One document matched: draft-urpalainen-simple-xml-patch-ops-00.txt
SIMPLE WG J. Urpalainen
Internet-Draft Nokia Research Center
Expires: September 2, 2005 March 2005
An Extensible Markup Language (XML) Patch Operations Framework
Utilizing XML Path Language (XPath) Selectors
draft-urpalainen-simple-xml-patch-ops-00
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Extensible Markup Language (XML) documents are widely used as
containers for the exchange and storage of arbitrary data in today's
systems. Updates to this data requires transporting of the entire
XML instance document between hosts, unless there's a mechanism that
allows transmitting only the changes of XML documents. This memo
describes a framework utilizing XML Path language (XPath) selectors
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which can be used to apply a set of patches to an existing XML
document.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of patch operations . . . . . . . . . . . . . . . . 4
3.1 <add> element . . . . . . . . . . . . . . . . . . . . . . 5
3.2 <replace> element . . . . . . . . . . . . . . . . . . . . 7
3.3 <remove> element . . . . . . . . . . . . . . . . . . . . . 8
4. Error handling . . . . . . . . . . . . . . . . . . . . . . . 9
5. Usage of patch operations . . . . . . . . . . . . . . . . . 9
6. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. XML Schema . . . . . . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
8.1 XML Schema Registration . . . . . . . . . . . . . . . . . 13
9. Security considerations . . . . . . . . . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1 Normative references . . . . . . . . . . . . . . . . . . 14
11.2 Informative references . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . 16
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1. Introduction
Extensible Markup Language (XML) [2] documents are widely used as
containers for the exchange and storage of arbitrary data in today's
systems. An example of such a system is the Common Presence Profile
(CPP) [11] compatible presence system, in which presence data is
represented using the XML based Presence Information Data Format
(PIDF) [12]. Updates to this data requires transporting of the
entire XML instance document between hosts, unless there's a
mechanism that allows transmitting only the changes of a document.
This memo describes a framework utilizing XML Path language (XPath)
[3] selectors which can be used to apply a set of patches to an
existing XML document.
These changes or synonymously patch operations as used in this memo,
are described by defining XML Schema element types which can be
embedded within an application specific XML diff document. The full
content of these application specific documents is not defined in
this document, but instead, applications utilizing these element
types MUST define the full format with a suitable MIME [7] type.
An XPath selector is used to locate a single unique node from the
existing XML document. Once the node which pinpoints the target for
the modification has been found, modifications like additions,
removals or substitutions of elements and attributes can be done.
As an example, in the Session Initiation Protocol (SIP) [13] based
presence system a partial PIDF XML document format [9] consists of
the existing PIDF document format combined with the patch operations
elements. In general, the patch operations can be used in any
application that exchanges XML documents, e.g. in the SIP Events [8]
framework.
The aim of this memo is to describe a deterministic framework where
only a single possible canonical form [4] of the XML document exists
once the patches have been applied onto it. Especially significant
whitespace text nodes MUST be processed properly in order to fulfil
this requirement.
2. Conventions
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [1] and
indicate requirement levels for compliant implementations.
The following terms are used in this document:
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XML diff document: This is the document that will carry all the patch
operations, namespace declarations and all the document content
changes. It is the frame XML document which includes all the
patch operation elements.
Patched document: This is the local copy of the XML document onto
which all the patch operations in the XML diff document are being
applied.
Patch operation: This is a single change that will be applied to the
patched document. Defined operations in this memo are: add,
remove and replace. Corresponding XML elements contain XPath
selector values and also added or changed document content.
3. Overview of patch operations
The XML diff document contains a collection of patch operations: add,
replace or remove which will be applied one-by-one in the given
order. Each of these XML Schema types contains a 'sel' attribute.
The value of this attribute is an XPath selector with a restricted
subset of the full XPath 1.0 recommendation. The following XPath 1.0
data model node types can be added, replaced or removed with this
framework: elements, attributes, namespaces, comments, texts and
processing instructions.
