SpiceDB is an open source, Google Zanzibar-inspired, database system for creating and managing security-critical application permissions.

SpiceDB

True, so here's an example more illustrative. Maybe not the best operation, but again I'm trying to build something general purpose my peers can use without friction.
```
# user.bestFriend = null at first
userA.bestFriend = userB # line 1
userA.bestFriend = userC # line 2
userA.bestFriend = userB # line 3
```
which results in
1. touch `user:A#bestFriend@user:B` and with precondition DOES_NOT_MATCH `user:A#bestFriend`
2. touch `user:A#bestFriend@user:C` and delete `user:A#bestFriend@user:B` with precondition MATCH `user:A#bestFriend@user:B`
3. touch `user:A#bestFriend@user:B` and delete `user:A#bestFriend@user:C` with precondition MATCH `user:A#bestFriend@user:C`

In one `WriteRelationships`, this requires deduplicating. In a remote transaction, spicedb could rollback if anything happened, but it would otherwise handle it:
```
with db.transaction():
    userA.bestFriend = userB # line 1
    userA.bestFriend = userC # line 2
    userA.bestFriend = userB # line 3
```