SERVER-71825 Add splitChangeEvent function and new 'fragmentNum' ResumeToken field

1 files changed, 122 insertions, 0 deletions

diff --git a/src/mongo/db/pipeline/change_stream_split_event_helpers.cpp b/src/mongo/db/pipeline/change_stream_split_event_helpers.cpp
new file mode 100644
index 00000000000..c39ae90e14e
--- /dev/null
+++ b/src/mongo/db/pipeline/change_stream_split_event_helpers.cpp
@@ -0,0 +1,122 @@
+/**
+ *    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/document_source_change_stream_ensure_resume_token_present.h"
+#include "mongo/db/pipeline/document_source_change_stream_handle_topology_change.h"
+
+namespace mongo {
+namespace change_stream_split_event {
+
+size_t getBsonSizeWithMetaData(const Document& doc) {
+    // TODO SERVER-72102: Calculate document BSON size without serializing the object.
+    return static_cast<size_t>(doc.toBsonWithMetaData().objsize());
+}
+
+size_t getFieldBsonSize(const Document& doc, const StringData& key) {
+    // TODO SERVER-72102: Calculate field BSON size without serializing the object.
+    return static_cast<size_t>(doc.toBson().getField(key).size());
+}
+
+std::list<Document> splitChangeEvent(const Document& event,
+                                     size_t maxFragmentBsonSize,
+                                     size_t skipFirstFragments) {
+    // Construct an 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, StringData>, Value> sortedFieldMap;
+    for (auto it = event.fieldIterator(); it.more();) {
+        auto&& [key, value] = it.next();
+        if (key != kIdField) {
+            sortedFieldMap.emplace(std::make_pair(getFieldBsonSize(event, key), 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 = getBsonSizeWithMetaData(fragment.peek());
+
+        // 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);
+
+        // TODO SERVER-71828: make sure we don't hit this spuriously for large documents.
+        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::list<Document> outputFragments;
+    for (auto [it, i] = std::make_pair(fragments.begin(), 0UL); it != fragments.end(); ++it, ++i) {
+        // Do not insert first 'skipFirstFragments' into the output.
+        if (i >= skipFirstFragments) {
+            it->setNestedField(totalFragmentsFieldPath, totalFragments);
+            outputFragments.push_back(it->freeze());
+        }
+    }
+
+    return outputFragments;
+}
+
+}  // namespace change_stream_split_event
+}  // namespace mongo