Introduction

Salve (Schema-Aware Library for Validation and Edition) is a TypeScript library which implements a validator able to validate an XML document on the basis of a subset of Relax NG (RNG). It is developed as part of the Buddhist Translators Workbench. It can be seen in action in wed.

Salve is used for validating XML with custom Relax NG schemas. We've also validated files that use the TEI standard and the DocBook v5.0 schema. We want to support as much Relax NG as reasonably possible, but salve currently has the following limitations:

• XML Schema types ENTITY and ENTITIES are treated as a string.

• None of the XML Schema types that deal with time allow the parameters minInclusive, minExclusive, maxInclusive and maxExclusive.

• Salve does not verify that numerical values validated as float or double fit within the limits of float or double. (This is a common limitation of validators. We tested with jing and xmllint --relaxng and found they do not raise errors if, for instance, a validation that expects a float is given a value that cannot be represented with a float.)

If someone wishes to use salve but needs support for any of the features that are missing, they may ask for the feature to be added. Submit an issue on GitHub for it. If you do submit an issue to add a feature, please make a case for it. Even better, if someone wishes for a feature to be added, they can contribute code to salve that will add the feature they want. A solid contribution is more likely to result in the feature being speedily added to salve than asking for us to add the feature, and waiting until we have time for it.

A full validation solution has the following components:

• A tokenizer: responsible for recognizing XML tokens, tag names, tag delimiters, attribute names, attribute values, etc.

• A parser: responsible for converting tokens to validation events (see below).

• A well-formedness checker. Please check the Events section for more information about what this concretely means.

• A validator: responsible for checking that validation events are valid against a schema, telling the parser what is possible at the current point in validation, and telling the parser what is possible in general (e.g., what namespace uris are used in the schema). This is what salve offers, and only this!

A good example of this division of labor can be found in bin/parse.js and in the test suite. In both cases the tokenizer function is performed by saxes, and the parser function is performed by a parser object that saxes creates, customized to call salve's Walker.fireEvent().

Salve has a sister library named salve-dom which uses the parsing facilities available in a browser to provide the tokenizer, well-formedness and parser components described above.

NOTE: If you are looking at the source tree of salve as cloned from GitHub, know that executables cannot be executed from bin. They can be executed after a build, from the build/dist/bin directory.

If you are looking at the files installed by npm when you install salve as a package, the files in bin are those you want to execute.

Basic Usage

A typical usage scenario would be as follows:

// Import the validation module
const salve = require("salve");

// Source is a URL to the Relax NG schema to use. A ``file://`` URL
// may be used to load from the local fs.
const grammar = salve.convertRNGToPattern(source).pattern;

// Get a walker on which to fire events.
const walker = grammar.newWalker();

Then the code that parses the XML file to be validated should call fireEvent() on walker. Remember to call the end() method on your walker at the end of validation to make sure that there are no unclosed tags, etc.

The file bin/parse.js (included in salve's source but not in the npm module) contains an example of a rudimentary parser runnable in Node.js::

$ node ....../parse.js [rng] [xml to validate]

The [rng] parameter is the Relax NG schema, recorded in the full XML format used by Relax NG (not the compact form). The [xml to validate] parameter is the XML file to validate against the schema.

Converting Schemas

As you can see above, a Relax NG schema, stored in XML, needs to be converted to salve's internal format before salve can use it. Internally this happens:

• The XML file recording the Relax NG schema is converted to a tree of objects representing the XML.

• The XML tree is validated against the Relax NG schema.

• The XML tree is simplified as described in the Relax NG specification.

• Constraints specified by the Relax NG specification are checked.

• The XML tree is converted to a "pattern", which is a structure internal to salve.

The simplest usage is like this::

const result = salve.convertRNGToPattern(source);

By default the conversion returns an object with a grammar stored on the pattern field, may reveal some simplification warnings on the warnings field, and provides a the simplified schema as an XML tree on the simplified field. In trivial use-case scenarios, only pattern and warnings are used.

