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VelocyPack (VPack)

Version 1

VelocyPack (VPack) is a fast and compact serialization format

Generalities

VPack is (unsigned) byte oriented, so VPack values are simply sequences of bytes and are platform independent. Values are not necessarily aligned, so all access to larger subvalues must be properly organised to avoid alignment assumptions of the CPU.

Value types

We describe a single VPack value, which is recursive in nature, but resides (with two exceptions, see below) in a single contiguous block of memory. Assume that the value starts at address A, the first byte V indicates the type (and often the length) of the VPack value at hand:

We first give an overview with a brief but accurate description for reference, for arrays and objects see below for details:

  • 0x00 : none - this indicates absence of any type and value, this is not allowed in VPack values
  • 0x01 : empty array
  • 0x02 : array without index table (all subitems have the same byte length), 1-byte byte length
  • 0x03 : array without index table (all subitems have the same byte length), 2-byte byte length
  • 0x04 : array without index table (all subitems have the same byte length), 4-byte byte length
  • 0x05 : array without index table (all subitems have the same byte length), 8-byte byte length
  • 0x06 : array with 1-byte index table offsets, bytelen and # subvals
  • 0x07 : array with 2-byte index table offsets, bytelen and # subvals
  • 0x08 : array with 4-byte index table offsets, bytelen and # subvals
  • 0x09 : array with 8-byte index table offsets, bytelen and # subvals
  • 0x0a : empty object
  • 0x0b : object with 1-byte index table offsets, sorted by attribute name, 1-byte bytelen and # subvals
  • 0x0c : object with 2-byte index table offsets, sorted by attribute name, 2-byte bytelen and # subvals
  • 0x0d : object with 4-byte index table offsets, sorted by attribute name, 4-byte bytelen and # subvals
  • 0x0e : object with 8-byte index table offsets, sorted by attribute name, 8-byte bytelen and # subvals
  • 0x0f : object with 1-byte index table offsets, not sorted by attribute name, 1-byte bytelen and # subvals
  • 0x10 : object with 2-byte index table offsets, not sorted by attribute name, 2-byte bytelen and # subvals
  • 0x11 : object with 4-byte index table offsets, not sorted by attribute name, 4-byte bytelen and # subvals
  • 0x12 : object with 8-byte index table offsets, not sorted by attribute name, 8-byte bytelen and # subvals
  • 0x13 : compact array, no index table
  • 0x14 : compact object, no index table
  • 0x15-0x16 : reserved
  • 0x17 : illegal - this type can be used to indicate a value that is illegal in the embedding application
  • 0x18 : null
  • 0x19 : false
  • 0x1a : true
  • 0x1b : double IEEE-754, 8 bytes follow, stored as little endian uint64 equivalent
  • 0x1c : UTC-date in milliseconds since the epoch, stored as 8 byte signed int, little endian, two's complement
  • 0x1d : external (only in memory): a char* pointing to the actual place in memory, where another VPack item resides, not allowed in VPack values on disk or on the network
  • 0x1e : minKey, nonsensical value that compares < than all other values
  • 0x1f : maxKey, nonsensical value that compares > than all other values
  • 0x20-0x27 : signed int, little endian, 1 to 8 bytes, number is V - 0x1f, two's complement
  • 0x28-0x2f : uint, little endian, 1 to 8 bytes, number is V - 0x27
  • 0x30-0x39 : small integers 0, 1, ... 9
  • 0x3a-0x3f : small negative integers -6, -5, ..., -1
  • 0x40-0xbe : UTF-8-string, using V - 0x40 bytes (not Unicode characters!), length 0 is possible, so 0x40 is the empty string, maximal length is 126, note that strings here are not zero-terminated
  • 0xbf : long UTF-8-string, next 8 bytes are length of string in bytes (not Unicode characters) as little endian unsigned integer, note that long strings are not zero-terminated and may contain zero bytes
  • 0xc0-0xc7 : binary blob, next V - 0xbf bytes are the length of blob in bytes, note that binary blobs are not zero-terminated
  • 0xc8-0xcf : positive long packed BCD-encoded float, V - 0xc7 bytes follow that encode in a little endian way the length of the mantissa in bytes. Directly after that follow 4 bytes encoding the (power of 10) exponent, by which the mantissa is to be multiplied, stored as little endian two's complement signed 32-bit integer. After that, as many bytes follow as the length information at the beginning has specified, each byte encodes two digits in big-endian packed BCD Example: 12345 decimal can be encoded as 0xc8 0x03 0x00 0x00 0x00 0x00 0x01 0x23 0x45 or 0xc8 0x03 0xff 0xff 0xff 0xff 0x12 0x34 0x50
  • 0xd0-0xd7 : negative long packed BCD-encoded float, V - 0xcf bytes follow that encode in a little endian way the length of the mantissa in bytes. After that, same as positive long packed BCD-encoded float above.
  • 0xd8-0xef : reserved
  • 0xf0-0xff : custom types

