Append Block

The Append Block operation commits a new block of data to the end of an existing append blob.

The Append Block operation is permitted only if the blob was created with x-ms-blob-type set to AppendBlob. Append Block is supported only on version 2015-02-21 version or later.

Request

You can construct the Append Block request as follows. HTTPS is recommended. Replace myaccount with the name of your storage account.

PUT method request URI	HTTP version
https://myaccount.blob.core.windows.net/mycontainer/myblob?comp=appendblock	HTTP/1.1

When you're making a request against the emulated storage service, specify the emulator hostname and Azure Blob Storage port as 127.0.0.1:10000, followed by the emulated storage account name.

PUT method request URI	HTTP version
http://127.0.0.1:10000/devstoreaccount1/mycontainer/myblob?comp=appendblock	HTTP/1.1

For more information, see Use the Azurite emulator for local Azure Storage development.

URI parameters

Parameter	Description
timeout	Optional. The timeout parameter is expressed in seconds. For more information, see Setting timeouts for Azure Blob Storage operations.

Request headers

The following table describes required and optional request headers.

Request header	Description
Authorization	Required. Specifies the authorization scheme, account name, and signature. See Authorize requests to Azure Storage for more information.
Date or x-ms-date	Required. Specifies the Coordinated Universal Time (UTC) for the request. For more information, see Authorize requests to Azure Storage.
x-ms-version	Required for all authorized requests. Specifies the version of the operation to use for this request. For more information, see Versioning for the Azure Storage services.
Content-Length	Required. The length of the block content in bytes. The block size must be less than or equal to 100 MiB (preview) in size for version 2022-11-02 and later. See the Remarks section for limits in older versions. When the length is not provided, the operation will fail with the status code 411 (Length Required).
Content-MD5	Optional. An MD5 hash of the block content. This hash is used to verify the integrity of the block during transport. When this header is specified, the storage service compares the hash of the content that has arrived with this header value. Note that this MD5 hash isn't stored with the blob. If the two hashes don't match, the operation will fail with error code 400 (Bad Request).
x-ms-content-crc64	Optional. A CRC64 hash of the append block content. This hash is used to verify the integrity of the append block during transport. When this header is specified, the storage service compares the hash of the content that has arrived with this header value. Note that this CRC64 hash isn't stored with the blob. If the two hashes don't match, the operation will fail with error code 400 (Bad Request). If both Content-MD5 and x-ms-content-crc64 headers are present, the request will fail with error code 400. This header is supported in versions 2019-02-02 or later.
x-ms-encryption-scope	Optional. Indicates the encryption scope to use to encrypt the request contents. This header is supported in versions 2019-02-02 or later.
x-ms-lease-id:<ID>	Required if the blob has an active lease. To perform this operation on a blob with an active lease, specify the valid lease ID for this header.
x-ms-client-request-id	Optional. Provides a client-generated, opaque value with a 1-kibibyte (KiB) character limit that's recorded in the logs when logging is configured. We highly recommend that you use this header to correlate client-side activities with requests that the server receives. For more information, see Monitor Azure Blob Storage.
x-ms-blob-condition-maxsize	Optional conditional header. Specifies the maximum length in bytes permitted for the append blob. If the Append Block operation causes the blob to exceed that limit, or if the blob size is already greater than the value specified in this header, the request fails with error code 412 (Precondition Failed).
x-ms-blob-condition-appendpos	Optional conditional header, used only for the Append Block operation. A number indicates the byte offset to compare. Append Block succeeds only if the append position is equal to this number. If it isn't, the request fails with error code 412 (Precondition Failed).

This operation supports the use of additional conditional headers to ensure that the API succeeds only if a specified condition is met. For more information, see Specifying conditional headers for Azure Blob Storage operations.

Request headers (customer-provided encryption keys)

Beginning with version 2019-02-02, you can specify the following headers on the request to encrypt a blob with a customer-provided key. Encryption with a customer-provided key (and the corresponding set of headers) is optional.

Request header	Description
x-ms-encryption-key	Required. The Base64-encoded AES-256 encryption key.
x-ms-encryption-key-sha256	Required. The Base64-encoded SHA256 hash of the encryption key.
x-ms-encryption-algorithm: AES256	Required. Specifies the algorithm to use for encryption. The value of this header must be AES256.

Request body

The request body contains the content of the block.

Sample request

Request Syntax:  
PUT https://myaccount.blob.core.windows.net/mycontainer/myblob?comp=appendblock  HTTP/1.1  
  
Request Headers:  
x-ms-version: 2015-02-21  
x-ms-date: <date>  
x-ms-blob-condition-appendpos: 2097152  
x-ms-blob-condition-maxsize: 4194304  
Authorization: SharedKey myaccount:J4ma1VuFnlJ7yfk/Gu1GxzbfdJloYmBPWlfhZ/xn7GI=  
Content-Length: 1048  
If-Match: "0x8CB172A360EC34B"  
  

Response

The response includes an HTTP status code and a set of response headers.

