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Does it means data get appended to commit log but not stored in sorted format immediately ?
What will happen to the client requests which comes before coalese/compaction process runs . When client/application tries to read same data which just got inserted but not got coalesced yet ? does query becomes expensive because it has to query and sort the data then serve/respond to the client which could consume more time and system resources ?
That's correct. By definition, "appends" mean that mutations (writes) are added to the end of the commitlog segment. This is done to make writes really fast in Cassandra by avoiding the overhead of disk seeks.
Cassandra does not just serve reads from disk -- data in memtables also get merged and are included in the result set returned to the clients.
For more info, see How data is read in Cassandra. Cheers!
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