流式读取

Cloud Storage 支持流式对象读取，这允许应用或进程通过网络连接实时直接使用对象字节流，而无需将数据副本保存到本地磁盘或永久性存储空间。

如果您想下载对象并将其保存到本地文件系统，请参阅将对象下载为文件。

流式传输读取时使用校验和验证

如果您需要在处理数据之前进行校验和验证，则不应使用流式读取。由于流式读取通常使用 HTTP Range 标头，因此 Cloud Storage 不会返回仅适用于任意所请求的对象数据部分的校验和。服务器校验和会验证整个对象，这意味着在完整的数据流完成并验证之前，可能会读取和处理损坏的数据。

所需的角色

为了获得流式读取所需的权限，请让您的管理员向您授予存储桶的 Storage Object Viewer (roles/storage.objectViewer) 角色。

此角色包含流式读取所需的权限。如需查看所需的确切权限，请展开所需权限部分：

所需权限

  • storage.objects.get

您也可以使用其他预定义角色或自定义角色来获取此权限。

如需了解如何授予存储桶的角色，请参阅为存储桶设置和管理 IAM 政策。

以流式传输方式读取对象

以下示例展示了如何执行从 Cloud Storage 对象到进程的流式读取：

控制台

Google Cloud 控制台不支持流式读取。请改用 Google Cloud CLI。

命令行

  1. 运行 gcloud storage cp 命令（注意使用短划线表示目标网址），然后将数据传输到该进程：

    gcloud storage cp gs://BUCKET_NAME/OBJECT_NAME - | PROCESS_NAME

    其中：

    • BUCKET_NAME 是包含对象的存储分区的名称。例如 my_app_bucket。
    • OBJECT_NAME 是您要流式传输到进程的对象的名称。例如 data_measurements。
    • PROCESS_NAME 是您要向其输送数据的进程的名称。例如 analyze_data。

您还可以将数据从 Cloud Storage 对象流式传输到标准 Linux 命令，比如 sort：

gcloud storage cp gs://my_app_bucket/data_measurements - | sort

客户端库

C++

如需了解详情，请参阅 Cloud Storage C++ API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 

namespace gcs = ::google::cloud::storage;
[](gcs::Client client, std::string const& bucket_name,
   std::string const& object_name) {
  gcs::ObjectReadStream stream = client.ReadObject(bucket_name, object_name);

  int count = 0;
  std::string line;
  while (std::getline(stream, line, '\n')) {
    ++count;
  }
  if (stream.bad()) throw google::cloud::Status(stream.status());

  std::cout << "The object has " << count << " lines\n";
}

C#

如需了解详情，请参阅 Cloud Storage C# API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 


using Google.Cloud.Storage.V1;
using System;
using System.IO;

public class DownloadFileSample
{
    public void DownloadFile(
        string bucketName = "your-unique-bucket-name",
        string objectName = "my-file-name",
        string localPath = "my-local-path/my-file-name")
    {
        var storage = StorageClient.Create();
        using var outputFile = File.OpenWrite(localPath);
        storage.DownloadObject(bucketName, objectName, outputFile);
        Console.WriteLine($"Downloaded {objectName} to {localPath}.");
    }
}

Go

如需了解详情，请参阅 Cloud Storage Go API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 


import (
	"context"
	"fmt"
	"io"
	"time"

	"cloud.google.com/go/storage"
)

// downloadFileIntoMemory downloads an object.
func downloadFileIntoMemory(w io.Writer, bucket, object string) ([]byte, error) {
	// bucket := "bucket-name"
	// object := "object-name"
	ctx := context.Background()
	client, err := storage.NewClient(ctx)
	if err != nil {
		return nil, fmt.Errorf("storage.NewClient: %w", err)
	}
	defer client.Close()

	ctx, cancel := context.WithTimeout(ctx, time.Second*50)
	defer cancel()

	rc, err := client.Bucket(bucket).Object(object).NewReader(ctx)
	if err != nil {
		return nil, fmt.Errorf("Object(%q).NewReader: %w", object, err)
	}
	defer rc.Close()

	data, err := io.ReadAll(rc)
	if err != nil {
		return nil, fmt.Errorf("io.ReadAll: %w", err)
	}
	fmt.Fprintf(w, "Blob %v downloaded.\n", object)
	return data, nil
}

Java

如需了解详情，请参阅 Cloud Storage Java API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 


import com.google.cloud.ReadChannel;
import com.google.cloud.storage.BlobId;
import com.google.cloud.storage.Storage;
import com.google.cloud.storage.StorageOptions;
import com.google.common.io.ByteStreams;
import java.io.IOException;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class StreamObjectDownload {

  public static void streamObjectDownload(
      String projectId, String bucketName, String objectName, String targetFile)
      throws IOException {
    // The ID of your GCP project
    // String projectId = "your-project-id";

    // The ID of your GCS bucket
    // String bucketName = "your-unique-bucket-name";

    // The ID of your GCS object
    // String objectName = "your-object-name";

    // The path to the file to download the object to
    // String targetFile = "path/to/your/file";
    Path targetFilePath = Paths.get(targetFile);

    Storage storage = StorageOptions.newBuilder().setProjectId(projectId).build().getService();
    try (ReadChannel reader = storage.reader(BlobId.of(bucketName, objectName));
        FileChannel targetFileChannel =
            FileChannel.open(targetFilePath, StandardOpenOption.WRITE)) {

      ByteStreams.copy(reader, targetFileChannel);

      System.out.println(
          "Downloaded object "
              + objectName
              + " from bucket "
              + bucketName
              + " to "
              + targetFile
              + " using a ReadChannel.");
    }
  }
}

