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Copy a blob with asynchronous scheduling using JavaScript or TypeScript

  • Artikkeli

This article shows how to copy a blob with asynchronous scheduling using the Azure Storage client library for JavaScript. You can copy a blob from a source within the same storage account, from a source in a different storage account, or from any accessible object retrieved via HTTP GET request on a given URL. You can also abort a pending copy operation.

The client library methods covered in this article use the Copy Blob REST API operation, and can be used when you want to perform a copy with asynchronous scheduling. For most copy scenarios where you want to move data into a storage account and have a URL for the source object, see Copy a blob from a source object URL with JavaScript.

Prerequisites

  • The examples in this article assume you already have a project set up to work with the Azure Blob Storage client library for JavaScript. To learn about setting up your project, including package installation, importing modules, and creating an authorized client object to work with data resources, see Get started with Azure Blob Storage and JavaScript.
  • The authorization mechanism must have permissions to perform a copy operation, or to abort a pending copy. To learn more, see the authorization guidance for the following REST API operation:

About copying blobs with asynchronous scheduling

The Copy Blob operation can finish asynchronously and is performed on a best-effort basis, which means that the operation isn't guaranteed to start immediately or complete within a specified time frame. The copy operation is scheduled in the background and performed as the server has available resources. The operation can complete synchronously if the copy occurs within the same storage account.

A Copy Blob operation can perform any of the following actions:

  • Copy a source blob to a destination blob with a different name. The destination blob can be an existing blob of the same blob type (block, append, or page), or it can be a new blob created by the copy operation.
  • Copy a source blob to a destination blob with the same name, which replaces the destination blob. This type of copy operation removes any uncommitted blocks and overwrites the destination blob's metadata.
  • Copy a source file in the Azure File service to a destination blob. The destination blob can be an existing block blob, or can be a new block blob created by the copy operation. Copying from files to page blobs or append blobs isn't supported.
  • Copy a snapshot over its base blob. By promoting a snapshot to the position of the base blob, you can restore an earlier version of a blob.
  • Copy a snapshot to a destination blob with a different name. The resulting destination blob is a writeable blob and not a snapshot.

To learn more about the Copy Blob operation, including information about properties, index tags, metadata, and billing, see Copy Blob remarks.

Copy a blob with asynchronous scheduling

This section gives an overview of methods provided by the Azure Storage client library for JavaScript to perform a copy operation with asynchronous scheduling.

The following methods wrap the Copy Blob REST API operation, and begin an asynchronous copy of data from the source blob:

The beginCopyFromURL method returns a long running operation poller that allows you to wait indefinitely until the copy is completed.

Copy a blob from a source within Azure

If you're copying a blob within the same storage account, the operation can complete synchronously. Access to the source blob can be authorized via Microsoft Entra ID, a shared access signature (SAS), or an account key. For an alterative synchronous copy operation, see Copy a blob from a source object URL with JavaScript.

If the copy source is a blob in a different storage account, the operation can complete asynchronously. The source blob must either be public or authorized via SAS token. The SAS token needs to include the Read ('r') permission. To learn more about SAS tokens, see Delegate access with shared access signatures.

The following example shows a scenario for copying a source blob from a different storage account with asynchronous scheduling. In this example, we create a source blob URL with an appended user delegation SAS token. The example shows how to generate the SAS token using the client library, but you can also provide your own. The example also shows how to lease the source blob during the copy operation to prevent changes to the blob from a different client. The Copy Blob operation saves the ETag value of the source blob when the copy operation starts. If the ETag value is changed before the copy operation finishes, the operation fails.

async function copyAcrossStorageAccountsAsync(sourceBlob, destinationBlob, blobServiceClient) {
  
  // Lease the source blob to prevent changes during the copy operation
  const sourceBlobLease = new BlobLeaseClient(sourceBlob);

