# MessagePack specification MessagePack is an object serialization specification like JSON. MessagePack has two concepts: **type system** and **formats**. Serialization is conversion from application objects into MessagePack formats via MessagePack type system. Deserialization is conversion from MessagePack formats into application objects via MessagePack type system. Serialization: Application objects --> MessagePack type system --> MessagePack formats (byte array) Deserialization: MessagePack formats (byte array) --> MessagePack type system --> Application objects This document describes the MessagePack type system, MessagePack formats and conversion of them. ## Table of contents * MessagePack specification * [Type system](#type-system) * [Limitation](#limitation) * [Extension types](#extension-types) * [Formats](#formats) * [Overview](#overview) * [Notation in diagrams](#notation-in-diagrams) * [nil format](#nil-format) * [bool format family](#bool-format-family) * [int format family](#int-format-family) * [float format family](#float-format-family) * [str format family](#str-format-family) * [bin format family](#bin-format-family) * [array format family](#array-format-family) * [map format family](#map-format-family) * [ext format family](#ext-format-family) * [Timestamp extension type](#timestamp-extension-type) * [Serialization: type to format conversion](#serialization-type-to-format-conversion) * [Deserialization: format to type conversion](#deserialization-format-to-type-conversion) * [Future discussion](#future-discussion) * [Profile](#profile) * [Implementation guidelines](#implementation-guidelines) * [Upgrading MessagePack specification](#upgrading-messagepack-specification) ## Type system * Types * **Integer** represents an integer * **Nil** represents nil * **Boolean** represents true or false * **Float** represents a IEEE 754 double precision floating point number including NaN and Infinity * **Raw** * **String** extending Raw type represents a UTF-8 string * **Binary** extending Raw type represents a byte array * **Array** represents a sequence of objects * **Map** represents key-value pairs of objects * **Extension** represents a tuple of type information and a byte array where type information is an integer whose meaning is defined by applications or MessagePack specification * **Timestamp** represents an instantaneous point on the time-line in the world that is independent from time zones or calendars. Maximum precision is nanoseconds. ### Limitation * a value of an Integer object is limited from `-(2^63)` upto `(2^64)-1` * maximum length of a Binary object is `(2^32)-1` * maximum byte size of a String object is `(2^32)-1` * String objects may contain invalid byte sequence and the behavior of a deserializer depends on the actual implementation when it received invalid byte sequence * Deserializers should provide functionality to get the original byte array so that applications can decide how to handle the object * maximum number of elements of an Array object is `(2^32)-1` * maximum number of key-value associations of a Map object is `(2^32)-1` ### Extension types MessagePack allows applications to define application-specific types using the Extension type. Extension type consists of an integer and a byte array where the integer represents a kind of types and the byte array represents data. Applications can assign `0` to `127` to store application-specific type information. An example usage is that application defines `type = 0` as the application's unique type system, and stores name of a type and values of the type at the payload. MessagePack reserves `-1` to `-128` for future extension to add predefined types. These types will be added to exchange more types without using pre-shared statically-typed schema across different programming environments. [0, 127]: application-specific types [-128, -1]: reserved for predefined types Because extension types are intended to be added, old applications may not implement all of them. However, they can still handle such type as one of Extension types. Therefore, applications can decide whether they reject unknown Extension