Sets in Active-Active databases

A Redis set is an unordered collection of strings. It is possible to add, remove, and test for the existence of members with Redis commands. A Redis set maintains a unique collection of elements. Sets can be great for maintaining a list of events (click streams), users (in a group conversation), products (in recommendation lists), engagements (likes, shares) and so on.

Sets in Active-Active databases behave the same and maintain additional metadata to achieve an “OR-Set” behavior to handle concurrent conflicting writes. With the OR-Set behavior, writes across multiple Active-Active database instances are typically unioned except in cases of conflicts. Conflicting instance writes can happen when a Active-Active database instance deletes an element while the other adds the same element. In this case and observed remove rule is followed. That is, remove can only remove instances it has already seen and in all other cases element add wins.

Here is an example of an “add wins” case:

Here is an example of an “observed remove” case.