Enriching data from external source with cache

[Derek VerLee]

My basic problem will sound familiar I think, I need to enrich incoming data using a REST call to an external system for slowly evolving metadata. and some cache based lag is acceptable, so to reduce load on the external system and to process more efficiently, I would like to implement a cache.  The cache would by key, and I am already doing a keyBy for the same key in the job. Please correct me if I'm wrong: * Keyed State would be great to store my metadata "cache", Async I/O is ideal for pulling from the external system, but AsyncFunction can not access keyed state ( "Exception: State is not supported in rich async functions.") and operators can not share state between them. This leaves me wondering, since side inputs are not here yet, what the best (and perhaps most idiomatic) way to approach my problem? I'd rather keep changes to existing systems minimal for this iteration and just minimize impact on them during peaks best I can... systemic refactoring and re-architecture will be coming soon (so I'm happy to hear thoughts on that as well). Approaches considered: 1. AsyncFunction with a transient guava cache.  Not ideal ... but maybe good enough to get by 2. Using compound message types (oh, if only java had real algebraic data types...) and send cache miss messages from some CacheEnrichmentMapper (keyed) to some AsyncCacheLoader (not keyed) which then backfeeds cache updates to the former via iteration ... i don't know why this couldn't work but it feels like a hot mess unless there is some way I am not thinking of to do it cleanly 3. One user mentioned on a similar thread loading the data in as another DataStream and then using joins, but I'm confused about how this would work, it seems to me that joins happen on windows, windows pertain to (some notion of) time, what would be my notion of time for the slow (maybe years old in some cases) meta-data? 4. Forget about async I/O 5. implement my own "async i/o" in using a process function or similar  .. is this a valid pattern