types, accept as opaque data, or transfer to another application without touching payload of them. Here is the list of predefined extension types. Formats of the types are defined at [Formats](#formats-timestamp) section. Name | Type --------- | ---- Timestamp | -1 ## Formats ### Overview format name | first byte (in binary) | first byte (in hex) --------------- | ---------------------- | ------------------- positive fixint | 0xxxxxxx | 0x00 - 0x7f fixmap | 1000xxxx | 0x80 - 0x8f fixarray | 1001xxxx | 0x90 - 0x9f fixstr | 101xxxxx | 0xa0 - 0xbf nil | 11000000 | 0xc0 (never used) | 11000001 | 0xc1 false | 11000010 | 0xc2 true | 11000011 | 0xc3 bin 8 | 11000100 | 0xc4 bin 16 | 11000101 | 0xc5 bin 32 | 11000110 | 0xc6 ext 8 | 11000111 | 0xc7 ext 16 | 11001000 | 0xc8 ext 32 | 11001001 | 0xc9 float 32 | 11001010 | 0xca float 64 | 11001011 | 0xcb uint 8 | 11001100 | 0xcc uint 16 | 11001101 | 0xcd uint 32 | 11001110 | 0xce uint 64 | 11001111 | 0xcf int 8 | 11010000 | 0xd0 int 16 | 11010001 | 0xd1 int 32 | 11010010 | 0xd2 int 64 | 11010011 | 0xd3 fixext 1 | 11010100 | 0xd4 fixext 2 | 11010101 | 0xd5 fixext 4 | 11010110 | 0xd6 fixext 8 | 11010111 | 0xd7 fixext 16 | 11011000 | 0xd8 str 8 | 11011001 | 0xd9 str 16 | 11011010 | 0xda str 32 | 11011011 | 0xdb array 16 | 11011100 | 0xdc array 32 | 11011101 | 0xdd map 16 | 11011110 | 0xde map 32 | 11011111 | 0xdf negative fixint | 111xxxxx | 0xe0 - 0xff ### Notation in diagrams one byte: +--------+ | | +--------+ a variable number of bytes: +========+ | | +========+ variable number of objects stored in MessagePack format: +~~~~~~~~~~~~~~~~~+ | | +~~~~~~~~~~~~~~~~~+ `X`, `Y`, `Z` and `A` are the symbols that will be replaced by an actual bit. ### nil format Nil format stores nil in 1 byte. nil: +--------+ | 0xc0 | +--------+ ### bool format family Bool format family stores false or true in 1 byte. false: +--------+ | 0xc2 | +--------+ true: +--------+ | 0xc3 | +--------+ ### int format family Int format family stores an integer in 1, 2, 3, 5, or 9 bytes. positive fixint stores 7-bit positive integer +--------+ |0XXXXXXX| +--------+ negative fixint stores 5-bit negative integer +--------+ |111YYYYY| +--------+ * 0XXXXXXX is 8-bit unsigned integer * 111YYYYY is 8-bit signed integer uint 8 stores a 8-bit unsigned integer +--------+--------+ | 0xcc |ZZZZZZZZ| +--------+--------+ uint 16 stores a 16-bit big-endian unsigned integer +--------+--------+--------+ | 0xcd |ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+ uint 32 stores a 32-bit big-endian unsigned integer +--------+--------+--------+--------+--------+ | 0xce |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+--------+--------+ uint 64 stores a 64-bit big-endian unsigned integer +--------+--------+--------+--------+--------+--------+--------+--------+--------+ | 0xcf |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+--------+--------+--------+--------+--------+--------+ int 8 stores a 8-bit signed integer +--------+--------+ | 0xd0 |ZZZZZZZZ| +--------+--------+ int 16 stores a 16-bit big-endian signed integer +--------+--------+--------+ | 0xd1 |ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+ int 32 stores a 32-bit big-endian signed integer +--------+--------+--------+--------+--------+ | 0xd2 |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+--------+--------+ int 64 stores a 64-bit big-endian signed integer +--------+--------+--------+--------+--------+--------+--------+--------+--------+ | 0xd3 |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| +--------+--------+--------+--------+--------+--------+--------+--------+--------+ ### float format family Float format family stores a floating point number in 5 bytes or 9 bytes. float 32 stores a floating point number in IEEE 754 single precision floating point number format: +--------+--------+--------+--------+--------+ | 0xca |XXXXXXXX|XXXXXXXX|XXXXXXXX|XXXXXXXX| +--------+--------+--------+--------+--------+ float 64 stores a floating point number in IEEE 754 double precision floating point number format: +--------+--------+--------+--------+--------+--------+--------+--------+--------+ | 0xcb |YYYYYYYY|YYYYYYYY|YYYYYYYY|YYYYYYYY|YYYYYYYY|YYYYYYYY|YYYYYYYY|YYYYYYYY| +--------+--------+--------+--------+--------+--------+--------+--------+--------+ where * XXXXXXXX_XXXXXXXX_XXXXXXXX_XXXXXXXX is a big-endian IEEE 754 single precision floating point number. Extension of precision from single-precision to double-precision does not lose precision. * YYYYYYYY_YYYYYYYY_YYYYYYYY_YYYYYYYY_YYYYYYYY_YYYYYYYY_YYYYYYYY_YYYYYYYY is a big-endian IEEE 754 double precision floating point number ### str format family Str format family stores a byte array in 1, 2, 3, or 5 bytes of extra bytes in addition to the size of the byte array. fixstr stores a byte array whose length is upto 31 bytes: +--------+========+ |101XXXXX| data | +--------+========+ str 8 stores a byte array whose length is upto (2^8)-1 bytes: +--------+--------+========+ | 0xd9 |YYYYYYYY| data | +--------+--------+========+ str 16 stores a byte array whose length is upto (2^16)-1 bytes: +--------+--------+--------+========+ | 0xda |ZZZZZZZZ|ZZZZZZZZ| data | +--------+--------+--------+========+ str 32 stores a byte array whose length is upto (2^32)-1 bytes: +--------+--------+--------+--------+--------+========+ | 0xdb |AAAAAAAA|AAAAAAAA|AAAAAAAA|AAAAAAAA| data | +--------+--------+--------+--------+--------+========+ where * XXXXX is a 5-bit unsigned integer which represents N * YYYYYYYY is a 8-bit unsigned integer which represents N * ZZZZZZZZ_ZZZZZZZZ is a 16-bit big-endian unsigned integer which represents N * AAAAAAAA_AAAAAAAA_AAAAAAAA_AAAAAAAA is a 32-bit big-endian unsigned integer which represents N * N is the length of data ### bin format family Bin format family stores an byte array in 2, 3, or 5 bytes of extra bytes in addition to the size of the byte array. bin 8 stores a byte array whose length is upto (2^8)-1 bytes: +--------+--------+========+ | 0xc4 |XXXXXXXX| data | +--------+--------+========+ bin 16 stores a byte array whose length is upto (2^16)-1 bytes: +--------+--------+--------+========+ | 0xc5 |YYYYYYYY|YYYYYYYY| data | +--------+--------+--------+========+ bin 32 stores a byte array whose length is upto (2^32)-1 bytes: +--------+--------+--------+--------+--------+========+ | 0xc6 |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| data | +--------+--------+--------+--------+--------+========+ where * XXXXXXXX is a 8-bit unsigned integer which represents N * YYYYYYYY_YYYYYYYY is a 16-bit big-endian unsigned integer which represents N * ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ is a 32-bit big-endian unsigned integer which represents N * N is the length of data ### array format family Array format family stores a sequence of elements in 1, 3, or 5 bytes of extra bytes in addition to the elements. fixarray stores an array whose length is upto 15 elements: +--------+~~~~~~~~~~~~~~~~~+ |1001XXXX| N objects | +--------+~~~~~~~~~~~~~~~~~+ array 16 stores an array whose length is upto (2^16)-1 elements: +--------+--------+--------+~~~~~~~~~~~~~~~~~+ | 0xdc |YYYYYYYY|YYYYYYYY| N objects | +--------+--------+--------+~~~~~~~~~~~~~~~~~+ array 32 stores an array whose length is upto (2^32)-1 elements: +--------+--------+--------+--------+--------+~~~~~~~~~~~~~~~~~+ | 0xdd |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| N objects | +--------+--------+--------+--------+--------+~~~~~~~~~~~~~~~~~+ where * XXXX is a 4-bit unsigned integer which represents N * YYYYYYYY_YYYYYYYY is a 16-bit big-endian unsigned integer which represents N * ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ is a 32-bit big-endian unsigned integer which represents N * N is the size of an array ### map format family Map format family stores a sequence of key-value pairs in 1, 3, or 5 bytes of extra bytes in addition to the key-value pairs. fixmap stores a map whose length is upto 15 elements +--------+~~~~~~~~~~~~~~~~~+ |1000XXXX| N*2 objects | +--------+~~~~~~~~~~~~~~~~~+ map 16 stores a map whose length is upto (2^16)-1 elements +--------+--------+--------+~~~~~~~~~~~~~~~~~+ | 0xde |YYYYYYYY|YYYYYYYY| N*2 objects | +--------+--------+--------+~~~~~~~~~~~~~~~~~+ map 32 stores a map whose length is upto (2^32)-1 elements +--------+--------+--------+--------+--------+~~~~~~~~~~~~~~~~~+ | 0xdf |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| N*2 objects | +--------+--------+--------+--------+--------+~~~~~~~~~~~~~~~~~+ where * XXXX is a 4-bit unsigned integer which represents N * YYYYYYYY_YYYYYYYY is a 16-bit big-endian unsigned integer which represents N * ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ is a 32-bit big-endian unsigned integer which represents N * N is the size of a map * odd elements in objects are keys of a map * the next element of a key is its associated value ### ext format family Ext format family stores a tuple of an integer and a byte array. fixext 1 stores an integer and a byte array whose length is 1 byte +--------+--------+--------+ | 0xd4 | type | data | +--------+--------+--------+ fixext 2 stores an integer and a byte array whose length is 2 bytes +--------+--------+--------+--------+ | 0xd5 | type | data | +--------+--------+--------+--------+ fixext 4 stores an integer and a byte array whose length is 4 bytes +--------+--------+--------+--------+--------+--------+ | 0xd6 | type | data | +--------+--------+--------+--------+--------+--------+ fixext 8 stores an integer and a byte array whose length is 8 bytes +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+ | 0xd7 | type | data | +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+ fixext 16 stores an integer and a byte array whose length is 16 bytes +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+ | 0xd8 | type | data +--------+--------+--------+--------+--------+--------+--------+--------+--------+--------+ +--------+--------+--------+--------+--------+--------+--------+--------+ data (cont.) | +--------+--------+--------+--------+--------+--------+--------+--------+ ext 8 stores an integer and a byte array whose length is upto (2^8)-1 bytes: +--------+--------+--------+========+ | 0xc7 |XXXXXXXX| type | data | +--------+--------+--------+========+ ext 16 stores an integer and a byte array whose length is upto (2^16)-1 bytes: +--------+--------+--------+--------+========+ | 0xc8 |YYYYYYYY|YYYYYYYY| type | data | +--------+--------+--------+--------+========+ ext 32 stores an integer and a byte array whose length is upto (2^32)-1 bytes: +--------+--------+--------+--------+--------+--------+========+ | 0xc9 |ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ|ZZZZZZZZ| type | data | +--------+--------+--------+--------+--------+--------+========+ where * XXXXXXXX is a 8-bit unsigned integer which represents N * YYYYYYYY_YYYYYYYY is a 16-bit big-endian unsigned integer which represents N * ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ_ZZZZZZZZ is a big-endian 32-bit unsigned integer which represents N * N is a length of data * type is a signed 8-bit signed integer * type < 0 is reserved for future extension including 2-byte type information ### Timestamp extension type Timestamp extension type is assigned to extension type `-1`. It defines 3 formats: 32-bit format, 64-bit format, and 96-bit format. timestamp 32 stores the number of seconds that have elapsed since 1970-01-01 00:00:00 UTC in an 32-bit unsigned integer: +--------+--------+--------+--------+--------+--------+ | 0xd6 | -1 | seconds in 32-bit unsigned int | +--------+--------+--------+--------+--------+--------+ timestamp 64 stores the number of seconds and nanoseconds that have elapsed since 1970-01-01 00:00:00 UTC in 32-bit unsigned integers: +--------+--------+--------+--------+--------+------|-+--------+--------+--------+--------+ | 0xd7 | -1 | nanosec. in 30-bit unsigned int | seconds in 34-bit unsigned int | +--------+--------+--------+--------+--------+------^-+--------+--------+--------+--------+ timestamp 96 stores the number of seconds and nanoseconds that have elapsed since 1970-01-01 00:00:00 UTC in 64-bit signed integer and 32-bit unsigned integer: +--------+--------+--------+--------+--------+--------+--------+ | 0xc7 | 12 | -1 |nanoseconds in 32-bit unsigned int | +--------+--------+--------+--------+--------+--------+--------+ +--------+--------+--------+--------+--------+--------+--------+--------+ seconds in 64-bit