I never understood why people would use sequential UUIDs. That rather defeats the purpose of a UUID. If you need something sequential then just use a much more simple number
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.
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). Assuming the variant indicators are present, there is are 122 random bits. Even if your system is up long enough to use 2^58 (2.8810^17 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. For most systems the chance of collision with sequential UUIDs is, while larger than with other types, still so small as to be inconsequential. These are "atoms in several galaxies" level numbers.
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.
> If you need something sequential then just use a much more simple number*
Sequential isn't their only property. UUIDs, including those that increment from the start point, are intended not to collide with those generated elsewhere.
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).
Each GUID generated by using NEWSEQUENTIALID is unique on that computer. GUIDs generated by using NEWSEQUENTIALID are unique across multiple computers only if the source computer has a network card.
Hard to achieve if everyone starts from 00000000-0000-0000-0000-000000000000.
UUIDs are generally not guaranteed to be universally unique. The name is misleading marketing.
If you generate them randomly, then the probability of collision is small enough not to matter, but the more you introduce deterministic elements, like generating them sequentially, the more your probability of collision increases.