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Implement a retry policy with Python

  • Artikkeli

Any application that runs in the cloud or communicates with remote services and resources must be able to handle transient faults. It's common for these applications to experience faults due to a momentary loss of network connectivity, a request timeout when a service or resource is busy, or other factors. Developers should build applications to handle transient faults transparently to improve stability and resiliency.

In this article, you learn how to use the Azure Storage client library for Python to set up a retry policy for an application that connects to Azure Blob Storage. Retry policies define how the application handles failed requests, and should always be tuned to match the business requirements of the application and the nature of the failure.

Configure retry options

Retry policies for Blob Storage are configured programmatically, offering control over how retry options are applied to various service requests and scenarios. For example, a web app issuing requests based on user interaction might implement a policy with fewer retries and shorter delays to increase responsiveness and notify the user when an error occurs. Alternatively, an app or component running batch requests in the background might increase the number of retries and use an exponential backoff strategy to allow the request time to complete successfully.

To configure a retry policy for client requests, you can choose from the following approaches:

  • Use the default values: The default retry policy for the Azure Storage client library for Python is an instance of ExponentialRetry with the default values. If you don't specify a retry policy, the default retry policy is used.
  • Pass values as keywords to the client constructor: You can pass values for the retry policy properties as keyword arguments when you create a client object for the service. This approach allows you to customize the retry policy for the client, and is useful if you only need to configure a few options.
  • Create an instance of a retry policy class: You can create an instance of the ExponentialRetry or LinearRetry class and set the properties to configure the retry policy. Then, you can pass the instance to the client constructor to apply the retry policy to all service requests.

The following table shows all the properties you can use to configure a retry policy. Any of these properties can be passed as keywords to the client constructor, but some are only available to use with an ExponentialRetry or LinearRetry instance. These restrictions are noted in the table, along with the default values for each property if you make no changes. You should be proactive in tuning the values of these properties to meet the needs of your app.

Property Type Description Default value ExponentialRetry LinearRetry
retry_total int The maximum number of retries. 3 Yes Yes
retry_connect int The maximum number of connect retries 3 Yes Yes
retry_read int The maximum number of read retries 3 Yes Yes
retry_status int The maximum number of status retries 3 Yes Yes
retry_to_secondary bool Whether the request should be retried to the secondary endpoint, if able. Only use this option for storage accounts with geo-redundant replication enabled, such as RA-GRS or RA-GZRS. You should also ensure your app can handle potentially stale data. False Yes Yes
initial_backoff int The initial backoff interval (in seconds) for the first retry. Only applies to exponential backoff strategy. 15 seconds Yes No
increment_base int The base (in seconds) to increment the initial_backoff by after the first retry. Only applies to exponential backoff strategy. 3 seconds Yes No
backoff int The backoff interval (in seconds) between each retry. Only applies to linear backoff strategy. 15 seconds No Yes
random_jitter_range int A number (in seconds) which indicates a range to jitter/randomize for the backoff interval. For example, setting random_jitter_range to 3 means that a backoff interval of x can vary between x+3 and x-3. 3 seconds Yes Yes

Note

The properties retry_connect, retry_read, and retry_status are used to count different types of errors. The remaining retry count is calculated as the minimum of the following values: retry_total, retry_connect, retry_read, and retry_status. Because of this, setting only retry_total might not have an effect unless you also set the other properties. In most cases, you can set all four properties to the same value to enforce a maximum number of retries. However, you should tune these properties based on the specific needs of your app.

The following sections show how to configure a retry policy using different approaches:

Use the default retry policy

The default retry policy for the Azure Storage client library for Python is an instance of ExponentialRetry with the default values. If you don't specify a retry policy, the default retry policy is used. You can also pass any configuration properties as keyword arguments when you create a client object for the service.

The following code example shows how to pass a value for the retry_total property as a keyword argument when creating a client object for the blob service. In this example, the client object uses the default retry policy with the retry_total property and other retry count properties set to 5:

# TODO: Replace <storage-account-name> with your actual storage account name
account_url = "https://<storage-account-name>.blob.core.windows.net"
credential = DefaultAzureCredential()

# Create the BlobServiceClient object with retry options
blob_service_client = BlobServiceClient(account_url, credential, retry_total=5,
                                        retry_connect=5, retry_read=5, retry_status=5)

Create an ExponentialRetry policy

You can configure a retry policy by creating an instance of ExponentialRetry, and passing the instance to the client constructor using the retry_policy keyword argument. This approach can be useful if you need to configure multiple properties or multiple policies for different clients.

The following code example shows how to configure the retry options using an instance of ExponentialRetry. In this example, we set initial_backoff to 10 seconds, increment_base to 4 seconds, and retry_total to 3 retries:

# TODO: Replace <storage-account-name> with your actual storage account name
account_url = "https://<storage-account-name>.blob.core.windows.net"
credential = DefaultAzureCredential()

# Specify retry policy parameters
retry = ExponentialRetry(initial_backoff=10, increment_base=4, retry_total=3)

# Create the BlobServiceClient object
blob_service_client = BlobServiceClient(account_url, credential, retry_policy=retry)

Create a LinearRetry policy

You can configure a retry policy by creating an instance of LinearRetry, and passing the instance to the client constructor using the retry_policy keyword argument. This approach can be useful if you need to configure multiple properties or multiple policies for different clients.

The following code example shows how to configure the retry options using an instance of LinearRetry. In this example, we set backoff to 10 seconds, retry_total to 3 retries, and retry_to_secondary to True:

# TODO: Replace <storage-account-name> with your actual storage account name
account_url = "https://<storage-account-name>.blob.core.windows.net"
credential = DefaultAzureCredential()

# Specify retry policy parameters
retry = LinearRetry(backoff=10, retry_total=3, retry_to_secondary=True)

# Create the BlobServiceClient object
blob_service_client = BlobServiceClient(account_url, credential, retry_policy=retry)

Next steps

  • This article is part of the Blob Storage developer guide for Python. See the full list of developer guide articles at Build your app.
  • For architectural guidance and general best practices for retry policies, see Transient fault handling.
  • For guidance on implementing a retry pattern for transient failures, see Retry pattern.