Node.js

如需了解详情，请参阅 Cloud Storage Node.js API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 

/**
 * TODO(developer): Uncomment the following lines before running the sample.
 */
// The ID of your GCS bucket
// const bucketName = 'your-unique-bucket-name';

// The ID of your GCS file
// const fileName = 'your-file-name';

// The filename and file path where you want to download the file
// const destFileName = '/local/path/to/file.txt';

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

// Creates a client
const storage = new Storage();

async function streamFileDownload() {
  // The example below demonstrates how we can reference a remote file, then
  // pipe its contents to a local file.
  // Once the stream is created, the data can be piped anywhere (process, sdout, etc)
  await storage
    .bucket(bucketName)
    .file(fileName)
    .createReadStream() //stream is created
    .pipe(fs.createWriteStream(destFileName))
    .on('finish', () => {
      // The file download is complete
    });

  console.log(
    `gs://${bucketName}/${fileName} downloaded to ${destFileName}.`
  );
}

streamFileDownload().catch(console.error);

PHP

如需了解详情，请参阅 Cloud Storage PHP API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 

use Google\Cloud\Storage\StorageClient;

/**
 * Download an object from Cloud Storage and save it as a local file.
 *
 * @param string $bucketName The name of your Cloud Storage bucket.
 *        (e.g. 'my-bucket')
 * @param string $objectName The name of your Cloud Storage object.
 *        (e.g. 'my-object')
 * @param string $destination The local destination to save the object.
 *        (e.g. '/path/to/your/file')
 */
function download_object(string $bucketName, string $objectName, string $destination): void
{
    $storage = new StorageClient();
    $bucket = $storage->bucket($bucketName);
    $object = $bucket->object($objectName);
    $object->downloadToFile($destination);
    printf(
        'Downloaded gs://%s/%s to %s' . PHP_EOL,
        $bucketName,
        $objectName,
        basename($destination)
    );
}

Python

如需了解详情，请参阅 Cloud Storage Python API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 

from google.cloud import storage


def download_blob_to_stream(bucket_name, source_blob_name, file_obj):
    """Downloads a blob to a stream or other file-like object."""

    # The ID of your GCS bucket
    # bucket_name = "your-bucket-name"

    # The ID of your GCS object (blob)
    # source_blob_name = "storage-object-name"

    # The stream or file (file-like object) to which the blob will be written
    # import io
    # file_obj = io.BytesIO()

    storage_client = storage.Client()

    bucket = storage_client.bucket(bucket_name)

    # Construct a client-side representation of a blob.
    # Note `Bucket.blob` differs from `Bucket.get_blob` in that it doesn't
    # retrieve metadata from Google Cloud Storage. As we don't use metadata in
    # this example, using `Bucket.blob` is preferred here.
    blob = bucket.blob(source_blob_name)
    blob.download_to_file(file_obj)

    print(f"Downloaded blob {source_blob_name} to file-like object.")

    return file_obj
    # Before reading from file_obj, remember to rewind with file_obj.seek(0).

Ruby

如需了解详情，请参阅 Cloud Storage Ruby API 参考文档。

如需向 Cloud Storage 进行身份验证，请设置应用默认凭证。如需了解详情，请参阅为客户端库设置身份验证。 

# Downloads a blob to a stream or other file-like object.

# The ID of your GCS bucket
# bucket_name = "your-unique-bucket-name"

# Name of a file in the Storage bucket
# file_name   = "some_file.txt"

# The stream or file (file-like object) to which the contents will be written
# local_file_obj = StringIO.new

require "google/cloud/storage"

storage = Google::Cloud::Storage.new
bucket  = storage.bucket bucket_name
file    = bucket.file file_name

file.download local_file_obj, verify: :none

# rewind the object before starting to read the downloaded contents
local_file_obj.rewind
puts "The full downloaded file contents are: #{local_file_obj.read.inspect}"

Rust

use google_cloud_storage::client::Storage;

pub async fn sample(client: &Storage, bucket_id: &str) -> anyhow::Result<()> {
    const NAME: &str = "object-to-download.txt";
    let mut reader = client
        .read_object(format!("projects/_/buckets/{bucket_id}"), NAME)
        .send()
        .await?;
    println!("counting newlines {NAME} in bucket {bucket_id}");
    let mut count = 0;
    while let Some(buffer) = reader.next().await.transpose()? {
        count += buffer.into_iter().filter(|c| *c == b'\n').count();
    }
    println!("object {NAME} in bucket {bucket_id} contains {count} newlines");
    Ok(())
}

REST API

JSON API

如需执行流式读取，请按照下载对象的说明操作，并注意以下几点：

  • 在开始读取之前，请检索对象的元数据并保存对象的世代编号。在您的每个请求中加入此世代编号，以确保在原始版本被覆盖时，您不会读取两个不同世代中的数据。

  • 在请求中使用 Range 标头来检索整个对象的一部分，您可以将该对象发送到本地进程。

  • 继续发出对象的后续部分请求，直到检索到整个对象为止。

XML API

如需执行流式读取，请按照下载对象的说明操作，并注意以下几点：

  • 在开始读取之前，请检索对象的元数据并保存对象的世代编号。在您的每个请求中加入此世代编号，以确保在原始版本被覆盖时，您不会读取两个不同世代中的数据。

  • 在请求中使用 Range 标头来检索整个对象的一部分，您可以将该对象发送到本地进程。

  • 继续发出对象的后续部分请求，直到检索到整个对象为止。

后续步骤