I have seen plenty of local-machine fast key-value stores, such as LevelDB (By Google), or RocksDB (By Facebook), but I have a hard time imagining what they are for.
What are the use cases for such a library?
What are the use cases for such a library?
One solution is to maintain such state in some key-value store. Different functions of your service can query this state within the data center and never have to suffer disk delays - which are often much longer than the internal network of your data center.
As mentioned by the comment you replied to, it's because FASTER (and similar libraries) allow persisting this to disk in an efficient manner. If it can fit in memory and your language/framework has an efficient hashmap implementation, then you're right.. it's probably not worth using something like this.