When the patched document has namespace declarations, QName [5]
expansion within the location step is done according to the namespace
declarations of the XML diff document. Thus the namespace URIs for
the prefixes within the evaluation step strings are easily found from
the XML diff document. If the XML diff document uses a default
namespace declaration and there is no prefix used in a node test then
that step is interpreted as if it had had a prefixed name associated
with this same namespace URI. This is of course only relevant when
there's a reference to an element within the location step. It
should be emphasized that prefix names within the XML diff document
do not have to be the same than that of the patched document as node
matching is based on the equivalent namespace URIs and local names.
While the XPath recommendation specifies that prefixes can be used in
location steps, it does not specify how associated namespace URIs are
to be found during the evaluations. However, it allows using
"namespace-uri()" and "local-name()" functions within XPath
predicates. In practice, these functions may then be utilized if
there are no other means to "register" prefixes with associated
namespace URIs. The Schema types defined in this memo do not allow
using these functions.
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When locating the target for a patch operation, all XPath selections
start from the root node of the document. Thus only relative
location paths are used. When locating elements in the document
tree, the node test can be either a "*" character or a QName. A "*"
character selects all element children of the context node.
Attribute value comparisons can be used as predicates. Also text
content of the current "." or a child element can alternatively be
used to identify elements in the tree. The character "." is an
abbreviated form of "self::node()". Finally positional constraints
can also be used as an additional predicate. Ordering of these
positional constraints and value comparisons can interchange.
As all the namespace declarations relevant to the patch operations
are within the XML diff document, the element names within the
optional data content are also fully namespace qualified. As with
location step strings the prefixes of these elements are not
significant only the namespace URIs MUST match. The process that
performs all these patch operations does not add new or change
existing namespace declarations based on the prefixes and
declarations of the XML diff document. Instead, it keeps the
declarations that already exist within the patched document and
matches XML nodes according to the local names and namespace URIs.
If the intention is to add new namespace declarations to the patched
document then their declaration MUST be within the added or changed
data content or they MUST be explicitly added by using XPath
namespace axis semantics shown later in this document.
The XML Schema defines element types for these patch operations. The
XML Schema is intended to be included into the other XML Schemas that
utilize these operations: e.g. partial PIDF [9]. It will provide a
relevant XML diff document context. As this Schema does not declare
a target namespace, elements defined according to these types inherit
the target namespace of the including Schema. Furthermore, it is
anticipated that applications using these types will define <add>,
<replace> and <remove> elements from the corresponding types defined
in this Schema. Also applications MAY either extend or restrict the
types described in this document as some applications MAY not need
all the provided features. The XML document fragments defined in
this draft MUST be well formed and valid.
3.1 <add> element
The <add> element type has three attributes: 'sel', 'type' and 'pos'.
The value of the 'sel' attribute is used to select a single unique
element from the document to be patched. It is an error condition if
multiple nodes will be found during the evaluation of this selector
value.
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The value of the optional 'type' attribute is used to describe the
type of the new data content. The new data content exists as child
node(s) of this <add> element. The value of 'type' attribute is also
an XPath 1.0 compatible selector with a very limited set of XPath
features. Once the new content within the <add> element contains
elements, they will include all the attribute, namespace and
descendant nodes. As the default value of the 'type' attribute is
"node()" the new content can be element, text, comment or processing
instruction nodes or a mixture of them. The "node()" selector value
is an abbreviated form of "child::node()". A positional constraint
can also be used with the "node()" selector when only a single node
needs to be added. If the value of the 'type' attribute equals
"@attr" the purpose is to add a new 'attr' attribute. The value of
the 'attr' attribute is then the text content of the <add> element.
The less frequently used, prefixed attributes can also be added. If
the value of the 'type' attribute equals "namespace::pref" the aim is
to add a new "pref" prefixed namespace declaration and the text
content of the <add> element is then the corresponding namespace URI.
The value of the optional 'pos' attribute indicates the positioning
of the new data content. As the default value is "to", the new
content is then simply added onto the found element based on the
value of 'sel' selector. For other node types than attribute and
namespace nodes new content is appended as the last child node(s).