Status code

A successful operation returns status code 201 (Created).

For information about status codes, see Status and error codes.

Response headers

The response for this operation includes the following headers. The response can also include additional standard HTTP headers. All standard headers conform to the HTTP/1.1 protocol specification.

Response header	Description
ETag	The ETag contains a value in quotes. The client can use the value to perform conditional PUT operations by using the If-Match request header.
Last-Modified	The date and time that the blob was last modified. The date format follows RFC 1123. For more information, see Representation of date-time values in headers. Any write operation on the blob (including updates on the blob's metadata or properties) changes the last-modified time of the blob.
Content-MD5	This header is returned so that the client can check for message content integrity. The value of this header is computed by Blob Storage. It isn't necessarily the same value specified in the request headers. For versions 2019-02-02 or later, this header is only returned when the request has this header.
x-ms-content-crc64	For versions 2019-02-02 or later, this header is returned so that the client can check for message content integrity. The value of this header is computed by Blob Storage. It isn't necessarily the same value specified in the request headers. This header is returned when the Content-md5 header isn't present in the request.
x-ms-request-id	This header uniquely identifies the request that was made, and can be used for troubleshooting the request.
x-ms-version	Indicates the version of Blob Storage used to run the request. This header is returned for requests made against version 2009-09-19 and later.
Date	A UTC date/time value that indicates the time at which the response was initiated. The service generates this value.
x-ms-blob-append-offset	This response header is returned only for append operations. It returns the offset at which the block was committed, in bytes.
x-ms-blob-committed-block-count	The number of committed blocks present in the blob. You can use this to control how many more appends can be done.
x-ms-request-server-encrypted: true/false	Version 2015-12-11 or later. The value of this header is set to true if the contents of the request are successfully encrypted by using the specified algorithm. Otherwise, the value is set to false.
x-ms-encryption-key-sha256	Version 2019-02-02 or later. This header is returned if the request used a customer-provided key for encryption. The client can then ensure the contents of the request are successfully encrypted by using the provided key.
x-ms-encryption-scope	Version 2019-02-02 or later. This header is returned if the request used an encryption scope. The client can then ensure the contents of the request are successfully encrypted by using the encryption scope.
x-ms-client-request-id	You can use this header to troubleshoot requests and corresponding responses. The value of this header is equal to the value of the x-ms-client-request-id header if it's present in the request. The value is at most 1024 visible ASCII characters. If the x-ms-client-request-id header isn't present in the request, this header isn't present in the response.

Sample response

Response Status:  
HTTP/1.1 201 Created  
  
Response Headers:  
Transfer-Encoding: chunked  
x-ms-content-crc64: 77uWZTolTHU  
Date: <date>  
Server: Windows-Azure-Blob/1.0 Microsoft-HTTPAPI/2.0  
x-ms-blob-append-offset: 2097152  
x-ms-blob-committed–block-count: 1000  
  

Authorization

Authorization is required when calling any data access operation in Azure Storage. You can authorize the Append Block operation as described below.

Important

Microsoft recommends using Microsoft Entra ID with managed identities to authorize requests to Azure Storage. Microsoft Entra ID provides superior security and ease of use compared to Shared Key authorization.

Microsoft Entra ID (recommended)

Azure Storage supports using Microsoft Entra ID to authorize requests to blob data. With Microsoft Entra ID, you can use Azure role-based access control (Azure RBAC) to grant permissions to a security principal. The security principal may be a user, group, application service principal, or Azure managed identity. The security principal is authenticated by Microsoft Entra ID to return an OAuth 2.0 token. The token can then be used to authorize a request against the Blob service.

To learn more about authorization using Microsoft Entra ID, see Authorize access to blobs using Microsoft Entra ID.

Permissions

Listed below are the RBAC action necessary for a Microsoft Entra user, group, managed identity, or service principal to call the Append Block operation, and the least privileged built-in Azure RBAC role that includes this action:

• Azure RBAC action: Microsoft.Storage/storageAccounts/blobServices/containers/blobs/add/action or Microsoft.Storage/storageAccounts/blobServices/containers/blobs/write
• Least privileged built-in role: Storage Blob Data Contributor

To learn more about assigning roles using Azure RBAC, see Assign an Azure role for access to blob data.

Shared access signatures (SAS)

A shared access signature (SAS) provides secure delegated access to resources in a storage account. With a SAS, you have granular control over how a client can access data. You can specify what resource the client may access, what permissions they have to those resources, and how long the SAS is valid.

The Append Block operation supports three types of shared access signatures: user delegation SAS, service SAS, and account SAS.

As a security best practice, we recommend that you use Microsoft Entra credentials rather than the storage account key, which can be more easily compromised. If your application design requires shared access signatures, use Microsoft Entra credentials to create a user delegation SAS, when possible.

