schematron

Schematron in pure Rust. Rule-based XML validation to ISO/IEC 19757-3, with no libxml2, no XSLT processor, no C toolchain, and no FFI.

Every other route to Schematron in Rust goes through C: bind to libxml2, or shell out to Saxon, or compile the schema into XSLT and find an XSLT engine to run it. This crate does none of that. It contains its own XML parser, its own XPath 1.0 engine, and its own validator, and it interprets a schema directly rather than transpiling it.

What Schematron is for

DTD, XML Schema, and RELAX NG describe the shape a document may take. Schematron describes the conditions a document must satisfy, written as XPath expressions — so it can express what a grammar cannot:

• co-occurrence rules — "if @type is invoice, then total is required"
• value relationships — "end must not precede start"
• cross-references between distant parts of a document
• cardinality that depends on content rather than position

Schematron is normally layered on top of a grammar, not used instead of one.

<schema xmlns="http://purl.oclc.org/dsdl/schematron">
  <pattern>
    <rule context="invoice">
      <assert test="total">An invoice must have a total.</assert>
      <report test="count(line) &gt; 100">This invoice has an unusual number of lines.</report>
    </rule>
  </pattern>
</schema>

Read that as: for every invoice element, assert that it has a total child. assert fires when its test is false; report fires when its test is true.

Install

cargo add schematron

Or the command line tool:

cargo install schematron

Library

use schematron::{Document, Schema};

fn main() -> schematron::Result<()> {
    // Compiling is the expensive step: it resolves includes, expands
    // abstractions, and parses every XPath expression in the schema.
    let schema = Schema::from_path("rules.sch")?;
    let document = Document::from_path("data.xml")?;
    let report = schema.validate(&document)?;

    if report.is_valid() {
        println!("valid");
    } else {
        for failure in report.failures() {
            println!("{}: {}", failure.location, failure.text);
        }
    }
    Ok(())
}

A report is data, not formatted text, so one run renders three ways:

let svrl = report.to_svrl();   // SVRL, for other Schematron tooling
let json = report.to_json()?;  // JSON, keeping the tree structure
let text = report.to_text();   // for a person

It can also be queried directly, instead of scraped back out of text:

report.is_valid();                  // no assert failed
report.count_failures();            // how many did
report.with_flag("error").count();  // findings the schema flagged as errors
report.count_fired_rules();         // zero here means NO context matched

Patterns are independent, so a schema with several of them can evaluate them on separate threads. The report is identical either way — this is a performance switch, not a behaviour switch:

let options = ValidateOptions::new().with_parallel_patterns(true);
let report = schema.validate_with(&document, &options)?;

Measure before turning it on: on a small document the threads cost more than they save. See spec/validation.md.

Schema is immutable and Send + Sync. Compile once, validate in parallel:

use std::sync::Arc;

let schema = Arc::new(Schema::from_path("rules.sch")?);

for path in paths {
    let schema = Arc::clone(&schema);
    std::thread::spawn(move || {
        let document = Document::from_path(path)?;
        schema.validate(&document)
    });
}

Command line

schematron --schema rules.sch data.xml
schematron -s rules.sch -p strict -f svrl -o report.svrl data.xml
schematron -s rules.sch --flag error docs/*.xml
cat data.xml | schematron -s rules.sch -

Two flags worth knowing when a schema is misbehaving:

schematron -s rules.sch --explain          # what the compiled schema will do
schematron -s rules.sch --verbose data.xml # which rules actually fired

Full option list: spec/cli.md.

The one rule to internalise

Within a single pattern, each node is processed by at most one rule: the first whose context matches it.

Rules in one pattern compete like the arms of a match expression. This is a feature — it is how you write "otherwise" branches:

<pattern>
  <rule context="line[@type='discount']">
    <assert test="number(@amount) &lt; 0">A discount must be negative.</assert>
  </rule>
  <rule context="line">
    <assert test="number(@amount) &gt;= 0">A normal line must not be negative.</assert>
  </rule>
</pattern>

— and it is also the most common way to write a schema that silently does nothing:

<pattern>
  <rule context="*">…</rule>
  <rule context="invoice">…</rule>  <!-- never runs: * claimed everything -->
</pattern>

To apply independent checks to the same node, put them in separate patterns. Patterns do not compete; each gets its own pass over the document.

The second-most common cause of a schema that does nothing is a missing namespace prefix. XPath 1.0 has no default namespace, so an unprefixed name matches elements in no namespace. Declare a prefix with <ns> and use it everywhere. --verbose shows an empty list of fired rules when this is what has happened.

What is implemented

A summary. spec/conformance.md is authoritative, and states the limits and deliberate divergences in full.

Beyond the standard, the crate lints a schema for constructs that are legal but almost certainly wrong — a rule shadowed by an earlier one in the same pattern, an unprefixed name in a namespaced schema — which are the two ways a Schematron schema silently does nothing:

schematron --schema rules.sch --lint

See spec/linting.md.

XPath 2.0 is a different language, not XPath 1.0 with extra functions. The crate implements a documented subset of it — regular expressions, conditionals, and the string and numeric functions schemas actually use — and makes everything outside that subset a hard error naming the construct, never a wrong answer. spec/xpath2.md is explicit about what is in, what is out, and the handful of places where a xslt2 schema still evaluates with XPath 1.0 semantics.

Security

The XML parser never resolves an external entity and never processes a DTD's entity declarations. XXE is therefore structurally impossible here rather than merely switched off; a reference to a DTD-declared entity is an error.

The default resolver reads local files and refuses http: and https: URIs. Network access is something an application opts into by supplying its own Resolver, never something the library does behind your back.

Parse depth, include depth, and expression nesting are all bounded, and exceeding a bound returns an error rather than exhausting the stack. Four cargo-fuzz targets exist to keep that true.

Performance

Every XPath expression is parsed once, when the schema is compiled, and the compiled schema is reused across documents and across threads. Rule contexts are evaluated once per document rather than tested node by node, so matching is linear rather than quadratic in document size. Patterns can optionally evaluate in parallel.

Indicative figures, and the benchmarks that produce them, are in spec/testing.md. Run them yourself with cargo bench — the numbers in any README are somebody else's hardware.

Documentation

index.md is the full map. AGENTS.md is the contributor and agent guide.

The specification is in spec/, is normative, and is written to be read:

If the code and the specification disagree, that is a defect in one of them.

Runnable examples:

cargo run --example validate_file        # the shortest useful program
cargo run --example report_formats       # SVRL, JSON, and text from one run
cargo run --example embedded_schema      # includes served from memory
cargo run --example parallel_validation  # one schema, eight threads
cargo run --example xpath_engine         # the XPath engine on its own

Development

cargo test --all-features                                   # everything
cargo clippy --all-targets --all-features -- -D warnings    # pedantic, and clean
cargo doc --no-deps --all-features                          # warning-free
cargo +1.94 test --all-features                             # the MSRV boundary

Slower, run when relevant: cargo bench, and cargo +nightly fuzz run fuzz_validate (needs cargo-fuzz).

The conformance suite lives in tests/corpus/. Each case is a directory holding schema.sch, input.xml, and expected.txt; adding a case means adding a directory, with no Rust to change.

The documentation is machine-checked: tests/docs.rs compiles every schema shown in the docs, resolves every relative link, and ties duplicated facts — the MSRV, the CLI flags, the XPath function list — back to their single source. See agents/testing.md.

License

Licensed under any of:

• MIT License
• Apache License, Version 2.0
• GNU General Public License version 2 only
• GNU General Public License version 3 only

at your option.