In some cases, the code using convertRNGToPattern may want to serialize the result of simplification for future use. To do this, it should use writeTreeToJSON and pass the value of the simplified field to serialize the simplified XML tree to JSON. The serialized JSON may then be read with readTreeFromJSON to create a structure identical to the original pattern. Consider the following code::

const result = salve.convertRNGToPattern(source);
const json = salve.writeTreeToJSON(result.simplified);
const x = salve.readTreeFromJSON(json);

After executing it, x contains a pattern which represents the same Relax NG schema as result.pattern.

Do note that that writeTreeToJSON takes an XML tree and produces JSON, whereas readTreeFromJSON reads a JSON and produces a pattern rather than an XML tree. There is currently no use-case scenario that requires the two functions to mirror one-another.

Optionally, you may pass an options object as the 2nd argument of convertRNGToPattern with createManifest set to true. If you do this, then you also get a manifest field which is an array of objects containing file paths and their corresponding hashes. Manifests are useful to allow systems that use salve to know whether a pattern needs to be regenerated with convertRNGToPattern. This is necessary if the system allows a schema to change after use. Consider the following scenario. Alice uses an XML editor that uses salve to perform on the fly validation.

1. Alice edits the file foo.xml with the schema foo.rng. It so happens that foo.rng imports math.rng. The editor will use convertRNGToPattern to convert foo.rng and math.rng into the format salve needs. It also uses writeTreeToJSON to cache the result. Alice sees a brief progress indicator while the editor converts the schema.

2. Over the span of a week, Alice continues editing foo.xml. Each time she opens the editor, the editor uses readTreeFromJSON to load the tree from cache instead of using convertRNGToPattern over and over. As far as Alice is concerned, the editor starts immediately. There's no progress indicator needed because readTreeFromJSON is super fast.

3. Alice then changes math.rng to add new elements. When Alice starts the XML editor to edit foo.xml again, the XML editor must be able to detect that the schema needs to go through convertRNGToPattern again because math.rng has changed.

The manifest is used to support the scenario above. If the XML editor stores the converted schema, and the manifest into its cache, then it can detect if and when it needs to convert the schema anew.

Security note: It is up to you to decide what strength hash you need. The manifest is not designed for the sake of providing security. So its hashes are not designed to detect willful tampering but rather to quickly determine whether a schema was edited. In the vast majority of real world usage scenarios, using a stronger hash would not provide better security because if an attacker can replace a schema with their own file, they also can access the manifest and replace the hash in the manifest.

Events

Salve expects that the events it receives are those that would be emitted when validating a well-formed document. That is, passing to salve the events emitted from a document that is malformed will cause salve to behave in an undefined manner. (It may crash. It may generate misleading errors. It may not report any errors.) This situation is due to the fact that salve is currently developed in a context where the documents it validates cannot be malformed (because they are represented as DOM trees). So salve contains no functionality to handle problems with well-formedness. Salve can be used on malformed documents, provided you take care of reporting malformedness issues yourself and strategize how you will pass events to salve.

Multiple strategies are possible for using salve in a context where well-formedness is not guaranteed. There is no one-size-fits-all solution here. A primitive parser could abort as soon as evidence surfaces that the document is malformed. A more sophisticated parser could process the problematic structure so as to generate an error but give salve something well-formed. For instance if parsing <foo></baz>, such parser could emit an error on encountering </baz> and replace the event that would be emitted for </baz> with the event that would be emitted for </foo>, and salve will happily validate it. The user will still get the error produced by the parser, and the parser will still be able to continue validating the document with salve.

The parser is responsible for calling fireEvent() on the walker returned by the tree created from the RNG. (See above.) The events currently supported by fireEvent() are defined below:

"enterStartTag", [uri, local-name] Emitted when encountering the beginning of a start tag (the string "<tag", where "tag" is the applicable tag name) or the equivalent. The qualified name should be resolved to its uri and local-name components.

"leaveStartTag", [] Emitted when encountering the end of a start tag (the string ">") or equivalent.

"endTag", [uri, local-name] Emitted when encountering an end tag.

"attributeName", [uri, local-name] Emitted when encountering an attribute name.

