SCALE Encoding

Substrate uses a lightweight and efficient encoding and decoding program to optimize how data is sent and received over the network. The program used to serialize and deserialize data is called the SCALE codec, with SCALE being an acronym for simple concatenated aggregate little-endian.

The SCALE codec is a critical component for communication between the runtime and the outer node.

It is designed for high-performance, copy-free encoding and decoding of data in resource-constrained execution environments like the Substrate WebAssembly runtime.

The SCALE codec is not self-describing in any way. It assumes the decoding context has all type knowledge about the encoded data. Front-end libraries maintained by Parity use the parity-scale-codec crate—which is a Rust implementation of the SCALE codec—to encode and decode interactions between RPCs and the runtime.

SCALE codec is advantageous for Substrate and blockchain systems because:

  • It is lightweight relative to generic serialization frameworks like serde, which add significant boilerplate that can bloat the size of the binary.

  • It does not use Rust libstd making it compatible with no_std environments that compile to Wasm, such as the Substrate runtime.

  • It is built to have great support in Rust for deriving codec logic for new types using: #[derive(Encode, Decode)].

It's important to define the encoding scheme used in Substrate rather than reuse an existing Rust codec library because this codec needs to be re-implemented on other platforms and languages that want to support interoperability among Substrate blockchains.

The following table shows how the Rust implementation of the Parity SCALE codec encodes different types.

SCALE codec examples of different types

Type
Description
Example SCALE decoded value
SCALE encoded value

Fixed-width integers

Basic integers are encoded using a fixed-width little-endian (LE) format.

signed 8-bit integer 69

0x45

unsigned 16-bit integer 42

0x2a00

unsigned 32-bit integer 16777215

0xffffff00

Compact/general

A "compact" or general integer encoding is sufficient for encoding large integers (up to 2**536) and is more efficient at encoding most values than the fixed-width version. (Though for single-byte values, the fixed-width integer is never worse.)

unsigned integer 0

0x00

unsigned integer 1

0x04

unsigned integer 42

0xa8

unsigned integer 69

0x1501

unsigned integer 65535

0xfeff0300

BigInt(100000000000000)

0x0b00407a10f35a

Boolean

Boolean values are encoded using the least significant bit of a single byte.

false

0x00

true

0x01

Results are commonly used enumerations which indicate whether certain operations were successful or unsuccessful.

Ok(42)

0x002a

Err(false)

0x0100

Options

One or zero values of a particular type.

Some

0x01 followed by the encoded value

None

0x00 followed by the encoded value

Vectors (lists, series, sets)

A collection of same-typed values is encoded, prefixed with a compact encoding of the number of items, followed by each item's encoding concatenated in turn.

Vector of unsigned 16-bit integers: [4, 8, 15, 16, 23, 42]

0x18040008000f00100017002a00

Strings

Strings are Vectors of bytes (Vec<u8>) containing a valid UTF8 sequence.

Tuples

A fixed-size series of values, each with a possibly different but predetermined and fixed type. This is simply the concatenation of each encoded value.

Tuple of compact unsigned integer and boolean: (3, false)

0x0c00

Structs

For structures, the values are named, but that is irrelevant for the encoding (names are ignored - only order matters). All containers store elements consecutively. The order of the elements is not fixed, depends on the container, and cannot be relied on at decoding. This implicitly means that decoding some byte-array into a specified structure that enforces an order and then re-encoding it could result in a different byte array than the original that was decoded.

A SortedVecAsc<u8> structure that always has byte-elements in ascending order: SortedVecAsc::from([3, 5, 2, 8])

[3, 2, 5, 8]

Enumerations (tagged-unions)

A fixed number of variants, each mutually exclusive and potentially implying a further value or series of values. Encoded as the first byte identifying the index of the variant that the value is. Any further bytes are used to encode any data that the variant implies. Thus, no more than 256 variants are supported.

Int(42) and Bool(true) where enum IntOrBool { Int(u8), Bool(bool),}

0x002a and 0x0101

SCALE Codec has been implemented in other languages, including:

PreviousState Transitions and StorageNextWeight and Fee

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