Crate serde_cbor

CBOR and serialization.

What is CBOR?

CBOR is a way to encode data in a space-efficient and fast binary format. CBORs data-model is a superset of the JSONs.

A simple object describing a person in diagnostic notation (it is actually JSON plus some annotations) looks like

{
    "FirstName": "John",
    "LastName": "Doe",
    "Age": 43,
    "Address": {
        "Street": "Downing Street 10",
        "City": "London",
        "Country": "Great Britain"
    },
    "PhoneNumbers": [
        "+44 1234567",
        "+44 2345678"
    ]
}

The CBOR encoded object with comments in hexadecimal notation looks like

a5                                      # map(5)
   69                                   # text(9)
      46697273744e616d65                # "FirstName"
   64                                   # text(4)
      4a6f686e                          # "John"
   68                                   # text(8)
      4c6173744e616d65                  # "LastName"
   63                                   # text(3)
      446f65                            # "Doe"
   63                                   # text(3)
      416765                            # "Age"
   18 2b                                # unsigned(43)
   67                                   # text(7)
      41646472657373                    # "Address"
   a3                                   # map(3)
      66                                # text(6)
         537472656574                   # "Street"
      71                                # text(17)
         446f776e696e6720537472656574203130 # "Downing Street 10"
      64                                # text(4)
         43697479                       # "City"
      66                                # text(6)
         4c6f6e646f6e                   # "London"
      67                                # text(7)
         436f756e747279                 # "Country"
      6d                                # text(13)
         4772656174204272697461696e     # "Great Britain"
   6c                                   # text(12)
      50686f6e654e756d62657273          # "PhoneNumbers"
   82                                   # array(2)
      6b                                # text(11)
         2b34342031323334353637         # "+44 1234567"
      6b                                # text(11)
         2b34342032333435363738         # "+44 2345678"

While the JSON encoding is 174 bytes long the CBOR representation is only 141 bytes long. This is 19% shorter! Sometimes compression will even better, but never CBOR will be longer than the corresponding JSON. More importantly CBOR supports binary data, custom data tyes, annotations for dates, times and expected encoding and is faster to serialize and deserialize. It can even be used on embedded devices.

Type-based Serialization and Deserialization

Serde provides a mechanism for low boilerplate serialization & deserialization of values to and from CBOR via the serialization API. To be able to serialize a piece of data, it must implement the serde::Serialize trait. To be able to deserialize a piece of data, it must implement the serde::Deserialize trait. Serde provides an annotation to automatically generate the code for these traits: #[derive(Serialize, Deserialize)].

The CBOR API also provides an enum serde_cbor::Value.

Packed Encoding

When serializing structs or enums in CBOR the keys or enum variant names will be serialized as string keys to a map. Esspecially in embedded environments this can increase the file size too much. In packed encoding the keys and variants will be serialized as variable sized integers. The first 24 entries in any struct consume only a single byte! To serialize a document in packed encoding use ser::to_(vec|writer)_packed, deserialization works without any changes.

Self describing documents

In some contexts different formats are used but there is no way to declare the format used out of band. For this reason CBOR has a magic number that may be added before any document. The _sd (for selfd*escribe) append the magic number before documents.

Examples

Read a CBOR value that is known to be a map of string keys to string values and print it.

use std::collections::BTreeMap;
use serde_cbor::from_slice;

let slice = b"\xa5aaaAabaBacaCadaDaeaE";
let value: BTreeMap<String, String> = from_slice(slice).unwrap();
println!("{:?}", value); // {"e": "E", "d": "D", "a": "A", "c": "C", "b": "B"}

Read a general CBOR value with an unknown content.

use serde_cbor::{from_slice, Value};

let slice = b"\x82\x01\xa1aaab";
let value: Value = from_slice(slice).unwrap();
println!("{:?}", value); // Array([U64(1), Object({String("a"): String("b")})])

Serialize an object.

use std::collections::BTreeMap;
use serde_cbor::to_vec;

let mut programming_languages = BTreeMap::new();
programming_languages.insert("rust", vec!["safe", "concurrent", "fast"]);
programming_languages.insert("python", vec!["powerful", "friendly", "open"]);
programming_languages.insert("js", vec!["lightweight", "interpreted", "object-oriented"]);
let encoded = to_vec(&programming_languages);
assert_eq!(encoded.unwrap().len(), 103);

Modules

de	Deserialization.
error	When serializing or deserializing CBOR goes wrong.
ser	Serialize a Rust data structure to CBOR data.
value	CBOR values, keys and serialization routines.

Structs

Deserializer	A Serde Deserializer of CBOR data.
Serializer	A structure for serializing Rust values to CBOR.

Enums

ObjectKey	A simplified CBOR value containing only types useful for keys.
Value	An enum over all possible CBOR types.

Functions

from_reader	Decodes a value from CBOR data in a reader.
from_slice	Decodes a value from CBOR data in a slice.
to_value	Convert a T into serde_cbor::Value which is an enum that can represent any valid CBOR data.
to_vec	Serializes a value to a vector.
to_writer	Serializes a value to a writer.