To make it easy to upload and download objects from Amazon S3, we provide a TransferUtility component with built-in support for background transfers, progress tracking, and MultiPart uploads. This section explains how to implement upload and download functionality and a number of additional storage use cases.

<Callout>

**Note:** If you use the transfer utility MultiPart upload feature, take advantage of automatic cleanup features by setting up the [AbortIncompleteMultipartUpload](https://docs.aws.amazon.com/AmazonS3/latest/dev/intro-lifecycle-rules.html) action in your Amazon S3 bucket life cycle configuration.

</Callout>

## Transfer Utility Options

You can use the `AWSS3TransferUtilityConfiguration` object to configure the operations of the `TransferUtility`.

### isAccelerateModeEnabled
The isAccelerateModeEnabled option allows you to upload and download content from a S3 bucket that has Transfer Acceleration enabled. This option is set to false by default. See [Transfer Acceleration](https://docs.aws.amazon.com/AmazonS3/latest/dev/transfer-acceleration.html) for information on how to enable transfer acceleration for your bucket.

_The code sample below manually sets up credentials for the TransferUtility. The best practice is to use the AWSMobileClient. See [Authentication](/sdk/auth/how-it-works) for more details_

```swift
//Setup credentials, see your awsconfiguration.json for the "YOUR-IDENTITY-POOL-ID"
let credentialProvider = AWSCognitoCredentialsProvider(regionType: YOUR-IDENTITY-POOL-REGION, identityPoolId: "YOUR-IDENTITY-POOL-ID")

//Setup the service configuration
let configuration = AWSServiceConfiguration(region: .USEast1, credentialsProvider: credentialProvider)

//Setup the transfer utility configuration
let tuConf = AWSS3TransferUtilityConfiguration()
tuConf.isAccelerateModeEnabled = true

//Register a transfer utility object asynchronously
AWSS3TransferUtility.register(
    with: configuration!,
    transferUtilityConfiguration: tuConf,
    forKey: "transfer-utility-with-advanced-options"
) { (error) in
     if let error = error {
         //Handle registration error.
     }
}

//Look up the transfer utility object from the registry to use for your transfers.
let transferUtility:(AWSS3TransferUtility?) = AWSS3TransferUtility.s3TransferUtility(forKey: "transfer-utility-with-advanced-options")
```

### retryLimit
The retryLimit option allows you to specify the number of times the TransferUtility will retry a transfer when it encounters an error. By default it is set to `0`.

```swift
tuConf.retryLimit = 5
```

### multiPartConcurrencyLimit
The multiPartConcurrencyLimit option allows you to specify the number of parts that will be uploaded in parallel for a MultiPart upload request. By default, this is set to `5`.

```swift
tuConf.multiPartConcurrencyLimit = 3
```

### timeoutIntervalForResource
The timeoutIntervalForResource parameter allows you to specify the maximum duration the transfer will run. The default value for this parameter is `50` minutes. This value is important if you use Amazon Cognito temporary credential because it aligns with the maximum span of time that those credentials are valid.

```swift
tuConf.timeoutIntervalForResource = 15*60 //15 minutes
```

## Upload a File

 The transfer utility provides methods for both single-part and multipart uploads. When a transfer uses multipart upload, the data is chunked into a number of 5 MB parts which are transferred in parallel for increased speed.

 The following example shows how to upload a file to an S3 bucket.

```swift
func uploadData() {

  let data: Data = Data() // Data to be uploaded

  let expression = AWSS3TransferUtilityUploadExpression()
     expression.progressBlock = {(task, progress) in
        DispatchQueue.main.async(execute: {
          // Do something e.g. Update a progress bar.
       })
  }

  var completionHandler: AWSS3TransferUtilityUploadCompletionHandlerBlock?
  completionHandler = { (task, error) -> Void in
     DispatchQueue.main.async(execute: {
        // Do something e.g. Alert a user for transfer completion.
        // On failed uploads, `error` contains the error object.
     })
  }

  let transferUtility = AWSS3TransferUtility.default()

  transferUtility.uploadData(data,
       bucket: "YourBucket",
       key: "YourFileName",
       contentType: "text/plain",
       expression: expression,
       completionHandler: completionHandler).continueWith {
          (task) -> AnyObject? in
              if let error = task.error {
                 print("Error: \(error.localizedDescription)")
              }

              if let _ = task.result {
                 // Do something with uploadTask.
              }
              return nil;
      }
}
```

