Hello list!

I have application with following characteristics:
I consider to use Cassandra. No other existing database (HBase, Riak, etc) seems well suited for this.

Questions:
  • Did I miss some others database that could work? Please suggest me if you know one.
  • What are benefits or drawbacks of leveled compaction for this workload?
  • Setting column TTL seems bad choice due to extra storage. Agree? Is efficient to run routine batch job to purge oldest data? Is there will be any gotcha with that (like fullscan of something instead of just oldest, maybe?)
  • Will column index beneficial? If reads are scans, does it matter, or is it just extra work and storage space to maintain, without much benefit especially since reads are rare?
  • How gc_grace_seconds impacts operations in this workload? Will purges of old data leave sstables mostly obsolete, rather than sparsely obsolete? I think they will. So, after purge, tombstones can be GC shortly, no need for default 10 days grace period. BUT, I read in docs that if gc_grace_seconds is short, then nodetool repair needs run quite often. Is that true? Why would that be needed in my use case?
  • Related question: is it sensible to set tombstone_threshold to 1.0 but tombstone_compaction_interval to something short, like 1 hour? I suppose this depends on whether I am correct that SSTables will be deleted entirely, instead of just getting sparse.
  • Should I disable row_cache_provider? It invalidates every row on update, right? I will be updating rows constantly, so it seems not benefitial.
  • Docs say "compaction_throughput_mb_per_sec" is per "entire system." Does that mean per NODE, or per ENTIRE CLUSTER? Will this cause trouble with periodic deletions of expired columns? Do I need to make sure my purges of old data are trickled out over time to avoid huge overhead of compaction? But in that case, SSTables will become sparsely deleted, right? And then re-compacted, which seems wasteful if the remaining data will soon be purged again and there will be another re-compaction. So this is partially why I asked about tombstone-threshold and compaction interval -- I think is best if I can purge data in such a way that Cassandra never recompacts SsTables, but just realizes "oh, whole thing is dead, I can delete, no work needed." But I am not sure if my considered settings will have unintended consequence.
  • Finally, with proposed workload, will there be troubles with flush_larges_memtables_at and reduce_cache_capacity_to, reduce_cache_sizes_at? These are describe as "emergency measures" in docs. If my workload is edge case that could trigger bad emergency-measure behavior I hope you can say me that :-)
Many thanks!

Boris