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Binary JData: A portable interchange format for complex binary data

  • Status of this document: Request for comments
  • Maintainer: Qianqian Fang <q.fang at>
  • License: Apache License, Version 2.0
  • Version: 1 (Draft 2)
  • Last Stable Release: Version 1 (Draft 2)
  • Abstract:

The Binary JData (BJData) Specification defines an efficient serialization protocol for unambiguously storing complex and strongly-typed binary data found in diverse applications. The BJData specification is the binary counterpart to the JSON format, both of which are used to serialize complex data structures supported by the JData specification ( The BJData spec is derived and extended from the Universal Binary JSON (UBJSON, specification (Draft 12). It adds supports for N-dimensional packed arrays and extended binary data types.

Table of Content


The Javascript Object Notation (JSON) format, formally known as the ECMA-404 or ISO21778:2017 standard, is ubiquitously used in today's web and native applications. JSON presents numerous advantages, such as human readability, generality for accommodating complex hierarchical data, self-documentation, and abundant existing free and commercial libraries. However, its utility is largely restricted to the storage of lightweight textural data, and has very limited presence in many data-intensive and performance-demanding applications, such as database backends, medical imaging, and scientific data storage.

The lack of support for strongly-typed and binary data has been one of the main barriers towards widespread adoption of JSON in these domains. In recent years, efforts to address these limitation have resulted in an array of versatile binary JSON formats, such as BSON (Binary JSON,, UBJSON (Universal Binary JSON,, MessagePack (, CBOR (Concise Binary Object Representation, [RFC 7049], etc. These binary JSON counterparts are broadly used in speed-sensitive data processing applications and address various needs from a diverse range of applications.

To better champion findable, accessible, interoperable, and reusable (FAIR principle) data in scientific data storage and management, we have created the OpenJData Initiative ( to develop a set of open-standards for portable, human-readable and high-performance data annotation and serialization aimed towards enabling scientific researchers, IT engineers, as well as general data users to efficiently annotate and store complex data structures arising from diverse applications.

The OpenJData framework first converts complex data structures, such as N-D arrays, trees, tables and graphs, into easy-to-serialize, portable data annotations via the JData Specification ( and then serializes and stores the annotated JData constructs using widely-supported data formats. To balance data portability, readability and efficiency, OpenJData defines a dual-interface: a text-based format syntactically compatible with JSON, and a binary-JSON format to achieve significantly smaller file sizes and faster encoding/decoding.

The Binary JData (BJData) format is the official binary interface for the JData specification. It is derived from the widely supported UBJSON Specification Draft 12 (, and adds native support for N-dimensional packed arrays - an essential data structure for scientific applications - as well as extended binary data types, including unsigned integer types and half-precision floating-point numbers. The new data constructs also allow a BJData file to store binary arrays larger than 4 GB in size, which is not currently possible with MessagePack (maximum data record size is limited to 4 GB) and BSON (maximum total file size is 4 GB).

A key rationale for basing the BJData format upon UBJSON as opposed to other more popular binary JSON-like formats, such as BSON, CBOR and MessagePack, is UBJSON's quasi-human-readability - a unique characteristic that is absent from almost all other binary formats. This is because all data semantic elements in a UBJSON/BJData file, e.g. the "name" fields and data-type markers, are defined in human-readable strings. The resulting binary files are not only capable of storing complex and hierarchical binary data structures, but also directly readable using an editor with minimal or no processing. We anticipate that such a unique capability, in combination with the highly portable JData annotation keywords, makes a data file self-explanatory, easy to reuse, and easy to inter-operate in complex applications.


The Binary JData Specification is licensed under the Apache 2.0 License.

Format Specification

Format overview

A single construct with two optional segments (length and data) is used for all types:

[type, 1-byte char]([integer numeric length])([data])

Each element in the tuple is defined as:

  • type - A 1-byte ASCII char (Marker) used to indicate the type of the data following it.

  • length (optional) - A positive, integer numeric type (uint8, int8, uint16, int16, uint32, int32, uint64 or int64) specifying the length of the following data payload.

  • data (optional) - A contiguous byte-stream containing serialized binary
    data representing the actual binary data for this type of value.


  • Some values are simple enough that just writing the 1-byte ASCII marker into the stream is enough to represent the value (e.g. null), while others have a type that is specific enough that no length is needed as the length is implied by the type (e.g. int32). Yet others still require both a type and a length to communicate their value (e.g. string). In addition, some values (e.g. array) have additional (optional) parameters to improve decoding efficiency and/or to reduce the size of the encoded value even further.

  • The BJData specification (since Draft-2) requires that all numeric values must be written in the Little-Endian order. This is a breaking change compared to BJData Draft-1 and UBJSON Draft-12, where numeric values are in Big-Endian order.

  • The array and object data types are container types, similar to JSON arrays and objects. They help partition and organize data records of all types, including container types, into composite and complex records.