When Shared Key access is disallowed for the storage account, a service SAS token or an account SAS token will not be permitted on a request to Blob Storage. To learn more, see Understand how disallowing Shared Key affects SAS tokens.

User delegation SAS

A user delegation SAS is secured with Microsoft Entra credentials and also by the permissions specified for the SAS.

Calling the Append Block operation using a SAS token delegated to a container or blob resource requires the Add (a) or Write (w) permission as part of the user delegation SAS token:

To learn more about the user delegation SAS, see Create a user delegation SAS.

Service SAS

A service SAS is secured with the storage account key. A service SAS delegates access to a resource in a single Azure Storage service, such as blob storage.

Calling the Append Block operation using a SAS token delegated to a container or blob resource requires the Add (a) or Write (w) permission as part of the service SAS token.

To learn more about the service SAS, see Create a service SAS.

Account SAS

An account SAS is secured with the storage account key. An account SAS delegates access to resources in one or more of the storage services. All of the operations available via a service or user delegation SAS are also available via an account SAS.

The following table indicates the signed resource type and signed permissions to specify when delegating access:

Operation	Signed service	Signed resource type	Signed permission
Append Block	Blob (b)	Object (o)	Add (a) or Write (w)

To learn more about the account SAS, see Create an account SAS.

Shared key

The Append Block operation supports Shared Key authorization. Authorizing with account keys is not recommended for most scenarios, as it's less secure.

Microsoft recommends disallowing Shared Key authorization for the storage account when possible. For more information, see Prevent authorization with Shared Key.

Remarks

Append Block uploads a block to the end of an existing append blob. The block of data is immediately available after the call succeeds on the server. A maximum of 50,000 appends are permitted for each append blob. Each block can be of different size.

The following table describes the maximum permitted block and blob sizes, by service version:

Service version	Maximum block size (via Append Block)	Maximum blob size
Version 2022-11-02 and later	100 MiB (Preview)	Approximately 4.75 TiB (100 MiB × 50,000 blocks)
Versions earlier than 2022-11-02	4 MiB	Approximately 195 gibibytes (GiB) (4 MiB × 50,000 blocks)

Append Block succeeds only if the blob already exists.

Blobs uploaded by using Append Block don't expose block IDs. You can't call Get Block List against an append blob. Doing so results in an error.

You can specify the following optional, conditional headers on the request:

• x-ms-blob-condition-appendpos: You can set this header to a byte offset at which the client expects to append the block. The request succeeds only if the current offset matches that specified by the client. Otherwise, the request fails with error code 412 (Precondition Failed).

  Clients that use a single writer can use this header to determine whether an Append Block operation succeeded, despite a network failure.

• x-ms-blob-condition-maxsize: Clients can use this header to ensure that append operations don't increase the blob size beyond an expected maximum size in bytes. If the condition fails, the request fails with error code 412 (Precondition Failed).

If you attempt to upload a block that is larger than the permitted size, the service returns error code 413 (Request Entity Too Large). The service also returns additional information about the error in the response, including the maximum block size permitted in bytes. If you attempt to upload more than 50,000 blocks, the service returns error code 409 (Conflict).

If the blob has an active lease, the client must specify a valid lease ID on the request in order to write a block to the blob. If the client doesn't specify a lease ID, or specifies an invalid lease ID, Blob Storage returns error code 412 (Precondition Failed). If the client specifies a lease ID, but the blob doesn't have an active lease, Blob Storage also returns error code 412.

If you call Append Block on an existing block blob or page blob, the service returns a conflict error. If you call Append Block on a non-existent blob, the service also returns an error.

Avoid duplicate or delayed appends

In a single writer scenario, the client can avoid duplicate appends or delayed writes by using the *x-ms-blob-condition-appendpos conditional header to check the current offset. The client can also avoid duplicates or delays by checking the ETag conditionally, by using If-Match.

In a multiple writer scenario, each client can use conditional headers, but this might not be optimal for performance. For the highest concurrent append throughput, applications should handle redundant appends and delayed appends in the application layer. For example, the application can add epochs or sequence numbers in the data being appended.

Billing

Pricing requests can originate from clients that use Blob Storage APIs, either directly through the Blob Storage REST API, or from an Azure Storage client library. These requests accrue charges per transaction. The type of transaction affects how the account is charged. For example, read transactions accrue to a different billing category than write transactions. The following table shows the billing category for Append Block requests based on the storage account type:

Operation	Storage account type	Billing category
Append Block	Premium block blob Standard general-purpose v2 Standard general-purpose v1	Write operations

Append Blocks do not support object level tiering but infer their access tier from the default account access tier setting and are billed accordingly. To learn more about the default account tier setting, see Access tiers for blob data - Azure Storage.

To learn about pricing for the specified billing category, see Azure Blob Storage Pricing.