The following example shows how to upload a file to an S3 bucket using multipart uploads.

```swift
func uploadData() {

   let data: Data = Data() // Data to be uploaded

   let expression = AWSS3TransferUtilityMultiPartUploadExpression()
      expression.progressBlock = {(task, progress) in
         DispatchQueue.main.async(execute: {
           // Do something e.g. Update a progress bar.
        })
   }

   var completionHandler: AWSS3TransferUtilityMultiPartUploadCompletionHandlerBlock
   completionHandler = { (task, error) -> Void in
      DispatchQueue.main.async(execute: {
         // Do something e.g. Alert a user for transfer completion.
         // On failed uploads, `error` contains the error object.
      })
   }

   let transferUtility = AWSS3TransferUtility.default()

   transferUtility.uploadUsingMultiPart(data:data,
        bucket: "YourBucket",
        key: "YourFileName",
        contentType: "text/plain",
        expression: expression,
        completionHandler: completionHandler).continueWith {
           (task) -> AnyObject! in
               if let error = task.error {
                  print("Error: \(error.localizedDescription)")
               }

               if let _ = task.result {
                  // Do something with uploadTask.
               }
               return nil;
       }
}
```

## Download a File

The following example shows how to download a file from an S3 bucket.

```swift
func downloadData() {
   let expression = AWSS3TransferUtilityDownloadExpression()
   expression.progressBlock = {(task, progress) in DispatchQueue.main.async(execute: {
      // Do something e.g. Update a progress bar.
      })
   }

   var completionHandler: AWSS3TransferUtilityDownloadCompletionHandlerBlock?
   completionHandler = { (task, URL, data, error) -> Void in
      DispatchQueue.main.async(execute: {
      // Do something e.g. Alert a user for transfer completion.
      // On failed downloads, `error` contains the error object.
      })
   }

   let transferUtility = AWSS3TransferUtility.default()
   transferUtility.downloadData(
         fromBucket: "YourBucket",
         key: "YourFileName",
         expression: expression,
         completionHandler: completionHandler
         ).continueWith {
            (task) -> AnyObject! in if let error = task.error {
               print("Error: \(error.localizedDescription)")
            }

            if let _ = task.result {
              // Do something with downloadTask.

            }
            return nil;
        }
}
```

## Track Transfer Progress

Implement progress and completion actions for transfers by passing a `progressBlock` and `completionHandler` blocks to the call to `AWSS3TransferUtility` that initiates the transfer.

The following example of initiating a data upload, shows how progress and completion handling is typically done for all transfers. The `AWSS3TransferUtilityUploadExpression`, `AWSS3TransferUtilityMultiPartUploadExpression` and `AWSS3TransferUtilityDownloadExpression` contains the `progressBlock` that gives you the progress of the transfer which you can use to update the progress bar.

```swift
// For example, create a progress bar
let progressView: UIProgressView! = UIProgressView()
progressView.progress = 0.0;

let data = Data() // The data to upload

let expression = AWSS3TransferUtilityUploadExpression()
expression.progressBlock = {(task, progress) in DispatchQueue.main.async(execute: {
        // Update a progress bar.
        progressView.progress = Float(progress.fractionCompleted)
    })
}

let completionHandler: AWSS3TransferUtilityUploadCompletionHandlerBlock = { (task, error) -> Void in DispatchQueue.main.async(execute: {
        if let error = error {
            NSLog("Failed with error: \(error)")
        }
        else if(self.progressView.progress != 1.0) {
            NSLog("Error: Failed.")
        } else {
            NSLog("Success.")
        }
    })
}

var refUploadTask: AWSS3TransferUtilityTask?
let transferUtility = AWSS3TransferUtility.default()
transferUtility.uploadData(data,
           bucket: "S3BucketName",
           key: "S3UploadKeyName",
           contentType: "text/plain",
           expression: expression,
           completionHandler: completionHandler).continueWith { (task) -> AnyObject! in
                if let error = task.error {
                    print("Error: \(error.localizedDescription)")
                }

                if let uploadTask = task.result {
                    // Do something with uploadTask.
                    // The uploadTask can be used to pause/resume/cancel the operation, retrieve task specific information
                    refUploadTask = uploadTask
                }

                return nil;
            }
```

## Pause a Transfer

To pause a transfer, retain references to `AWSS3TransferUtilityUploadTask`, `AWSS3TransferUtilityMultiPartUploadTask` or `AWSS3TransferUtilityDownloadTask` .