  • Using a strongly-typed container construct can further reduce data file sizes as it extracts common data type markers to the header. It also helps a parser to pre-allocate necessary memory buffers before reading the data payload.

In the following sections, we use a block-notation to illustrate the layout of the encoded data. In this notation, the data type markers and individual data payloads are enclosed by a pair of [], strictly for illustration purposes. Both illustration markers [ and ] as well as the whitespaces between these data elements, if present, shall be ignored when performing the actual data storage.

Type summary

Type Total size ASCII Marker(s) Length required Data (payload)
null 1 byte Z No No
no-op 1 byte N No No
true 1 byte T No No
false 1 byte F No No
int8 2 bytes i No Yes
uint8 2 bytes U No Yes
int16 3 bytes I (upper case i) No Yes
uint16* 3 bytes u No Yes
int32 5 bytes l (lower case L) No Yes
uint32* 5 bytes m No Yes
int64 9 bytes L No Yes
uint64* 9 bytes M No Yes
float16/half* 3 bytes h No Yes
float32/single 5 bytes d No Yes
float64/double 9 bytes D No Yes
high-precision number 1 byte + int num val + string byte len H Yes Yes
char 2 bytes C No Yes
string 1 byte + int num val + string byte len S Yes Yes (if not empty)
array 2+ bytes [ and ] Optional Yes (if not empty)
object 2+ bytes { and } Optional Yes (if not empty)

* Data type markers that are not defined in the UBJSON Specification (Draft 12)

Value types


The null value is equivalent to the null value from the JSON specification.



    "passcode": null

In BJData (using block-notation):



There is no equivalent to the no-op value in the original JSON specification. When decoding, No-Op values should be skipped.

The intended usage of the no-op value is as a valueless signal between a producer (most likely a server) and a consumer (most likely a client) to indicate activity, for example, as a keep-alive signal so that a client knows a server is still working and hasn't hung or timed out.


A Boolean type is equivalent to the Boolean value (true or false) defined in the JSON specification.



    "authorized": true,
    "verified": false

In BJData (using block-notation):



Unlike in JSON, which has a single Number type (used for both integers and floating point numbers), BJData defines multiple types for integers. The minimum/maximum of values (inclusive) for each integer type are as follows:

Type Signed Minimum Maximum
int8 Yes -128 127
uint8 Yes 0 255
int16 No -32,768 32,767
uint16 Yes 0 65,535
int32 No -2,147,483,648 2,147,483,647
uint32 Yes 0 4,294,967,295
int64 No -9,223,372,036,854,775,808 9,223,372,036,854,775,807
uint64 Yes 0 18,446,744,073,709,551,615
float16/half Yes See IEEE 754 Spec See IEEE 754 Spec
float32/single Yes See IEEE 754 Spec See IEEE 754 Spec
float64/double Yes See IEEE 754 Spec See IEEE 754 Spec
high-precision number Yes Infinite Infinite


  • Numeric values of +infinity, -infinity and NaN are to be encoded using their respective IEEE 754 binary form; this is different from the UBJSON specification where NaN and infinity are converted to null.
  • It is advisable to use the smallest applicable type when encoding a number.


All integer types (uint8, int8, uint16, int16, uint32, int32, uint64 and int64) are written in Little-Endian order (this is different from UBJSON, where all integers are written in Big-Endian order).


All float types (half, single, double are written in Little-Endian order (this is different from UBJSON which does not specify the endianness of floats).


These are encoded as a string and thus are only limited by the maximum string size. Values must be written out in accordance with the original JSON number type specification.


Numeric values in JSON:

    "int8": 16,
    "uint8": 255,
    "int16": 32767,
    "uint16": 32768,
    "int32": 2147483647,
    "int64": 9223372036854775807,
    "uint64": 9223372036854775808,
    "float32": 3.14,
    "float64": 113243.7863123,
    "huge1": "3.14159265358979323846",
    "huge2": "-1.93+E190",
    "huge3": "719..."

In BJData (using block-notation):



The char type in BJData is an unsigned byte meant to represent a single printable ASCII character (decimal values 0-127). It must not have a decimal value larger than 127. It is functionally identical to the uint8 type, but semantically is meant to represent a character and not a numeric value.


Char values in JSON:

    "rolecode": "a",
    "delim": ";",

BJData (using block-notation):



The string type in BJData is equivalent to the string type from the JSON specification apart from the fact that BJData string value requires UTF-8 encoding.


String values in JSON:

    "username": "andy",
    "imagedata": "...huge string payload..."

BJData (using block-notation):

    [i][9][imagedata][S][l][2097152][...huge string payload...]

Container types

See also optimized format below.


The array type in BJData is equivalent to the array type from the JSON specification.

The child elements of an array are ordered and can be accessed by their indices.


Array in JSON:


BJData (using block-notation):



The object type in BJData is equivalent to the object type from the JSON specification. Since value names can only be strings, the S (string) marker must not be included since it is redundant.

