .createStatement()
              .executeQuery(
                  "SELECT singer_id, album_id, album_title "
                      + "FROM albums "
                      + "ORDER BY singer_id, album_id")) {
        while (resultSet.next()) {
          System.out.printf(
              "%d %d %s\n",
              resultSet.getLong("singer_id"),
              resultSet.getLong("album_id"),
              resultSet.getString("album_title"));
        }
      }
      try (ResultSet resultSet =
          connection
              .createStatement()
              .executeQuery(
                  "SELECT singer_id, album_id, album_title "
                      + "FROM albums "
                      + "ORDER BY album_title")) {
        while (resultSet.next()) {
          System.out.printf(
              "%d %d %s\n",
              resultSet.getLong("singer_id"),
              resultSet.getLong("album_id"),
              resultSet.getString("album_title"));
        }
      }
      // End the read-only transaction by calling commit().
      connection.commit();
    }
  }
}

Node.js

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

// Imports the Google Cloud client library
const {Spanner} = require('@google-cloud/spanner');

/**
 * TODO(developer): Uncomment the following lines before running the sample.
 */
// const projectId = 'my-project-id';
// const instanceId = 'my-instance';
// const databaseId = 'my-database';

// Creates a client
const spanner = new Spanner({
  projectId: projectId,
});
let database;
try {
  // Gets a reference to a Cloud Spanner instance and database
  const instance = spanner.instance(instanceId);
  database = instance.database(databaseId);

  // Gets a transaction object that captures the database state
  // at a specific point in time
  const [transaction] = await database.getSnapshot();

  try {
    const queryOne = 'SELECT SingerId, AlbumId, AlbumTitle FROM Albums';

    // Read #1, using SQL
    const [qOneRows] = await transaction.run(queryOne);

    qOneRows.forEach(row => {
      const json = row.toJSON();
      console.log(
        `SingerId: ${json.SingerId}, AlbumId: ${json.AlbumId}, AlbumTitle: ${json.AlbumTitle}`
      );
    });

    const queryTwo = {
      columns: ['SingerId', 'AlbumId', 'AlbumTitle'],
    };

    // Read #2, using the `read` method. Even if changes occur
    // in-between the reads, the transaction ensures that both
    // return the same data.
    const [qTwoRows] = await transaction.read('Albums', queryTwo);

    qTwoRows.forEach(row => {
      const json = row.toJSON();
      console.log(
        `SingerId: ${json.SingerId}, AlbumId: ${json.AlbumId}, AlbumTitle: ${json.AlbumTitle}`
      );
    });

    console.log('Successfully executed read-only transaction.');
  } finally {
    // Ensure the transaction is released
    transaction.end();
  }
} catch (err) {
  console.error('ERROR:', err);
} finally {
  // Close the database when finished.
  await database.close();
}

Node.js

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

import { Client } from 'pg';

async function readOnlyTransaction(host: string, port: number, database: string): Promise<void> {
  const connection = new Client({
    host: host,
    port: port,
    database: database,
  });
  await connection.connect();

  // Start a transaction.
  await connection.query("begin");
  // This SQL statement instructs the PGAdapter to make it a read-only transaction.
  await connection.query("set transaction read only");

  const albumsOrderById = await connection.query(
      "SELECT singer_id, album_id, album_title "
      + "FROM albums "
      + "ORDER BY singer_id, album_id");
  for (const row of albumsOrderById.rows) {
    console.log(`${row["singer_id"]} ${row["album_id"]} ${row["album_title"]}`);
  }
  const albumsOrderByTitle = await connection.query(
      "SELECT singer_id, album_id, album_title "
      + "FROM albums "
      + "ORDER BY album_title");
  for (const row of albumsOrderByTitle.rows) {
    console.log(`${row["singer_id"]} ${row["album_id"]} ${row["album_title"]}`);
  }
  // End the read-only transaction by executing commit.
  await connection.query("commit");

  // Close the connection.
  await connection.end();
}

PHP

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

function read_only_transaction(string $host, string $port, string $database): void
{
    $dsn = sprintf("pgsql:host=%s;port=%s;dbname=%s", $host, $port, $database);
    $connection = new PDO($dsn);

    // Start a transaction.
    $connection->beginTransaction();
    // Change the current transaction to a read-only transaction.
    // This statement can only be executed at the start of a transaction.
    $connection->exec("set transaction read only");

    // The following two queries use the same read-only transaction.
    $statement = $connection->query(
        "select singer_id, album_id, album_title "
        ."from albums "
        ."order by singer_id, album_id"
    );
    $rows = $statement->fetchAll();
    foreach ($rows as $album)
    {
        printf("%s\t%s\t%s\n", $album["singer_id"], $album["album_id"], $album["album_title"]);
    }