"attributeValue", [value] Emitted when encountering an attribute value

"text", [value] Emitted when encountering text. This event must be fired for all instances of text, including white space. Moreover, salve requires that you fire one text event per consecutive sequence of text. For instance, if you have the text foo bar you may not fire one event for foo and another for bar. Or if you have a sequence of lines, you may not fire one event per line. You have to concatenate the lines and fire a single text event.

Do not generate text events with an empty string as the value. (Conversely, a valid document must have an attributeValue for all attributes, even those that have empty text as a value.)

Salve support a couple of compact events that serve to pass as one event data that would normally be passed as multiple events:

"attributeNameAndValue", [uri, local-name, value] Combines the attributeName and attributeValue events into one event.

"startTagAndAttributes", [uri, local-name, [attribute-data...]] Combines the enterStartTag, attributeNameAndValue and leaveStartTag events. The attribute-data part of the event must be a sequence of uri, local-name, value as would be passed to with attributeNameAndValue.

For instance if an element named foo has the attribute a with the value valA, the event would be: "startTagAndAttributes", "", foo, "", "a", "valA".

.. note:: The compact events do not allow salve to be very precise with reporting errors. It is recommended to use them only when optimizing for speed, at the expense of precision.

.. note:: When reporting possible events, salve never returns compact events in the list.

The reason for the set of events supported is that salve is designed to handle not only XML modeled as a DOM tree but also XML parsed as a text string being dynamically edited. The best and closest example of this would be what nxml-mode does in Emacs. If the user starts a new document and types only the following into their editing buffer::

<html

then what the parser has seen by the time it gets to the end of the buffer is an enterStartTag event with an empty uri and the local-name "html". The parser will not see a leaveStartTag event until the user enters the greater-than symbol ending the start tag.

You must call enterContext() or enterContextWithMapping each time you encounter a start tag that defines namespaces and call leaveContext() when you encounter its corresponding end tag. You must also call definePrefix(...) for each prefix defined by the element. Example::

<p xmlns="q" xmlns:foo="foons">...

would require calling::

enterContext()
definePrefix("", "q")
definePrefix("foo", "foons")

Presumably, after the above, your code would call resolveName("p") on your walker to determine what namespace p is in, which would yield the result "q". And then it would fire the enterStartTag event with q as the namespace and p as the local name of the tag::

"enterStartTag", ["q", "p"]

Note the order of the events. The new context must start before salve sees the enterStartTag event because the way namespaces work, a start tag can declare its own namespace. So by the time enterStartTag is issued, salve must know what namespaces are declared by the tag. If the events were not issued this way, then the start tag p in the example would be interpreted to be in the default namespace in effect before it started, which could be other than q. Similarly, leaveContext must be issued after the corresponding endTag event.

Note on performance: if you already have a simple JavaScript object that maps prefixes to URIs it is better to call enterContextWithMapping and pass your object to this method. enterContextWithMapping enters a new context and immediately initializes it with the mapping you pass. This is faster than calling enterContext and calling definePrefix a bunch of times.

For the lazy: it is possible to call enterContext() for each start tag and leaveContext() for each end tag irrespective of whether or not the start tag declares new namespaces. The test suite does it this way. Note, however, that performance will be affected somewhat because name resolution will have to potentially search a deeper stack of contexts than would be strictly necessary.

Support for Guided Editing

Calling the possible() method on a walker will return the list of valid Event objects that could be fired on the walker, given what the walker has seen so far. If the user is editing a document which contains only the text::

<html

and hits a function key which makes the editor call possible(), then the editor can tell the user what attributes would be possible to add to this element. In editing facilities like nxml-mode in Emacs this is called completion. Similarly, once the start tag is ended by adding the greater-than symbol::

and the user again asks for possibilities, calling possible() will return the list of Event objects that could be fired. Note here that it is the responsibility of the editor to translate what salve returns into something the user can use. The possible() function returns only Event objects.

Editors that would depend on salve for guided editing would most likely need to use the clone() method on the walker to record the state of parsing at strategic points in the document being edited. This is to avoid needless reparsing. How frequently this should happen depends on the structure of the editor. The clone() method and the code it depends on has been optimized since early versions of salve, but it is possible to call it too often, resulting in a slower validation speed than could be attained with less aggressive cloning.

