This page gives background information and describes the issues that
are addressed in the BitstreamFormat Renovation project page.
It is also a manifesto of sorts that presents some of the reasoning behind
the new format architecture.
As we discuss it here, a "data format" is the description of how
intellectual content (i.e. the abstract meaning) is encoded in
a digital byte-stream to form what we call a digital object.
The format description is technical metadata, since it describes
how e.g. software would interpret the contents of a Bitstream to
deliver the intellectual content.
In Automatic Format Identification using PRONOM and DROID,
Adrian Brown defines a "data format" succinctly as:
The internal structure and encoding of a digital object,
which allows it to be processed, or to be rendered in human-accessible
Why Data Formats Matter
The purpose of a digital archive or repository like DSpace is to find
and deliver digital objects for users. However, the real goal of the
users is to get at the intellectual content encoded in those digital
objects. Providing a dissemination format that can readily be
interpreted by common desktop applications and accurately naming it
is an integral part of the job. If you doubt that, try changing the
BitstreamFormats of an Item's contents to Unknown and see how useful it is.
Now that we have established that some knowledge of data formats and
technical metadata is necessary to fulfill DSpace's mission, we'll
break down the various uses of technical metadata, and show
how much there is to know about data formats.
Not all uses of formats are relevant to every DSpace installation. For
example, preservation of digital objects has specific demands of formats,
but not every DSpace administrator is concerned with preservation.
However, the architecture has to support preservation activities
for those users that need them (and others who may be surprised at how
relevant preservation is to them after all).
What Goes Into a Data Format
Typically, the description of a format may include:
- A descriptive name, e.g.
- Identifiers, including:
- Rigorous definition of the format's encoding, and documentation showing how to interpret it.
- References to other formats upon which it is built, e.g. XML depends on the ASCII (or UNICODE) character set and UTF-8 encoding.
- Relationships to other formats as subtypes, families, etc.
- References to application code that generates or interprets the format.
- Classification descriptive metadata about the nature of the intellectual content or encoding.
Not all of this metadata about formats is relevant to the mission of DSpace.
And, fortunately, there are public format registries such as
PRONOM which are
already collecting and maintaining it.
DSpace Use of Data Formats
What does DSpace do with formats, and what should it be doing?
Each kind of use has its own requirements of the format technical metadata:
When disseminating a Bitstream e.g. through a Web-based UI, DSpace
needs a MIME type for format technical metadata. The only problem with
this is that MIME types are poorly standardized, so there might be
several valid identifiers for the same format. The recipient of the
dissemination can only handle certain MIME types, so if DSpace gives it
an unfamiliar one it will not render the content correctly.
To correct problems of patron's browsers not recognizing the MIME types
that DSpace sends, it may be more practical to adjust DSpace than
the browser. This is possible (and has been done) now, in DSpace 1.4.
Does anyone search on actual Bitstream formats? The DC type
element is similar to a format but not quite the same. Has anyone
configured DSpace to put the types present in member Bitstreams into
a search index for Items (or wanted to)?
The search criteria would probably be formats at a very coarse-grained
level, e.g. "image", "audio", "text".
DSpace uses format metadata to locate applications relevant to a Bitstream.
For example, the
MediaFilter mechanism processes a Bitstream
by getting its format, and looking for filters that accept that format
as their input.
This depends on the format of the Bitstream being accurately and
completely identified. If a Bitstream's format was set to the
wrong one or is completely unknown, it will
not be processed.
There must also be a way to describe the formats each application
This requires describing ranges of acceptable formats
in the DSpace configuration.
If we have thousands of formats available from a registry, it ought to
also offer some simpler means of describing the range of formats
accepted by an application, ideally without listing every individual
format. For example, if formats are modeled in a hierarchy or by
family groups, perhaps a range of fine-grained formats can be indicated
by naming its superclass or parent.
Data formats are also critical to all digital preservation operations:
To validate whether a Bitstream conforms to its identified format,
it is first necessary to know its format in precise (fine-grained) detail.
For example, if its format has distinct versions, the particular version
(e.g. "PDF 1.2") must be identified, so the validation is meaningful.
Obsolete Format Detection
This also requires fine-grained format identification, because some
versions of a format within a family of formats will usually go obsolete
before the rest, so formats must be identified down to the version.
Migration is very similar to the existing
application, since it is done by a collection of filters that translate
obsolete or unpreservable formats into formats more suited to digital
A fine-grained identification of formats is necessary so we can choose only the
Bitstreams in immediate danger of obsolesence, but then there has to
be a way to match that specific format against a possibly-coarser-grained
declaration of the formats accepted by a migration tool.
Why the DSpace Format Model is Inadequate
The data model and implementation of
originally intended as a placeholder to be supplanted by
an externally-developed format registry such as
the GDFR. Unfortunately,
progress has been slow in the field of format technical metadata,
so there hasn't been any obvious need to revisit the original design
decision because of the ultimate format registry coming on the scene.
There are some external format registries available now, however, and
a prototype implementation of the GDFR itself is under active development.