signed int | +--------+--------+--------+--------+--------+--------+--------+--------+ * Timestamp 32 format can represent a timestamp in [1970-01-01 00:00:00 UTC, 2106-02-07 06:28:16 UTC) range. Nanoseconds part is 0. * Timestamp 64 format can represent a timestamp in [1970-01-01 00:00:00.000000000 UTC, 2514-05-30 01:53:04.000000000 UTC) range. * Timestamp 96 format can represent a timestamp in [-292277022657-01-27 08:29:52 UTC, 292277026596-12-04 15:30:08.000000000 UTC) range. * In timestamp 64 and timestamp 96 formats, nanoseconds must not be larger than 999999999. Pseudo code for serialization: struct timespec { long tv_sec; // seconds long tv_nsec; // nanoseconds } time; if ((time.tv_sec >> 34) == 0) { uint64_t data64 = (time.tv_nsec << 34) | time.tv_sec; if (data64 & 0xffffffff00000000L == 0) { // timestamp 32 uint32_t data32 = data64; serialize(0xd6, -1, data32) } else { // timestamp 64 serialize(0xd7, -1, data64) } } else { // timestamp 96 serialize(0xc7, 12, -1, time.tv_nsec, time.tv_sec) } Pseudo code for deserialization: ExtensionValue value = deserialize_ext_type(); struct timespec result; switch(value.length) { case 4: uint32_t data32 = value.payload; result.tv_nsec = 0; result.tv_sec = data32; case 8: uint64_t data64 = value.payload; result.tv_nsec = data64 >> 34; result.tv_sec = data64 & 0x00000003ffffffffL; case 12: uint32_t data32 = value.payload; uint64_t data64 = value.payload + 4; result.tv_nsec = data32; result.tv_sec = data64; default: // error } ## Serialization: type to format conversion MessagePack serializers convert MessagePack types into formats as following: source types | output format ------------ | --------------------------------------------------------------------------------------- Integer | int format family (positive fixint, negative fixint, int 8/16/32/64 or uint 8/16/32/64) Nil | nil Boolean | bool format family (false or true) Float | float format family (float 32/64) String | str format family (fixstr or str 8/16/32) Binary | bin format family (bin 8/16/32) Array | array format family (fixarray or array 16/32) Map | map format family (fixmap or map 16/32) Extension | ext format family (fixext or ext 8/16/32) If an object can be represented in multiple possible output formats, serializers SHOULD use the format which represents the data in the smallest number of bytes. ## Deserialization: format to type conversion MessagePack deserializers convert MessagePack formats into types as following: source formats | output type -------------------------------------------------------------------- | ----------- positive fixint, negative fixint, int 8/16/32/64 and uint 8/16/32/64 | Integer nil | Nil false and true | Boolean float 32/64 | Float fixstr and str 8/16/32 | String bin 8/16/32 | Binary fixarray and array 16/32 | Array fixmap map 16/32 | Map fixext and ext 8/16/32 | Extension ## Future discussion ### Profile Profile is an idea that Applications restrict the semantics of MessagePack while sharing the same syntax to adapt MessagePack for certain use cases. For example, applications may remove Binary type, restrict keys of map objects to be String type, and put some restrictions to make the semantics compatible with JSON. Applications which use schema may remove String and Binary types and deal with byte arrays as Raw type. Applications which use hash (digest) of serialized data may sort keys of maps to make the serialized data deterministic. ## Implementation guidelines ### Upgrading MessagePack specification MessagePack specification is changed at this time. Here is a guideline to upgrade existent MessagePack implementations: * In a minor release, deserializers support the bin format family and str 8 format. The type of deserialized objects should be same with raw 16 (== str 16) or raw 32 (== str 32) * In a major release, serializers distinguish Binary type and String type using bin format family and str format family * At the same time, serializers should offer "compatibility mode" which doesn't use bin format family and str 8 format ___ MessagePack specification Last modified at 2017-08-09 22:42:07 -0700 Sadayuki Furuhashi © 2013-04-21 21:52:33 -0700