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If you require a streaming update,
I would take a generic approach, not specific to Elektron, rather analogous to other use cases for asynchronous processing of multiple parallel requests. I would keep a map of requested symbols, and mark completion on each receipt of either data refresh or negative status. Once all the responses have been received, I would unitialize the consumer and exit.
If you are only looking for a single refresh for each symbol, I would consider a snapshot request, interest after refresh = false.