Overbroad Possibilities

possible() may at times report possibilities that allow for a document structure that is ultimately invalid. This could happen, for instance, where the Relax NG schema uses data to specify that the document should contain a positiveInteger between 1 and 10. The possible() method will report that a string matching the regular expression /^\+?\d+$/ is possible, when in fact the number 11 would match the expression but be invalid. The software that uses salve should be prepared to handle such a situation.

Name Classes

.. note:: The symbol ns used in this section corresponds to uri elsewhere in this document and name corresponds to local-name elsewhere. We find the uri, local-name pair to be clearer than ns, name. Is ns meant to be a namespace prefix? A URI? Is name a qualified name, a local name, something else? So for the purpose of documentation, we use uri, local-name wherever we can. However, the Relax NG specification uses the ns, name nomenclature, which salve also follows internally. The name class support is designed to be a close representation of what is described in the Relax NG specification. Hence the choice of nomenclature in this section.

The term "name class" is defined in the Relax NG specification, please refer to the specification for details.

Support for Relax NG's name classes introduces a few peculiarities in how possibilities are reported to clients using salve. The three events that accept names are affected: enterStartTag, endTag, and attributeName. When salve returns these events as possibilities, their lone parameter is an instance of name_patterns.Base class. This object has a .match method that takes a namespace and a name and will return true if the namespace and name match the pattern, or false if not.

Client code that wants to provide a sophisticated analysis of what a name class does could use the .toObject() method to get a plain JavaScript object from such an object. The returned object is essentially a syntax tree representing the name class. Each pattern has a unique structure. The possible patterns are:

• Name, a pattern with fields ns and name which respectively record the namespace URL and local name that this object matches. (Corresponds to the <name> element in the simplified Relax NG syntax.)

• NameChoice, a pattern with fields a and b which are two name classes. (Corresponds to a <choice> element appearing inside a name class in the simplified Relax NG syntax.)

• NsName, a pattern with the field ns which is the namespace that this object would match. The object matches any name. It may have an optional except field that contains a name class for patterns that it should not match. The lack of name field distinguishes it from Name. (Corresponds to an <nsName> element in the simplified Relax NG syntax.)

• AnyName, a pattern. It has the pattern field set to AnyName. We use this pattern field because AnyName does not require any other fields so {} would be its representation. This representation would too easily mask possible coding errors. AnyName matches any combination of namespace and name. May have an optional except field that contains a name class for patterns it should not match. It corresponds to an <anyName> element in the simplified Relax NG syntax.

.. note:: We do not use the pattern field for all patterns above because the only reason to do so would be to distinguish ambiguous structures. For instance, if Relax NG were to introduce a <superName> element that also needs ns and name fields then it would look the same as <name> and we would not be able to distinguish one from the other. However, Relax NG is stable. In the unlikely event a new version of Relax NG is released, we'll cross whatever bridge needs to be crossed.

Note that the <except> element from Relax NG does not have a corresponding object because the presence of <except> in a name class is recorded in the except field of the patterns above.

Here are a couple of examples. The name class for::

element (foo | bar | foo:foo) { ... }

would be recorded as (after partial beautification)::

{
    a: {
        a: {ns: "", name: "foo"},
        b: {ns: "", name: "bar"}
    },
    b: {ns: "foo:foo", name: "foo"}
}

The name class for::

element * - (foo:* - foo:a) { ... }

would be recorded as (after partial beautification)::

{
    pattern: "AnyName",
    except: {
        ns: "foo:foo",
        except: {ns: "foo:foo", name: "a"}
    }
}

Clients may want to call the .simple() method on a name pattern to determine whether it is simple or not. A pattern is deemed "simple" if it is composed only of Name and NameChoice objects. Such a pattern could be presented to a user as a finite list of possibilities. Otherwise, if the pattern is not simple, then either the number of choices is unbounded or it not a discrete list of items. In such a case, the client code may instead present to the user a field in which to enter the name of the element or attribute to be created and validate the name against the pattern. The method .toArray() can be used to reduce a pattern which is simple to an array of Name objects.

