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/**
* Copyright (C) 2018-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 "mongo/platform/basic.h"
#include <boost/intrusive_ptr.hpp>
#include <boost/multi_index/hashed_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/multi_index/sequenced_index.hpp>
#include <boost/multi_index_container.hpp>
#include <boost/optional.hpp>
#include <functional>
#include <vector>
#include "mongo/base/string_data_comparator_interface.h"
#include "mongo/db/exec/document_value/document.h"
#include "mongo/db/exec/document_value/value.h"
#include "mongo/db/exec/document_value/value_comparator.h"
namespace mongo {
using boost::multi_index_container;
using boost::multi_index::hashed_unique;
using boost::multi_index::indexed_by;
using boost::multi_index::member;
using boost::multi_index::sequenced;
/**
* A least-recently-used cache from key to a vector of values. It does not implement any default
* size limit, but includes the ability to evict down to both a specific number of elements, and
* down to a specific amount of memory. Memory usage includes only the size of the elements in the
* cache at the time of insertion, not the overhead incurred by the data structures in use.
*/
class LookupSetCache {
public:
struct Cached {
Value key;
std::vector<Document> docs;
// This includes the size of both key and the documents.
size_t approxCacheEntrySize = 0;
};
// boost::multi_index_container provides a system for implementing a cache. Here, we create
// a container of 'Cached', that is both sequenced, and has a unique index on the 'Cached::key'.
// From this, we are able to evict the least-recently-used member, and maintain key uniqueness.
using IndexedContainer =
multi_index_container<Cached,
indexed_by<sequenced<>,
hashed_unique<member<Cached, Value, &Cached::key>,
ValueComparator::Hasher,
ValueComparator::EqualTo>>>;
/**
* Constructs the underlying cache data structure in such a way that respects the
* ValueComparator. This requires instantiating the multi_index_container with comparison and
* hasher functions obtained from the comparator.
*/
explicit LookupSetCache(const ValueComparator& comparator)
: _container(boost::make_tuple(IndexedContainer::nth_index<0>::type::ctor_args(),
boost::make_tuple(0,
member<Cached, Value, &Cached::key>(),
comparator.getHasher(),
comparator.getEqualTo()))) {}
/**
* Insert "value" into the set with key "key". If "key" is already present in the cache, move it
* to the middle of the cache. Otherwise, insert a new key in the middle of the cache.
*
* Note: In this case, "middle" refers to the sequence of the cache, where "first" is the item
* most recently used, and "last" is the item least recently used.
*
* We insert and update in the middle because when a key is updated, we can't assume that it's
* important to keep in the cache (i.e., that we should put it at the front), but it's also
* likely we don't want to evict it (i.e., we want to make sure it isn't at the back).
*/
void insert(Value key, Document doc) {
// Get an iterator to the middle of the container.
size_t middle = size() / 2;
auto it = _container.begin();
std::advance(it, middle);
const auto keySize = key.getApproximateSize();
auto cacheEntrySizeIncreaseBy = doc.getApproximateSize();
// Find the cache entry, or create one if it doesn't exist yet.
auto insertionResult = _container.insert(it, {std::move(key), {}, 0});
if (insertionResult.second) {
cacheEntrySizeIncreaseBy += keySize;
} else {
// We did not insert due to a duplicate key. Update the cached doc, moving it to the
// middle of the cache.
_container.relocate(it, insertionResult.first);
}
// Add the doc to the cache entry.
_container.modify(insertionResult.first, [&doc, cacheEntrySizeIncreaseBy](Cached& entry) {
entry.docs.push_back(std::move(doc));
entry.approxCacheEntrySize += cacheEntrySizeIncreaseBy;
});
_memoryUsage += cacheEntrySizeIncreaseBy;
}
/**
* Evict the least-recently-used item.
*/
void evictOne() {
if (_container.empty()) {
return;
}
const auto cacheEntrySize = _container.back().approxCacheEntrySize;
tassert(5429600,
str::stream() << "The cache entry size: " << cacheEntrySize
<< ", cannot be larger than entire cache size: " << _memoryUsage,
cacheEntrySize <= _memoryUsage);
_memoryUsage -= cacheEntrySize;
_container.erase(std::prev(_container.end()));
}
/**
* Evicts from the cache until there are 'num' items remaining.
*/
void evictUntilSize(size_t num) {
while (size() > num) {
evictOne();
}
}
/**
* Returns the number of elements in the cache.
*/
size_t size() const {
return _container.size();
}
/**
* Evict items in LRU order until the cache's size is less than or equal to "maximum".
*/
void evictDownTo(size_t maximum) {
while (_memoryUsage > maximum && !_container.empty()) {
evictOne();
}
}
/**
* Clear the cache, resetting the memory usage.
*/
void clear() {
_container.clear();
_memoryUsage = 0;
}
/**
* Retrieve the vector of values with key "key". Returns nullptr if not found.
*/
const std::vector<Document>* operator[](const Value& key) {
auto it = boost::multi_index::get<1>(_container).find(key);
if (it != boost::multi_index::get<1>(_container).end()) {
boost::multi_index::get<0>(_container)
.relocate(boost::multi_index::get<0>(_container).begin(),
boost::multi_index::project<0>(_container, it));
return &it->docs;
}
return nullptr;
}
auto getMemoryUsage() const {
return _memoryUsage;
}
private:
IndexedContainer _container;
size_t _memoryUsage = 0;
};
} // namespace mongo
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