Enabled with the tablets_rack_aware_view_pairing cluster feature
rack-aware pairing pairs base to view replicas that are in the
same dc and rack, using their ordinality in the replica map
We distinguish between 2 cases:
- Simple rack-aware pairing: when the replication factor in the dc
is a multiple of the number of racks and the minimum number of nodes
per rack in the dc is greater than or equal to rf / nr_racks.
In this case (that includes the single rack case), all racks would
have the same number of replicas, so we first filter all replicas
by dc and rack, retaining their ordinality in the process, and
finally, we pair between the base replicas and view replicas,
that are in the same rack, using their original order in the
tablet-map replica set.
For example, nr_racks=2, rf=4:
base_replicas = { N00, N01, N10, N11 }
view_replicas = { N11, N12, N01, N02 }
pairing would be: { N00, N01 }, { N01, N02 }, { N10, N11 }, { N11, N12 }
Note that we don't optimize for self-pairing if it breaks pairing ordinality.
- Complex rack-aware pairing: when the replication factor is not
a multiple of nr_racks. In this case, we attempt best-match
pairing in all racks, using the minimum number of base or view replicas
in each rack (given their global ordinality), while pairing all the other
replicas, across racks, sorted by their ordinality.
For example, nr_racks=4, rf=3:
base_replicas = { N00, N10, N20 }
view_replicas = { N11, N21, N31 }
pairing would be: { N00, N31 }\*, { N10, N11 }, { N20, N21 }
\* cross-rack pair
If we'd simply stable-sort both base and view replicas by rack,
we might end up with much worse pairing across racks:
{ N00, N11 }\*, { N10, N21 }\*, { N20, N31 }\*
\* cross-rack pair
Fixesscylladb/scylladb#17147
* This is an improvement so no backport is required
Closesscylladb/scylladb#21453
* github.com:scylladb/scylladb:
network_topology_strategy_test: add tablets rack_aware_view_pairing tests
view: get_view_natural_endpoint: implement rack-aware pairing for tablets
view: get_view_natural_endpoint: handle case when there are too few view replicas
view: get_view_natural_endpoint: track replica locator::nodes
locator: topology: consult local_dc_rack if node not found by host_id
locator: node: add dc and rack getters
feature_service: add tablet_rack_aware_view_pairing feature
view: get_view_natural_endpoint: refactor predicate function
view: get_view_natural_endpoint: clarify documentation
view: mutate_MV: optimize remote_endpoints filtering check
view: mutate_MV: lookup base and view erms synchronously
view: mutate_MV: calculate keyspace-dependent flags once
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