Arrays

Empty arrays are simply a single byte 0x01.

We next describe the type cases 0x02 to 0x09, see below for the special compact type 0x13.

Nonempty arrays look like one of the following:

one of 0x02 to 0x05
BYTELENGTH
OPTIONAL UNUSED: 1, 3 or 7 zero bytes
sub VPack values

or

0x06
BYTELENGTH in 1 byte
NRITEMS in 1 byte
OPTIONAL UNUSED: 6 bytes equal to 0
sub VPack values
INDEXTABLE with 1 byte per entry

or

0x07
BYTELENGTH in 2 bytes
NRITEMS in 2 bytes
OPTIONAL UNUSED: 4 bytes equal to 0
sub VPack values
INDEXTABLE with 4 byte per entry

or

0x08
BYTELENGTH in 4 bytes
NRITEMS in 4 bytes
sub VPack values
INDEXTABLE with 4 byte per entry

or

0x09
BYTELENGTH in 8 bytes
sub VPack values
INDEXTABLE with 8 byte per entry
NRITEMS in 8 bytes

Numbers (for byte length, number of subvalues and offsets in the INDEXTABLE) are little endian unsigned integers, using 1 byte for types 0x02 and 0x06, 2 bytes for types 0x03 and 0x07, 4 bytes for types 0x04 and 0x08, and 8 bytes for types 0x05 and 0x09.

NRITEMS is a single number as described above.

The INDEXTABLE consists of:

  • for types 0x06-0x09 an array of offsets (unaligned, in the number format described above) earlier offsets reside at lower addresses. Offsets are measured from the start of the VPack value.

Nonempty arrays of types 0x06 to 0x09 have a small header including their byte length, the number of subvalues, then all the subvalues and finally an index table containing offsets to the subvalues. To find the index table, find the number of subvalues, then the end, and from that the base of the index table, considering how wide its entries are.

For types 0x02 to 0x05 there is no offset table and no number of items. The first item begins at address A+2, A+3, A+5 or respectively A+9, depending on the type and thus the width of the byte length field. Note the following special rule: The actual position of the first subvalue is allowed to be further back, provided there are zero bytes at the earlier possible positions. For example, if 4 bytes are used for the byte length, then the first item may be at A+9, provided A[5], is a zero byte. This is to give a program that builds a VPack value the opportunity to reserve 8 bytes in the beginning and only later find out that fewer bytes suffice to write the byte length. One can determine the number of subvalues by finding the first subvalue, its byte length, and dividing the amount of available space by it.

For types 0x06 to 0x09 the offset table describes where the subvalues reside. It is not necessary for the subvalues to start immediately after the number of subvalues field. As above, it is allowed to start at a later position, provided there are zero bytes in the earlier possible positions. For example, the first subvalue could be at address A+9, although the number of bytes of the byte length and number of subvalues is only 2, in that case, A[5] must be a zero byte. This rule is devised, because for performance reasons when building the value, it could be desirable to reserve 8 bytes for the byte length and number of subvalues and not fill the gap, even though it turns out later that offsets and thus the byte length only uses 2 bytes, say.

There is one exception for the 8-byte numbers case: In this case the number of elements is moved behind the index table. This is to get away without moving memory when one has reserved 8 bytes in the beginning and later noticed that all 8 bytes are needed for the byte length.

All offsets are measured from base A.

