Module msgpack¶

The msgpack module takes strings in MsgPack format and decodes them, or takes a series of non-MsgPack values and encodes them.

msgpack.encode(lua_value)¶

Convert a Lua object to a MsgPack string.

Parameters:	lua_value – either a scalar value or a Lua table value.
Return:	the original value reformatted as a MsgPack string.
Rtype:	string

msgpack.decode(string)¶

Convert a MsgPack string to a Lua object.

Parameters:	string – a string formatted as MsgPack.

the original contents formatted as a Lua table;
the number of bytes that were decoded.

Rtype:	lua object

msgpack.NULL¶: A value comparable to Lua “nil” which may be useful as a placeholder in a tuple.

Example¶

tarantool> msgpack = require('msgpack')
---
...
tarantool> y = msgpack.encode({'a',1,'b',2})
---
...
tarantool> z = msgpack.decode(y)
---
...
tarantool> z[1], z[2], z[3], z[4]
---
- a
- 1
- b
- 2
...
tarantool> box.space.tester:insert{20, msgpack.NULL, 20}
---
- [20, null, 20]
...

The MsgPack output structure can be specified with __serialize:

__serialize = "seq" or "sequence" for an array
__serialize = "map" or "mapping" for a map

Serializing ‘A’ and ‘B’ with different __serialize values causes different results. To show this, here is a routine which encodes {‘A’,’B’} both as an array and as a map, then displays each result in hexadecimal.

function hexdump(bytes)
    local result = ''
    for i = 1, #bytes do
        result = result .. string.format("%x", string.byte(bytes, i)) .. ' '
    end
    return result
end

msgpack = require('msgpack')
m1 = msgpack.encode(setmetatable({'A', 'B'}, {
                             __serialize = "seq"
                          }))
m2 = msgpack.encode(setmetatable({'A', 'B'}, {
                             __serialize = "map"
                          }))
print('array encoding: ', hexdump(m1))
print('map encoding: ', hexdump(m2))

Result:

array encoding: 92 a1 41 a1 42
map encoding:   82 01 a1 41 02 a1 42

The MsgPack Specification page explains that the first encoding means:

fixarray(2), fixstr(1), "A", fixstr(1), "B"

and the second encoding means:

fixmap(2), key(1), fixstr(1), "A", key(2), fixstr(2), "B".

Here are examples for all the common types, with the Lua-table representation on the left, with the MsgPack format name and encoding on the right.

Common Types and MsgPack Encodings

{}	‘fixmap’ if metatable is ‘map’ = 80 otherwise ‘fixarray’ = 90
‘a’	‘fixstr’ = a1 61
false	‘false’ = c2
true	‘true’ = c3
127	‘positive fixint’ = 7f
65535	‘uint 16’ = cd ff ff
4294967295	‘uint 32’ = ce ff ff ff ff
nil	‘nil’ = c0
msgpack.NULL	same as nil
[0] = 5	‘fixmap(1)’ + ‘positive fixint’ (for the key) + ‘positive fixint’ (for the value) = 81 00 05
[0] = nil	‘fixmap(0)’ = 80 – nil is not stored when it is a missing map value
1.5	‘float 64’ = cb 3f f8 00 00 00 00 00 00

Also, some MsgPack configuration settings for encoding can be changed, in the same way that they can be changed for JSON.

Version:

Module msgpack¶

Example¶