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rpc: make blocking queries for non-existent items more efficient #12110

Merged
merged 3 commits into from
Feb 17, 2022
Merged

rpc: make blocking queries for non-existent items more efficient #12110

merged 3 commits into from
Feb 17, 2022

Commits on Feb 15, 2022

  1. Add a test for blocking query on non-existent entry 

    This test shows how blocking queries are not efficient when the query
returns no results.  The test fails with 100+ calls instead of the
expected 2.

This test is still a bit flaky because it depends on the timing of the
writes. It can sometimes return 3 calls.

A future commit should fix this and make blocking queries even more
optimal for not-found results.
    @dnephin
    dnephin committed Feb 15, 2022
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  2. Make blockingQuery efficient with 'not found' results. 

    By using the query results as state.

Blocking queries are efficient when the query matches some results,
because the ModifyIndex of those results, returned as queryMeta.Mindex,
will never change unless the items themselves change.

Blocking queries for non-existent items are not efficient because the
queryMeta.Index can (and often does) change when other entities are
written.

This commit reduces the churn of these queries by using a different
comparison for "has changed". Instead of using the modified index, we
use the existence of the results. If the previous result was "not found"
and the new result is still "not found", we know we can ignore the
modified index and continue to block.

This is done by setting the minQueryIndex to the returned
queryMeta.Index, which prevents the query from returning before a state
change is observed.
    @dnephin
    dnephin committed Feb 15, 2022
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  3. rpc: add errNotFound to all Get queries 

    Any query that returns a list of items is not part of this commit.
    @dnephin
    dnephin committed Feb 15, 2022
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