Proposal status: Implemented

Summary

Add additional RPC to regatta.v1.KV API that would provide a way to query the large dataset from the server. Currently, the client has to rely on Range RPC paging but that is not providing consistent view in time as well as is rather unwieldy.

Motivation

• To provide point in time view over the large dataset.

Design

• Introduce new RPC to regatta.proto

service KV {
  // Cursor gets the keys in the range from the key-value store.
  rpc Cursor(RangeRequest) returns (stream RangeResponse);
}

• On client call server creates iterator according to RangeRequest
• Server batches responses in returned stream from the iterator until iterator exhaustion.
• Batch size should be "reasonable" (needs to be clarified).
• RangeResponse should set more to true unless last (or only) message in the stream.

Alternatives

• Use of regular Range queries in series - Unfeasible due to consistency reasons, every query would be executed on potentially different state of the database.

Unresolved Questions

• Name of the RPC, the proposal is Cursor other alternatives might be RangeStream, Stream, Query etc.
• Default batch (stream response message) size, should be probably lower than max response size 4MB, prosed value is 512KB as tradeoff between latency and throughput.