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raft: retain existing voters across data centers and racks

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Fix an issue in the voter calculator where existing voters were not
retained across data centers and racks in certain scenarios. This
occurred when voters were distributed across more data centers and racks
than the maximum allowed number of voters.

Previously, the prioritization logic for data centers and racks did not
consider the number of existing assigned voters. It only prioritized
nodes within a single data center or rack, which could result in
unnecessary reassignment of voters.

Improved the prioritization logic to account for the number of existing
voters in each data center and rack.

This change ensures a more stable voter distribution and reduces
unnecessary voter reassignments.

Fixes: scylladb/scylladb#23950
This commit is contained in:
Emil Maskovsky
2025-04-28 19:19:11 +02:00
parent 018fb63305
commit 2ae59e8a87
2 changed files with 136 additions and 13 deletions
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70
test/boost/group0_voter_calculator_test.cc
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@@ -869,4 +869,74 @@ BOOST_AUTO_TEST_CASE(dcs_preferred_over_racks) {
}
BOOST_AUTO_TEST_CASE(existing_voters_are_retained_across_dcs) {
// Arrange: Set the voters limit and create the voter calculator.
constexpr size_t max_voters = 3;
const service::group0_voter_calculator voter_calc{max_voters};
// Act: Add the nodes (3 of them are voters).
const std::array ids = {raft::server_id::create_random_id(), raft::server_id::create_random_id(), raft::server_id::create_random_id(),
raft::server_id::create_random_id(), raft::server_id::create_random_id()};
const service::group0_voter_calculator::nodes_list_t nodes = {
{ids[0], {.datacenter = "dc-1", .rack = "rack", .is_voter = true, .is_alive = true}},
{ids[1], {.datacenter = "dc-2", .rack = "rack", .is_voter = false, .is_alive = true}},
{ids[2], {.datacenter = "dc-3", .rack = "rack", .is_voter = true, .is_alive = true}},
{ids[3], {.datacenter = "dc-4", .rack = "rack", .is_voter = false, .is_alive = true}},
{ids[4], {.datacenter = "dc-5", .rack = "rack", .is_voter = true, .is_alive = true}},
};
const auto& voters = voter_calc.distribute_voters(nodes);
// Assert: Existing voters are retained in the voter set.
BOOST_CHECK_EQUAL(voters.size(), max_voters);
for (const auto& [id, node] : nodes | std::views::filter([](const auto& node) {
return node.second.is_voter;
})) {
BOOST_CHECK(voters.contains(id));
}
}
BOOST_AUTO_TEST_CASE(existing_voters_are_kept_across_racks) {
// Arrange: Set the voters limit and create the voter calculator.
constexpr size_t max_voters = 3;
const service::group0_voter_calculator voter_calc{max_voters};
// Act: Add the nodes (3 of them are voters).
const std::array ids = {raft::server_id::create_random_id(), raft::server_id::create_random_id(), raft::server_id::create_random_id(),
raft::server_id::create_random_id(), raft::server_id::create_random_id()};
const service::group0_voter_calculator::nodes_list_t nodes = {
{ids[0], {.datacenter = "dc", .rack = "rack-1", .is_voter = true, .is_alive = true}},
{ids[1], {.datacenter = "dc", .rack = "rack-2", .is_voter = false, .is_alive = true}},
{ids[2], {.datacenter = "dc", .rack = "rack-3", .is_voter = true, .is_alive = true}},
{ids[3], {.datacenter = "dc", .rack = "rack-4", .is_voter = false, .is_alive = true}},
{ids[4], {.datacenter = "dc", .rack = "rack-5", .is_voter = true, .is_alive = true}},
};
const auto& voters = voter_calc.distribute_voters(nodes);
// Assert: Existing voters are retained in the voter set.
BOOST_CHECK_EQUAL(voters.size(), max_voters);
for (const auto& [id, node] : nodes | std::views::filter([](const auto& node) {
return node.second.is_voter;
})) {
BOOST_CHECK(voters.contains(id));
}
}
BOOST_AUTO_TEST_SUITE_END()