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1.15. Модуль msgpack

Замечание

Документация находится в процессе перевода и может отставать от английской версии.

Модуль 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.

Параметры:
  • 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.

Параметры:
  • 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.

Пример

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.