The child elements of an object are ordered and can be accessed by their names.


Object in JSON:

    "post": {
        "id": 1137,
        "author": "Andy",
        "timestamp": 1364482090592,
        "body": "The quick brown fox jumps over the lazy dog"

BJData (using block-notation):

        [i][4][body][S][i][43][The quick brown fox jumps over the lazy dog]

Optimized Format

Both container types (array and object) support optional parameters that can help optimize the container for better parsing performance and smaller size.

Type - $

When a type is specified, all value types stored in the container (either array or object) are considered to be of that singular type and, as a result, type markers are omitted for each value within the container. This can be thought of as providing the ability to create a strongly-typed container in BJData.

A major different between BJData and UBJSON is that the type in a BJData strongly-typed container is limited to non-zero-fixed-length data types, therefore, only integers (i,U,I,u,l,m,L,M), floating-point numbers (h,d,D) and char (C) are qualified. All zero-length types (T,F,Z,N), variable-length types(S, H) and container types ([,{) shall not be used in an optimized type header. This restriction is set to reduce the security risks due to potentials of buffer-overflow attacks using zero-length markers, hampered readability and dimished benefit using variable/container types in an optimized format.

The requirements for type are

  • If a type is specified, it must be one of i,U,I,u,l,m,L,M,h,d,D,C.
  • If a type is specified, it must be done so before a count.
  • If a type is specified, a count must be specified as well. (Otherwise it is impossible to tell when a container is ending, e.g. did you just parse ] or the int8 value of 93?)

Example (uint8 type):


Count - #

When a count is followed by a single non-negative integer record, i.e. one of i,U,I,u,l,m,L,M, it specifies the total child element count. This allows the parser to pre-size any internal construct used for parsing, verify that the promised number of child values were found, and avoid scanning for any terminating bytes while parsing.

  • A count can be specified without a type.

Example (count of 64):


Optimized N-dimensional array of uniform type

When both type and count are specified and the count marker # is followed by [, the parser should expect the following sequence to be a 1-D array with zero or more (Ndim) integer elements (Nx, Ny, Nz, ...). This specifies an Ndim-dimensional array of uniform type specified by the type marker after $. The array data are serialized in the row-major format.

For example, the below two block sequences both represent an Nx*Ny*Nz*... array of uniform numeric type:

[[] [$] [type] [#] [[] [$] [Nx type] [#] [Ndim type] [Ndim] [Nx Ny Nz ...]  [Nx*Ny*Nz*...*sizeof(type)]
[[] [$] [type] [#] [[] [Nx type] [nx] [Ny type] [Ny] [Nz type] [Nz] ... []] [Nx*Ny*Nz*...*sizeof(type)]

where Ndim is the number of dimensions, and Nx, Ny, and Nz ... are all non-negative numbers specifying the dimensions of the N-dimensional array. Nz/Ny/Nz/Ndim types must be one of the integer types (i,U,I,u,l,m,L,M). The binary data of the N-dimensional array is then serialized into a 1-D vector in the row-major element order (similar to C, C++, Javascript or Python) .

Example (a 2x3x4 uint8 array):

The following 2x3x4 3-D uint8 array


shall be stored as

 [[] [$][U] [#][[] [$][U][#][3] [2][3][4]
    [1][9][6][0] [2][9][3][1] [8][0][9][6] [6][4][2][7] [8][5][1][2] [3][3][2][6]

Additional rules

  • A count must be >= 0.
  • A count can be specified by itself.
  • If a count is specified, the container must not specify an end-marker.
  • A container that specifies a count must contain the specified number of child elements.
  • If a type is specified, it must be done so before count.
  • If a type is specified, a count must also be specified. A type cannot be specified by itself.
  • A container that specifies a type must not contain any additional type markers for any contained value.

Array Examples

Optimized with count

[[][#][i][5] // An array of 5 elements.
// No end marker since a count was specified.

Optimized with both type and count

[[][$][d][#][i][5] // An array of 5 float32 elements.
    [29.97] // Value type is known, so type markers are omitted.
// No end marker since a count was specified.

Object Examples

Optimized with count

[{][#][i][3] // An object of 3 name:value pairs.
// No end marker since a count was specified.

Optimized with both type and count

[{][$][d][#][i][3] // An object of 3 name:float32-value pairs.
    [i][3][lat][29.976] // Value type is known, so type markers are omitted.
// No end marker since a count was specified.

Recommended File Specifiers

For Binary JData files, the recommended file suffix is ".bjd". The MIME type for a Binary JData document is "application/jdata-binary"


The BJData spec is derived from the Universal Binary JSON (UBJSON, specification (Draft 12) developed by Riyad Kalla and other UBJSON contributors.

The initial version of this MarkDown-formatted specification was derived from the documentation included in the Py-UBJSON repository (Commit 5ce1fe7).

This specification was developed as part of the NeuroJSON project ( with funding support from the US National Institute of Health (NIH) under grant U24-NS124027.