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/**
* Copyright (C) 2019-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.
*/
#pragma once
#include <functional>
#include <vector>
#include "mongo/client/read_preference.h"
#include "mongo/client/sdam/sdam_configuration.h"
#include "mongo/client/sdam/sdam_datatypes.h"
#include "mongo/client/sdam/server_description.h"
#include "mongo/client/sdam/topology_description.h"
#include "mongo/platform/random.h"
namespace mongo::sdam {
/**
* This is the interface that allows one to select a server to satisfy a DB operation given a
* TopologyDescription and a ReadPreferenceSetting.
*/
class ServerSelector {
public:
/**
* Finds a list of candidate servers according to the ReadPreferenceSetting.
*/
virtual boost::optional<std::vector<ServerDescriptionPtr>> selectServers(
TopologyDescriptionPtr topologyDescription, const ReadPreferenceSetting& criteria) = 0;
/**
* Select a single server according to the ReadPreference and latency of the
* ServerDescription(s). The server is selected randomly from those that match the criteria.
*/
virtual boost::optional<ServerDescriptionPtr> selectServer(
const TopologyDescriptionPtr topologyDescription,
const ReadPreferenceSetting& criteria) = 0;
virtual ~ServerSelector();
};
using ServerSelectorPtr = std::unique_ptr<ServerSelector>;
class SdamServerSelector : public ServerSelector {
public:
explicit SdamServerSelector(const ServerSelectionConfiguration& config);
boost::optional<std::vector<ServerDescriptionPtr>> selectServers(
const TopologyDescriptionPtr topologyDescription,
const ReadPreferenceSetting& criteria) override;
boost::optional<ServerDescriptionPtr> selectServer(
const TopologyDescriptionPtr topologyDescription,
const ReadPreferenceSetting& criteria) override;
// remove servers that do not match the TagSet
void filterTags(std::vector<ServerDescriptionPtr>* servers, const TagSet& tagSet);
private:
void _getCandidateServers(std::vector<ServerDescriptionPtr>* result,
const TopologyDescriptionPtr topologyDescription,
const ReadPreferenceSetting& criteria);
bool _containsAllTags(ServerDescriptionPtr server, const BSONObj& tags);
ServerDescriptionPtr _randomSelect(const std::vector<ServerDescriptionPtr>& servers) const;
// staleness for a ServerDescription is defined here:
// https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#maxstalenessseconds
Milliseconds _calculateStaleness(const TopologyDescriptionPtr& topologyDescription,
const ServerDescriptionPtr& serverDescription) {
if (serverDescription->getType() != ServerType::kRSSecondary)
return Milliseconds(0);
const Date_t lastWriteDate = serverDescription->getLastWriteDate()
? *serverDescription->getLastWriteDate()
: Date_t::min();
if (topologyDescription->getType() == TopologyType::kReplicaSetWithPrimary) {
// (S.lastUpdateTime - S.lastWriteDate) - (P.lastUpdateTime - P.lastWriteDate) +
// heartbeatFrequencyMS
// topologyType == kReplicaSetWithPrimary implies the validity of the primary server
// description.
invariant(topologyDescription->getPrimary());
const auto& primaryDescription = *topologyDescription->getPrimary();
const auto primaryLastWriteDate = primaryDescription->getLastWriteDate()
? *primaryDescription->getLastWriteDate()
: Date_t::min();
auto result = (serverDescription->getLastUpdateTime() - lastWriteDate) -
(primaryDescription->getLastUpdateTime() - primaryLastWriteDate) +
_config.getHeartBeatFrequencyMs();
return duration_cast<Milliseconds>(result);
} else if (topologyDescription->getType() == TopologyType::kReplicaSetNoPrimary) {
// SMax.lastWriteDate - S.lastWriteDate + heartbeatFrequencyMS
Date_t maxLastWriteDate = Date_t::min();
// identify secondary with max last write date.
for (const auto& s : topologyDescription->getServers()) {
if (s->getType() != ServerType::kRSSecondary)
continue;
const auto sLastWriteDate =
s->getLastWriteDate() ? *s->getLastWriteDate() : Date_t::min();
if (sLastWriteDate > maxLastWriteDate) {
maxLastWriteDate = sLastWriteDate;
}
}
auto result = (maxLastWriteDate - lastWriteDate) + _config.getHeartBeatFrequencyMs();
return duration_cast<Milliseconds>(result);
} else {
// Not a replica set
return Milliseconds(0);
}
}
bool recencyFilter(const ReadPreferenceSetting& readPref, const ServerDescriptionPtr& s);
// A SelectionFilter is a higher order function used to filter out servers from the current
// Topology. It's return value is used as input to the TopologyDescription::findServers
// function, and is a function that takes a ServerDescriptionPtr and returns a bool indicating
// whether to keep this server or not based on the ReadPreference, server type, and recency
// metrics of the server.
using SelectionFilter = unique_function<std::function<bool(const ServerDescriptionPtr&)>(
const ReadPreferenceSetting&)>;
const SelectionFilter secondaryFilter = [this](const ReadPreferenceSetting& readPref) {
return [&](const ServerDescriptionPtr& s) {
return (s->getType() == ServerType::kRSSecondary) && recencyFilter(readPref, s);
};
};
const SelectionFilter primaryFilter = [this](const ReadPreferenceSetting& readPref) {
return [&](const ServerDescriptionPtr& s) {
return (s->getType() == ServerType::kRSPrimary) && recencyFilter(readPref, s);
};
};
const SelectionFilter nearestFilter = [this](const ReadPreferenceSetting& readPref) {
return [&](const ServerDescriptionPtr& s) {
return (s->getType() == ServerType::kRSPrimary ||
s->getType() == ServerType::kRSSecondary) &&
recencyFilter(readPref, s);
};
};
ServerSelectionConfiguration _config;
mutable PseudoRandom _random;
};
// This is used to filter out servers based on their current latency measurements.
struct LatencyWindow {
const IsMasterRTT lower;
const IsMasterRTT upper;
explicit LatencyWindow(const IsMasterRTT lowerBound, const IsMasterRTT windowWidth)
: lower(lowerBound), upper(lowerBound + windowWidth) {}
bool isWithinWindow(IsMasterRTT latency);
// remove servers not in the latency window in-place.
void filterServers(std::vector<ServerDescriptionPtr>* servers);
static bool rttCompareFn(const ServerDescriptionPtr& a, const ServerDescriptionPtr& b) {
return a->getRtt() < b->getRtt();
}
};
} // namespace mongo::sdam
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