HTTP::Upload::FlowJs - handle resumable multi-part HTTP uploads with flowjs


This synopsis assumes a Plack/PSGI-like environment. There are plugins for Dancer and Mojolicious planned. See HTTP::Upload::FlowJs::Examples for longer examples.

The flow.js workflow assumes that your application handles two kinds of requests, POST requests for storing the payload data and GET requests for retrieving information about uploaded parts. You will have to make various calls to the HTTP::Upload::FlowJs object to validate the incoming request at every stage.

  use HTTP::Upload::FlowJs;

  my $uploads = '/tmp/flowjs_uploads/';
  my $flowjs = HTTP::Upload::FlowJs->new(
      incomingDirectory => $uploads,
      allowedContentType => sub { $_[0] =~ m!^image/! },

  my @parameter_names = $flowjs->parameter_names();

  # In your POST handler for /upload:
  sub POST_upload {
    my $params = params();

    my %info;
    @info{ @parameter_names } = @{$params}{@parameter_names};
    $info{ localChunkSize } = -s $params{ file };
    # or however you get the size of the uploaded chunk

    # you might want to set this so users don't clobber each others upload
    my $session_id = '';
    my @invalid = $flowjs->validateRequest( 'POST', \%info, $session_id );
    if( @invalid ) {
        warn 'Invalid flow.js upload request:';
        warn $_ for @invalid;
        return [500,[],["Invalid request"]];

    if( $flowjs->disallowedContentType( \%info, $session_id )) {
        # We can determine the content type, and it's not an image
        return [415,[],["File type disallowed"]];

    my $chunkname = $flowjs->chunkName( \%info, undef );

    # Save or copy the uploaded file

    # Now check if we have received all chunks of the file
    if( $flowjs->uploadComplete( \%info, undef )) {
        # Combine all chunks to final name

        my $digest = Digest::SHA256->new();

        my( $content_type, $ext ) = $flowjs->sniffContentType();
        my $final_name = "file1.$ext";
        open( my $fh, '>', $final_name )
            or die $!;
        binmode $fh;

        my( $ok, @unlink_chunks )
            = $flowjs->combineChunks( \%info, undef, $fh, $digest );
        unlink @unlink_chunks;

        # Notify backend that a file arrived
        print sprintf "File '%s' upload complete\n", $final_name;

    # Signal OK
    return [200,[],[]]

  # This checks whether a file has been received completely or
  # needs to be uploaded again
  sub GET_upload {
    my $params = params();
    my %info;
    @info{ @parameter_names} = @{$params}{@parameter_names};

    my @invalid = $flowjs->validateRequest( 'GET', \%info, session->{connid} );
    if( @invalid ) {
        warn 'Invalid flow.js upload request:';
        warn $_ for @invalid;
        return [500, [], [] ];

    } elsif( $flowjs->disallowedContentType( \%info, $session_id)) {
        # We can determine the content type, and it's not an image
        return [415,[],["File type disallowed"]];

    } else {
        my( $status, @messages )
            = $flowjs->chunkOK( $uploads, \%info, $session_id );
        if( $status != 500 ) {
            # 200 or 416
            return [$status, [], [] ];
        } else {
            warn $_ for @messages;
            return [$status, [], [] ];


flow.js is a client-side Javascript upload library that uploads a file in multiple parts. It requires two API points on the server side, one GET API point to check whether a part already has been uploaded completely and one POST API point to send the data of each partial upload to. This Perl module implements the backend functionality for both endpoints. It does not implement the handling of the HTTP requests themselves, but you likely already use a framework like Mojolicious or Dancer for that.



  my $flowjs = HTTP::Upload::FlowJs->new(
      maxChunkCount => 1000,
      maxFileSize => 10_000_000,
      maxChunkSize => 1024*1024,
      simultaneousUploads => 3,
      allowedContentType => sub {
          my($type) = @_;
          $type =~ m!^image/!; # we only allow for cat images

    incomingDirectory - path for the temporary upload parts


    maxChunkCount - hard maximum chunks allowed for a single upload

    Default 1000

    maxFileSize - hard maximum total file size for a single upload

    Default 10_000_000

    maxChunkSize - hard maximum chunk size for a single chunk

    Default 1048576

    minChunkSize - hard minimum chunk size for a single chunk

    Default 1024

    The minimum chunk size is required since the file type detection works on the first chunk. If the first chunk is too small, its file type cannot be checked.

    forceChunkSize - force all chunks to be less or equal than maxChunkSize

    Default: true

    Otherwise, the last chunk will be greater than or equal to maxChunkSize (the last uploaded chunk will be at least this size and up to two the size).

    Note: when forceChunkSize is false it only make chunkSize value in "jsConfig" equal to maxChunkSize/2.

    simultaneousUploads - simultaneously allowed uploads per file

    Default 3

    This is just an indication to the Javascript flow.js client if you pass it the configuration from this object. This is not enforced in any way yet.

    allowedContentType - subroutine to check the MIME type

    The default is to allow any kind of file

    If you need more advanced checking, do so after you've determined a file upload as complete with $flowjs->uploadComplete.

    fileParameterName - The name of the multipart POST parameter to use for the file chunk

    Default file

More interesting limits would be hard maxima for the number of pending uploads or the number of outstanding chunks per user/session. Checking these would entail a call to glob for each check and thus would be fairly disk-intensive on some systems.


Return the incoming directory name.


Return the MIME::Detect instance.



