ZhaoYang created CASSANDRA-15774:
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Summary: Improve range reads to query by endpoints instead of
vnodes to reduce number of remote requests
Key: CASSANDRA-15774
URL: https://issues.apache.org/jira/browse/CASSANDRA-15774
Project: Cassandra
Issue Type: Improvement
Components: Legacy/Coordination
Reporter: ZhaoYang
Currently, range read queries in batches, see
{{StorageProxy.RangeCommandIterator#sendNextRequests()}}. For each batch, it
computes a list of merged vnode ranges up to concurrency factor and query each
merged vnode range asynchronously. (note: consecutive vnode ranges can be
merged if they share enough replicas to satisfy consistency level requirement)
This works fine in general, but when concurrency factor is high because
returned row count is small comparing to query limit or index filtering is
used, coordinator may send too many concurrent remote range requests in a batch.
We can improve it by grouping remote range requests by endpoints where each
endpoint will return response corresponding to multiple non-consecutive ranges.
With endpoint grouping, number of remote range requests should largely reduced
and it's always capped by number of nodes in the cluster instead of number of
ranges which is capped by concurrency factor.
Let's look at an example on a 5-node cluster with 10
ranges(a,b,c,d,e,f,g,h,i,h) and rf3.
Following is the range to replica mapping using round robin that should work
well with consecutive range merger (consecutive range merger doesn't work well
with fully random replica mapping, because it's less likely to have overlapping
replicas for consecutive ranges)
{code:java}
range-a replicas: 1, 2, 3
range-b replicas: 2, 3, 4
range-c replicas: 3, 4, 5
range-d replicas: 1, 4, 5
range-e replicas: 1, 2, 5
range-f replicas: 1, 2, 3
range-g replicas: 2, 3, 4
range-h replicas: 3, 4, 5
range-i replicas: 1, 4, 5
range-j replicas: 1, 2, 5
{code}
With default range read implementation and consecutive range merger, we need 10
replica read requests(2 for each merged range) for quorum:
{code:java}
range (a,b] on node [2, 3]
range (c,d] on node [4, 5]
range (e,f] on node [1, 2]
range (g,h] on node [3, 4]
range (i,j] on node [1, 5]
{code}
With group query by endpoints, we only need 4 replica read requests for quorum:
{code:java}
* node 1: a, d, e, f, i, j
* node 2: a, b, e, f, g, j
* node 3: b, c, g, h
* node 4: c, d, h, i
{code}
Note that there are some complexities around short-read protection which needs
to know whether replica has more rows available for current range.
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