AWS S3 TransferObserver Doesn’t Give Any Errors When Internet is Not Available

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Summary

The AWS S3 TransferObserver is designed to handle file transfers to Amazon S3, but it has a notable issue when the internet is not available. Specifically, the TransferState remains in WAITING_FOR_NETWORK and never transitions to FAILED, and the onError() callback is not invoked. This behavior can be confusing and may lead to difficulties in handling network errors.

Root Cause

The root cause of this issue lies in the way the TransferObserver handles network connectivity. When the internet is not available, the TransferObserver does not immediately fail the transfer. Instead, it waits for the network to become available, which can lead to an indefinite wait. The key causes of this issue are:

  • The TransferObserver is designed to be resilient to temporary network outages
  • The WAITING_FOR_NETWORK state is intended to handle situations where the network is temporarily unavailable
  • The onError() callback is only invoked when a fatal error occurs, such as a permissions issue or a server error

Why This Happens in Real Systems

This issue can occur in real-world systems due to various reasons, including:

  • Network connectivity issues: Poor or intermittent network connectivity can cause the TransferObserver to wait indefinitely for the network to become available
  • Mobile network limitations: Mobile networks may have limited bandwidth or coverage, leading to frequent disconnections and WAITING_FOR_NETWORK states
  • Server-side issues: Server-side problems, such as maintenance or outages, can also cause the TransferObserver to wait for the network to become available

Real-World Impact

The impact of this issue can be significant, including:

  • Delayed or failed transfers: Transfers may be delayed or fail due to the TransferObserver waiting indefinitely for the network to become available
  • Poor user experience: Users may experience frustration or confusion due to the lack of feedback or error messages
  • Increased support requests: The lack of clear error messages or feedback can lead to increased support requests and debugging efforts

Example or Code

val observer = transferUtility.upload(
    fileName, 
    context.contentResolver.openInputStream(uri)
)
observer.setTransferListener(object : TransferListener {
    override fun onStateChanged(id: Int, state: TransferState?) {
        when (state) {
            TransferState.WAITING_FOR_NETWORK -> {
                // Handle waiting for network state
            }
            TransferState.FAILED -> {
                // Handle failed state
            }
        }
    }

    override fun onProgressChanged(id: Int, bytesCurrent: Long, bytesTotal: Long) {
        // Handle progress changed
    }

    override fun onError(id: Int, e: Exception) {
        // Handle error
    }
})

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Implementing a timeout: Setting a timeout for the TransferObserver to wait for the network to become available
  • Handling the WAITING_FOR_NETWORK state: Implementing a handler for the WAITING_FOR_NETWORK state to provide feedback or error messages to the user
  • Using a retry mechanism: Implementing a retry mechanism to retry the transfer after a certain period of time

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience: Limited experience with the AWS S3 TransferObserver and its behavior
  • Insufficient testing: Inadequate testing of the TransferObserver in different network scenarios
  • Poor understanding of network connectivity: Limited understanding of network connectivity issues and how they can impact the TransferObserver

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