Since the FACADE project is dedicated to
improving DSpace's digital preservation capabilities,
resources are also available to upgrade DSpace's use of data formats.
Here are some particular issues to be addressed, explained
in greater detail below:
BitstreamFormatshort names are meaningless (as standard identifiers) outside of DSpace.
- Difficult to use external preservation tools without common format names.
- No fine-grained format representation.
- Lack of technical metadata useful in preservation (format documentation, links to tools, etc.)
- Cannot leverage format work done for other format registries.
Granularity of Data Format Technical Metadata
The granularity of a data format definition refers to how broad or
narrow its concept of the format is. A fine-grained format description
is limited to a particular indivisible version or subset of a format,
e.g. "PDF 1.2". A coarse-grained format encompasses an entire family
of formats, e.g. "PDF" (all versions and subsets), or "PostScript".
DSpace 1.4 includes descriptions of fewer than 40 coarse-grained formats.
Also, its method of identifying formats is crude and extremely
vulnerable to error and failure, since it depends on recognizing filename
extensions of uploaded content files.
A serious preservation effort demands fine-grained indentification of
file formats, and more reliable format identification.
We acknowledge that not every DSpace installation is required to
implement preservation on its contents, so it will still be possible
to configure simpler and coarser format identification tools.
Note that coarse-grained format descriptions can be useful as well,
so long as there is a way to discover the fine-grained formats that conform to each coarse format.
It is easier to cite a coarse-grained format when specifying the
formats acceptable to an application, or when searching for Items
with components of a given format.
Data Format Identifiers
Identifiers are names for data formats. There is obvious value in
having a set of identifiers that:
- Are globally recognized, can be exchanged meaninfully between Dspace, other archives, and applications.
- Each uniquely identify a data format, i.e. correspond 1:1 with formats.
- Indexes an entry in a publically-available format registry, which contains metadata about the formats.
What is Wrong With MIME Types
Many of today's most popular multimedia applications, such as Web browsers
and email user agents, recognize
as data format identifiers. I believe they employ MIME types because
they are already widely implemented as part of the mail system, not because
they are a good solution.
In practice, MIME types often fail to get
content rendered correctly, requiring adjustments and human intervention.
Problems with MIME Types:
- There is no authoritative standard that actually gets followed, resulting in multiple "valid" identifiers for the same format (e.g.
application/igeshave both been officially recommended at various times; unofficial usage is all over the map.)
- IANA attempts to provide a central registry.
- Applications receiving MIME types are wont to be configured with the types they commonly receive, regardless of the "standard".
- The same format can be legally described at different levels of granularity, e.g. these are all valid descriptions of RDF/XML:
Content-type: application/rdf+xml(specifies RDF encoded as XML)
Content-type: text/xml(only acknowledges the XML level of encoding)
- In practice, it is never used for fine-grained format identification. Each format has its own rules (if any) to distinguish versions within a MIME type.
It's worth noting that Apple Computer felt it necessary to
invent their own Uniform Type Identifiers
to take over the role of MIME types on MacOS X. The UTIs are
hiearchical, and cleanly represent families of formats and various granularities.
They give each vendor (or organization) a private namespace to control,
for unambiguous extensibility. It is not a file format
registry, however, since there is no metadata, just names.
DSpace Abuse of "License" Format Name
The current (1.5) DSpace data model applies the special
BitstreamFormat named "License" to all Bitstreams in the "LICENSE" bundle.
This is a convention in the data model that identifies the Bistreams
as, effectively, rights metadata. It has two serious flaws:
- Because it overloads the purpose of the BitstreamFormat metadata, there is no way to describe the actual data format of the License Bitstream.
- It is unnecessary and redundant, since all License Bitstreams are stored in the "LICENSE" bundle anyway.
Here is a case study that clearly illustrates the hazard of stealing a
Bitstream's technical metadata:
course of moving from a Windows to Unix platform,
this DSpace administrator has discovered
a digital preservation problem.
Many existing License Bitstreams need to be either converted in place or
labelled correctly on dissemination so they are understandable. If they
had honest BitstreamFormat metadata accurately identifying their format,
it would be straightforward to pick out the problem Bitstreams. As it is,
there may be License bitstreams representing XML and URIs for CreativeCommons,
so any mechanism to detect or fix them has to identify the file format
from the contents of the Bitstream. Also, given the existing format model,
the only way to repair this problem is to rewrite the License Bitstream in
the new preferred character set – which is also undocumented!
Value of External Format Registries
There are many advantages to using a data format registry outside
of DSpace as the main source of format technical metadata, so long
as it is stable and well-maintained.
- Formats have globally-unique, persistent identifiers
- Some other application talking to DSpace can refer precisely to a format with this external identifier, so it is mutually understandable.
- It provides technical metadata (and possibly tools) describing how to identify formats by internal and external characteristics.
- Includes references to standards documents and other descriptions.
- Registries typically include descriptive metadata showing relationships between formats, allowing fine-grained definitions to be grouped under coarse-grained formats.
- A central registry gets support and attention from other users, which we can leverage.
- Adding documentation of new formats
- Catching and correcting mistakes
- Monitoring formats for obsolescence