Proposal: Binary Large Objects

Motivation

DFDL needs an extension that allows data much larger than memory to be manipulated.

A variety of data formats such as for image and video files, consist of fields of what is effectively metadata, surrounding large blocks of data containing compressed image or video data.

An important use case for DFDL is to expose this metadata for easy use, and to provide access to the large data via a streaming mechanism akin to opening a file, rather than including large chunks of a hexBinary string in the infoset, as is common today.

In RDBMS systems, BLOB (Binary Large Object) and CLOB (Character large object) are the types used when the data row returned from an SQL query will not contain the actual value data, but rather a handle that can be used to open/read/write/close the BLOB or CLOB.

DFDL needs at least BLOB capability. This would enable processing of images or video of arbitrary size without the need to every hold all the data in memory.

This also eliminates the limitation on object size.

Requirements

Rather than hexBinary data appearing in the infoset, the infoset should include some unique identifier to a location external to the infoset where the hexBinary byte data can be found.
The unique identifier must be expressed in DFDL's subset of XML Schema (e.g. things like an attribute containing the unique identifer is not allowed, as attributes aren't part of the DFDL schema language).
Accessing, modifying, or creating custom BLOB resources should be possible without the use of Daffodil (e.g. no GUID mappaings stored in Daffodil memory).
New API calls or changes to existing API calls should not be required if the BLOB feature is not enabled in a schema (e.g. maintain source backwads compatibility).
Suport is only needed for wirting BLOB's out to files, one per BLOB. Future enhancements can be made to support alternative BLOB storage mediums (e.g. databases, offset into original file, API for custom stroage) if necessary.

Proposed Changes

DFDL Extension 'blob' Simple Type

A new type is defined internal to Daffodil in the DFDL exension namespace like so:

dfdlx.xsd

<xs:schema
  targetNamespace="http://www.ogf.org/dfdl/dfdl-1.0/extensions"
  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  ...

  <xs:simpleType name="blob">
    <xs:restriction base="xs:anyURI">
  </xs:simpleType>

</xs:schema>

This defines a new simple type, dfdlx:blob, that can be used in a DFDL schema to specify that an element should use this new BLOB feature. An element with type dfdlx:blob has the exact same restrictions and avialable properties as elements with the xs:hexBinary type. The only difference is that rather than the hexBinary data being output to the infoset, the hexBinary data will be written to a file (as bytes rather than a hexBinary string), and a URI that that identifies file will be inserted into the infoset.

An example of this usage in a DFDL schema may look something like this:

<xs:schema
  xmlns:dfdlx="http://www.ogf.org/dfdl/dfdl-1.0/extensions"
  xmlns:dfdl="http://www.ogf.org/dfdl/dfdl-1.0/">

  <xs:element name="data" type="dfdlx:blob" dfdl:lengthKind="explicit" dfdl:lengthUnits="bytes" dfdl:length="1024" />

</xs:schema>

The resulting infoset will look something like this:

<data>file:///path/to/blob/data</data>

With the 1024 bytes of data being written to a file at location /path/to/blob/data.

Because the dfdlx:blob type is based on the xs:anyURI type, non DFDL-aware schema validators are still able to validate that the BLOB elements are a valid URI. However, for this initial propsoal, the BLOB URI will always use the file scheme. Although this may be a restrictive limitation for some usecases, the flexibilty and generality of URI's allows for future enhancement to support different or even custom schemes if needed.

A benefit of this proposal is its simplicity and non-reliance on other DFDL extensions (e.g. one does not need to implement the DFDL layer extension to support this).

Regarding compatability, because the dfdlx:blob type is defined internal to Daffodil (in dfdlx.xsd), other DFDL implementations are likely to error when this type is used in a schema because it is an uknown type to them. However, because the properties and restrictions are exactly the same as those for xs:hexBinary, some solutions for to allow compatability:

Implementations or users can define the "blob" DFDL extension simple type with a restriction of xs:hexBinary
DFDL schema aurthors could change all occurrences of dfdlx:blob in a schema to xs:hexBinary

Each of these solutions provide the current DFDL behavior of outputting hexBinary data to the infoset as is standard.

Daffodil API

With a new simple type defined, some changes to the API are needed to specify where Daffodil should write these new BLOB files. A likely usecase is a need to define a different BLOB output directory for each call to parse(). Thus, changes to the API must be made to define the output diretory either directly to the parse() function or to a paramter already passed to the parse function. Since the InfosetOutputter is related parse output, and the BLOB file is a sort of output, it makes the most sense for such definitions that control BLOB file output to added to the InfosetOutputter.

Two functions are added to the InfosetOutputter.

The first API function allows a way to set the properties used when creating BLOB files, including the output directory, and prefix/suffix for the BLOB file.

/**
 * Set an output directory and file prefix/suffix to be used when creating
 * files containing BLOB data. These values will only be used when Daffodil
 * parses an element with the dfdlx:blob type to determine which file to write
 * the blob data.
 *
 * The directory, prefix, and suffix parameters will be passed directly to
 * File.createTempFile to create a new empty file.
 *
 * @param directory The output directory to write BLOB files to. If None, any
 *                  attempts to parse dfdlx:blob data will result in a Schema
 *                  Definition Error.
 * @param prefix    The prefix string to be used in generating the BLOB file name.
 *                  Must be at least three characters long.
 * @param suffix    The suffix string to be used in generating the BLOB file name.
 */

final def setBlobOutputFileProperties(directory: Option[File], prefix: String, suffix: String)

The second API function allows a way for the API user to get a list of all BLOB files that were created during parse().