As described in the previous section :ref:`native-track-progress-and-completion-of-a-transfer`, the variable `refUploadTask` is a reference to the `UploadTask` object that is retrieved from the `continueWith` block of an upload operation that is invoked through `transferUtility.uploadData`.

To pause a transfer, use the `suspend` method:

```swift
refUploadTask.suspend()
```

<Callout>

Note that pause internally uses the `suspend` api of `NSURLSessionTask`, which suspends the task temporarily and does not fully pause the transfer. Complete pausing of task is tracked in this Github issue - https://github.com/aws-amplify/aws-sdk-ios/issues/3668

</Callout>

## Resume a Transfer

To resume a transfer, use the `resume` method:

```swift
refUploadTask.resume()
```

## Cancel a Transfer

To cancel a transfer, use the `cancel` method:

```swift
refUploadTask.cancel()
```

## Background Transfers

All transfers performed by TransferUtility for iOS happen in the background using NSURLSession background sessions. Once a transfer is initiated, it will continue regardless of whether the initiating app moves to the foreground, moves to the background, is suspended, or is terminated by the system. Note that this doesn't apply when the app is force-closed. Transfers initiated by apps that are force-closed are terminated by the operating system at the NSURLSession level. For regular uploads and downloads you will have to re-initiate the transfer. For multi-part uploads, the TransferUtility will resume automatically and will continue the transfer.

To wake an app that is suspended or in the background when a transfer is completed, implement the handleEventsForBackgroundURLSession method in the AppDelegate and have it call the interceptApplication method of AWSS3TransferUtility as follows.

```swift
func application(
    _ application: UIApplication,
    handleEventsForBackgroundURLSession identifier: String,
    completionHandler: @escaping () -> Void
) {
    // Store the completion handler.
    AWSS3TransferUtility.interceptApplication(
        application,
        handleEventsForBackgroundURLSession: identifier,
        completionHandler: completionHandler
    )
}
```

## Managing Transfers When an App Restarts

When an app that has initiated a transfer restarts (if it has been terminated by the system and not force-closed), it is possible that the transfer may still be in progress or completed. Tasks for uploads, downloads and multipart uploads can be monitored with progress and completion handlers. In the code below accessing the default instance of the Transfer Utility has a completion handler which is run once the recovery process has completed. Any network operations which were still running and were persisted by Transfer Utility are restored. Once the completion handler is done it is ready to attach handlers for progress and completion updates.

```swift
init() {
    transferUtility = AWSS3TransferUtility.default(completionHandler: { error in
        if let error = error {
            print("Error: \(error)")
            return
        }
        self.reattachHandlers()
    })
}
func reattachHandlers() {
    let blocks = AWSS3TransferUtilityBlocks(
        uploadProgress: nil,
        multiPartUploadProgress: handleProgress(task:progress:),
        downloadProgress: nil,
        uploadCompleted: nil,
        multiPartUploadCompleted: handleCompletion(task:error:),
        downloadCompleted: nil
    )
    transferUtility.enumerateToAssign(blocks: blocks)
}
 ```

 This code is only attaching handlers for multipart uploads. The other handlers are left as `nil` but they could be defined as well depending on your needs. Handlers are given the `task` which includes the `transferID` property which can be used to associate with network operations. Place this initialization routine at the start of your app launch lifecycle.