Event Asymmetry

Note that the events returned by possible() are not identical to the events that fireEvent() expects. While most events returned are exactly those that would be passed to fireEvent(), there are three exceptions: the enterStartTag, endTag and attributeName events returned by possible() will have a single parameter after the event name which is an object of name_patterns.Base class. However, when passing a corresponding event to fireEvent(), the same events take two string parameters after the event name: a namespace URL and a local name. To spell it out, they are of this form::

event_name, [uri, local-name]

where event_name is the string which is the name of the event to fire, uri is the namespace URI and local-name is the local name of the element or attribute.

Error Messages

Error messages that report attribute or element names use the name_patterns.Name class to record names, even in cases where patterns.EName would do. This is for consistency purposes, because some error messages must use name_patterns objects to report their errors. Rather than have some error messages use EName and some use the object in name_patterns they all use the objects in name_patterns, with the simple cases using name_patterns.Name.

In most cases, in order to present the end user of your application with error messages that make sense to the user, you will need to process error messages. This is because error messages generated by salve provide in the error object (ns, local name) pairs. A user would most likely like to see a namespace prefix rather than URI (ns). However, since namespace prefixes are a matter of user preference, and there may be many ways to decide how to associate a namespace prefix with a URI, salve does not take a position in this matter and lets the application that uses it decide how it wants to present URIs to users. The application also has to determine what strategy to use to present complex (i.e., non-simple) name patterns to the user. Again, there is no one-size-fits-all solution.

Misplaced Elements

A problem occurs when validating an XML document that contains an unexpected element. In such case, salve will issue an error but then what should it do with the contents of the misplaced element? Salve handles this in two ways:

1. If the unexpected element is known in the schema and has only one definition, then salve will assume that the user meant to use the element defined in the schema and will validate it as such.

2. Otherwise, salve will turn off validation until the element is closed.

Consider the following case::

<p>Here we have a <name><first>John</first><last>Doe</last></name>
because the <emph>person's name</emph> is not known.</p>

If name cannot appear in p but name has only one definition in the schema, then salve will emit an error upon encountering the enterStartTag event for name, and then validate name as if it had been found in a valid place. If it turns out that the schema defines one name element which can appear inside a person element and another name element which can appear inside a location element (which would be possible with Relax NG), then salve will emit an error but won't perform any validation inside name. Validation will resume after the endTag event for name. (Future versions of salve may implement logic to figure out ambiguous cases such as this one.) This latter scenario also occurs if name is not defined at all by the schema.

Documentation

The code is documented using typedoc. The following command will generate the documentation::

$ gulp doc

You may need to create a gulp.local module to tell gulp where to get rst2html. (Defaults are such that gulp will use your PATH to locate such tools.) The formatted documentation will appear in the build/api/ subdirectory, and the README.html in the root of the source tree.

NOTE: All the public interfaces of salve are available through the validate module. However, validate is a facade that exposes interfaces that are implemented in separate modules like patterns and formats.

Dependencies

In Node

Whenever you call on salve's functionalities to read a Relax NG schema, the fetch function must be available in the global space for salve to use. On Node, this means you must load a polyfill to provide this function.

Running salve's tests additionally requires that the development dependencies be installed. Please see the package.json file for details regarding these dependencies. Note that gulp should be installed so that its executable is in your path. Either this, or you will have to execute ./node_modules/.bin/gulp

If you want to contribute to salve, your code will have to pass the checks listed in .glerbl/repo_conf.py. So you either have to install glerbl to get those checks done for you or run the checks through other means. See Contributing.

In The Browser

The following lists the most prominent cases. It is not practical for us to keep track of every single feature that old browsers like IE11 don't support.

• fetch must be present.

• Promise must be present.

• Object.assign must be present.

• URL must be present.

• Symbol [and Symbol.iterator] must be present.

• The String methods introduced by ES6 (includes, endsWith, etc.)

• Array.prototype.includes

• Old Set and Map implementations like those in IE11 are either broken or incomplete.