/**
 * Get the list of all BLOB files creates during this call to parse. It is the
 * responsibility of the caller to delete these files when appropriate.
 *
 * @return Sequence of File's containing BLOB data
 */
final def getBlobFiles(): Seq[File]

Note that no changes to the unparse() API are required, since the BLOB URI provides all the necessary information to retrieve files contianing BLOB data.

Schema Compilation

Schema compilation remains mostly the same. Daffodil will treat elements with the special dfdlx:blob type exactly the same as if the type were xs:hexBinary. The only difference being that a new flag will be passed to the existing HexBinary primitives/processor to alter their behavior to output to a file rather than to the infoset.

Parser Runtime

To support BLOBs, the BLOB parse logic follows these steps:

As with hexBinary, determine the starting bitPosition and length of the hexBinary content
Create a new BLOB file using directory/prefix/suffix information set in the InfosetOutputter. If the directory is None or creation of the temp file fails, throw a Schema Defintion Error.
Open the newly created file using using a FileOutputStream. If opening of the file fails, throw a Schema Definition Error.
Read length bytes of data from the ParseState dataInputStream and write them out to the FileOutputStream. Chunk the reads into smaller byte lengths to minimize total memory required and to support >2GB of data. If at any point no more bytes are available, throw a PENotEnoughBits parse error. If there is an IOException, throw a Schema Definition Error.
Close the file stream.
Set the value of the current element to the URI of the File.

Additionaly, logic must be created to remove BLOB files if Daffodil backtracks past an already created BLOB. This involves modifying the InfosetImpl so that when we restore state we inspect all removed children (potentially recursively?) for any that are BLOB types and delete the file specified by its URI. If we fail to delete a file, a Schema Defintion Warning should be emitted to alert users. This file should still be returned when getBlobFiles is called, even though the file will not be referened in the infoset. The caller is responsible for deleting this file since Daffodil could not remove it.

Unparser Runtime

To support BLOBs, the BLOB unparse logic follows these steps:

Get the URI from the infoset and the file length. If the length cannot be determined, throw an UnparseError.
As with hexBinary, determine the length of the hexBinary content and error if the BLOB file length is larger than the content length
Open the File using a FileInputStream. If opening of the file falis, throw an UnparseError
Read bytes from the FileInputStream and write them to the UState dataOutputStream. Chunk the reads into smaller byte lengths to minimize total memory required and to support >2GB of data. If at any point there is an IOException, throw an UnparseError.
As with hexBinary, write skip bits if the content length is not filled completely.

Note that we are explicitly not removing files after unparsing their data. It is the responsibility of the API user to determine when files are no longer needed and remove them.

TBD: We may need to add a feature so that if we unparse to a data output stream that is not direct (i.e. the backing OutputStream is a ByteArrayOutputStream with a 2GB limit), we should split it a new buffered output stream and continue writing to that.

DFDL Expression

There are going to be cases where expressions may want to reference elements with type dfdlx:blob.

This proposal adds the restriction that any expression access to the data of a BLOB element is not be allowed. This limitation is really for practical purposes. Presumably, the dfdlx:blob type is only to be used because the data is very large or meaningless, and so accessing the data is unnessary. This restriction minimizes comlexity since expression do not need to worry about converting blobs to byte arrays. If it is later determined that such a feature is needed, this restriction may be lifted. Any access to the data of a BLOB will result in a Schema Definition Error during schema compilation.

This proposal does allow for access to the length of a BLOB element. This is almost certainly needed since it is very common in data formats to include both a BLOB payload and the length of that payload. On unparse, we certainly almost certainly need the ability to calculate the length of the BLOB data so that the value can be output in a length field in the data. Fortunately, the content/valueLength functions to not actually query the data, but instead query bitPositions in stored in the infoset. Thus, no changes should be necessary to support this.

Minimal changes my be necessary to make the expression language aware of the dfdlx:blob type and act accordingly.

TDML Runner

Because the parsing of BLOBs results in a random URI in the infoset, this provides challenges to the TDML runners ability to compare expected and actual infosets. To resolve this, the TDML Runner will be modified in the following ways:

Use the new API to specify a temp directory for BLOBs to be stored
Perform type aware comparisions for the dfdlx:blob type, similar to what we do now for xs:date, xs:dateTime, and xs:time. Type awareness will be enable by using the xsi:type attribute on the expected infoset, since Daffodil does not currently supprt adding xsi:type information to the actual infoset as of yet. And example looks something like:
```
<tdml:dfdlInfoset>
  <data xsi:type="dfdlx:blob">file:///path/to/blob/data</data>
</tdml:dfdlInfoset>
```
During type aware comparisons, the TDML Runner will extract and modify the path (e.g. treat it as not absolute) to be suitable for use in logic similar to finding files using the type="file" attribute for expected infosets. Once the expected file is found, it will compare the contents of that file with the contents of the URI specified in the actual infoset and report any differences as usual.
After a test completes, delete all BLOB files listed in the InfosetOutputter

Command Line Interface

A new -b/--blob-dir option will be added to specify a custom blob directory, defaulting to "daffodil-blobs" in the current working directory if not specified. The directory should only be created when blobs are created.

The CLI will be modified to use the new BLOB API on the InfosetInputter to set the BLOB directory appropriately.

The CLI will never delete blob files.

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