## Manage a Transfer when a Suspended App Returns to the Foreground

When an app that has initiated a transfer becomes suspended and then returns to the foreground, the transfer may still be in progress or may have completed. In both cases, use the following code to re-establish the progress and completion handler blocks of the app.

```swift
if let uploadTasks = transferUtility.getUploadTasks().result {
   for task in uploadTasks {
         task.setCompletionHandler(completionHandler)
         task.setProgressBlock(progressBlock)
   }
}

if let downloadTasks = transferUtility.getDownloadTasks().result {
   for task in downloadTasks {
         task.setCompletionHandler(completionHandler)
         task.setProgressBlock(progressBlock)
   }
}

if let multiPartUploadTasks = transferUtility.getMultiPartUploadTasks().result {
   for task in multiPartUploadTasks {
         task.setCompletionHandler(completionHandler)
         task.setProgressBlock(progressBlock)
   }
}
```

## Transfer with Object Metadata

The `AWSS3TransferUtilityUploadExpression` and `AWSS3TransferUtilityMultiPartUploadExpression` classes contain the method `setValue:forRequestHeader` where you can pass in metadata to Amazon S3. This example demonstrates passing in the Server-side Encryption Algorithm as a request header in uploading data to S3 using MultiPart. See [Object Key and Metadata](https://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html) for more information.

```swift

let data: Data = Data() // The data to upload

let uploadExpression = AWSS3TransferUtilityMultiPartUploadExpression()
uploadExpression.setValue("AES256", forRequestHeader: "x-amz-server-side-encryption-customer-algorithm")
uploadExpression.progressBlock = {(task, progress) in
    DispatchQueue.main.async(execute: {
        // Do something e.g. Update a progress bar.
    })
}

let transferUtility = AWSS3TransferUtility.default()

transferUtility.uploadUsingMultiPart(
    data:data,
    bucket: "S3BucketName",
    key: "S3UploadKeyName",
    contentType: "text/plain",
    expression: uploadExpression,
    completionHandler: nil
).continueWith { (task) -> AnyObject! in
    if let error = task.error {
        print("Error: \(error.localizedDescription)")
    }

    return nil;
}

```

## Working with access levels in your app code

All Amazon S3 resources are private by default. If you want your users to have access to S3 buckets or objects, you can assign appropriate permissions with an [IAM policy](http://docs.aws.amazon.com/IAM/latest/UserGuide/PoliciesOverview.html).

### IAM Policy Based Permissions

When you upload objects to the S3 bucket the Amplify CLI creates, you must manually prepend the appropriate access-level information to the `key`. The correct prefix - `public/`, `protected/` or `private/` - will depend on the access level of the object as documented in the [Storage Access section](/sdk/storage/getting-started#storage-access).

## Transfer Utility and Pre-Signed URLS

The TransferUtility generates Amazon S3 pre-signed URLs to use for background data transfer. The best practice is to use Amazon Cognito for credentials with Transfer Utility. With Amazon Cognito Identity, you receive AWS temporary credentials that are valid for up to 60 minutes. The pre-signed URLs built using these credentials are bound by the same time limit, after which the URLs will expire.

Because of this limitation, when you use TransferUtility with Amazon Cognito, the expiry on the Pre-Signed URLs generated internally is set to **50 minutes**. Transfers that run longer than the 50 minutes will fail.  If you are transferring a large enough file where this becomes a constraint, you should create static credentials using AWSStaticCredentialsProvider ( _see [Authentication](/sdk/auth/how-it-works) for more details on how to do this_)  and increase the expiry time on the Pre-Signed URLs by increasing the value for the `timeoutIntervalForResource` in the Transfer Utility Options.  Note that the max allowed expiry value for a Pre-Signed URL is 7 days.

## Additional Resources

* [Amazon Simple Storage Service Getting Started Guide](http://docs.aws.amazon.com/AmazonS3/latest/gsg/GetStartedWithS3.html)
* [Amazon Simple Storage Service API Reference](http://docs.aws.amazon.com/AmazonS3/latest/API/Welcome.html)
* [Amazon Simple Storage Service Developer Guide](http://docs.aws.amazon.com/AmazonS3/latest/dev/Welcome.html)
* [Identity and Access Management Console](https://console.aws.amazon.com/iam/home)
* [Granting Access to an Amazon S3 Bucket](http://blogs.aws.amazon.com/security/post/Tx3VRSWZ6B3SHAV/Writing-IAM-Policies-How-to-grant-access-to-an-Amazon-S3-bucket)
* [Protecting data using customer provided encryption keys](https://docs.aws.amazon.com/AmazonS3/latest/dev/ServerSideEncryptionCustomerKeys.html)
