Nice I was looking for a jira. So I agree we should justify why we are building something. Now to that direction here is what I have seen from my experience.
People quite often use state within their streaming app and may have large states (TBs). Shortening the pipeline by not having to copy data (to Cassandra for example for serving) is an advantage, in terms of at least latency and complexity. 
This can be true if we advantage of state checkpointing (locally could be RocksDB or in general HDFS the latter is currently supported)  along with an API to efficiently query data. 
Some use cases I see:

- real-time dashboards and real-time reporting, the faster the better
- monitoring of state for operational reasons, app health etc...
- integrating with external services via an API eg. making accessible  aggregations over time windows to some third party service within your system

Regarding requirements here are some of them:
- support of an API to expose state (could be done at the spark driver), like rest.
- supporting dynamic allocation (not sure how it affects state management) 
- an efficient way to talk to executors to get the state (rpc?)
- making local state more efficient and easier accessible with an embedded db (I dont think this is supported from what I see, maybe wrong)?
Some people are already working with such techs and some stuff could be re-used:


On Fri, Dec 8, 2017 at 10:32 PM, Michael Armbrust <> wrote:

I don't believe anyone is working on it yet.  I think the most useful thing is to start enumerating requirements and use cases and then we can talk about how to build it.

On Fri, Dec 8, 2017 at 10:47 AM, Stavros Kontopoulos <> wrote:
Cool Burak do you have a pointer, should I take the initiative for a first design document or Databricks is working on it?


On Fri, Dec 8, 2017 at 8:40 PM, Burak Yavuz <> wrote:
Hi Stavros,

Queryable state is definitely on the roadmap! We will revamp the StateStore API a bit, and a queryable StateStore is definitely one of the things we are thinking about during that revamp.


On Dec 8, 2017 9:57 AM, "Stavros Kontopoulos" <> wrote:
Just to re-phrase my question: Would query-able state make a viable SPIP?


On Thu, Dec 7, 2017 at 1:34 PM, Stavros Kontopoulos <> wrote:

Maybe this has been discussed before. Given the fact that many streaming apps out there use state extensively, could be a good idea to make Spark expose streaming state with an external API like other systems do (Kafka streams, Flink etc), in order to facilitate interactive queries?