Example:

[1,2,3] has the hex dump

02 05 31 32 33

in the most compact representation, but the following are equally possible, though not necessarily advised to use:

Examples:

03 06 00 31 32 33

04 08 00 00 00 31 32 33

05 0c 00 00 00 00 00 00 00 31 32 33

06 09 03 31 32 33 03 04 05

07 0e 00 03 00 31 32 33 05 00 06 00 07 00

08 18 00 00 00 03 00 00 00 31 32 33 09 00 00 00 0a 00 00 00 0b 00 00 00

09
2c 00 00 00 00 00 00 00
31 32 33
09 00 00 00 00 00 00 00
0a 00 00 00 00 00 00 00
0b 00 00 00 00 00 00 00
03 00 00 00 00 00 00 00

Note that it is not recommended to encode short arrays in too long a format.

We now describe the special type 0x13, which is useful for a particularly compact array representation. Note that to some extent this goes against the principles of the VelocyPack format, since quick access to subvalues is no longer possible, all items in the array must be scanned to find a particular one. However, there are certain use cases for VelocyPack which only require sequential access (for example JSON dumping) and have a particular need for compactness.

The overall format of this array type is

0x13 as type byte BYTELENGTH sub VPack values NRITEMS

There is no index table at all, although the sub VelocyPack values can have different byte sizes. The BYTELENGTH and NRITEMS are encoded in a special format, which we describe now.

The BYTELENGTH consists of 1 to 8 bytes, of which all but the last one have their high bit set. Thus, the high bits determine, how many bytes are actually used. The lower 7 bits of all these bits together comprise the actual byte length in a little endian fashion. That is, the byte at address A+1 contains the least significant 7 bits (0 to 6) of the byte length, the following byte at address A+2 contains the bits 7 to 13, and so on. Since the total number of bytes is limited to 8, this encodes unsigned integers of up to 56 bits, which is the overall limit for the size of such a compact array representation.

The NRITEMS entry is encoded essentially the same, except that it is laid out in reverse order in memory. That is, one has to use the BYTELENGTH to find the end of the array value and go back bytes until one finds a byte with high bit reset. The last byte (at the highest memory address) contains the least significant 7 bits of the NRITEMS value, the second one bits 7 to 13 and so on.

Here is an example, the array [1, 16] can be encoded as follows:

13 06
31 28 10
02

Objects

Empty objects are simply a single byte 0x0a.

We next describe the type cases 0x0b to 0x12, see below for the special compact type 0x14.

Nonempty objects look like this:

one of 0x0b - 0x12 BYTELENGTH optional NRITEMS sub VPack values as pairs of attribute and value optional INDEXTABLE NRITEMS for the 8-byte case

Numbers (for byte length, number of subvalues and offsets in the INDEXTABLE) are little endian unsigned integers, using 1 byte for types 0x0b and 0x0f, 2 bytes for types 0x0c and 0x10, 4 bytes for types 0x0d and 0x11, and 8 bytes for types 0x0e and 0x12.

NRITEMS is a single number as described above.

The INDEXTABLE consists of:

  • an array of offsets (unaligned, in the number format described above) earlier offsets reside at lower addresses. Offsets are measured from the beginning of the VPack value.

Nonempty objects have a small header including their byte length, the number of subvalues, then all the subvalues and finally an index table containing offsets to the subvalues. To find the index table, find number of subvalues, then the end, and from that the base of the index table, considering how wide its entries are.

For all types the offset table describes where the subvalues reside. It is not necessary for the subvalues to start immediately after the number of subvalues field. For performance reasons when building the value, it could be desirable to reserve 8 bytes for the byte length and the number of subvalues and not fill the gap, even though it turns out later that offsets and thus the byte length only uses 2 bytes, say.

There is one special case: the empty object is simply stored as the single byte 0x0a.

There is another exception: For 8-byte numbers the number of subvalues is stored behind the INDEXTABLE. This is to get away without moving memory when one has reserved 8 bytes in the beginning and later noticed that all 8 bytes are needed for the byte length.

All offsets are measured from base A.

Each entry consists of two parts, the key and the value, they are encoded as normal VPack values as above, the first is always a short or long UTF-8 string starting with a byte 0x40-0xbf as described below. The second is any other VPack value.

