![]() ![]() Sequential UUIDs are a compromise - giving away a small amount of collision protection in order to gain what could be a significant efficiency boost in some circumstances (DB indexes being the main one). UUIDs, including those that increment from the start point, are intended not to collide with those generated elsewhere. > If you need something sequential then just use a much more simple number* ![]() You don't want to use sequential UUIDs in place of v4 UUIDs where security matters, or course, as it is easy to work the next in sequence. These are "atoms in several galaxies" level numbers. The fourth number preserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time). For most systems the chance of collision with sequential UUIDs is, while larger than with other types, still so small as to be inconsequential. MySQL uses UUID version 1 which is a 128-bit number represented by a utf8 string of five hexadecimal numbers The first three numbers are generated from a timestamp. Even if your system is up long enough to use 2^58 (2.88 10^17 if you want that in decimal) IDs generated this way, you still effectively have 64-bits of randomness even if the variant bits are present. The endianness is in terms of the fields of the UUID, not the environment. Assuming the variant indicators are present, there is are 122 random bits. MS SQL Server's NEWSEQUENTIALID function returns something akin to a v4 UUID (fully random, aside from variant indicator bits, I'm not sure if the variant bits are respected in NEWSEQUENTIALIDs output or if it just returns a 128-bit number) but after the first is generated the rest follow in sequence until the sequence is reset (by a reboot). IIRC this is often a mix of hardware network address and a portion that is either random or based on a high-precision time, so it is still unlikely that you'll see collisions between machines. It is a unique identifier that is used to identify objects, without relying on any external reference. It is a 128-bit long value that is unique across the globe and is generated using an algorithm. It depends on how the non-sequential part is derived. UUID stands for Universally Unique Identifier and is specified by the Request for Comments 4122 or RFC 4122. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |