[CASSANDRA-2540] Data reads by default

• Type: Improvement
• Status: Resolved
• Priority: Low
• Resolution: Won't Fix
• Assignee: Peter Schuller
• Reporter: Stu Hood

Authors

Peter Schuller

• Created: 22/Apr/11 05:53
• Updated: 16/Apr/19 09:33
• Resolved: 24/Sep/12 13:57

Description

The intention of digest vs data reads is to save bandwidth in the read path at the cost of latency, but I expect that this has been a premature optimization.

• Data requested by a read will often be within an order of magnitude of the digest size, and a failed digest means extra roundtrips, more bandwidth
• The digest reads but not your data read problem means failing QUORUM reads because a single node is unavailable, and would require eagerly re-requesting at some fraction of your timeout
• Saving bandwidth in cross datacenter usecases comes at huge cost to latency, but since both constraints change proportionally (enough), the tradeoff is not clear

Some options:

1. Add an option to use digest reads
2. Remove digest reads entirely (and/or punt and make them a runtime optimization based on data size in the future)
3. Continue to use digest reads, but send them to N - R nodes for (somewhat) more predicatable behavior with QUORUM

The outcome of data-reads-by-default should be significantly improved latency, with a moderate increase in bandwidth usage for large reads.