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/**
* Copyright (C) 2023-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 "mongo/db/pipeline/change_stream_split_event_helpers.h"
#include "mongo/db/pipeline/field_path.h"
#include "mongo/db/pipeline/resume_token.h"
namespace mongo {
namespace change_stream_split_event {
std::pair<Document, size_t> processChangeEventBeforeSplit(const Document& event,
bool withMetadata) {
if (withMetadata) {
auto eventBson = event.toBsonWithMetaData<BSONObj::LargeSizeTrait>();
return {Document::fromBsonWithMetaData(eventBson), eventBson.objsize()};
} else {
// Serialize just the user data, and add the metadata fields separately.
auto eventBson = event.toBson<BSONObj::LargeSizeTrait>();
MutableDocument mutDoc(Document{eventBson});
mutDoc.copyMetaDataFrom(event);
return {mutDoc.freeze(), eventBson.objsize()};
}
}
std::queue<Document> splitChangeEvent(const Document& event,
size_t maxFragmentBsonSize,
size_t skipFirstFragments) {
// Extract the underlying BSON. We expect the event to be trivially convertible either with
// or without metadata, so we attempt to optimize the serialization here.
auto eventBson =
(event.isTriviallyConvertible() ? event.toBson<BSONObj::LargeSizeTrait>()
: event.toBsonWithMetaData<BSONObj::LargeSizeTrait>());
// Construct a sorted map of fields ordered by size and key for a deterministic greedy strategy
// to minimize the total number of fragments (the first fragment contains as many fields as
// possible). Don't include the original '_id' field, since each fragment will have its own.
std::map<std::pair<size_t, std::string>, Value> sortedFieldMap;
for (auto it = event.fieldIterator(); it.more();) {
auto&& [key, value] = it.next();
if (key != kIdField) {
sortedFieldMap.emplace(std::make_pair(eventBson[key].size(), key), value);
}
}
uassert(7182502,
"Cannot split an empty event or an event containing solely '_id' field",
!sortedFieldMap.empty());
auto resumeTokenData =
ResumeToken::parse(event.metadata().getSortKey().getDocument()).getData();
std::list<MutableDocument> fragments;
for (auto it = sortedFieldMap.cbegin(); it != sortedFieldMap.cend();) {
// Update the resume token with the index of the fragment we're about to add.
resumeTokenData.fragmentNum = fragments.size();
// Add a new fragment at the end of the fragments list.
auto& fragment = fragments.emplace_back();
// Add fields required by all fragments.
ResumeToken token(resumeTokenData);
fragment.metadata().setSortKey(Value(token.toDocument()), true);
fragment.addField(kIdField, fragment.metadata().getSortKey());
fragment.addField(kSplitEventField,
Value(Document{{kFragmentNumberField, static_cast<int>(fragments.size())},
{kTotalFragmentsField, 0}}));
auto fragmentBsonSize = static_cast<size_t>(fragment.peek().toBsonWithMetaData().objsize());
// Fill the fragment with as many fields as we can until we run out or exceed max size.
// Always make sure we add at least one new field on each iteration.
do {
fragment.addField(it->first.second /* field name */, it->second /* field value */);
fragmentBsonSize += it->first.first /* field size */;
} while (++it != sortedFieldMap.cend() &&
fragmentBsonSize + it->first.first /* field size */ <= maxFragmentBsonSize);
uassert(7182500,
str::stream() << "Splitting change event failed: fragment size " << fragmentBsonSize
<< " is greater than maximum allowed fragment size "
<< maxFragmentBsonSize,
fragmentBsonSize <= maxFragmentBsonSize);
}
// Iterate over the fragments to populate the 'kTotalFragmentsField' field and freeze the final
// events.
const auto totalFragments = Value(static_cast<int>(fragments.size()));
const auto totalFragmentsFieldPath =
FieldPath::getFullyQualifiedPath(kSplitEventField, kTotalFragmentsField);
std::queue<Document> outputFragments;
for (auto [it, i] = std::make_pair(fragments.begin(), 0ULL); it != fragments.end(); ++it, ++i) {
// Do not insert first 'skipFirstFragments' into the output.
if (i >= skipFirstFragments) {
it->setNestedField(totalFragmentsFieldPath, totalFragments);
outputFragments.push(it->freeze());
}
}
return outputFragments;
}
} // namespace change_stream_split_event
} // namespace mongo
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