There is one extension: For the key it is possible to use the positive small integer values 0x30-0x39 or an unsigned integer starting with a type byte of 0x28-0x2f. Any such integer value is an index into an outside-given table of attribute names. These are convenient when only very few attribute names occur or some are repeated very often. The standard way to encode such an attribute name table is as a VPack array of strings as specified here.

Objects are always stored with sorted key/value pairs, sorted by bytewise comparions of the keys on each nesting level. Sorting has some overhead but will allow looking up keys in logarithmic time later. Note that only the index table needs to be sorted, it is not required that the offsets in these tables are increasing. Since the index table resides after the actual subvalues, one can build up a complex VPack value by writing linearly.

Example: the object {"a": 12, "b": true, "c": "xyz"} can have the hexdump:

0b
13 03
41 62 1a
41 61 28 0c
41 63 43 78 79 7a
06 03 0a

The same object could have been done with an index table with longer entries, as in this example:

0d
22 00 00 00
03 00 00 00
41 62 1a
41 61 28 0c
41 63 43 78 79 7a
0c 00 00 00 09 00 00 00 10 00 00 00

Similarly with type 0x0c and 2-byte offsets, byte length and number of subvalues, or with type 0x0e and 8-byte numbers.

Note that it is not recommended to encode short arrays with too long index tables.

Special compact objects

We now describe the special type 0x14, which is useful for a particularly compact object representation. Note that to some extent this goes against the principles of the VelocyPack format, since quick access to subvalues is no longer possible, all key/value pairs in the object must be scanned to find a particular one. However, there are certain use cases for VelocyPack which only require sequential access (for example JSON dumping) and have a particular need for compactness.

The overall format of this object type is

0x14 as type byte BYTELENGTH sub VPack key/value pairs NRPAIRS

There is no index table at all, although the sub VelocyPack values can have different byte sizes. The BYTELENGTH and NRPAIRS are encoded in a special format, which we describe now. It is the same as for the special compact array type 0x13, which we repeat here for the sake of completeness.

The BYTELENGTH consists of 1 to 8 bytes, of which all but the last one have their high bit set. Thus, the high bits determine, how many bytes are actually used. The lower 7 bits of all these bits together comprise the actual byte length in a little endian fashion. That is, the byte at address A+1 contains the least significant 7 bits (0 to 6) of the byte length, the following byte at address A+2 contains the bits 7 to 13, and so on. Since the total number of bytes is limited to 8, this encodes unsigned integers of up to 56 bits, which is the overall limit for the size of such a compact array representation.

The NRPAIRS entry is encoded essentially the same, except that it is laid out in reverse order in memory. That is, one has to use the BYTELENGTH to find the end of the array value and go back bytes until one finds a byte with high bit reset. The last byte (at the highest memory address) contains the least significant 7 bits of the NRPAIRS value, the second one bits 7 to 13 and so on.

Here is an example, the object {"a":1, "b":16} can be encoded as follows:

14 0a
41 61 31 42 62 28 10
02

Doubles

Type 0x1b indicates a double IEEE-754 value using the 8 bytes following the type byte. To guarantee platform-independentness the details of the byte order are as follows. Encoding is done by using memcpy to copy the internal double value to an uint64_t. This 64-bit unsigned integer is then stored as little endian 8 byte integer in the VPack value. Decoding works in the opposite direction. This should sort out the undetermined byte order in IEEE-754 in practice.

Dates

Type 0x1c indicates a signed 64-int integer stored in 8 bytes little endian two's complement notation directly after the type. The value means a universal UTC-time measured in milliseconds since the epoch, which is 00:00 on 1 January 1970 UTC.

External VPack values

This type is only for use within memory, not for data exchange over disk or network. Therefore, we only need to specify that the following k bytes are the memcpy of a char* on the current architecture. That char* points to the actual VPack value elsewhere in memory.

Artifical minimal and maximal keys

These values of types 0x1e and 0x1f have no meaning other than comparing smaller or greater respectively than any other VPack value. The idea is that these can be used in systems that define a total order on all VPack values to specify left or right ends of infinite intervals.

