roxmltree

Represents an XML 1.0 document as a read-only tree.

// Find element by id.
let doc = roxmltree::Document::parse("<rect id='rect1'/>").unwrap();
let elem = doc.descendants().find(|n| n.attribute("id") == Some("rect1")).unwrap();
assert!(elem.has_tag_name("rect"));

Why read-only?

Because in some cases all you need is to retrieve some data from an XML document. And for such cases, we can make a lot of optimizations.

As for roxmltree, it's fast not only because it's read-only, but also because it uses xmlparser, which is many times faster than xml-rs. See the Performance section for details.

Parsing behavior

Sadly, XML can be parsed in many different ways. roxmltree tries to mimic the behavior of Python's lxml. But unlike lxml, roxmltree does support comments outside the root element.

For more details see docs/parsing.md.

Alternatives

Feature/Crate	roxmltree	libxml2	xmltree	sxd-document	minidom
Element namespace resolving	✓	✓	✓	~¹	✓
Attribute namespace resolving	✓	✓		✓	✓
Entity references	✓	✓	×	×	×
Character references	✓	✓	✓	✓	✓
Attribute-Value normalization	✓	✓
Comments	✓	✓		✓
Processing instructions	✓	✓	✓	✓
UTF-8 BOM	✓	✓	×	×	✓
Non UTF-8 input		✓
Complete DTD support		✓
Position preserving²	✓	✓
HTML support		✓
Tree modification		✓	✓	✓	✓
Writing		✓	✓	✓	✓
No unsafe	✓		✓		~³
Language	Rust	C	Rust	Rust	Rust
Size overhead⁴	~64KiB	~1.4MiB⁵	~118KiB	~138KiB	~62KiB
Dependencies	1	?⁵	2	2	2
Tested version	0.11.0	2.9.8	0.10.0	0.3.2	0.12.0
License	MIT / Apache-2.0	MIT	MIT	MIT	MIT

Legend:

✓ - supported
× - parsing error
~ - partial
nothing - not supported

Notes:

No default namespace propagation.
roxmltree keeps all node and attribute positions in the original document, so you can easily retrieve it if you need it. See examples/print_pos.rs for details.
In the memchr crate.
Binary size overhead according to cargo-bloat.
Depends on build flags.

There is also elementtree and treexml crates, but they are abandoned for a long time.

Performance

Parsing

test large_roxmltree     ... bench:   3,123,941 ns/iter (+/- 19,992)
test large_minidom       ... bench:   4,969,218 ns/iter (+/- 163,727)
test large_sdx_document  ... bench:   7,266,856 ns/iter (+/- 26,998)
test large_xmltree       ... bench:  21,354,608 ns/iter (+/- 136,311)

test medium_roxmltree    ... bench:     547,522 ns/iter (+/- 5,956)
test medium_minidom      ... bench:   1,223,620 ns/iter (+/- 16,180)
test medium_sdx_document ... bench:   2,470,063 ns/iter (+/- 24,159)
test medium_xmltree      ... bench:   8,083,860 ns/iter (+/- 25,363)

test tiny_roxmltree      ... bench:       4,170 ns/iter (+/- 41)
test tiny_minidom        ... bench:       7,495 ns/iter (+/- 81)
test tiny_sdx_document   ... bench:      17,411 ns/iter (+/- 203)
test tiny_xmltree        ... bench:      29,522 ns/iter (+/- 223)

roxmltree uses xmlparser internally, while sdx-document uses its own implementation, xmltree uses the xml-rs and minidom uses quick-xml. Here is a comparison between xmlparser, xml-rs and quick-xml:

test large_quick_xml     ... bench:   1,286,273 ns/iter (+/- 27,174)
test large_xmlparser     ... bench:   1,742,202 ns/iter (+/- 11,616)
test large_xmlrs         ... bench:  19,615,797 ns/iter (+/- 105,848)

test medium_quick_xml    ... bench:     248,169 ns/iter (+/- 3,885)
test medium_xmlparser    ... bench:     386,658 ns/iter (+/- 1,721)
test medium_xmlrs        ... bench:   7,387,753 ns/iter (+/- 18,668)

test tiny_quick_xml      ... bench:       2,382 ns/iter (+/- 29)
test tiny_xmlparser      ... bench:       2,788 ns/iter (+/- 20)
test tiny_xmlrs          ... bench:      27,619 ns/iter (+/- 262)

Iteration

test xmltree_iter_descendants_expensive     ... bench:     436,684 ns/iter (+/- 7,851)
test roxmltree_iter_descendants_expensive   ... bench:     470,459 ns/iter (+/- 6,233)
test minidom_iter_descendants_expensive     ... bench:     785,847 ns/iter (+/- 51,495)

test roxmltree_iter_descendants_inexpensive ... bench:      36,759 ns/iter (+/- 684)
test xmltree_iter_descendants_inexpensive   ... bench:     168,541 ns/iter (+/- 1,885)
test minidom_iter_descendants_inexpensive   ... bench:     215,615 ns/iter (+/- 38,101)

Where expensive refers to the matching done on each element. In these benchmarks, expensive means searching for any node in the document which contains a string. And inexpensive means searching for any element with a particular name.

Notes

You can try running the benchmarks yourself by running cargo bench in the benches dir.

Since all libraries have a different XML support, benchmarking is a bit pointless.
Tree crates may use different xml-rs crate versions.
We do not bench the libxml2, because xmlReadFile() will parse only an XML structure, without attributes normalization and stuff. So it's hard to compare. And we have to use a separate benchmark utility.
quick-xml is faster than xmlparser because it's more forgiving for the input, while xmlparser is very strict and does a lot of checks, which are expensive. So performance difference is mainly due to validation.

Safety

This library must not panic. Any panic should be considered a critical bug and reported.
This library forbids unsafe code.

Non-goals

Complete XML support.
Tree modification and writing.
XPath/XQuery.

API

This library uses Rust's idiomatic API based on iterators. In case you are more familiar with browser/JS DOM APIs - you can check out tests/dom-api.rs to see how it can be converted into a Rust one.

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

jfrimmel / roxmltree