Binary Formats

Facet supports several binary serialization formats for compact, efficient data exchange.

facet-msgpack

MessagePack is a compact binary format that's "like JSON but fast and small."

use facet::Facet;

#[derive(Facet)]
struct Message {
    id: u64,
    content: String,
    tags: Vec<String>,
}

let msg = Message {
    id: 42,
    content: "Hello, world!".into(),
    tags: vec!["greeting".into()],
};

// Serialize to bytes
let bytes = facet_msgpack::to_vec(&msg)?;

// Deserialize from bytes
let decoded: Message = facet_msgpack::from_slice(&bytes)?;

When to use messagePack

• Network protocols: More compact than JSON, widely supported
• Cross-language data exchange: Libraries exist for most languages
• Caching: Efficient binary storage for Redis, etc.
• Logs and metrics: Smaller than JSON, still human-debuggable with tools

Features

• Full serialization and deserialization
• Streaming support
• Extension types for dates, bytes, etc.
• Compatible with other MessagePack implementations

facet-postcard

Postcard is a no_std-friendly binary format designed for embedded systems and high-performance applications.

use facet::Facet;

#[derive(Facet)]
struct SensorReading {
    sensor_id: u16,
    temperature: f32,
    humidity: f32,
}

let reading = SensorReading {
    sensor_id: 1,
    temperature: 23.5,
    humidity: 65.0,
};

// Serialize to bytes
let bytes = facet_postcard::to_vec(&reading)?;

// Deserialize from bytes
let decoded: SensorReading = facet_postcard::from_slice(&bytes)?;

When to use postcard

• Embedded systems: no_std compatible, minimal overhead
• High-performance: Variable-length encoding, no schema overhead
• Rust-to-Rust: Native Rust types, no schema negotiation
• Fixed-size buffers: Can serialize directly into [u8; N]

Features

• no_std support (with alloc feature)
• Variable-length integer encoding (varint)
• Zero-copy deserialization where possible
• Compact encoding for common patterns

Fixed-size serialization

Postcard can serialize to fixed-size arrays:

use facet_postcard::to_slice;

let mut buf = [0u8; 64];
let used = to_slice(&reading, &mut buf)?;
// `used` contains the actual bytes written

Comparison

Size comparison (approximate)

For a typical struct with a few fields:

Other binary formats

facet-asn1

ASN.1 DER encoding, used in cryptography and X.509 certificates. Serialization only.

facet-xdr

XDR (External Data Representation), used in NFS and some RPC protocols. Serialization only.

Next steps

• See Format comparison matrix for feature support
• Check Ecosystem for no_std support details
• Read the postcard crate docs for advanced usage