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DXR: declarative XML-RPC

crates.io crates.io crates.io docs.rs

The dxr project provides crates for writing XML-RPC API clients and servers in Rust. The goal is to match the XML-RPC Specification -- even though some parts of it are under-specified -- and provide optional support for some common non-standard extensions.

Documentation of the public API and a tutorial-style introduction are available on the docs.rs page for this crate. Additionally, there are a few example binaries in dxr/examples.

Features

  • (de)serialization support for converting XML-RPC XML strings into strongly-typed Rust values
  • conversion traits between XML-RPC values and Rust primitives, arrays, slices, byte arrays, tuples, hashmaps, and custom structs (via derive macros)
  • built-in XML-escaping and un-escaping of string arguments
  • built-in date & time parsing for the dateTime.iso8861 value type
  • built-in base64 en- and decoding of byte vectors for the base64 type
  • optional support for (non-standard) <i8> (64-bit unsigned integer) and <nil/> values
  • support for arbitrary method call argument types without needing to convert values first (for up to 8 arguments; support for more could be implemented, if needed)
  • basic support for both XML-RPC clients (with reqwest) and servers (with axum)

All conversion methods (both between Rust XML-RPC values and XML strings, and between Rust primitives and Rust XML-RPC values) are extensively checked for correctness by unit tests and property-based tests using quickcheck.

Limitations

The implementation of XML-RPC provided by dxr also has a few limitations (which might or might not be deal-breakers for specific use cases):

  • Only valid UTF-8 is currently supported in both XML-RPC requests and responses. This is a limitation of the serde support of quick-xml. Support for other encodings could be added by implementing custom clients or servers which handle other encodings transparently.
  • All dateTime.iso8861 values are assumed to be UTC, as the dateTime.iso8861 type of XML-RPC does not include a timezone. Clients will need to adjust these values according to the server timezone.
  • The default client implementation (based on reqwest) is currently async-only. However, adding a "blocking" client implementation based on reqwest::blocking should be relatively straightforward.
  • The default server implementation (based on axum) and associated traits are async-only.

Architecture

  • dxr: top-level crate that exposes all publicly available functionality
  • dxr_shared: implementation of XML-RPC types, conversion traits between XML-RPC types and Rust types, and (de)serialization implementations for converting between XML strings and XML-RPC values
  • dxr_derive: ToDXR and FromDXR derive macros for custom data types
  • dxr_client: XML-RPC client implementation using reqwest
  • dxr_server: generic XML-RPC server functionality
  • dxr_server_axum: XML-RPC server implementation using axum

It is recommended to only add a direct dependency on dxr and to enable the required features.

Why another crate for XML-RPC?

Searching for xml-rpc on crates.io yields a few results, but they all did not fit my use case, or were very hard to use. Either they didn't support implementing both clients and servers, or no easy conversion methods from Rust types to XML-RPC types was available. And none of the crates supports (de)serializing both Rust types and custom user-defined types by using derive macros.

Goals

Because of this state of the XML-RPC crate ecosystem in Rust, the defining purpose of the dxr crate is that it should be opinionated, but also very easy to use, for implementing both XML-RPC clients and servers, with first-class support for (de)serializing custom types, in addition to built-in support for transparently converting Rust primitives to XML-RPC values.

Additionally, the crate is built on top of best-in-class (in my opinion) libraries for (de)serializing XML (quick-xml), HTTP client side (reqwest), HTTP server side (axum).

Examples

The /examples/ directory contains implementations of two simple clients and a simple server for demonstration purposes. They use the tokio runtime, which works great with both reqwest and axum.

Note that the amount of code that is required for writing simple XML-RPC clients and servers is very small. The client example has only ~10 LOC, and the server example only needs ~15 LOC, but both examples even include error handling.