# uuid7
[](https://crates.io/crates/uuid7)
[](https://github.com/LiosK/uuid7-rs/blob/main/LICENSE)
A Rust implementation of the proposed UUID Version 7
```rust
let uuid = uuid7::uuid7();
println!("{uuid}"); // e.g. "01809424-3e59-7c05-9219-566f82fff672"
println!("{:?}", uuid.as_bytes()); // as 16-byte big-endian array
let uuid_string: String = uuid7::uuid7().to_string();
```
See [draft-ietf-uuidrev-rfc4122bis-01](https://www.ietf.org/archive/id/draft-ietf-uuidrev-rfc4122bis-01.html).
## Field and bit layout
This implementation produces identifiers with the following bit layout:
```text
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| unix_ts_ms | ver | counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|var| counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| rand |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
```
Where:
- The 48-bit `unix_ts_ms` field is dedicated to the Unix timestamp in
milliseconds.
- The 4-bit `ver` field is set at `0111`.
- The 42-bit `counter` field accommodates the sequence counter that ensures the
monotonic order of IDs generated within the same millisecond. The counter is
incremented by one for each new ID generated within the same timestamp and is
randomly initialized whenever the `unix_ts_ms` changes.
- The 2-bit `var` field is set at `10`.
- The remaining 32 `rand` bits are filled with a cryptographically strong random
number.
In the rare circumstances where the 42-bit `counter` field reaches its maximum
value and can no more be incremented within the same timestamp, this library
increments the `unix_ts_ms`; therefore, the `unix_ts_ms` may have a larger value
than that of the real-time clock. This library goes on with such larger
`unix_ts_ms` values caused by counter overflows and system clock rollbacks as
long as the difference from the system clock is small enough. If the system
clock moves back by ten seconds or more, this library resets the generator state
and thus breaks the monotonic order of generated identifiers.
## Crate features
Default features:
- `std` enables the primary `uuid7()` function. Without `std`, this crate
provides limited functionality available under `no_std` environments.
Optional features:
- `serde` enables the serialization and deserialization of `Uuid` objects.
- `uuid` (together with `std`) enables the `new_v7()` function that returns the
popular [uuid](https://crates.io/crates/uuid) crate's `Uuid` objects.
## Other functionality
This library also supports the generation of UUID version 4:
```rust
let uuid = uuid7::uuid4();
println!("{uuid}"); // e.g. "2ca4b2ce-6c13-40d4-bccf-37d222820f6f"
```
`gen7::Generator` provides a flexible interface to customize the various aspects
of the UUIDv7 generation:
```rust
use uuid7::gen7::{Generator, Status};
let mut g = Generator::new(rand::rngs::OsRng);
let unix_ts_ms = 0x0123_4567_8901u64;
let (uuid, status) = g.generate_core(unix_ts_ms);
if status == Status::ClockRollback {
panic!("clock moved back; monotonic order was broken");
} else {
println!("{uuid}");
}
```
## License
Licensed under the Apache License, Version 2.0.
## See also
- [docs.rs/uuid7](https://docs.rs/uuid7)