Ernie

Simple binary data exchange format that is based on External Term Format in Erlang. This is update on BERT which is a little bit outdated right now.

Name

Ernie is fun over the original BERT name, from the Sesame Street Bert & Ernie couple.

RPC

No, this document will not define RPC syntax because of 2 reasons:

1. This is out of scope of this document. It is meant to define only data encoding format, not services one will use it for.
2. Authors of this document believes that RPC are bad, and you should not use them. Instead use message passing between your services.

Schema/structured data

This is meant to encode free-form data, if you want to encode structured data with schema then we highly suggest you to take look into other formats like:

• ASN.1 and any of it's encodings
• ProtoBuffers
• Apache Thrift
• FlatBuffers
• Cap'n'Proto

Specification

Introduction

1. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14.

Types

1. Integer

   Binary representation of integers.

   Implementation MUST support at least 8-bit unsigned and 32-bit signed integers with full precision.

2. 64-bit Float

   IEEE 754 double precision floats.

   Implementation MUST support all non-NaN, non-infinite, non-subnormal values described in IEEE 754 binary64 spec. Implementations SHOULD NOT encode these values.

3. Tuple

   Fixed length sequence of values.

4. List

   Dynamic length sequence of values.

5. Binary

   Representation of bytes with arbitrary length. If used to store human-readable strings it MUST be encoded as correct UTF-8 string.

6. Map

   List of key-value pairs in arbitrary order. Keys MUST NOT appear twice in the same map.

Encoding

1. Top level

   Binary encoded data MUST start with magic byte of decimal value of 131 followed by single byte Tag value followed by arbitrary length data:

    0        1       2
    +--------+-------+-------+-------
    | 131    | Tag   | Data ...
    +--------+-------+-------+-------
   

2. Integer

   Integer can be encoded in 3 different forms:

   • Values in range 0..255 (aka byte)

       0       1        2
       +-------+--------+
       | 97    | Data   |
       +-------+--------+
     

     Where Data is unsigned byte representation of value.

   • Values in range -4 294 967 295..4 294 967 294 (signed 32-bit value)

       0       1       2      3      4      5
       +-------+-------+------+------+------+
       | 98    | Data                       |
       +-------+-------+------+------+------+
     

     Where Data is signed 32-bit representation of integer in U2 big-endian encoding.

   • "Short" arbitrary long signed integers (up to 2040-bit)

       0       1       2       3     ...     N+3
       +-------+-------+-------+------+------+
       | 110   | N     | Sign  | Data        |
       +-------+-------+-------+------+------+
     

     N is big-endian 16-bit unsigned integer. Data represents little-endian encoded N byte long integer. Sign is 0 for positive integer and 1 for negative one.

   • "Long" arbitrary long signed integers (from 2041 to 524 288-bit long)

       0       1       2       3       4     ...     N+4
       +-------+-------+-------+-------+------+------+
       | 111   | N             | Sign  | Data        |
       +-------+-------+-------+-------+------+------+
     

     N is big-endian 16-bit unsigned integer. Data represents little-endian encoded N byte long integer. Sign is 0 for positive integer and 1 for negative one.

3. Float

    0       1       2        3       4       5       6       7       8       9
    +-------+-------+--------+-------+-------+-------+-------+-------+-------+
    | 70    | Data                                                           |
    +-------+-------+--------+-------+-------+-------+-------+-------+-------+
   

   Where Data is IEEE 754 double precision floating-point number encoded as big-endian.

4. Tuple

   • Tuples up to 255 elements

       0       1       2       ...       N+2
       +-------+-------+--------+--------+
       | 104   | Arity | Data ...        |
       +-------+-------+--------+--------+
     

   • Tuples up to 4 294 967 295 elements

       0       1       2        3        4        5       ...       N+5
       +-------+-------+--------+--------+--------+--------+--------+
       | 105   | Arity                            | Data ...        |
       +-------+-------+--------+--------+--------+--------+--------+
     

   Where Arity is big-endian unsigned integer declaring amount of elements in Data.

5. List

   • Byte list

       0       1       2       3      ...      N+3
       +-------+-------+-------+-------+-------+
       | 107   | N             | Data ...      |
       +-------+-------+-------+-------+-------+
     

     This is optimisation for lists that contains only values in range 0..255, inclusive.

   • General list

       0       1       2       3       4       5      ...      N+5     N+6
       +-------+-------+-------+-------+-------+-------+-------+-------+
       | 108   | Arity                         | Data ...      | 106   |
       +-------+-------+-------+-------+-------+-------+-------+-------+
     

     Where Arity is big-endian unsigned 32-bit integer declaring amount of elements in Data.

   • Empty list

       0       1
       +-------+
       | 106   |
       +-------+
     

6. Binary

    0       1       2       3       4       5      ...      N+5
    +-------+-------+-------+-------+-------+-------+-------+
    | 109   | N                             | Data ...      |
    +-------+-------+-------+-------+-------+-------+-------+
   

7. Map

    0       1       2       3       4       5      ...      N+5
    +-------+-------+-------+-------+-------+-------+-------+
    | 116   | Arity                         | Data ...      |
    +-------+-------+-------+-------+-------+-------+-------+
   

   The Arity field is an unsigned 32-bit integer in big-endian format that determines the number of key-value pairs in the map. Key and value pairs (Ki => Vi) are encoded in section Data in the following order: K1, V1, K2, V2,..., Kn, Vn.

Open questions

1. Should we support atoms?
2. Should we allow defining improper lists?

License

This document is released on Creative Commons Attribution-ShareAlike 4.0 International License.