On old browsers, we recommend using core-js to take care of many of these in one fell swoop. You'll have to provide polyfills for fetch and URL from other sources.

Note that we do not support old browsers. Notably, salve won't run on any version of IE.

Build System

Salve uses gulp. Salve's build setup gets the values for its configuration variables from three sources:

• Internal default values.

• From an optional gulp.local.js module that can override the internal defaults.

• From command line options that can override everything above.

The variables that can be set are:

+-----------------------+------------------------------------------------------+ |Name | Meaning | +=======================+======================================================+ |doc_private | Whether to produce documentation for private | | | entities. You can set doc_private to false | | | using no_doc_private. | +-----------------------+------------------------------------------------------+ |mocha_grep | --grep parameter for Mocha | +-----------------------+------------------------------------------------------+ |rst2html | rst2html command to run | +-----------------------+------------------------------------------------------+

Note that when used on the command line, underscores become dashes, thus --mocha-grep and --doc-private.

The gulp.local.js file is a module. You must export values like this::

exports.doc_private = true

Building

Run::

$ gulp

This will create a build/dist/ subdirectory in which the JavaScript necessary to validate XML files against a prepared Relax NG schema. You could copy what is in build/dist> to a server to serve these files to a client that would then perform validation.

Deploying

When you install salve through npm, you get a package that contains:

• a hierarchy of CommonJS modules in lib,
• a minified UMD build as salve.min.js.

The UMD build can be loaded in a CommonJS environment, in a AMD environment or as "plain scripts" in a browser. If you use the latter, then salve will be accessible as the salve global.

Testing

Running the following command from the root of salve will run the tests::

$ gulp test

Running mocha directly also works, but this may run the test against stale code, whereas gulp test always runs a build first.

Contributing

Contributions must pass the commit checks turned on in glerbl/repo_conf.py. Use glerbl install to install the hooks. Glerbl itself can be found at https://github.com/lddubeau/glerbl. It will eventually make its way to the Python package repository so that pip install glerbl will work.

Schema File Format

writeTreeToJSON converts a Relax NG file formatted in XML into a more compact format used by salve at validation time. Salve supports version 3 of this file format. Versions 0 to 2 are now obsolete. The structure is::

{"v":<version>,"o":<options>,"d":[...]}

The v field gives the version number of the data. The o field is a bit field of options indicating how the file was created. Right now the only thing it records is whether or not element paths are present in the generated file. The d field contains the actual schema. Each item in it is of the form::

[, ...]

The first element, <array type>, determines how to interpret the array. The array type could indicate that the array should be interpreted as an actual array or that it should be interpreted as an object of type Group or Choice, etc. If it is an array, then <array type> is discarded and the rest of the array is the converted array. If it is another type of object then again the <array type> is discarded and an object is created with the rest of the array as its constructor's parameters. All the array's elements after <array type> can be JSON primitive types, or arrays to be interpreted as actual arrays or as objects as described above.

License

Original Code

Code completely original to salve is released under the Mozilla Public License version 2.0. Copyright 2013-2016 Mangalam Research Center for Buddhist Languages, Berkeley, CA.

RNG Simplification Code

The RNG simplification files coded in XSL were adapted from Nicolas Debeissat's code. These files were originally released under the CeCILL license. Nicolas in March 2016 then changed the license to the Apache License 2.0.

In the version of these files bundled with salve, multiple bugs have been corrected, some minor and some major, and some changes have been made for salve's own internal purposes. For the sake of simplicity, these changes are also covered by the original licenses that apply to Nicolas' code.

Credits

Salve is designed and developed by Louis-Dominique Dubeau, Director of Software Development for the Buddhist Translators Workbench project, Mangalam Research Center for Buddhist Languages.

Jesse Bethel has contributed to salve's documentation, and migrated salve's build system from Make to Grunt.

This software has been made possible in part by a Level I Digital Humanities Start-up Grant and a Level II Digital Humanities Start-up Grant from the National Endowment for the Humanities (grant numbers HD-51383-11 and HD-51772-13). Any views, findings, conclusions, or recommendations expressed in this software do not necessarily represent those of the National Endowment for the Humanities.

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