Generic Transfer Value (GTV)

The Generic Transfer Value (GTV) protocol serves as the foundation for data serialization in Postchain. It supports a comprehensive range of data types:

• Primitive types: integers, big integers, strings and byte arrays
• Complex types: arrays and dictionaries

All GTV data is serialized using the ASN.1 DER format for transmission.

GtvMessages DEFINITIONS ::= BEGIN

DictPair ::= SEQUENCE {
    name UTF8String,
    value RawGtv
}

RawGtv ::= CHOICE {
    null [0] NULL,
    byteArray [1] OCTET STRING,
    string [2] UTF8String,
    integer [3] INTEGER,
    dict [4] SEQUENCE OF DictPair,
    array [5] SEQUENCE OF RawGtv,
    bigInteger [6] INTEGER
}

END

Type conversions

The following table shows how Rell types are converted to and from GTV types in different contexts:

Rell type	Operation input	Query input	Query Output
rowid	GtvInteger	GtvInteger	GtvInteger
entity (becomes rowid)	GtvInteger	GtvInteger	GtvInteger
enum	GtvInteger	GtvInteger (can also be GtvString)	GtvString
struct	GtvArray	GtvArray (can also be GtvDict)	GtvDict
integer	GtvInteger	GtvInteger	GtvInteger
big_integer	GtvBigInteger	GtvBigInteger	GtvBigInteger
decimal	GtvString	GtvString	GtvString
boolean	GtvInteger	GtvInteger	GtvInteger
json	GtvString	GtvString	GtvString
text	GtvString	GtvString	GtvString
byte_array	GtvByteArray	GtvByteArray	GtvByteArray
nullable	GtvNull or Type	GtvNull or Type	GtvNull or Type
list	GtvArray	GtvArray	GtvArray
set	GtvArray	GtvArray	GtvArray
text map	GtvDict	GtvDict	GtvDict
non-text map (e.g., [[k1, v1], [k2, v2], ...])	GtvArray	GtvArray	GtvArray
named tuple (e.g., (x = 1, y = 2, z = 3))	GtvDict	GtvDict	GtvDict
unnamed tuple (e.g., (1, 2, 3))	GtvArray	GtvArray	GtvArray

Strict type conversion

With Rell Version 0.13.9, type conversion has become stricter by default. The following table shows the differences between strict and non-strict type handling:

Operation input Rell type	Accepted types in strict	Accepted types in non-strict
byte_array	GtvByteArray	GtvByteArray
		GtvString
integer	GtvInteger	GtvInteger
		GtvBigInteger
big_integer	GtvBigInteger	GtvBigInteger
		GtvInteger
decimal	GtvString	GtvString
		GtvInteger
		GtvBigInteger