With the value of "before" the new content MUST be the closest
preceding sibling node(s) and with "after" the closest following
sibling node(s). Naturally the usage of "before" and "after" is only
allowed with other types than attributes and namespaces. They can be
used e.g. when a comment node is added just before or after a
particular element which was found based on the 'sel' selector value.
Appending elements with all the descendant and attribute nodes is one
of the most typical operations. The default values of 'type' and
'pos' attributes allow that the <add> element MAY then only contain
the 'sel' attribute with the added content.
Some examples without any namespaces in XPath selectors or elements
and patch operation elements are also not having any namespaces
attached:
<add sel="root"><elem id="ert4773">This is a new child</elem></add>
Once the <root> element has been found from the document, an <elem>
element is appended as the last child of the <root> element.
<add sel="root/elem[@id='ert4773']" type="@user">Bob</add>
This operation adds a new 'user' attribute to the <elem> element.
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The value of this attribute is "Bob".
It should be noted that as the 'sel' selector value MAY contain
quotation marks, escaped forms: " or ' can then be used.
However, it is often more appropriate to use the apostrophe (')
character as shown in these examples. An alternative is also to
interchange the apostrophes and quotation marks.
<add sel="root" type="namespace::pref">urn:ns:xxx</add>
This operation adds a new namespace declaration to the <root>
element. The prefix name of a new namespace node is thus "pref" and
the namespace URI "urn:ns:xxx".
<add sel="root/elem[@id='ert4773']" pos="before"><!-- comment
--></add>
This operation adds a new comment node just before the <elem> element
as the closest preceding sibling node.
Some complexity arises when so called whitespace text nodes exist
within the patched document. The XPath 1.0 data model requires that
a text node can not have another text node as a sibling node. For
instance, if an add operation is like this:
<add sel="root">
<elem id="ert4773">This is a new child</elem></add>
The <add> element in this example has then two child nodes: a
whitespace text node and an <elem> element. If the last child of the
<root> element is a text node, it's content and the whitespace text
node content MUST be catenated together. Otherwise whitespace text
nodes can be added just like elements and thus the canonical form of
the patched XML document easily remains deterministic.
3.2 <replace> element
The <replace> element type has only one attribute: 'sel'. The value
of this attribute is used to select a single unique node from the
document. If the target node which was found based on the 'sel'
selector value is an element, then the child of the <replace> element
MUST also be an element. Otherwise the <replace> element MUST have
text content. Examples for replace operations:
<replace sel="root/elem[@a='1']"><update a="2"/></replace>
This will replace the <elem> with the <update> element.
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<replace sel="root/@a">new content</replace>
This will replace the attribute 'a' of the <root> element with the
value "new content".
<replace sel="root/namespace::pref">urn:new:xxx</replace>
This will replace the URI value of 'pref' prefixed namespace node
with "urn:new:xxx".
<replace sel="root/comment()[1]"> This is the new content </replace>
This will replace the comment node content with the above string.
<replace
sel="root/processing-instruction('foo')">bar="foobar"</replace>
This will replace the content of the processing instruction node
"foo".
<replace sel="root/elem/text()[1]">This is the new text
content</replace>
This will replace the first text node content of the <elem> element.
Usually the positional constraint e.g. "[1]" is not needed as the
element content is rarely of mixed type [6].
3.3 <remove> element
The <remove> element type has two attributes: 'sel' and 'ws'. The
value of the 'sel' attribute is used to select a single unique node
from the document. The value of the optional 'ws' attribute is used
to remove the possible whitespace text nodes that are either the
closest following or preceding sibling nodes of the found node. The
usage of 'ws' attribute is only meaningful when removing other types
than text, attribute and namespace nodes. As the default value of
'ws' attribute is "none", removal of whitespace nodes is thus not
requested. If the value of 'ws' is "before", the purpose is to
remove the closest preceding sibling node which MUST be a whitespace
text node and if the value is "after", the corresponding following
node. If the 'ws' value is "both", both the preceding and following
whitespace text nodes MUST be removed. Examples for remove
operations:
<remove sel="root/elem[@a='1']" ws="after"/>
This will remove the <elem> element as well as the closest following
sibling node of the <elem> element. This sibling node MUST be a
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whitespace text node.
