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, MesagePack formats and conversion of them.
Table of contents
- MessagePack specification
Type system
- Types
- Integer represents an integer
- Nil represents nil
- Boolean represents true or false
- Float represents a floating point number
- 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
- Extended implements Extension interface: represents a tuple of type information and a byte array where type information is an integer whose meaning is defined by applications
- Interfaces
- Extension represents a tuple of an integer and a byte array where the integer represents type information and the byte array represents data. The format of the data is defined by concrete types
Limitation
- a value of an Integer object is limited from
-(2^63)upto(2^64)-1 - a value of a Float object is IEEE 754 single or double precision floating-point number
- 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 type
MessagePack allows applications to define application-specific types using the Extended type. Extended 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.
MessagePack reserves -1 to -128 for future extension to add predefined types which will be described in separated documents.
[0, 127]: application-specific types
[-1, -128]: reserved for predefined types
Formats
Overview
format namefirst byte (in binary)first byte (in hex) positive fixint0xxxxxxx0x00 - 0x7f fixmap1000xxxx0x80 - 0x8f fixarray1001xxxx0x90 - 0x9f fixstr101xxxxx0xa0 - 0xbf nil110000000xc0 (never used)110000010xc1 false110000100xc2 true110000110xc3 bin 8110001000xc4 bin 16110001010xc5 bin 32110001100xc6 ext 8110001110xc7 ext 16110010000xc8 ext 32110010010xc9 float 32110010100xca float 64110010110xcb uint 8110011000xcc uint 16110011010xcd uint 32110011100xce uint 64110011110xcf int 8110100000xd0 int 16110100010xd1 int 32110100100xd2 int 64110100110xd3 fixext 1110101000xd4 fixext 2110101010xd5 fixext 4110101100xd6 fixext 8110101110xd7 fixext 16110110000xd8 str 8110110010xd9 str 16110110100xda str 32110110110xdb array 16110111000xdc array 32110111010xdd map 16110111100xde map 32110111110xdf negative fixint111xxxxx0xe0 - 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 fixnum stores 7-bit positive integer
+--------+
|0XXXXXXX|
+--------+
negative fixnum 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 an 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
* 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 an 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 a 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
Serialization: type to format conversion
MessagePack serializers convert MessagePack types into formats as following:
source typesoutput format Integerint format family (positive fixint, negative fixint, int 8/16/32/64 or uint 8/16/32/64) Nilnil Booleanbool format family (false or true) Floatfloat format family (float 32/64) Stringstr format family (fixstr or str 8/16/32) Binarybin format family (bin 8/16/32) Arrayarray format family (fixarray or array 16/32) Mapmap format family (fixmap or map 16/32) Extendedext 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 convert MessagePack formats into types as following:
source formatsoutput type positive fixint, negative fixint, int 8/16/32/64 and uint 8/16/32/64Integer nilNil false and trueBoolean float 32/64Float fixstr and str 8/16/32String bin 8/16/32Binary fixarray and array 16/32Array fixmap map 16/32Map fixext and ext 8/16/32Extended
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 2013-04-21 21:52:33 -0700
Sadayuki Furuhashi © 2013-04-21 21:52:33 -0700