  # Perl HASH
  my $config = $flowjs->jsConfig(
      target => '/upload',

  # JSON string
  my $config = $flowjs->jsConfigStr(
      target => '/upload',

Create a JSON string that encapsulates the configuration of the Perl object for inclusion with the JS side of the world.


    my $params = params();                 # request params
    my @parameter_names = $flowjs->parameter_names; # params needed by Flowjs

    my %info;
    @info{ @parameter_names } = @{$params}{@parameter_names};

    $info{ file }           = $params{ file };
    $info{ localChunkSize } = -s $params{ file };

    my @invalid = $flowjs->validateRequest( 'POST', \%info );

Returns needed params for validating request.


    my $session_id = '';
    my @invalid = $flowjs->validateRequest( 'POST', \%info, $session_id );
    if( @invalid ) {
        warning 'Invalid flow.js upload request:';
        warning $_ for @invalid;
        status 500;

Does formal validation of the request HTTP parameters. It does not check previously stored information.

Note when POST there are addition required params localChunkSize and $self-{fileParameterName}> (default 'file').


    my $expectedSize = $flowJs->expectedChunkSize( $info, $chunkIndex );

Returns the file size we expect for the chunk $chunkIndex. The index starts at 1, if it is not passed in or zero, we assume it is for the current chunk as indicated by $info.


    if( $firstrun or $wipe ) {
        $flowJs->resetUploadDirectories( $wipe )

Creates the directory for incoming uploads. If $wipe is passed, it will remove all partial files from the directory.


    my $target = $flowjs->chunkName( $info, $sessionid );

Returns the local filename of the chunk described by $info and the $sessionid if given. An optional index can be passed in as the third parameter to get the filename of another chunk than the current chunk.

    my $target = $flowjs->chunkName( $info, $sessionid, 1 );
    # First chunk


    my( $status, @messages ) = $flowjs->chunkOK( $info, $sessionPrefix );
    if( $status == 500 ) {
        warn $_ for @messages;
        return [ 500, [], [] ]

    } elsif( $status == 200 ) {
        # That chunk exists and has the right size
        return [ 200, [], [] ]

    } else {
        # That chunk does not exist and should be uploaded
        return [ 416, [],[] ]

$flowjs->uploadComplete( $info, $sessionPrefix=undef )

  if( $flowjs->uploadComplete($info, $sessionPrefix) ) {
      # do something with the chunks


  my $fh = $flowjs->chunkFh( $info, $sessionid, $index );

Returns an opened filehandle to the chunk described by $info. The session and the index are optional.


  my $content = $flowjs->chunkContent( $info, $sessionid, $index );

Returns the content of a chunk described by $info. The session and the index are optional.

$flowjs->disallowedContentType( $info, $session )

    if( $flowjs->disallowedContentType( $info, $session )) {
        return 415, "This type of file is not allowed";

Checks that the subroutine validator passed in the constructor allows this MIME type. Unrecognized files will be blocked.

$flowjs->sniffContentType( $info, $session )

    my( $content_type, $image_ext ) = $flowjs->sniffContentType( $info, $session );
    if( !defined $content_type ) {
        # we need more chunks uploaded to check the content type

    } elsif( $content_type eq '' ) {
        # We couldn't determine what the content type is?!
        return 415, "This type of upload is not allowed";

    } elsif( $content_type !~ m!^image/(jpeg|png|gif)$!i ) {
        return 415, "This type of upload is not allowed";

    } else {
        # We allow this upload to continue, as it seems to have
        # an appropriate content type

This allows for finer-grained checking of the MIME-type. See also the allowedContentType argument in the constructor and ->disallowedContentType for a more convenient way to quickly check the upload type.

$flowjs->combineChunks $info, $sessionPrefix, $target_fh, $digest )

  if( not $flowjs->uploadComplete($info, $sessionPrefix) ) {
      print "Upload not yet completed\n";

  open my $file, '>', 'user_upload.jpg'
      or die "Couldn't create final file 'user_upload.jpg': $!";
  binmode $file;
  my $digest = Digest::SHA256->new();
  my($ok,@unlink) = $flowjs->combineChunks( $info, undef, $file, $digest );
  close $file;

  if( $ok ) {
      print "Received upload OK, removing temporary upload files\n";
      unlink @unlink;
      print "Checksum: " . $digest->md5hex;
  } else {
      # whoops
      print "Received upload failed, removing target file\n";
      unlink 'user_upload.jpg';


  my $uploading = $flowjs->pendingUploads();

In scalar context, returns the number of pending uploads. In list context, returns the list of filenames that belong to the pending uploads. This list can be larger than the number of pending uploads, as one upload can have more than one chunk.

$flowjs->staleUploads( $timeout, $now )

  my @stale_files = $flowjs->staleUploads(3600);

In scalar context, returns the number of stale uploads. In list context, returns the list of filenames that belong to the stale uploads.

An upload is considered stale if no part of it has been written to since $timeout seconds ago.

The optional $timeout parameter is the minimum age of an incomplete upload before it is considered stale.

The optional $now parameter is the point of reference for $timeout. It defaults to time.

$flowjs->purgeStaleOrInvalid( $timeout, $now )

    my @errors = $flowjs->purgeStaleOrInvalid();

Routine to delete all stale uploads and uploads with an invalid file type.

This is mostly a helper routine to run from a cron job.

Note that if you allow uploads of multiple flowJs instances into the same directory, they need to all have the same allowed file types or this method will delete files from another instance.


The public repository of this module is


The public support forum of this module is


Please report bugs in this module via the RT CPAN bug queue at or via mail to


Max Maischein


Copyright 2009-2018 by Max Maischein


This module is released under the same terms as Perl itself.