<remove sel="root/@a"/>
This will remove the 'a' attribute node from the <root> element.
<remove sel="root/namespace::pref"/>
This will remove the 'pref' prefixed namespace node from the <root>
element. Naturally this prefix MAY not be used by any node prior to
the removal of this namespace node.
<remove sel="root/comment()[1]"/>
This will remove the first comment node from the <root> element.
<remove sel="root/processing-instruction('foo')"/>
This will remove the processing instruction node "foo" from the
<root> element.
<remove sel="root/elem/text()[1]"/>
This will remove the first text node content from the <elem> element.
If for example an element, a comment node or a processing instruction
node which has a whitespace text node as both the closest preceding
and following node, is removed without a request to remove
whitespaces, the content of these two whitespace nodes MUST then be
catenated together to the remaining single whitespace node.
4. Error handling
It is an error condition if any of the given operations can not be
unambiguously fulfilled. However, it is beyond the scope of this
document to describe a generic error response.
5. Usage of patch operations
The XML diff document SHOULD contain only those nodes which have been
modified. However, when there's a large collection of changes it MAY
be desirable to transport the full document content instead. How
this will be done in practice is beyond the scope of this document.
6. Examples
A document to be patched:
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<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="urn:ietf:params:xml:ns:xxx"
xmlns:z="urn:ietf:params:xml:ns:yyy">
<note>This is a sample document</note>
<elem a="foo">
<child/>
</elem>
<elem a="bar">
<z:child/>
</elem>
</root>
An imaginary XML diff document where prefix "p" corresponds to the
targetNamespace of this imaginary XML Schema:
<?xml version="1.0" encoding="UTF-8"?>
<p:diff xmlns="urn:ietf:params:xml:ns:xxx"
xmlns:y="urn:ietf:params:xml:ns:yyy"
xmlns:p="urn:ietf:params:xml:ns:diff">
<p:add sel="root/elem[@a='foo']"> <!-- This is a new child -->
<child id="ert4773">
<y:node>
</child>
</p:add>
<p:replace sel="root/note/text()">Patched doc</p:replace>
<p:remove sel="*/elem[@a='bar']/y:child" ws="both"/>
<p:add sel="*/elem[@a='bar']" type="@b">new attr</p:add>
</p:diff>
One possible form of the resulting document after applying the
patches:
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<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="urn:ietf:params:xml:ns:xxx"
xmlns:z="urn:ietf:params:xml:ns:yyy">
<note>Patched doc</note>
<elem a="foo">
<child/>
<!-- This is a new child -->
<child id="ert4773">
<z:node/>
</child>
</elem>
<elem a="bar" b="new attr"/>
</root>
7. XML Schema
The XML schema types for the patch operations.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE schema [
<!ENTITY ncname "[^:\I][^:\C]*">
<!ENTITY qname "(&ncname;:)?&ncname;">
<!ENTITY aname "@&qname;">
<!ENTITY pos_t "\[\d+\]">
<!ENTITY attr_t "\[&aname;=('|")(.)*('|")\]">
<!ENTITY name_t "\[(&qname;|\.)=('|")(.)*('|")\]">
<!ENTITY cond "(&attr_t;|&name_t;)?(&pos_t;)?|(&pos_t;)?(&attr_t;|&name_t;)?">
<!ENTITY step "(&qname;|\*)(&cond;)?">
<!ENTITY pi "processing-instruction\((('|")&qname;('|"))?\)">
<!ENTITY comm "comment\(\)">
<!ENTITY text "text\(\)">
<!ENTITY nspace "namespace::&ncname;">
<!ENTITY last "&step;|&aname;|&nspace;|(&comm;(&pos_t;)?)|&text;(&pos_t;)?|π(&pos_t;)?">
]>
<xsd:schema
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
elementFormDefault="qualified">
<xsd:simpleType name="xpath">
<xsd:restriction base="xsd:string">
<xsd:pattern value="(&step;/)*(&last;)"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="xpath-elem">
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<xsd:restriction base="xsd:string">
<xsd:pattern value="(&step;/)*(&step;)"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="pos">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="to"/>