Integer types

There are different ways to specify integers. For small values -6 to 9 inclusively there are specific type bytes in the range 0x30 to 0x3f to allow for storage in a single byte. After that there are signed and unsigned integer types that can code in the type byte the number of bytes used (ranges 0x20-0x27 for signed and 0x28-0x2f for unsigned).

Null and boolean values

These three values use a single byte to store the corresponding JSON values.

Strings

Strings are stored as UTF-8 encoded byte sequences. There are two variants, a short one and a long one. In the short one, the byte length (not the number of UTF-8 characters) is directly encoded in the type, and this works up to and including byte length 126. Types 0x40 to 0xbe are used for this and the byte length is V - 0x3f, if V is the type byte. For strings longer than 126 bytes, the type byte is 0xbf and the byte length of the string is stored in the first 8 bytes after the type byte, using a little endian unsigned integer representation. The actual string follows after these 8 bytes. There is no terminating zero byte in either case and the string may contain zero bytes.

Binary data

The type bytes 0xc0 to 0xc7 allow to store arbitrary binary byte sequences as a VPack value. The format is as follows: If V is the type byte, then V - 0xbf bytes follow it to make a little endian unsigned integer representing the length of the binary data, which directly follows these length bytes. No alignment is guaranteed. The content is entirely up to the user.

Packed BCD long floats

These types are used to represent arbitrary precision decimal numbers. There are different types for positive and negative numbers. The overall format of these values is:

one of 0xc8 - 0xcf (positive) or of 0xd0 - 0xd7 (negative)
LENGTH OF MANTISSA in bytes
EXPONENT (as 4-byte little endian signed two's complement integer)
MANTISSA (as packed BCD-encoded integer, big-endian)

The type byte describes the sign of the number as well as the number of bytes used to specify the byte length of the mantissa. As usual, if V is the type byte, then V - 0xc7 (in the positive case) or V - 0xcf (in the negative case) bytes are used for the length of the mantissa, stored as little endian unsigned integer directly after the byte length. After this follow exactly 4 bytes (little endian signed two's complement integer) to specify the exponent. After the exponent, the actual mantissa bytes follow.

Packed BCD is used, so that each byte stores exactly 2 decimal digits as in 0x34 for the decimal digits 34. Therefore, the mantissa always has an even number of decimal digits. Note that the mantissa is stored in big endian form, to make parsing and dumping efficient. This leads to the "unholy nibble problem": When a JSON parser sees the beginning of a longish number, it does not know whether an even or odd number of digits follow. However, for efficiency reasons it wants to start writing bytes to the output as it reads the input. This is, where the exponent comes to the rescue, which is illustrated by the following example:

12345 decimal can be encoded as:

0xc8 0x03 0x00 0x00 0x00 0x00 0x01 0x23 0x45
0xc8 0x03 0xff 0xff 0xff 0xff 0x12 0x34 0x50

The former encoding puts a leading 0 in the first byte and uses exponent 0, the latter encoding directly starts putting two decimal digits in one byte and then in the end has to "erase" the trailing 0 by using exponent -1, encoded by the 4 byte sequence 0xff 0xff 0xff 0xff.

There for the unholy nibble problem is solved and parsing (and indeed dumping) can be efficient.

Custom types

Note that custom types should usually not be used for data exchange but only internally in systems. Nevertheless, the design of this part of the specification is made such that it is possible by generic methods to derive the byte length of each custom data type.

The following user-defined types exist:

  • 0xf0 : 1 byte payload, directly following the type byte
  • 0xf1 : 2 bytes payload, directly following the type byte
  • 0xf2 : 4 bytes payload, directly following the type byte
  • 0xf3 : 8 bytes payload, directly following the type byte
  • 0xf4-0xf6 : length of the payload is described by a single further unsigned byte directly following the type byte, the payload of that many bytes follows
  • 0xf7-0xf9 : length of the payload is described by two bytes (little endian unsigned integer) directly following the type byte, the payload of that many bytes follows
  • 0xfa-0xfc : length of the payload is described by four bytes (little endian unsigned integer) directly following the type byte, the payload of that many bytes follows
  • 0xfd-0xff : length of the payload is described by eight bytes (little endian unsigned integer) directly following the type byte, the payload of that many bytes follows

Note: In types 0xf4 to 0xff the "payload" refers to the actual data not including the length specification.