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/**
* Copyright (C) 2017 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* 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 GNU Affero General 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.
*/
#define MONGO_LOG_DEFAULT_COMPONENT ::mongo::logger::LogComponent::kControl
#include "mongo/platform/basic.h"
#include "mongo/db/logical_session_cache.h"
#include "mongo/util/duration.h"
#include "mongo/util/log.h"
#include "mongo/util/periodic_runner.h"
namespace mongo {
constexpr int LogicalSessionCache::kLogicalSessionCacheDefaultCapacity;
constexpr Minutes LogicalSessionCache::kLogicalSessionDefaultTimeout;
constexpr Minutes LogicalSessionCache::kLogicalSessionDefaultRefresh;
LogicalSessionCache::LogicalSessionCache(std::unique_ptr<ServiceLiason> service,
std::unique_ptr<SessionsCollection> collection,
Options options)
: _refreshInterval(options.refreshInterval),
_sessionTimeout(options.sessionTimeout),
_service(std::move(service)),
_sessionsColl(std::move(collection)),
_cache(options.capacity) {
PeriodicRunner::PeriodicJob job{[this] { _refresh(); },
duration_cast<Milliseconds>(_refreshInterval)};
_service->scheduleJob(std::move(job));
}
LogicalSessionCache::~LogicalSessionCache() {
try {
_service->join();
} catch (...) {
// If we failed to join we might still be running a background thread,
// log but swallow the error since there is no good way to recover.
severe() << "Failed to join background service thread";
}
}
StatusWith<LogicalSessionRecord::Owner> LogicalSessionCache::getOwner(LogicalSessionId lsid) {
// Search our local cache first
auto owner = getOwnerFromCache(lsid);
if (owner.isOK()) {
return owner;
}
// Cache miss, must fetch from the sessions collection.
auto res = _sessionsColl->fetchRecord(lsid);
// If we got a valid record, add it to our cache.
if (res.isOK()) {
auto& record = res.getValue();
record.setLastUse(_service->now());
auto oldRecord = _addToCache(record);
// If we had a conflicting record for this id, and they aren't the same record,
// it could mean that an interloper called endSession and startSession for the
// same lsid while we were fetching its record from the sessions collection.
// This means our session has been written over, do not allow the caller to use it.
// Note: we could find expired versions of our same record here, but they'll compare equal.
if (oldRecord && *oldRecord != record) {
return {ErrorCodes::NoSuchSession, "no matching session record found"};
}
return record.getSessionOwner();
}
return res.getStatus();
}
StatusWith<LogicalSessionRecord::Owner> LogicalSessionCache::getOwnerFromCache(
LogicalSessionId lsid) {
stdx::unique_lock<stdx::mutex> lk(_cacheMutex);
auto it = _cache.find(lsid);
if (it == _cache.end()) {
return {ErrorCodes::NoSuchSession, "no matching session record found in the cache"};
}
// Do not use records if they have expired.
auto now = _service->now();
if (_isDead(it->second, now)) {
return {ErrorCodes::NoSuchSession, "no matching session record found in the cache"};
}
// Update the last use time before returning.
it->second.setLastUse(now);
return it->second.getSessionOwner();
}
Status LogicalSessionCache::startSession(LogicalSessionRecord authoritativeRecord) {
// Make sure the timestamp makes sense
auto now = _service->now();
authoritativeRecord.setLastUse(now);
// Attempt to insert into the sessions collection first. This collection enforces
// unique session ids, so it will act as concurrency control for us.
auto res = _sessionsColl->insertRecord(authoritativeRecord);
if (!res.isOK()) {
return res;
}
// Add the new record to our local cache. If we get a conflict here, and the
// conflicting record is not dead and is not equal to our record, an interloper
// may have ended this session and then created a new one with the same id.
// In this case, return a failure.
auto oldRecord = _addToCache(authoritativeRecord);
if (oldRecord) {
if (*oldRecord != authoritativeRecord) {
if (!_isDead(*oldRecord, now)) {
return {ErrorCodes::DuplicateSession, "session with this id already exists"};
}
}
}
return Status::OK();
}
void LogicalSessionCache::_refresh() {
SessionList activeSessions;
SessionList deadSessions;
auto now = _service->now();
// We should avoid situations where we have records in the cache
// that have been expired from the sessions collection. If they haven't been
// used in _sessionTimeout, we should just remove them.
// Assemble a list of active session records in our cache
std::vector<decltype(_cache)::ListEntry> cacheCopy;
{
stdx::unique_lock<stdx::mutex> lk(_cacheMutex);
cacheCopy.assign(_cache.begin(), _cache.end());
}
for (auto& it : cacheCopy) {
auto record = it.second;
if (!_isDead(record, now)) {
activeSessions.push_back(record.getLsid());
} else {
deadSessions.push_back(record.getLsid());
}
}
// Append any active sessions from the service. We should always have
// cache entries for active sessions. If we don't, then it is a sign that
// the cache needs to be larger, because active session records are being
// evicted.
// Promote our cached entries for all active service sessions to be recently-
// used, and update their lastUse dates so we don't lose them to eviction. We
// do not need to do this with records from our own cache, which are being used
// regularly. Sessions for long-running queries, however, must be kept alive
// by us here.
auto serviceSessions = _service->getActiveSessions();
{
stdx::unique_lock<stdx::mutex> lk(_cacheMutex);
for (auto lsid : serviceSessions) {
auto it = _cache.promote(lsid);
if (it != _cache.end()) {
// If we have not found our record, it may have been removed
// by another thread.
it->second.setLastUse(now);
}
}
}
activeSessions.splice(activeSessions.begin(), serviceSessions);
// Query into the sessions collection to do the refresh. If any sessions have
// failed to refresh, it means their authoritative records were removed, and
// we should remove such records from our cache as well.
auto failedToRefresh = _sessionsColl->refreshSessions(std::move(activeSessions));
deadSessions.splice(deadSessions.begin(), failedToRefresh);
// Prune any dead records out of the cache. Dead records are ones that failed to
// refresh, or ones that have expired locally. We don't make an effort to check
// if the locally-expired records still have live authoritative records in the
// sessions collection. We also don't attempt to resurrect our expired records.
{
stdx::unique_lock<stdx::mutex> lk(_cacheMutex);
for (auto deadId : deadSessions) {
_cache.erase(deadId);
}
}
}
bool LogicalSessionCache::_isDead(const LogicalSessionRecord& record, Date_t now) const {
return record.getLastUse() + _sessionTimeout < now;
}
boost::optional<LogicalSessionRecord> LogicalSessionCache::_addToCache(
LogicalSessionRecord record) {
stdx::unique_lock<stdx::mutex> lk(_cacheMutex);
return _cache.add(record.getLsid(), std::move(record));
}
} // namespace mongo
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