<xsd:enumeration value="before"/>
<xsd:enumeration value="after"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="type">
<xsd:restriction base="xsd:string">
<xsd:pattern value="node\(\)(&pos_t;)?"/>
<xsd:pattern value="&aname;"/>
<xsd:pattern value="&nspace;"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="add">
<xsd:complexContent mixed="true">
<xsd:restriction base="xsd:anyType">
<xsd:sequence>
<xsd:any processContents="lax" namespace="##any"
minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
<xsd:attribute name="sel" type="xpath-elem"
use="required"/>
<xsd:attribute name="pos" type="pos"
default="to"/>
<xsd:attribute name="type" type="type"
default="node()"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
<xsd:complexType name="replace">
<xsd:complexContent mixed="true">
<xsd:restriction base="xsd:anyType">
<xsd:sequence>
<xsd:any processContents="lax" namespace="##any"
minOccurs="0" maxOccurs="1"/>
</xsd:sequence>
<xsd:attribute name="sel" type="xpath"
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use="required"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
<xsd:simpleType name="ws">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="none"/>
<xsd:enumeration value="before"/>
<xsd:enumeration value="after"/>
<xsd:enumeration value="both"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:complexType name="remove">
<xsd:attribute name="sel" type="xpath" use="required"/>
<xsd:attribute name="ws" type="ws" default="none"/>
</xsd:complexType>
</xsd:schema>
8. IANA Considerations
8.1 XML Schema Registration
This section registers a new XML Schema.
URI:
urn:ietf:params:xml:schema:xml-patch-ops
Registrant Contact:
IETF, SIMPLE working group, <simple@ietf.org>
Jari Urpalainen, <jari.urpalainen@nokia.com>
9. Security considerations
Information transported within these patch operations can be highly
sensitive. Thus systems need to protect the integrity and
confidentiality of this data. Especially, the transport protocol
SHOULD have capabilities to protect from possible threats.
10. Acknowledgments
The author would like to thank Eva Leppanen, Mikko Lonnfors, Aki
Niemi, Jonathan Rosenberg and Miguel A. Garcia for their valuable
comments.
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11. References
11.1 Normative references
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] "Extensible Markup Language (XML) 1.0 (Third Edition)", W3C
Recommendation REC-xml-20040204 , February 2004.
[3] "XML Path Language (XPath) Version 1.0", W3C Recommendation
REC-xpath-19991116 , November 1999.
[4] "Canonical XML 1.0", W3C Recommendation REC-xml-c14n-20010315 ,
March 2001.
[5] "Namespaces in XML", W3C Recommendation REC-xml-names-19990114 ,
January 1999.
[6] "XML Schema Part 1: Structures Second Edition", W3C
Recommendation REC-xmlschema-1-20041028 , October 2004.
11.2 Informative references
[7] Murata, M., "XML media types", RFC 3023, January 2001.
[8] Roach, A., "Session Initiation Protocol (SIP)-Specific Event
Notification", RFC 3265, June 2002.
[9] Lonnfors, M., Leppanen, E., Khartabil, H. and J. Urpalainen,
"Presence Information Data format (PIDF) Extension for Partial
Presence", draft-ietf-simple-partial-pidf-format-04 (work in
progress), February 2005.
[10] Lonnfors, M., Costa-Requena, J., Leppanen, E. and H. Khartabil,
"Session Initiation Protocol (SIP) extension for Partial
Notification of Presence Information",
draft-ietf-simple-partial-notify-04 (work in progress),
February 2005.
[11] Peterson, J., "Common Profile for Presence (CPP)", RFC 3859,
August 2004.
[12] Sugano, H., "CPIM presence information data format", RFC 3863,
May 2003.
[13] Niemi, A., "Session Initiation Protocol (SIP) Extension for
Event State Publication", RFC 3903, October 2004.
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Author's Address
Jari Urpalainen
Nokia Research Center
Itamerenkatu 11-13
Helsinki 00180
Finland
Phone: +358 7180 37686
Email: jari.urpalainen@nokia.com
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