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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#include <cmath>
#include <cstdlib>
#include "mongo/db/mirroring_sampler.h"
namespace mongo {
std::vector<HostAndPort> MirroringSampler::getMirroringTargets(
std::shared_ptr<const repl::IsMasterResponse> isMaster,
const double ratio,
RandomFunc rnd,
const int rndMax) noexcept {
invariant(ratio >= 0 && ratio <= 1);
invariant(isMaster);
if (!isMaster->isMaster()) {
return {};
}
/**
* `ratio == 0` disables mirroring
* Also, mirroring requires at least one active secondary.
*/
if (ratio == 0 || isMaster->getHosts().size() < 2) {
return {};
}
/**
* The goal is to mirror every request to approximately `ratio x secondariesCount`.
*/
const auto secondariesCount = isMaster->getHosts().size() - 1;
const auto secondariesRatio = ratio * secondariesCount;
// Mirroring factor is the number of secondaries that will receive the command.
auto mirroringFactor = std::ceil(secondariesRatio);
invariant(mirroringFactor > 0 && mirroringFactor <= secondariesCount);
size_t randVar = rnd();
const auto normalizedRatio = static_cast<size_t>(secondariesRatio * rndMax / mirroringFactor);
if (randVar > normalizedRatio) {
return {};
}
auto getEligibleSecondaries = [](auto isMaster) noexcept->std::vector<HostAndPort> {
auto self = isMaster->getPrimary();
auto hosts = isMaster->getHosts();
invariant(hosts.size() > 1);
std::vector<HostAndPort> potentialTargets;
for (auto host : hosts) {
if (host != self) {
potentialTargets.push_back(host);
}
}
return potentialTargets;
};
auto eligibleSecondaries = getEligibleSecondaries(isMaster);
invariant(!eligibleSecondaries.empty());
std::mt19937 twisterEngine(randVar);
std::shuffle(eligibleSecondaries.begin(), eligibleSecondaries.end(), twisterEngine);
auto it = eligibleSecondaries.begin();
return std::vector<HostAndPort>(it, it + mirroringFactor);
}
} // namespace mongo
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