(Tcl) S3 Resume Download

Suppose an S3 download of a very large file failed for some reason and you have a partial file on disk. Rather than restart the entire download, you wish to download the remaining portion. This example demonstrates how to finish a previously failed download.

Note: This example requires Chilkat v9.5.0.83 or above.

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load ./chilkat.dll

# This example requires the Chilkat API to have been previously unlocked.
# See Global Unlock Sample for sample code.

set rest [new_CkRest]

# Connect to the Amazon AWS REST server.
set bTls 1
set port 443
set bAutoReconnect 1
set success [CkRest_Connect $rest "s3.amazonaws.com" $port $bTls $bAutoReconnect]

# ---------------------------------------------------------------------------
# Important: For buckets created in regions outside us-east-1,
# there are three important changes that need to be made.
# See Working with S3 Buckets in Non-us-east-1 Regions for the details.
# ---------------------------------------------------------------------------

# Provide AWS credentials for the REST call.
set authAws [new_CkAuthAws]

CkAuthAws_put_AccessKey $authAws "AWS_ACCESS_KEY"
CkAuthAws_put_SecretKey $authAws "AWS_SECRET_KEY"
CkAuthAws_put_ServiceName $authAws "s3"
set success [CkRest_SetAuthAws $rest $authAws]

# Set the bucket name via the HOST header.
# In this case, the bucket name is "chilkat100".
CkRest_put_Host $rest "chilkat100.s3.amazonaws.com"

# We want to continue downloading a file.
# The relative local filepath of our previously partially downoaded file is: qa_output/hamlet.xml
# Let's find out how many bytes are already downloaded.
set fac [new_CkFileAccess]

set localFilepath "qa_output/hamlet.xml"
# Note: The FileSize method returns a signed 32-bit integer.  If the file is potentially larger than 2GB, call FileSizeStr instead to return
# the size of the file as a string, then convert to an integer value.
set sz [CkFileAccess_FileSize $fac $localFilepath]
if {$sz > 0} then {
    puts "sz = $sz"

    # If the sz equals 42375 bytes, then we want to add a Range header that looks like this:
    # Range: bytes=42375-
    set sbRange [new_CkStringBuilder]

    CkStringBuilder_Append $sbRange "bytes="
    CkStringBuilder_AppendInt $sbRange $sz
    CkStringBuilder_Append $sbRange "-"
    CkRest_AddHeader $rest "Range" [CkStringBuilder_getAsString $sbRange]
    puts "Added Range: [CkStringBuilder_getAsString $sbRange]"
}

# Send the request to download the remainder of the file.
set success [CkRest_SendReqNoBody $rest "GET" "/hamlet.xml"]
if {$success != 1} then {
    puts [CkRest_lastErrorText $rest]
    delete_CkRest $rest
    delete_CkAuthAws $authAws
    delete_CkFileAccess $fac
    delete_CkStringBuilder $sbRange
    exit
}

# Read the response header.
set responseStatusCode [CkRest_ReadResponseHeader $rest]
if {$responseStatusCode < 0} then {
    puts [CkRest_lastErrorText $rest]
    delete_CkRest $rest
    delete_CkAuthAws $authAws
    delete_CkFileAccess $fac
    delete_CkStringBuilder $sbRange
    exit
}

puts "Response status code = $responseStatusCode"

# We expect a 200 or 206 response status if the file data is coming.
# Otherwise, we'll get a string response body with an error message(or no response body).
if {expr [$responseStatusCode == 200]  ||  [$responseStatusCode == 206]} then {

    set bodyStream [new_CkStream]

    # The stream's sink will be a file.
    # We will append to the file..
    CkStream_put_SinkFile $bodyStream $localFilepath

    # Indicate that we wish to append to the output file.
    # The SinkFileAppend property was added in Chilkat v9.50.83
    CkStream_put_SinkFileAppend $bodyStream 1

    # Read the response body to the stream.  Given that we've
    # set the stream's sink to a file, it will stream directly
    # to the file.
    set success [CkRest_ReadRespBodyStream $rest $bodyStream 1]
    if {$success != 1} then {
        puts [CkRest_lastErrorText $rest]
        delete_CkRest $rest
        delete_CkAuthAws $authAws
        delete_CkFileAccess $fac
        delete_CkStringBuilder $sbRange
        delete_CkStream $bodyStream
        exit
    }

    puts "Successfully downloaded the file."

} else {
    set errResponse [CkRest_readRespBodyString $rest]
    if {[CkRest_get_LastMethodSuccess $rest] != 1} then {
        puts [CkRest_lastErrorText $rest]
    }     else {
        puts "$errResponse"
    }

}


delete_CkRest $rest
delete_CkAuthAws $authAws
delete_CkFileAccess $fac
delete_CkStringBuilder $sbRange
delete_CkStream $bodyStream