  // Create a SAS token that's valid for 1 hour
  const sasToken = await generateUserDelegationSAS(sourceBlob, blobServiceClient);
  const sourceBlobSASURL = sourceBlob.url + "?" + sasToken;

  try {
    await sourceBlobLease.acquireLease(-1);

    // Start the copy operation and wait for it to complete
    const copyPoller = await destinationBlob.beginCopyFromURL(sourceBlobSASURL);
    await copyPoller.pollUntilDone();
  } catch (error) {
    // Handle the exception
  } finally {
    // Release the lease once the copy operation completes
    await sourceBlobLease.releaseLease();
  }
}

async function generateUserDelegationSAS(sourceBlob, blobServiceClient) {
  
  // Get a user delegation key for the Blob service that's valid for 1 hour, as an example
  const delegationKeyStart = new Date();
  const delegationKeyExpiry = new Date(Date.now() + 3600000);
  const userDelegationKey = await blobServiceClient.getUserDelegationKey(
    delegationKeyStart,
    delegationKeyExpiry
  );

  // Create a SAS token that's valid for 1 hour, as an example
  const sasTokenStart = new Date();
  const sasTokenExpiry = new Date(Date.now() + 3600000);
  const blobName = sourceBlob.name;
  const containerName = sourceBlob.containerName;
  const sasOptions = {
    blobName,
    containerName,
    permissions: BlobSASPermissions.parse("r"),
    startsOn: sasTokenStart,
    expiresOn: sasTokenExpiry,
    protocol: SASProtocol.HttpsAndHttp
  };

  const sasToken = generateBlobSASQueryParameters(
    sasOptions,
    userDelegationKey,
    blobServiceClient.accountName
  ).toString();

  return sasToken.toString();
}

Note

User delegation SAS tokens offer greater security, as they're signed with Microsoft Entra credentials instead of an account key. To create a user delegation SAS token, the Microsoft Entra security principal needs appropriate permissions. For authorization requirements, see Get User Delegation Key.

Copy a blob from a source outside of Azure

You can perform a copy operation on any source object that can be retrieved via HTTP GET request on a given URL, including accessible objects outside of Azure. The following example shows a scenario for copying a blob from an accessible source object URL.

async function copyFromExternalSource(sourceURL, destinationBlob) {
  
  const copyPoller = await destinationBlob.beginCopyFromURL(sourceURL);
  await copyPoller.pollUntilDone();
}

Check the status of a copy operation

To check the status of an asynchronous Copy Blob operation, you can poll the getProperties method and check the copy status.

The following code example shows how to check the status of a pending copy operation:

async function checkCopyStatus(destinationBlob) {
  
  const properties = await destinationBlob.getProperties();
  console.log(properties.copyStatus);
}

Abort a copy operation

Aborting a pending Copy Blob operation results in a destination blob of zero length. However, the metadata for the destination blob has the new values copied from the source blob or set explicitly during the copy operation. To keep the original metadata from before the copy, make a snapshot of the destination blob before calling one of the copy methods.

To abort a pending copy operation, call the following operation:

This method wraps the Abort Copy Blob REST API operation, which cancels a pending Copy Blob operation. The following code example shows how to abort a pending Copy Blob operation:

async function abortCopy(destinationBlob) {
  
  const properties = await destinationBlob.getProperties();

  // Check the copy status and abort if pending
  if (properties.copyStatus === "pending") {
    await destinationBlob.abortCopyFromURL(properties.copyId);
  }
}

Resources

To learn more about copying blobs with asynchronous scheduling using the Azure Blob Storage client library for JavaScript, see the following resources.

Code samples

REST API operations

The Azure SDK for JavaScript contains libraries that build on top of the Azure REST API, allowing you to interact with REST API operations through familiar JavaScript paradigms. The client library methods covered in this article use the following REST API operations:

Client library resources

Related content

  • This article is part of the Blob Storage developer guide for JavaScript/TypeScript. To learn more, see the full list of developer guide articles at Build your JavaScript/TypeScript app.