Support AWS_S3_PREFERRED_TRANSFER_CLIENT environment variable for TransferConfig

[crowecawcaw]

Describe the feature

Add support for an AWS_S3_PREFERRED_TRANSFER_CLIENT environment variable that allows users to configure the
preferred_transfer_client setting for S3 transfers without modifying application code.

The environment variable would accept the same values as TransferConfig.preferred_transfer_client:

• auto (default)
• crt
• classic

When set, this environment variable would be used as the default value for TransferConfig.preferred_transfer_client if no explicit value is provided to the TransferConfig constructor.

Use Case

I need to enable CRT transfers for an application that uses boto3 for S3 operations (condo), but I do not wan to modify the library's source code.

Current situation:

• The library uses boto3's default TransferConfig, which means preferred_transfer_client='auto'
• On non-optimized instances, this doesn't use CRT even though I have awscrt installed
• To use CRT, I would need to fork the library or submit a PR to add TransferConfig configuration support

This feature would benefit:

1. Using third-party libraries/applications without code modifications
2. Operational flexibility (enable/disable CRT via environment without redeployment)

Proposed Solution

Above

Other Information

n/a

Acknowledgements

• I may be able to implement this feature request
• This feature might incur a breaking change

SDK version used

1.42.39

Environment details (OS name and version, etc.)

n/a

[786imran786]

I can work on implementing this feature if you’re open to assigning the issue to me.

[crowecawcaw]

I see this received a p3 designation. The PR would be very small and I'm perfectly happy to put it up. The actual work is 1) deciding if this is a reasonable thing to add and 2) deciding if AWS_S3_PREFERRED_TRANSFER_CLIENT is the right name for it. If you'd consider a PR if it were up, I can make one.

[RyanFitzSimmonsAK]

HI @crowecawcaw, thanks for reaching out. I've marked this issue for review by the team to determine if this is a feature we'd want to pursue. We will also determine if we'd accept a public PR for this, and update this issue in the near future.

[RyanFitzSimmonsAK]

Thanks for your patience. The team approved this feature request, and it will be left open for tracking. However, because we currently do not support configuring TransferConfig through environment variables or the config file, we cannot accept an external PR on this. The design for this feature needs to be done internally.