    $statement = $connection->query(
        "select singer_id, album_id, album_title "
        ."from albums "
        ."order by album_title"
    );
    $rows = $statement->fetchAll();
    foreach ($rows as $album)
    {
        printf("%s\t%s\t%s\n", $album["singer_id"], $album["album_id"], $album["album_title"]);
    }

    # Read-only transactions must also be committed or rolled back to mark
    # the end of the transaction. There is no semantic difference between
    # rolling back or committing a read-only transaction.
    $connection->commit();

    $rows = null;
    $statement = null;
    $connection = null;
}

Python

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

def read_only_transaction(instance_id, database_id):
    """Reads data inside of a read-only transaction.

    Within the read-only transaction, or "snapshot", the application sees
    consistent view of the database at a particular timestamp.
    """
    spanner_client = spanner.Client()
    instance = spanner_client.instance(instance_id)
    database = instance.database(database_id)

    with database.snapshot(multi_use=True) as snapshot:
        # Read using SQL.
        results = snapshot.execute_sql(
            "SELECT SingerId, AlbumId, AlbumTitle FROM Albums"
        )

        print("Results from first read:")
        for row in results:
            print("SingerId: {}, AlbumId: {}, AlbumTitle: {}".format(*row))

        # Perform another read using the `read` method. Even if the data
        # is updated in-between the reads, the snapshot ensures that both
        # return the same data.
        keyset = spanner.KeySet(all_=True)
        results = snapshot.read(
            table="Albums", columns=("SingerId", "AlbumId", "AlbumTitle"), keyset=keyset
        )

        print("Results from second read:")
        for row in results:
            print("SingerId: {}, AlbumId: {}, AlbumTitle: {}".format(*row))

Ruby

Informationen zum Installieren und Verwenden der Clientbibliothek für Cloud Spanner finden Sie unter Cloud Spanner-Clientbibliotheken.

Richten Sie zur Authentifizierung bei Cloud Spanner die Standardanmeldedaten für Anwendungen ein. Weitere Informationen finden Sie unter Authentifizierung für eine lokale Entwicklungsumgebung einrichten. 

# project_id  = "Your Google Cloud project ID"
# instance_id = "Your Spanner instance ID"
# database_id = "Your Spanner database ID"

require "google/cloud/spanner"

spanner = Google::Cloud::Spanner.new project: project_id
client  = spanner.client instance_id, database_id

client.snapshot do |snapshot|
  snapshot.execute("SELECT SingerId, AlbumId, AlbumTitle FROM Albums").rows.each do |row|
    puts "#{row[:AlbumId]} #{row[:AlbumTitle]} #{row[:SingerId]}"
  end

  # Even if changes occur in-between the reads, the transaction ensures that
  # both return the same data.
  snapshot.read("Albums", [:AlbumId, :AlbumTitle, :SingerId]).rows.each do |row|
    puts "#{row[:AlbumId]} #{row[:AlbumTitle]} #{row[:SingerId]}"
  end
end

Rust

use google_cloud_spanner::client::DatabaseClient;
use google_cloud_spanner::key::KeySet;
use google_cloud_spanner::read::ReadRequest;
use google_cloud_spanner::statement::Statement;

pub async fn sample(client: &DatabaseClient) -> anyhow::Result<()> {
    let transaction = client.read_only_transaction().build().await?;

    // 1. Execute a query using the read-only transaction
    let statement = Statement::builder("SELECT SingerId, AlbumId, AlbumTitle FROM Albums").build();
    let mut result_set = transaction.execute_query(statement).await?;
    println!("Results from query:");
    while let Some(row) = result_set.next().await.transpose()? {
        let singer_id: i64 = row.get(0);
        let album_id: i64 = row.get(1);
        let album_title: String = row.get(2);
        println!("{singer_id} {album_id} {album_title}");
    }

    // 2. Execute a read using the same read-only transaction
    let read_request = ReadRequest::builder("Albums", ["SingerId", "AlbumId", "AlbumTitle"])
        .with_keys(KeySet::all())
        .build();
    let mut result_set = transaction.execute_read(read_request).await?;
    println!("Results from read:");
    while let Some(row) = result_set.next().await.transpose()? {
        let singer_id: i64 = row.get(0);
        let album_id: i64 = row.get(1);
        let album_title: String = row.get(2);
        println!("{singer_id} {album_id} {album_title}");
    }

    Ok(())
}

Weitere Informationen

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