Hashes in an Active-Active databases
Hashes are great for structured data that contain a map of fields and values. They are used for managing distributed user or app session state, user preferences, form data and so on. Hash fields contain string type and string types operate just like the standard Redis string types when it comes to CRDTs. Fields in hashes can be initialized as a string using HSET or HMSET or can be used to initialize counter types that are numeric integers using HINCRBY or floats using HINCRBYFLOAT.
Hashes 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 to add new fields across multiple Active-Active database instances are typically unioned except in cases of conflicts. Conflicting instance writes can happen when an Active-Active database instance deletes a field while the other adds the same field. In this case and observed remove rule is followed. That is, remove can only remove fields it has already seen and in all other cases element add/update wins.
Field values behave just like CRDT strings. String values can be types string, counter integer based on the command used for initialization of the field value. See “String Data Type in Active-Active databases” and “String Data Type with Counter Value in Active-Active databases” for more details.
Here is an example of an “add wins” case:
|Time||CRDB Instance1||CRDB Instance2|
|t1||HSET key1 field1 “a”|
|t2||HSET key1 field2 “b”|
|t4||- Sync -||- Sync -|