XDM Text Serialization

A proposal for text serialization of XDM for purposes of data interchange and interoperability.

Abstract

The XQuery 1.0 and XPath 2.0 Data Model (XDM) references a Serialization format for XDM. This format is "lossy" and does not reconstruct into the original XDM. In particular, for purposes of data interchange, the losses are severe; sequences are "normalized" and type information is discarded.
This proposal is to define a standard format for serializing XDM data for purposes of data interchange and interoperability preserving more of the XDM information.

Intent

The intent of this specification is to provide a standard format for interchange of data between XML tools which produce and consume XDM data. Examples of XML tools which produce or consume XDM data include XPath (2.0), XSLT , XQuery, but also include other tools such as XProc, xmlsh and countless custom written programs using the XDM data model.

In current implementations there is no standard model for XDM data either within the same environment and language, or across languages and environements. For example, suppose an XQuery operation produces a sequence and it is desired to provide that sequence as a parameter to XSLT transformation, there is no standardized way to exchange the data. In practice in order to accomplish this, either the same vendor tools must be used within the same language and process, or the results must be serialized in a proprietary format and reconstituted in the target using the same proprietary format. Even with the same vendors implementations interchange is not always easy due to differences in API layers, languages, or transferring data across process or machine boundaries.

This proposal provides for a standard serialization format so that XDM data can be interchanged across tools, vendors, languages, environments and machines.

Definitions

For the purposes of this document the following definitions apply

XDM Tool A program, module, function or API which can produce or consume XDM Data in some form.
XDM Consumer An XDM Tool which can consume (allow as input, arguments) XDM Data.
XDM Producer An XDM Tool which can produce XDM data (on output, return or output variables)
Environment An instance of the runtime of a single language with native language types, typically a single process.
XDM Stream An in-memory or on-"disk" (file) representation of a single instance of XDM (a sequence zero or more items)

Goals

This proposal expresses multiple goals, not all of which may be possible to achieve. The use cases describe concrete examples of many of the goals, while this summary provides the intent.

A standardized text representation of XDM data preserving as much of the XDM model as reasonable.
- Individuality of sequence items
- Types of atomic items
A representation that can be easily implemented using existing vendors XML technology.

Some purposes for which this standard could be used include

Exchange of XDM data between XDM Tools in different environments
Exchange of XDM data between XDM Tools from different vendors in the same environment
Exchange of XDM data between XDM Tools from the same vendor in the same environment where it is difficult to preserve the vendors native data structure
Exchange of XDM Data between XDM Tools and tools which are not XDM capable, or with limited XDM capability.
Provide a human readable output of XDM data
Output of XDM data from test cases using XDM Tools with the purposes of validation and compare
Standardization of a format for use in XML Pipeline Processors so that steps can be implemented by different vendors or in different languages.
Incremental appending of XDM data to a file or stream without having to go back and rewrite or remove end markers.
Incremental reading of XDM data. Each item in the sequence should be readable or skipped without consuming the entire stream.

XDM Information Preserved and Lost

Preserving all of the information in the XDM is very difficult, and likely why a serialization model for XDM has not been specified. This proposal recognizes that not all XDM information is equally important. In the context of the Use Cases, and with the goals of reasonable implementation with existing vendor libraries this proposal aims at preserving some XDM information at the expense of others.

XDM Information preserved

The XDM Model defines values as a sequence of zero or more items. Each item is one of the following types

Atomic Type
Node type
- document, element,attribute, text, namespace, processing , instruction, comment.

Each type has a value. Atomic types have string values, and node types have XML values.

An XDM serialization format should preserve the following attributes

Sequences
- Sequences should not be normalized. Sequences should preserve the individuality, count and type of items. Adjacent atomic item should not be contented (normalized).
Atomic Types and values
- Atomic types are preserved with the expanded QName for the type and the string value
Nodes
- Nodes values are preserved for each 8 of the XDM Node types

Serialized XDM will retain information about the descendants of nodes in the sequence being serialized, but it will not retain information about their ancestors.

XDM Information NOT preserved

Ancester information. An XDM Node serialized will NOT maintain information about its ancestors. For example if a node $a is serliazed then $a/.. is NOT maintained.
Serialized XDM will not retain information about node identity: that is, the recipient of the serialized XDM will not be able to determine whether two serialized elements originated from the same node or merely from two nodes that were deep-equal to each other.

Schema and Type information. XDM Serialization does NOT transfer type defininitions. The consumer of the serialized XDM is assumed to have access to the same schema as the producer of the serialized XDM: that is, a QName identifying a type is assumed to have the same meaning to both the producer and consumer.

Use Cases

Use cases are concrete examples that demonstrate the goals.

Use Case 1 Exchange of XDM data between XDM Tools in different environments
Use Case 2 Exchange of XDM data between XDM Tools from different vendors in the same environment
Use Case 3 Exchange of XDM data between XDM Tools from the same vendor in the same environment where it is difficult to preserve the vendors native data structure
Use Case 4 Exchange of XDM Data between XDM Tools and tools which are not XDM capible.
Use Case 5 Provide a human readable output of XDM data
Use Case 6 Output of XDM data from test cases using XDM Tools with the purposes of validation and compare
Use Case 7 Standardization of a format for use in XML Pipeline Processors so that steps can be implemented by different vendors or in different languages.
Use Case 8 Creating and reading expanding files, such as logging data.

Serialization Format

Serialization Format Properties

The Goals and Use Cases provide rationale for which the desired properties of a serialization format can be derived.

An In-memory and on-"disk" (file) XDM Stream should be easily convertible. Use Case 1 Use Case 4
An XDM Stream must be be able to represent all unicode codepoints expressed in XDM (and by inference XML).
An XDM stream containing one sequence of 1 item should be represented the same as an XDM stream containing 1 item.
A text or binary concatenation of two XDM streams should produce the same format as the equivalent XDM Stream composed of a concatenation of the 2 XDM sequences Use Case 8 .
Serialized XDM must be viewable in standard text file tools. Use Case 5
Serialized XDM must survive line ending character sequence changes which may be introduced when copying the file or saving the file from a text editor. Use Case 1 Use Case 5
An XDM Stream must be able to be incrementally generated without writing an end marker (indicating end of the entire stream), and read without having to wait for an EOF marker Use Case 8.
An XDM Stream should be able to be parsed such that each item can be efficiently recognized and extracted without having to parse the content of the item. Use Case 8.

Abstract Form

The Serialization Format Properties suggest a possible Abstract Form comprised of a sequence of zero 0r more of the following markup.

ITEM END_MARKER

The empty sequence being represented by an empty stream.

Where "ITEM" is the serialized form of a single XDM Item, and "END_MARKER" is a character sequence which is not allowed within the serialized form of "ITEM". The END_MARKER is an item terminator not a separator. This allows concatenation of zero or more XDM Streams to be an XDM Stream without having to insert data Use Case 8.

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